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Analytical and Synthetic Studies on Anodic Alkoxylation of Homo Aromatic Compounds 1

Ph. D. Thesis – D. Abirami

CHAPTER I

INTRODUCTION

1.1 HISTORICAL PERSPECTIVE

Actually, Organic electrochemistry was among the first general

techniques to be employed at a time when organic chemistry itself was

in its infancy. The foundation was laid down by Faraday and Nernst in

the nineteenth century. Historically, synthetic electro organic chemistry

made its presence felt in 1801, when anodic oxidation of alcohol was

reported 1. Several works have been compiled, detailing many

electroorganic reactions which were considered as unique reactions,

identified as proceeding from ion radical intermediates 2-5. Subsequently,

the importance of electrode potential in controlling the course of

electrolytic reaction was expounded by Haber 6 in 1898. The concept of

controlled potential electrolysis has had a great impact on modern

electroorganic synthesis. But the fact that electro organic synthetic

technique could be an important tool in the hands of synthetic organic

chemists was made to realize when Kolbe proposed the mechanism of

electrolysis of Carboxylate ion, which envisaged the generation of



radicals under electrolytic conditions 7-10. Subsequently considerable

data had been accumulated by 1940 as documented by Fitchter in his

excellent monograph 11. It is easily in the last few decades that electro

organic chemistry has assumed a character distinct from usual

electrochemistry, which occupies a well defined position in inorganic

chemistry. During 1955 – 65, great efforts were made to introduce

electrochemical concepts into synthetic organic chemistry.

It is increasingly apparent that the electrochemical method offers

the most convenient general technique for generating ion radicals. The

study of these interesting species is fast becoming a new frontier for

organic chemists 12. However majority of organic chemists were still

unaware of its potentiality. Only a few pioneering synthetic chemists took

full advantage of the novel and versatile methods of electrochemistry.

Much has been learnt about the reaction pathways of cation radicals,

carbonium ions or uncharged radicals formed at the anode and anion

radicals, carbanions or uncharged radicals formed at the cathode,

through product isolation, polarography 13,14, coupled electro analytical

and spectroscopic techniques 15 and a host of other methods.

Apart from the advances in the field of electrosyntheses made in

1970’s, various other novel concepts and methodologies for organic

synthesis were developed during this period. These include the concept

”dipole inversion” which is of vital importance and has been widely

accepted 16. For example, it is possible to generate a cationic species in

a basic medium or anionic species in an acidic medium and to obtain



nucleophilic attack at the -position of a carbonyl group. In such ways,

the potentiality of electrosyntheses can be expected to have a profound

impact on research in organic chemistry in the 1980’s.

Cell design and scale-up including dimensionally stable anodes,

electroanalytical studies to elucidate the mechanisms of electroorganic

reactions, the effect of absorption and diffusion controls on the electrode

reactions, functionalization of organics, electro generation of unusual

valence states, electro initiated polymerization, electro bio-chemical

processes and environmental control by electrochemical methods have

subsequently gained increasing attention by electroorganic researchers.

Electrosynthetic reactions began to attract much attention among

synthetic chemists due to their high-energy efficiency and cleanliness

with its wide applications spread into all fields of organic chemistry 17.

Strong emphasis on the elucidation of product compositions being

replaced to some extent by greater investment in designing more

acceptable electrolysis systems and optimizing the electrolysis

conditions to obtain high yields of the desired products. In this sense,

electroorganic synthesis in the 1970’s was able to emerge from its

infancy and be more fully assimilated into routine synthetic organic

chemistry.

The development in this branch of chemical technology has been

substantial in recent years 18-25. Capillary gap cell, packed bed cell,

undivided foam cell, tubular flow cell, three component cell, etc. are

some of the developments observed in this period. In addition, paired



reaction, emulsion method and two phase electrolysis are worth

mentioning in the context of complex transformation 26-30. Technical

developments in electrochemical instrumentation 31, the use of non-

aqueous electrolytes 32 and the digital control of experiments 33, led to

the spread of electroanalytical techniques. Cyclic voltammograms are

exhaustively used to define the redox behaviors of newly synthesized

organic molecules similar to the use of spectral data for structural

characterization. Numerical simulation of the experiments 34 became

increasingly available during 1980’s. Ultra micro electrodes opened the

way not only to ever faster time scales but also to finer lateral resolution,

when characterizing electrode processes. Combinations with

spectroscopic and mass-sensitive devices opened new ways to augment

information available from molecular electrochemical experiments.

1.2 ELECTROORGANIC PROCESSES

The evolution of new types of electroorganic reactions based on

coupling and substitution reactions, cyclization and elimination reactions,

electrochemically promoted rearrangements, selective electrochemical

fluorination, electrochemical versions of the classical synthetic reactions

and exploitation of these reactions in multi-step targeted synthesis allow

the synthetic chemists to consider electrochemical methods as one of

the powerful tools of organic synthesis.

A number of text-books 4, 11, 35-44 which deal with laboratory scale

preparations for electroorganic synthesis, in a comprehensive and



exhaustive manner are available. As a matter of fact, these

electrochemical processes find a close similarity with conventional

organic synthetic methods except for the fact that current is a reactive

input in the place of Redox reagents.

1.2.1 Electrode process and its classification

Over the past 25 to 30 years, the use of electrochemistry as the

synthetic tool in organic chemistry has increased remarkably. Reductive

dimerization of acrylonitrile, hydrogenation of heterocyclics,

pinacolization, reduction of nitro aromatics, the Kolbe reaction, siemon’s

fluorination, methoxylation, epoxidation of olefins, oxidation of aromatic

hydrocarbons etc. are some of the synthetic Electroorganic processes

which have been piloted at levels ranging from a few tons up to

105 tons 45-49. There are many excellent reviews, monographs and

publications, which review the use of electrochemical methods as a tool

in laboratory scale synthesis solving R&D objectives for a multi-step

targeted synthesis and cover a broad spectrum of applications of

electrochemical methods in organic synthesis, including their use in

pharmaceutical industry 50-58.

Majority of electroorganic reactions have their chemical analogies

however, there exists a growing body of reactions, which remain unique

because of the nature of the products formed and their mode of

formation. A number of these unique reactions have been identified as

proceeding from ion radical intermediates. It is increasingly apparent that



the electrochemical method offers the most convenient general

technique for generating ion radicals 12.

In addition to direct oxidation or reduction, a number of other

chemical processes such as addition, elimination, dimerization, etc., may

also be achieved by electrochemical method [Table.1.1].

Table 1.1: Direct Electrochemical Reactions

a Electrochemical

Conversion

R - NO2 R - NHOH R - NH2

R - CH2 - OH R - CHO R - COOH

CH3 - CHO - COOH

R - CO - NH2 R - CH2 - NH2

R - CN R - CH2 - NH2

b Electrochemical

Substitution

R – E + Nu - R – Nu + E

+ + 2e

-

– H + CN - – CN + H

+ + 2e

-

R – Nu + E+ + 2e

- R – E + Nu

-

c Electrochemical Addition l l >C = C< + 2Nu

- Nu–C–C–Nu + 2e

-

l l

I i

>C = C< + 2E+ + 2e

- E–C–C–E

l l

d Electrochemical Elimination l l X–C–Y–X >C=Y + 2X

+ + 2e

-

l l

l l X–C–C–X + 2e

- >C=C< + 2X

-

l l

e Electrochemical Coupling 2 R – E R – R + 2E+ + 2e

-

2 R – Nu + 2e- R – R + 2Nu

–

l l l l

2 >C=C< +2Nu - Nu–C–C–C–C–Nu + 2e

-

l l l l

l l l l

2 >C=C< + 2E+

+ 2e- E–C–C–C–C–E

l l l l

f Electrochemical Cleavage R+ + X

. Product

RH . X RHX+.

-H+ E OS

-

RX . R-X+ ROS + X .

g Electro Polymerization



For difficultly reducible or oxidizable compounds, inorganic

mediators can be employed [Table.1.2]

Table 1.2: Indirect Electrochemical Reactions

No Redox Couple Conversions

a Ti4+

/ Ti3+

Nitro aromatics to Aniline

b Fe3+

/ Fe2+

Acrylonitrile Polymerization

c Fe(CN)6 3 -

/ Fe(CN)6 4 -

Benzene Oxidation

d MnO4 - / MnO4

2 - Oxidation of aromatics

e Ni 3 +

/ NiF6 2 -

Electroflourination

f Tl 3 +

/ Tl 1- Butene to Butanone

g Co 3 +

/ Co 2 +

Oxidation of aromatics

h Sn 4 +

/ Sn 2 +

Reduction of Nitro compounds

i Ce 2 +

/ Ce 3 +

Anthracene to Anthraquinone

j Cu 2 +

/ Cu + Hydroxylation of aromatics

k HIO4 / HIO3 Dialdehyde Starch Process

l Na-Hg / Hg Hydromerization

m OBr - / Br

- Oxidation of Sugars

Electrochemical synthesis can be seriously considered when

a. there is no known chemical procedures

b. the known chemical procedure is multi-staged and / or gives poor

yield

c. a scale up of a reaction is necessary - cathodic reduction instead of

metallic reduction, which is notoriously difficult to run in large

batches



d. an inorganic reagent is known to work well, but for scale up, the

cost of the reagent used stoichiometrically is prohibitive and

e. the present chemical process leads to pollution problem because a

spent reagent can not economically be recovered or eliminated

However the difficulties encountered in electroorganic processes

can be brushed aside. The organic electro-chemists of yester years,

were improperly equipped-both because of inadequacies in the theory

and in their instrumentation - to stimulate growth in the electroorganic

research and to achieve the still largely unexploited potential of

electrochemistry in organic synthesis. Cell design, cell voltage, electrode

materials, membrane materials, pH control, solvent, supporting

electrolyte, temperature etc., can pose problems when an

electrochemical process is scaled up.

Commencing with the extensive work of Tafel 59 on the

description of the phenomenon of irreversible electron transfer reactions

and the subsequent advanced theoretical studies of Frumkin, Temkin,

Delahay, Bockris and others, understanding of the electrochemical

aspects of irreversibility and the effect of the so called ‘double-layer’ and

adsorption, is now much deeper and on firmer ground than previously.

The development of electrochemical techniques and instrumentation,

permit the present day worker in organic electrochemistry, to study, in as

sophisticated a manner as he requires, the details of electroorganic

reaction mechanisms. More recent developments in this field are

accounted in an excellent book by Adams 14. The efforts of Wawzonek60,



Swann 5 and others 61-63

to promote and to encourage activity and

interest in organic electrochemistry are worth to be mentioned.

1.2.2 Merits and Demerits

One may list a number of advantageous features for

electrochemical route 21, 64-66. Nevertheless, the electrochemical

processes have their own misgivings.

a. Disadvantages

Electrochemical reactions are usually relatively slow. i.e., high

current densities cannot be used, when compared to typical

inorganic electrolytes or conventional homogeneous reactions.

The reputed slowness can usually be attributed to the low surface

to volume ratio of most preparative electrochemical cells.

Cell designs for synthetic usage are not standard, nor are such

apparatus generally available from commercial sources. The

worker is faced with a compromise between designing a cell with

maximum flexibility (electrode replacement, reference electrode

accessibility, cell divider, temperature control and agitations) and

one which maximizes electrode area and minimizes electrode

interspaces. The cell divider, when needed, gives rise to

experimentally awkward construction, can pose a maintenance

problem and usually results in high voltage drops. Also, the

number of available types of dividers is limited.



The requirement that the solvent be inert as well as capable of

ionizing a suitable electrolyte and dissolving the organic substrate

is restrictive, although several choices such as DMF, dioxan,

acetonitrile, ethanol, pyridine and others have been used

extensively. Choices of electrolytes in non-aqueous media are

usually also limited; tetra alkyl ammonium slats being the most

generally used in organic solvents. Also the electrolyte must be

separated from the product in the work up.

In oxidative studies, the numbers of stable anode materials are

limited, since most metals themselves get easily oxidized.

b. Advantages

Precise control of the electrode potential, and hence of product

selectivity is easily attainable when required.

The reducing or oxidizing energy and the reaction rate may be

increased or decreased to any extent by simply varying the

electrode potential. This certainly avoids the high temperature -

high pressure experimental conditions employed. Example, in the

catalytic hydrogenation route.

Electrochemical reactions do not require thermal energy to

overcome activation barriers, and hence are applicable to

thermally sensible compounds. The driving force is the electrode

potential.



Stoichiometric amounts of oxidants and reductants are not

required, and their bye products are thus avoided. This has

interesting implications, when one considers the current pressure

to avoid pollution by discarded bye products.

Some chemical oxidizing or reducing agents, although available

at cheaper rates, pose a formidable pollution problem. The zinc

and iron sludge from chemical reduction routes, for example, are

likely to face stiffer public resistance in the years to come.

Electrochemical routes either avoid the use of such reagents or

recycle them effectively in indirect electrochemical processes thus

providing a clean and pollution free alternative route.

Although the cost of most materials has increased steadily over

the years, the cost of electricity has remained remarkably stable,

and is thus becoming an ever more attractive reagent for large

scale reactions.

Electrochemical synthesis, by its very nature and by ease of

instrumentation, is eminently suitable for continuous and

automatic operations, another industrially attractive feature.

Although many oxidizing and reducing agents employed in the

chemical processes are cheaper ones, some costly reagents

such as lithium aluminium hydride are required for some specific

reductions, such as the reduction of anthranilic acid to o-

aminobenzyl alcohol. The same process may be cost effectively

achieved by electrochemical processing.



Even in chemical processes where a competitive chemical

process exists, the electrochemical process may consume less

energy when compared to the chemical ones.

The ease of quantitatively monitoring the course of the reactions

by Coulometry, using electronic or electrochemical coulometer, is

unsurpassed compared to most other general synthetic

techniques. The current itself is the measure of the rate of the

reaction.

Number of unit operations may be less in electrochemical process

when compared to a chemical process.

Some specific organic synthetic reactions can be carried out by

electrochemical means alone. Anodic fluorination and other

anodic substitution reactions and electrochemical reduction of

phthalic acid to dihydrophthalic acid are some examples of such

processes.

When a multifunctional molecule is oxidized or reduced,

electrochemical route can show selectivity which is not easy

achieve by other means. For example, in a molecule containing –

Br and –C=O groups, -Br species may be eliminated keeping –

C=O group intact 67

There are some specific occasions where very high purity of

products is required. In such occasions, the electrochemical

routes are the invariable options.



Petrochemical feed-stocks are becoming increasingly costly and

scarce. Electrochemical routes are being developed to use non-

conventional feed stocks such as carbon dioxide, coal and lignin.

Electrochemical method is the suitable one for small scale

production. Even the tonnage chemicals can be manufactured in

small quantities for captive use to reduce transportation and

inventory costs and

Desired or useful products on both the electrodes can be

produced by electrochemical method only.

The development of a unified approach for electrochemical

process development comprising of basic electrochemistry, synthetic

electrochemistry and electrochemical engineering would greatly

enhance the scope for the technology development in electroorganic

chemistry. Organic chemists would also have an opportunity to obtain a

comprehensive view of all aspects of electroorganic chemistry. Basic

studies on even the already established processes can lead to further

improvement in such processes. Some of the problems in an already

developed synthesis may also be solved by electrochemical basic

studies.

1.3 ELECTROSYNTHETIC ROUTES

Electrochemical reactions are intrinsically more complex than

typical chemical transformations. Transport of the substrate from the

bulk of the electrolyte to the electrode plays an important role. The



Ele

ctro

de

electron transfer step occurs at the interface. The product of the redox

reaction is transported back to the bulk. Purely chemical reactions may

precede or follow these steps. Specific interactions of any species

present in the electrolyte with the electrode surface leads to adsorption

which may considerable influence the overall process.

1.3.1 Transportation Modes

During an electrode process, electrons are transferred between

the substrate and the electrode.

Fig. 1.1 Modes of Transportation

Adsorption Diffusion layer Bulk

E E E’

Transport

E Electron Chemical

Transfer reactions

P

Transport P P P’

There are three modes of transportation which may occur at the

electrode [Fig. 1.1].

a. Diffusion: It is observed if the solution near the electrode is

depleted from a substrate or a product is accumulated. Diffusion

is characterized by a diffusion coefficient and extends over a



diffusion layer that develops from the electrode into the

electrolyte. The concentrations approach their bulk values as one

gets away from the electrode.

b. Migration: It occurs in the electrical field between the anode and

the cathode and contributes to the movement of charged species.

In most of the electrolytic processes, the concentration of the

supporting electrolyte ions is much higher than that of other ions.

Hence the migration of other ions is suppressed. Thus migration

becomes important at modified electrodes or in electrolytes of low

ion concentration 68.

c. Convection: The movement of the electrolyte liquid phase as a

whole either spontaneously can occur due to thermal effects and

density gradients, or by forced hydrodynamic techniques. Under

these circumstances also, a diffusion layer develops closer to the

electrode surface.

The electron transfer at the interface between electrode and electrolyte

is the criterion of an electrode reaction, electron pass through the

interface. As a result, current is observed macroscopically. The transfer

of an electron to (reduction) or from (oxidation) the substrate is an

activated process, characterized by a rate constant, defined as the

standard potential, and the transfer coefficient, reversible electron

transfer obliging Nernst equation, irreversible electron transfer in

conformation with Butler volmer equation and quasi reversible electron

transfer characterized by mixed controlled situations are the three



modes of electron transfers, depending on the experimental conditions,

in particular on the external control of mass transfer 69-71.

1.3.2 Electrogenerated Species [Intermediates]

As a result of electron transfers, unlike chemical processes

electrochemical reactions set in, involving neither bond forming nor bond

breaking steps as an initial process. On the other hand, electron transfer

from substrate at the electrode is characterized by the generation of the

reactive intermediate and subsequent reactions typical for the species.

The oxidation or reduction step initiates the follow up chemistry to the

reaction products 72. The active species may be the radical [S]. , cation

[S]+, cation radical [S]+. and dication [S]++ at the anode and radical,

anion[S]- , anion radicals[S]-. and dianion [S]-- around the cathode. In

addition to these common reaction intermediate species, those with

unusual oxidation states like metal complexes with low or high valent

central atoms are also produced at the electrodes 73-74.

Electrochemical generation of such intermediates may be

advantageous because of the mild reaction conditions employed and the

additional selectivity introduced in controlled potential experiments. Such

activated intermediates are generally derived from carboxylates,

alcoholates, phenolates, thiolates, halides, etc., by one or two electron

discharge around the electrodes.



1.3.3 Direct and Indirect Electrode Processes

Indirect electrooxidation procedures are currently the subject of

intensive study, especially for the development of innovative synthetic

methods in the industrial sector. An indirect route may be required when

the direct procedure is unsuitable because (i) the desired reaction does

not proceed sufficiently due to extremely slow reaction or very low

current efficiency (ii) the electrolysis lacks product selectivity and thus

offers only a low yield and (iii) tars and products cover the surface of the

electrodes, halting the electrolysis. Indirect electrooxidation techniques

involve the recycle use of electron carriers or mediators as a redox

system. The recycle use of a suitable redox electron carrier is otherwise

referred to as Indirect electroxidation 75-77.

The oxidation consists of two well defined processes namely

electrooxidation of the mediator to a higher oxidation state and chemical

oxidation of the organic substrate with the mediator. The technique in

which these two processes are carried out together in a single

electrolysis cell is called the “in-cell method” while the technique

involving separated electrochemical and chemical steps is called the

“ex-cell method”. Oxidizing redox carriers include a variety of metals,

non-metals and organic reduction system.

There are two types of electroorganic processes, which can be

described as direct or indirect electrode processes. In direct

electrochemical processes, a substrate [S] either gives up electrons to

the anode affording a reactive intermediate [S] +. or takes up the electron



from the cathode bringing up [S] -.. On the other hand the direct

electrode process involves electron transfer between [S] and a carrier

(mediator) which initially gets discharged at the electrode generating a

reactive oxidizing / reducing carrier capable of accepting / relieving

electrons from / to the substrate molecules in the medium. In case a

reactive intermediate such as radical ion produced around the electrode

or products formed by a follow up reaction can undergo a reverse

reaction at the opposite electrode, a two-compartment cell divided by

appropriate micro porous separators like fritted glass or porous ceramic

or ion exchange membranes are being used.

1.3.4 E and C Processes

The reaction mechanisms of electroorganic reactions are

henceforth composed of atleast one electron transfer steps at the

electrode as well as proceeding and follow up bond breaking, bond

forming and / or structural rearrangement steps 78-79. Most

electroxidation of organic compounds proceed in a step fashion through

the loss of electrons by electrolysis [E process] and subsequent

chemical reaction [C process] 80. In the E process, desired reactive

species are selectively produced and in the C process the reactivity of

the intermediates are controlled by designing a situation in which they

are directed in the desired ways. The importance of the C process is

therefore clear as even starting from the same reactive intermediate it is

possible to obtain widely desperate results by changing the constituents



of the electrolysis media. In other words, the fate of the reactive

intermediates is always affected by the solvents, electrolytes, additives

and electrodes. Most of the electrochemical reactions involving organic

compounds do not terminate in the E process. But the reactive

intermediates further undergo chemical reactions leading eventually to

stable products after substitution, elimination, addition, degradation,

recombination, fission or rearrangement reactions 81-83.

For a substrate [S-H] an ECEC mechanism may be proposed as

Today’s concept of electroorganic synthesis is necessarily an

extension of that of classical electrode reactions and embraces all

chemical and physical phenomena involved in the electrolysis system.

The E process in the electroorganic synthesis comprises all the possible

electron transfer processes which may exist not only around the

electrode surface but also in the diffusion layer or even in the bulk

solution.

In order to classify the various mechanisms of organic electrode

reactions, a specific nomenclature has been developed 84. It is often

extended in an informal way to accommodate particular reaction

features and one may find additional or deviant symbols.

Usually, however, electron transfers at the electrode are denoted

by ‘E’, while chemical steps not involving the electrode are denoted by

[S-H] – e [S-H] + . – H+ [S] . – e [S] + Y- [S-Y] E C E C

[S-H] – e [S-H] + . – H+ [S] . – e [S] + Y- [S-Y] E C E C



‘C’. The electron transfer may further be characterized as ‘Er’, ‘Eqr’ or ‘Ei’

in the reversible, quasi-reversible or irreversible case. It is usually not

indicated how transport occurs. If the ‘C’ step is a dimerization, the

symbol ‘D’ is common while an electron transfer between two species in

a solution is denoted as ‘SET’ (solution electron transfer) 85-86.

For more complex mechanisms picturesque names such as

square, ladder, fence or cubic schemes have been selected 87. In redox

polymer films additional transport of counter ions, solvation and polymer

reconfiguration are important and four dimensional hyper cubes are

needed to describe the reactions 88.

The E and C processes can very well be controlled by distinctly

different parameters. The E processes are controlled by maintaining

constant potential where the potentials are externally controlled by

means of the applied voltage through a potentiostat or by maintaining

constant value of current density throughout the electrolysis. The C

processes are dependent on the microscopic reaction sites under which

electrogenerated active species come in contact with the solvents,

electrolytes, additives, etc. to undergo subsequent chemical reactions.

Hence the control of the C process depends on optimizing the functions

including the solvents, electrolytes, additives, electrode materials,

current density, pH and temperature.

Modern electroorganic synthesis characteristically prefers on the

product selectivity. A careful provision of electrolytic conditions and

system can led to product selectivity. The difference from the traditional



reagent based organic reactions is that the solvent-electrolyte-electrode

system and the electrolysis conditions such as current density, potential,

pH, etc., play an important role in the outcome of the C process.

The electroanalytical method is conveniently classified on the

basis of current control or potential controlled electrolysis. Alternately the

electrochemical techniques can also be distinguished depending on the

stationary or non stationary diffusion of mass transportation.

1.4 FACTORS CONTROLLING ELECTRODE PROCESSES

Electrolytic processes characteristically are controlled and

affected by many variables – mechanical, electrical, chemical and a

combination of these. The better realization of electroorganic synthetic

process can be achieved by the proper application and control of these

variables.

The following experimental variables are of importance for the

outcome of an organic electro synthesis.

1.4.1 Cell Designs

Until very recently the electrosynthetic processes were developed

and used, based on the preparative scale information alone without

much detailed considerations of electrochemical engineering aspects.

Recently however, many changes have taken place. Chemical

engineering concepts are now widely employed in electrochemical

processes. Cost effectiveness is, of course, the overall objective of



process development. This can be achieved by optimizing a number of

valuable parameters. Highest yield and current efficiencies must be

achieved in the electrochemical process. In addition, high space-time

yield, which is a measure of productivity in time and space (YST), must

also be achieved.

YST = Amount of product formed / Time of electrolysis x Cell Volume

High specific electrode area (AS) and high electrode area volume ratio

(Ae) also contribute to high space time yield and higher productivity.

AS = Electrode area / Cell Volume

Ae = Electrode area / Electrode Volume

Minimizing energy consumption is another requirement. This is achieved

by minimizing ohmic contact loses, increasing electrolytic conductivity

and minimizing inter electrode gap. Uniform current density distribution

on the electrode surface is another requirement to ensure product

selectivity.

Based on energy and production considerations on ideal

electrolytic cell should satisfy the following requirements 89, 90. The cell

should be operated at a voltage very near to the theoretical voltage

a. The electrodes should be dimensionally stable and the

designs of the same should facilitate minimum losses in

current efficiency

b. Provision for easier separation of anodic and cathodic

products should be made



c. Adequate circulation of the electrolytes should be maintained

to realize uniform concentration within the cell and

d. At a given volume, it is always advantageous to have the

maximum electrode area.

No simple cell design can satisfy all these criteria for all types of

electrochemical processes. A wide variety of electrochemical cells have

been designed. Divided or undivided cells of monopolar or bipolar

configurations consisting 2-D as well as 3-D electrodes where the

electrodes as well as electrolytes may be stationary or dynamic have

been developed.

An excellent comprehensive book and a number of classical

reviews by experts 91-95, on cell designs and electrode geometry are

available.

A. Batch type cells with Stationary Electrodes

i. Undivided cell: All cell reactions must involve atleast two opposite

primary reactions such as an electronation or reduction, and a

deelectronation or oxidation. If these reactions do not interfere in any

detrimental way (sometimes the interference may be beneficial or even

necessary), a one compartment or undivided cell can be used for the

electrolysis.

The simple undivided cell is always more desirable for large –

scale synthesis from both a technical and an economic aspects.



ii. Divided cell: A two compartments or divided cell is mostly required for

best results. An H-type electrochemical cell with three electrode

assembly, was employed for both preparative and voltammetric studies.

In cases where gases that may form explosive mixtures (e.g. H2,

O2, Cl2) are produced during the electrolysis, provisions must be made

to minimize their mixing. Divided cells can be constructed so as to keep

such gases separate. Cylindrical porous pots dipped in cylindrical cell

containers. These have low space-time yields and energy losses.

B. Rotating cylindrical electrode cells

A series of rotating cylindrical electrodes operated by motors, are

suspended in a reactor vessel inside porous pots amidst a number of

auxiliary electrode strips. They can give higher current densities and

hence higher production rates. These cells are developed by Central

Electro Chemical Research Institute, Karaikudi, India 96, and are known

as Udupa cells. The minor handicap to be experienced in these cells, is

that the inter electrode distances cannot be minimized beyond a point

and hence the energy consumption is higher.

C. Flow cells with 2-D electrodes

Electrolyte continuously flows through the cell. A diaphragm is

introduced into an electrolytic cell when it is necessary to keep the

contents of the anodic and cathodic sides of the cell separate. This

condition arises when



i. The starting material and / or products of the electrode process

being investigated could be destroyed could be destroyed by

reaction at / or contaminated with reaction products of the counter

electrode.

ii. It is required to maintain different electrolyte compositions at the

anode from those at the cathode.

iii. There is a need to exercise careful control of pH in the vicinity of

the working electrode.

Cell dividers:

A suitable barrier material, to separate anolytes (the solution in the

compartment containing the anode) and catholyte (the solution in the

compartment containing the cathode), called the cell divider. The ideal

divider would be chemically inert and totally impenetrable by solvent,

reactants and products, but would allow free passage of atleast one ionic

species. The divider separates the system into two chambers, one in

which the anolyte comes into contact only with anode and the other in

which the catholyte contacts only the cathode. The membrane type cell

dividers made of thin polymeric material offer better selectivity and less

diffusion of neutral species, but are less chemically resistant than the

ceramic type.

The cell dividers must be selected according to the following

requirements.



(a) it must be permeable to ions, preferably impermeable to other

species

(b) it must be stable to the electrolytic medium at the experimental

temperature

(c) it must be continuous and mechanically strong enough to

withstand any pressure differences encountered

The diaphragms are of two types – permeable membrane composed of

porous matrix and semi permeable or ion exchange membranes consist

of a resin material. Refractory material, cellophane, ion exchange resins

are some of the diaphragms, being employed in electrochemical

processes, depending upon the requirements. The ceramic and fritted

glass types have excellent chemical resistance, but have less selectivity

with respect to diffusion of solvent, starting material, products and

electrolytes.

1.4.2 Nature of Electrode Material

Electrode material plays a crucial role in the electrode process.

The choice of the electrode was arrived at mainly on the basis of the

potential range within which the electrolyte-solvent did not undergo

electrolysis. However, the catalytic features of the electrodes and their

designs must also be taken into account when selecting a particular

electrode material 97, 98. The greatest stimulus for research in electro-

catalysis has come from the search for cheaper catalytic electrode

material 99-100. An electrode process involves atleast three steps



a. Adsorption of reactant [chemical step],

b. charge transfer [electrochemical step] and

c. desorption of products [chemical step]

The characteristics of the electrode becomes more important as the

changes occur around the electrode during electrolysis are influenced by

electrode potential and the structure of the double layer at the electrode,

which is itself being affected by electrode potential 101-103.

The nature of the electrode used for electrolysis is therefore

becoming a key variable. To the extent that adsorption and double layer

effects may play a role in a given system, the electrode may have a

profound influence on the system and could in fact change the entire

nature of the product 104. As a pertinent aspect, to realize the importance

of the electrode material, a number of electroorganic reactions may be

referred. Kolbe reaction is run at a platinum anode in order to optimize

the yield of product formed via radical pathway but takes a different

course on carbon electrode where carbonium ion pathway is followed

105. Similarly in the oxidation of sodium acetate during the preparation of

aromatic compound the use of the carbon anode produced mainly

methylated products with some acetoxylation, while the use of platinum

anode afforded the acetoxylated product exclusively 106. The use of

carbon generally affords carbonium ions while that of platinum gives

radical intermediates 107. It has been suggested that the ability of the

carbon anode to promote the generation of carbonium ions is due to the

presence of paramagnetic centres at its surface which would impede the



desorption of the initially formed radicals. In general, carbon electrodes

are suited to two electron oxidation, providing cationic species.

Several considerations should be applied to the selection of

electrode material for use in organic synthesis. Following are the

parameters to be observed while selecting an electrode

1. It should be a good conductor

2. The surface should be an effective catalyst if possible

3. It should not suffer from chemical or electrochemical attack and

4. It should be of rigid construction.

Platinum, carbon and lead dioxide are the most widely used

electrodes for the anodic process. However, titanium dioxide, ruthenium

oxide, gold, and modified electrodes have gained importance in the

present electrosynthetic and analytical works 108-110.

(a) Platinum

Platinum has been the most widely used anode material because

of its “inertness” in most electrolyte environments, and its high oxygen

over potential in aqueous media. Platinum in aqueous media forms an

oxide layer when subjected to electrode potentials above about 0.8V.

This oxide layer can also be produced chemically if the platinum surface

is treated with an oxidizing agent such as acidified potassium

dichromate, which might be used as an electrode cleansing agent. If the

cycling of electrode potential is continued, the alternate formation and

removal of the oxide layer will result in the “platinization” of the electrode

surface, and this may alter the characteristics of the electrode process



under investigation. A discussion of oxide-layer formation on platinum is

given by Gilman. Other platinum group metals and gold also exhibit this

behavior. A condition of platinum that is used to give high surface area

electrodes or electrodes with particular catalytic properties is the so-

called platinized platinum 111.

The relatively high cost of platinum deters the use of the bulk

metal in large scale applications. This problem is often overcome by

deposition of finely divided platinum on base metal 112. At strongly

anodic potentials or with strong oxidizing agents platinum is set to form

an oxide film which is observable on Voltammetric curves. On the other

hand, at cathodic potentials or with strong reducing agents hydrogen is

reported to get adsorbed on the surface. Platinum electrodes are liable

to bring about one-electron oxidation, and are available for holding a

cation radical or radical stage 105. For industrial scale electrolysis,

platinum – plated titanium electrodes are often used.

(b) Carbon (Graphite)

Both carbon and graphite have been used extensively as anode,

because of their relative resistance to oxidation. Carbon paste

electrodes have been described in detail by Adams 113. Graphite

electrodes are porous to some extent, which may be an advantage, if

high surface area is desired, but porosity may cause problem due to

clogging with insoluble residues 114.

There are various types of graphite manufactured which differ

from each other in the method of fabrication and temperature treatment.



They are chemically resistant and reasonably good conductors. Also a

black sludge is formed as a result of mechanical degradation. At high

electrode potentials the crystal structure is chemically attacked by

electrolysis in the pores, leading to mechanical breakdown. Methods of

improving the corrosion resistance of graphite have been reported.

A form of graphite known as pyrolytic graphite, although at

present expensive, has the desirable properties of being impervious to

liquids and gases and inert to most forms of chemical attack. It is a

highly ordered crystalline from of graphite and as a consequence is

anisotropic. Pyrolytic graphite has been used successfully in a number

of organic electrochemical investigations 64, 65.

Other special graphite materials namely glassy carbon 21 and a

similar material called vitreous carbon 66 have been used as anodes.

They are slightly porous and therefore may suffer degradation similar to

that of the more common graphite.

1.4.3 Nature of Solvents – Aqueous and Non-aqueous solvents

Electrolytic media consists normally of a solvent for the organic

reactions and a supporting electrolyte to enable the current to pass

through the medium without too great an ohmic resistance. In some

cases, the reactant or a solvent or a combination of the two, may be

sufficiently conducting to avoid the need for a supporting electrolyte.

This can be important since the recovery of products from the reaction

mixture will be considerably easier. The solvent and electrolyte perform



important functions in the electrolytic work, and should be selected only

after considering several factors.

The general considerations to be applied to the selection of

solvent and electrolyte may be summarized as follows:

1. Should have a high conductivity

2. Stability towards electrolysis conditions

3. Solubility of starting material

4. Inert toward products or intermediates

5. Ease of purification and separation.

6. Adsorption

7. Toxicity and ease of handling

8. High dielectric constant to ionize the electrolyte

In some cases, the solvent or electrolyte itself can be chosen

purposely to participate in the reaction. Proton availability can also be an

important factor, particularly when carbanions or radical anions are

involved.

Aqueous Solvents

Suitable choice of the solvent is very important in almost every

instance for obtaining product selectivity. Water itself is used as a

solvent. And also mixed aqueous – organic solvents are used to carry

out effective electrolysis. Difficulties encountered with aqueous media

can be two fold. First, when quantities of non-ionic organic solvents are

used insufficiently, conductivities are often too low. Second, particularly

in anolytes, reaction of the solvent at the electrode is sometimes difficult

to avoid.



Aqueous electrolytes are simply prepared by the dissolution of

solvent, supporting electrolyte and reactants in triply distilled water 115 if

particularly accurate kinetic studies are contemplated, further purification

of the distilled water and, additionally, pre-electrolysis 116, 117 might be

employed.

Non-aqueous Solvents

Many non-aqueous solvents are far more effective than water in

dissolving organic reactants. Generally, the solvents other than water

come into the category of non-aqueous solvents. The use of non-

aqueous solvents in electrochemistry has increased dramatically over

the past 20 years as interest in organic systems has grown. This subject

has been reviewed in a chapter by Mann 32. Some of the non-aqueous

solvents which find wide application in this field are acetonitrile,

ammonia, pyridine, THF, DMF, DMSO, etc.

The selection of a suitable medium can be made on the grounds

of atleast four criteria:

1. solubility of the reactant

2. range of electrode potential

3. suitability for a desired reaction path and

4. Degree of conductivity

Acetonitrile is one of the most used polar aprotic solvents, both

for anodic and cathodic reactions. It is an excellent solvent for many

organic substrates and quite a few organic and inorganic salts; it is



miscible with water. Salt solutions show a reasonably high conductivity

due to its rather high dielectric constant [= 37].

Acetonitrile has a wide, usable potential range both in anodic and

cathodic directions. The limit is set by the electrode reaction of the

supporting electrolyte in both directions 118. Transfer of protons in

acetonitrile is rather a slow process. Useful supporting electrolytes in

acetonitrile are sodium perchlorate, lithium perchlorate, tetra butyl

ammonium salts, such as the chloride, bromide, iodide, perchlorate and

tetrafluoro borate. Commercial acetonitrile contains usually impurities

such as acrylonitrile, acetic acid, aldehydes, amines and water. Several

methods of purification have been proposed in literature 119-123.

Electrolysis of acetonitrile, in the absence of added proton donor,

may produce the anion of acetonitrile, CH2CN: -, which may act as

nucleophile towards electrophilic centres 124. Acetonitrile reacts with

perchlorate radical to form succinonitrile and perchloric acid 125 as well

as acetamide. Carbanions can abstract a proton from acetonitrile 126.

1.4.4 Electrolytes

The electrolyte usually exhibits good conductivity. Their prime

function is to provide the source of ions to conduct current across the

cell. In general, electrolytic media of high conductivity are desirable.

Many electrolytes have a tendency to form ion pairs at high

concentrations in organic media. The ability of certain electrolytes to

adsorb on the electrode and to influence the double layer structure can



play a key role in determining the course of the reaction. In some cases,

the electrolyte may be chosen to serve as a buffer when acids or bases

are formed during electrolysis, and the electrolyte may also be chosen to

serve as a reactant.

1.4.5 Supporting electrolytes

The supporting electrolyte may actually participate in an electrode

process by attacking intermediate species or alter product distribution by

changing the acid-base character of the solution 32.

The criteria for selection of a suitable electrolyte are as follows:

(i) Must dissolve and ionize in solvent

(ii) Must be inert to starting materials, intermediates and

products.

(iii) Must be inert over the potential range of interest

(iv) Should be easily removed on product work-up

Cations: Tetra alkyl ammonium salts, ammonium salts, alkali metal cations.

Anions: Perchlorates, Halides, tetra phenyl borates, Acetates, sulfonates.

1.4.6. Solvent – Supporting electrolyte – Electrode combination [SSE]

Of all the factors involved, the SSE plays one of the most decisive

roles for the synthetic result of an organic electrode process. The SSE

systems must be chosen as per the following criteria,

a. It should possess good solvent properties with respect to the

substrate, as it serves as a medium for the reaction.



b. It should have good conductivity for the electric current. That is,

the solvent should have reasonably high dielectric constant and

the supporting electrolyte be present in fairly high concentration

(0.1M), and

c. It may serve as a source of reactant in the chemical processes

following electron transfer.

The anodic and cathodic limit of a particular SSE depends on an

electrochemical process involving either solvent or supporting

electrolyte. In acetonitrile, the anode limit is dependent on the nature of

the anion and the cathodic limit on the nature of the cation.

Anions can be ordered in series of increasing resistance towards

anodic oxidation as

I - < Br

- < Cl

- < NO3

- < CH3COO

- < ClO4

- < BF4

- < PF6

–

and cations in series of increasing resistance towards cathodic reduction

Na +

< K +

< R4N +

< Li +

In aqueous or aqueous-organic SSEs, the accessible potential

range is dependent on the electrochemical oxidation and reduction of

water with formation of oxygen and hydrogen respectively. The

potentials at which these processes take place are different for different

electrode materials 2. The anodic limit for aqueous systems moves to

more anodic potentials in the series

Ni < Pb < Ag < Pd < Pt < Au

and the cathodic limit moves to more cathodic potential in the series

Pd > Au > Pt > Ni > Cu > Sn > Pb > Zn > Hg



Thus to perform a reduction of difficultly reducible substances in a water

containing system, one would normally prefer a metal like Pb, Zn or Hg,

where as for difficult oxidations Au or Pt would be the anode material of

choice. In non-aqueous SSEs, the choice of the electrode material is not

critical from this point of view.

Generally, inert SSEs tend to favor coupling reaction between two

or more substrate molecules whereas those, with nucleophilic or

electrophilic properties favor substitution or addition reactions. As an

example, the anodic oxidation of Durene 127 on platinum can be

controlled to give substitution product only, in a strongly nucleophilic

SSE and coupling product only, in non-nucleophilic SSE. In SSEs of

intermediate nucleophilicity, both types of products are formed.

Even if a consideration of macroscopic properties of the SSE

many times is useful as a first approximation for predicting the outcome

of an unknown electroorganic reaction, it must be borne in mind that the

composition of the electrolyte at the electrode surface and its immediate

vicinity might be completely different from that of the bulk of the solution.

Current theory 81,128 assumes that the electrode surface is covered by an

adsorbed layer of ions and neutral molecules during electrolysis. The

thickness of this layer, the electrical double layer is of the order of 10A.

The region between the electrical double layer and the bulk of the

solution is denoted as the diffuse layer (50-100A in thickness) in which

concentrations gradually change from those of the double layer to those

of the bulk of the solution. Since the electron transfer process



necessarily must take place with the substrate molecule situated very

close to the electrode surface, and short lived intermediate formed will

undergo further chemical reactions in a medium with properties differing

from those of the bulk of the solution 129.

The ions of the SSE can exert a powerful influence on the

chemical follow-up reactions. For example, in the anodic oxidation of

hexamethyl benzene in acetonitrile-water in the presence different

supporting electrolytes, different product distribution is observed 130.

Acetamidation and hydroxylation at the side chain occurs. It is observed

at an anode potential, being kept the same, use of tetrabutyl ammonium

perchlorate favors 95% Acetamidation, while tetrabutyl ammonium

fluoborate favors 95% hydroxylation. This indicates the possibility that

some species of the SSE is available in much higher or lower

concentration at the electrode solution interface, which may decide the

product distribution.

Having chosen the solvent, a suitable supporting electrolyte,

which is soluble in the solvent, is to be selected in such a way that the

resultant solution may have adequate conductivity 131. In general, the

desirable solubility expected for an electrolyte is in the range of 0.05 –

0.3M and, under such conditions the solution should exhibit a usable

current-density. The actual procedures of electroorganic synthesis can

involve various electrolytes which include not only stable strong

electrolytes, but frequently also electrolytically labile electrolytes. For

example, most halides salts as well as some acids and bases can be



oxidized through electrolysis to give reactive species, i.e., halonium ion

or molecular halogen, which may react with substrates or solvents to

yield a variety of in situ-generated active species and products which are

otherwise inaccessible 43, 132.

When the electrolysis is carried out with electrolyte NaNO3, the

electro oxidation of nitrate ion provides [NO3]. by one electron oxidation

on the anode and this radical may abstract a reactive hydrogen atom

from the substrate 133. A sandwich-type arrangement such as electrode-

substrate-electrolyte can also be realized which function like an

‘electrolyte push-electrode pull’ electron transfer system 134.

A reasonable choice of suitable electrolysis solvent cannot be

expected without due consideration of what combination of electrolyte,

electrode material and additive will be best for achieving the desired

product-selectivity in relation to the chemical nature of the substrate. In

other words, adequate selection of the above variables is closely related

with the nature of the desired functionalization of the substrate. The

solvent-electrolyte system required for electroorganic synthesis is quite

different from that employed in conventional electrochemical

measurements. In electroorganic synthesis, emphasis should be placed

on the C process which may settle the fate of active intermediates

stemming from the E process. As a result, most efforts have tended to

concentrate on finding the best suited solvent-electrolyte-electrode

system, so that various possible combinations of such variables have

been examined experimentally. On the contrary, the aim of the later



investigation centers on the establishment of a theoretical basis and its

application to the analysis of electrode reactions, on the basis of

measurements of various parameters such as oxidation potential,

current, scanning rate etc.

1.4.7. Nature of Substrates

The substrate must be reducible or oxidizable within the

accessible potential range (-3.3 to +3.7 V). This is best done by studying

the electrochemical behavior of the compound using any of the simple

voltammetric techniques, for a cathode reaction at mercury cathode and

for an anodic reaction at the platinum.

As anodic electronic transfer occurs from the HOMO of the

substrate, groups which raise the energy of this molecular orbital will

lower their oxidation potentials. These are the same groups that stabilize

cationic centers inductively and/or by conjugation. Example: alkyl, aryl,

alkoxy, hydroxyl, amino and halogen. On the contrary, since the cathodic

electron transfer occurs to the lowest empty molecular orbital,

substituents which lower the energy of this molecular orbital will raise

reduction potentials. These are the electron withdrawing substituents

such as nitro, carbonyl, cyano, etc., which stabilize carbanionic centers.

1.4.8. Concentration

Generally, it is desirable to provide highest concentrations of the

substrate and electrolyte within the constraint of solubility. Higher



currents and hence greater rate of production of product can be

achieved. However, several other factors should also be considered in

assessing the effect of concentration on a particular reaction. In organic

electrochemical reactions, the first step is often the formation of a

relatively highly reactive, and perhaps unstable, intermediate such as

radical or ion. As such, this intermediate can decompose, react with

solvent or condense with another species. Decomposition or reaction

with solvent follows first order kinetics. On the other hand, condensation

is usually second order and will be relatively more effective at higher

concentrations. Thus, the distribution of the products from parallel

reactions of this type may be expected to depend on concentration.

1.4.9. Temperature

An increase of temperature increases the rate of diffusion of

electro-active material to the electrode and therefore allows for higher

currents to be passed through the electrolyte. An increase in cell

temperature has the distinct advantage of shortening electrolysis

periods. Only in rare cases, does the product composition change

considerably with temperature.

1.4.10. Additives

To increase the yield and efficiency in some electrochemical

processes, certain additives may be added. These so called oxygen and

hydrogen carriers have been used mainly in aqueous media. Their



action, in these cases results from a mechanism, referred to as electro

regeneration. These materials are potential oxidizing or reducing agents,

which get regenerated as per the mechanism,

Substrate + O f a s t product + R [Chemical]

R f a s t O + e- [Electrochemical]

where O and R are oxidized and reduced forms of the additive

respectively. This scheme reminiscent of catalysis, can be illustrated in

the electrochemical oxidation of anthracene to anthraquinone, promoted

by cerium salts 135, in the oxidation of toluene to benzoic acid by

chromium salts 136 and m-Phenoxy toluene to m-Phenoxy benzaldehyde

by ceric trifluoro acetate 137.

1.4.11. Electrolysis condition – Agitation / Rotation

Because electrolysis is a heterogeneous reaction, mass transport

of material toward and away from the electrode is an important

consideration. In most cases, it will be desirable to agitate the solution in

order to speed up mass transport. The most common and convenient

practice is to stir the bulk of the solution. But, it is not the most effective,

as a definite stationary boundary layer around the electrode will persist.

Alternately, the electrolyte is made to circulate, by means of an external

pump. More effective stirring can be achieved by moving the electrode

itself, such as with rotating or vibrating electrodes.

In some cases, it may be desirable to prevent disruption of the

depletion region, in order to take advantage of the high-concentration



gradients at the electrode surface. For example, if a desired reaction

involves a second order process (EE process) between reactants, the

higher concentrations near the electrode may result in better yield of the

product.

1.4.12. Electrode Potential

The electrode potential is the most important electrolysis variable,

since it essentially controls the type of the reaction and its rate. Many

compounds undergo only a single-electron-transfer reaction, so that

electrode potential becomes important only in governing the rate of the

reaction. When more than one electron-transfer reaction is possible,

proper control of electrode potential is crucial and is the basis for the

high selectivity of the electrolysis process. Though a good number of

electrode reactions are available, to illustrate the influence of electrode

potential on the product selectivity, the anodic oxidation of phenol may

be considered. At lower anodic potentials (0.8-0.9V vs SCE), phenol

undergoes dimerization through cyclohexadienone radical to give

4, 4’-Diphenols, whereas, at slightly higher potentials (0.9-0.95V vs

SCE), p-Benzoquinone is formed through cyclohexadienyl cation 138.

1.5 ELECTROCHEMICAL TECHNIQUES

Many electrode reactions have been performed successfully in an

undivided cell which is a simple beaker type apparatus. As a matter of

fact, numerous electroorganic reactions have been carried out in



undivided cells equipped with 2 electrodes, a thermometer, a gas outlet

and a magnetic stirrer.

For small scale experiments in the laboratories run with 1 - 10g of

substrates, compact DC suppliers are commercially available which can

be operated at a standard output of 0.5 to 2.5A / 0 - 35V. These are

connected with ammeter and voltmeter in series. A magnetic stirring

system is convenient and is used in almost all cases. The reference

electrodes are installed through the wall of the cell through a luggin

capillary inorder to monitor the operating potential.

1.5.1 Galvanostatic and Potentiostatic Methods

Electrochemical measurements commonly involve the three

variables namely electrode potential (E), current density (i) and time (t).

In order to investigate the relationship between any pair of these

variables, the potential of the working electrode is measured against a

reference electrode. A three electrode called counter electrode, to

complete the electric circuit is required. The investigation of these

parameters can be made by the application of steady and non-steady

signals to the cell. If the signal applied to the cell is a controlled current,

then the method is described as galvanostatic and if the signal to the

working electrode is a controlled electrode potential, then the technique

is described as potentiostatic. Both galvanostatic and potentiostatic can

be sued to obtain the current voltage and time characteristics of an

electrode reaction.



The galvanostatic method is unique in chemical kinetic

measurement in which the reaction is forced to proceed at a given rate

by the application of steady current. The free energy of activation under

these circumstance changes to adapt to this rate. Thus in this method, a

known current is applied to the cell and the resulting variation in the

electrode potential is observed. Measurements are usually made by both

increasing and decreasing current density values.

In a potentiostatic experiment, the electrode potential of the

working electrode is controlled and the resulting current is observed.

This control in the electrode potential can be achieved by an instrument

known as potentiostat. The potentiostatic method has several

advantages over the galvanostatic methods. The potentiostat is useful

for investigating parallel electrode reactions. In a situation where more

than one product is possible, suitable control of the electrode potential

can produce at a time. In the study of anodic reactions where electrode

processes are stopped due to the formation of passive oxide film at

specific potential regions, potentiostatic control can prevent entering this

region. However, both potentiostatic and galvanostatic techniques

produce identical polarization curves.

1.5.2 Electrochemical process development – Laboratory scale

preparations

Preliminary preparative works may be initiated using the

electrodes and the medium selected. A few preparative experiments



would indicate whether the electrodes really influence the reaction

process. The optimum conditions are chosen as the ones which give the

best yield (moles of the product x 100 / moles of the reactant used up)

and current efficiency (charge required theoretically x 100 / charge

consumed experimentally). Deposited electrode and even a few closely

related electrode materials may also be tried for obtaining better yield

and efficiency.

The first major decision to be taken is whether or not to use a

diaphragm to separate the anode and cathode compartments. If the

product formed at one electrode is likely to be destroyed at the other

electrode, divided becomes necessary. However, it is always advisable

to see if the diaphragm can be avoided, because it is much easier and

economical to devise and operate an undivided cell. If a diaphragm is a

necessity, then the choice for it must be made. A variety of materials,

such as unglazed porous ceramic pot, cloth, paper, asbestos and the

like have been employed as membrane materials. At present, a number

of ion exchange membranes like Nefion cation exchange membranes

are being employed. Optimizing the proper medium composition is

another important objective in preparative scale experiments. Since

organic compounds are generally poorly conducting, addition of large

quantities of such reactants in a single step itself might cause an

increase in cell resistance as well as electrode poisoning. Stepwise

addition may be resorted to in such occasions. The temperature range,

at which the preparative work is to be carried out, is another important



experimental parameter. Temperature in fact can have a number of

effects in an electrochemical process. It would increase the chemical

reaction rate as well as diffusion rate. It would enable melting of a solid

reactant and enhance its mixing with the solvent. It would also result in

volatilization of chemicals. If the reaction intermediates is a gas, higher

temperature would enhance its loss. Hence it is always necessary to

optimize the correct temperature, taking into consideration all these

favorable and adverse effects.

1.6 ELECTROANALYTICAL TECHNIQUES

Electrochemical measurements on chemical systems are made

for variety of reasons. For obtaining thermodynamic data about a

reaction, to generate an unstable intermediate such as radical ion, and

to study its rate of decay or its spectroscopic properties and for the

analysis of a solution for trace amounts of metal ions or organic species,

electrochemical methods are employed as tools in the study of chemical

systems in just the way spectroscopic methods are frequently applied. A

number of electrochemical methods have been devised for these

investigations. Phenomena such as the formation of unstable

intermediates, multi-step electron transfers, potential dependent

adsorption, competing electrolytic reactions and so on, can be known by

the proper application of suitable voltammetric experiments. As for the

anodic process, current-potential studies, cyclic voltammetry,



Coulometry, controlled potential electrolysis etc., may provide the

mechanistic information on electrode processes.

1.6.1 Current-Potential Studies

The electrode potential of the working electrode is controlled and

the resulting current is observed. The electrode potential is normally

measured between the working electrode and a reference electrode

coupled with it through a luggin capillary. The current is usually

monitored by a multi-range ammeter. Electrode potential measurements

and voltage measurements across circuits in which small currents are

flowing, require the use of a voltammeter, which draws essentially zero

current. Usually a vacuum tube voltmeter (VTVM) is employed to

monitor the potentials maintained.

The plots of electrode potential and the current density or current

for the solvent and the solution systems provide useful and essential

informations on the electrolytic processes. The shifts in the polarization

curves corresponding to the substrates chosen are often exploited to

offer interpretations on the mechanistic routes of the processes

undertaken.

The ampere-voltage relations for the solvent and the solutions

containing the substrate and electrolyte/supporting electrolyte could be

graphically represented to obtain polarization curves and their

corresponding decompositions potentials. The deviation from the regular

trend in the relations within the polarization curve and any shift in



between the polarization curves may well be utilized for appropriate

interpretations to fix the experimental conditions like the compatibility of

the solvent-electrolyte-electrode combinations, the maximization in the

working potential levels, the feasibility of the electrode process, etc. and

to substantiate the mechanistic pathways.

1.6.2 Cyclic Voltammetry

Cyclic voltammetry is a modern electrochemical technique and

because of its relative experimental simplicity, is perhaps the most

readily applied of the techniques available. Potential sweep voltammetry

is divided into linear sweep and cyclic voltammetries 139-141. Scanning of

the working electrode potential linearly with time in only one direction is

called Linear potential sweep voltammetry 14,142,143. Cyclic voltammetry is

an extension of the linear potential sweep method in which the direction

of the potential is reversed periodically so that electrolysis of the

products of the forward sweep occurs on sweep reversal. Cyclic

voltammetry of linear sweep voltammetry is more sensitive and faster

than polarography. In studying the mechanism of electrode reactions,

the use of stationary electrodes with cyclic potential scan makes it

possible to investigate the products of the electrode reaction and to

detect the electro active intermediates. In cyclic voltammetry, current-

potential curves are recorded on an X-Y recorder or oscilloscope. Unlike

polarography, the current-potential curves in cyclic voltammetry are in

the shape of peak. This is a consequence of the use of stationary



electrode. As the potential moves into the region where the substrate is

reduced or oxidized, the region adjacent to the electrode becomes

depleted of material and the current decreases.

Cyclic voltammetric behavior can exhibit a variety of forms. Each

peak observed on the current-potential curve corresponds to a separate

electrode process. The shape of the cyclic voltammogram is highly

dependent on the coupled chemical reactions occurring at the electrode

surface, this enables one to deduce a great deal about the course of an

electrode process from cyclic voltammogram. Reversible processes give

corresponding anodic and cathodic peak currents at peak potentials,

while irreversible processes exhibit only one of these peaks. Scan rates

can be varied over a wide range (Ca c.d. 10,000 V/sec) providing and

extremely useful experimental parameter. Useful information may be

obtained about the occurrence of chemical reactions subsequent to

charge transfer by comparing curves obtained at different scan rates. A

peak or an anodic/cathodic complementary pair of peaks, appearing only

in sweeps after the first sweep, indicate the presence in the system after

the first charge transfer of an electroactive species not originally present.

If the peak does not correspond simply to the reverse of the original

charge transfer process (a reduction peak from the product of an initial

oxidation step), then it arises from a species produced in a chemical

reaction following the initial charge transfer.

Cyclic voltammetry is a multisweep technique. The potential

sweep may be continued for as many cycles as desired. A distinction



should be made between the first two or three cycles and continuing

cycles. The theory for cyclic voltammetry was originally derived for the

steady state conditions, observed after many potential cycles 144. Most

cyclic voltammetric experiments are carried out for two to three cycles.

The theory of cyclic voltammetry for the important first cycle has been

given by Nicholson and Shain 145. In CV, the potential can be cycled over

the same range many times. Three potential parameters, the initial

potential (Initial E) and the two switching potentials (i.e., the potential at

which the direction of the scan is reversed) High E and Low E, are

required.

The asymmetry of the curve is due to the diffusional mass

transport. However, there are many other parameters that can affect the

shape of this curve; for example, slow heterogeneous transfer kinetics,

instability of the oxidized or reduced species, and adsorption. If the

heterogeneous electron transfer is rapid (relative to the timescale of the

experiment) and both the oxidized and reduced species are stable

(again, on the time scale of the experiment), then the redox process is

said to be electrochemically reversible. The standard redox potential is

the mean of the two peak potentials (Epa and Ep

c) and the separation of

the peak potentials is 57/n mV (n = number of electrons transferred per

molecule).The peak current for a reversible process is given by the

Randles - Sevcik equation:

ip = 2.69 x 105 n3/2AD1/2 C1/2



where

ip = peak current (A)

n = number of electrons transferred per molecule

A = electrode surface area (cm2)

D = diffusion coefficient (cm2/s)

C = concentration (mol/cm3)

V = scan rate (V/s)

Therefore, for a reversible process, ip is proportional to the

concentration (C), and the square root of the scan rate (1/2). There are

many parameters that can affect the shape of the CV curve. Slow

electron transfer kinetics can increase the separation of the peak

potentials (Ep), and the rate constant for electron transfer can be

calculated by examining the variation of (Ep) with scan rate. However,

uncompensated resistance between the working and reference

electrodes can also increase (Ep). The effect of uncompensated

resistance can be lowered or eliminated using electronic iR

compensation. Another application for CV is the study of the reactions of

electrolyzed species. These are generated on the forward scan, and

their reactivity can be examined on the reverse and subsequent scans.

Qualitative estimates of reaction rates can be obtained by varying the

scan rates.

Cyclic voltammetry is an excellent technique for the quantitative

study of the stability and the homogeneous reactions of species which

may be produced in an electrode reaction. This can be effectively



employed to observe the formation and decay of reactive intermediates

and even to identify such intermediates and/ or products 146.

The voltammetric experiment is designed so that the mode of

electroactive species to the electrode surface is well defined. The three

important mass transport processes are migration, convection and

diffusion, in stationary voltammetric studies, diffusion is considered the

only means of mass transport. During the reaction at an electrode

surface, material is depleted and a concentration gradient is set up.

Reactant from the bulk of the solution then diffuses toward the electrode

surface in response to this gradient. In a similar manner, products of the

electrode reaction diffuse away from the electrode. The diffusion

equations, used to describe the concentration gradient of the reactant

species as a function of time and distance from the electrode surface,

are the same as those in heat transfer.

Nicholson and Shain have considered eight cases of reaction

mechanisms.

a. Reversible charge transfers

b. Irreversible charge transfers

c. Chemical reaction proceeding a reversible charge transfer

d. Chemical reaction proceeding a irreversible charge transfer

e. Reversible charge transfer followed by reversible chemical

reaction

f. Reversible charge transfer followed by irreversible chemical

reaction



g. Catalytic reaction with reversible charge transfer and

h. Catalytic reaction with irreversible charge transfer.

The cells employed in cyclic voltammetry require provisions for a

working electrode, inert gas purge, and auxiliary electrode, a reference

electrode and thermostatic maintenance. It is desirable to minimize

solution resistances by keeping the supporting electrolyte concentrations

up to atleast 0.1M in solvents such as acetonitrile, DMF and methanol

and higher in solvents of lower dielectric constants 147. Electrodes that

are used include platinum, pyrolytic graphite, vitreous carbon and carbon

paste. The electrode surface should be renewed prior to each run. The

accessible potential range depends upon the selection of the solvent

and the supporting electrolyte. Increasing interest has been shown in

this technique, as it helps in analyzing the reaction process occurring at

the electrode. This fact has initiated several workers towards

fundamental electrochemical studies 148-150.

1.6.3 Controlled Potential Electrolysis

Once voltammetry has provided information about the electrode

process possible in given system, it remains to find out how product

formation is related with these. This done by controlled potential

electrolysis 151, by way of running a macro scale electrolysis in which the

working electrode is kept at a constant potential, this being chosen in a

range, where only one process occurs. During electrolysis i-t response



may be recorded until the current drops to zero. ie., all the electro active

species are removed from the bulk solution.

Controlled potential electrolysis offers a technique for the study of

slow chemical reactions 152. The i-t data are re-plotted in the form of log

i-t or i-q graphs. Such plots are linear for uncomplicated electrode

reactions. For systems where the product of the electron transfer step

can undergo a slow chemical reaction, the plots are more complex.

The advantages of the controlled potential electrolysis are

considerable. Through its proper use, undesired side reactions may be

eliminated, specific functional groups may be electrolyzed in the

presence of other electro active groups, or multi-step electrolysis may be

controlled to produce an intermediate product. In all of these cases, it

may not be possible to obtain comparable results by the use of

conventional oxidants and reductants. Controlled potential electrolysis

may be considered a readily available ‘store’ of a vast number of

different oxidizing and reducing agents.

1.6.4 Coulometry

For coulometry 142,152, a macro working electrode is employed.

The working electrode is placed in a separate compartment to prevent

interference from the reaction occurring at other electrodes. A controlled

potential is applied to the working electrode and i-t response recorded

until the current drops to zero. Controlled potential Coulometry is

principally used to confirm the overall number of electrons transferred in



the electrode reaction. This can be done simply by plotting i-t and

estimating the area under the curve. From knowledge of the balance

reactant material, the current yield of the products can be calculated.

1.7. ELECTRODE REACTION MECHANISMS

Examination of the behavior of a dilute solution of the substrate at

a small electrode is preliminary step towards electrochemical

transformation of an organic compound. The electrode potential is swept

in a linear fashion and the current recorded. This experiment shows the

potential range where the substrate is electroactive and information

about the mechanism of the electrochemical process can be deduced

from the shape of the voltammetric response curve 153.

1.7.1 Investigation of Electrode Reaction Mechanisms

Two extreme forms of mechanistic investigations in organic

electrochemistry are frequently applied:

1. Qualitative analysis has the main objective of confirming a given

mechanistic hypothesis by rejection of conflicting alternatives.

This may be applied to single elementary steps, the

intermediates, or how the steps are liked together.

2. Quantitative analysis relies on a highly probable mechanistic

hypothesis and determines as many as possible kinetic,

thermodynamic, and/or transport parameters for the various

steps. This is often a complex problem, since the values of the



parameters are usually correlated, their relation to experimental

data is nonlinear, and the data contain artifacts and statistical

errors 154,155.

Both types of mechanistic analysis are supported by the instrumental

techniques.

1.7.2 Mechanistic Analysis

In general, for a mechanistic analysis, as many facts as possible

of the investigated electrode reaction should be taken into account and

the various experimental parameters be varied as widely as possible.

Among these are

Time scale: This is particularly important for kinetic studies and

the determination of rate constants.

Concentration: The dependence of results on concentration

indicates chemical reactions of an order higher than unity.

Presence of Reagents: Formation of intermediates may be

proven by their reaction with intentionally added reagents, for

example, nucleophiles to quench electrogenerated carbonium

ions. Characteristic changes are expected, for example, peaks in

CV may disappear.

Usually, the experimental results are compared with the

theoretical model stimulations. Again, it is important to consider wide

ranges of experimental conditions that have to be adequately modeled

using a single set of parameters. Comparison is done by



Data Transformation: Suitable transformations of the

experimental data lead to straight lines (e.g. Anson plot in Chrono

Coulometry) or similar simple curves (semi-integration or

differentiation)156.

Feature Analysis: The experimental curves exhibit features (viz.

peaks in CV) that change characteristically with the experimental

conditions. The results are usually compared to working curves 157

or surfaces 158, 159.

Full Curve Analysis: Global analysis of experimental and

theoretical data is applied by comparing entire curves. This is

used to great advantage in simulation procedures 160,161.

Of course, experimental artifacts should be avoided. In particular, in

mechanistic electroorganic work these are

Background currents are current components related to the

ET of substrates or products, but rather to impurities or are

caused by non-Faradic processes (charging of the double

layer). They are atleast approximately corrected by subtraction

of a blank curve recorded in the electrolyte without substrate.

iR drop is caused by the resistance R between the reference

and the working electrode in a three-electrode cell. It is

particularly awkward in low-conductivity electrolytes and

distorts curves in a nonlinear way. Compensation in

commercial instruments is often possible, and procedures for

correction have also been given 162,163. However, it is best to



avoid an i R drop by decreasing i or R (increasing conductivity

or decreasing distance between reference and working

electrodes).

1.8 ELECTROCHEMICAL OXIDATIONS

Anodic electrochemistry offers an unique opportunity for initiating

reactions that construct new bonds while either increasing or preserving

the functionality needed to further manipulate the product generated. For

this reason, a number of groups have focused on the development of

anodically initiated synthetic methods. Much of this work had been

recently reviewed 46,164. Works on this study have much emphasis on the

nature and stability of cationic intermediates generated at the electrode.

Reviews on various aspects of anodic oxidation reactions of aromatic

compounds are now available 3, 138, 165-169.

1.8.1 General Aspects

Electrochemical oxidation reactions are well understood through

studies on aromatic compounds. Any one of the following routes has

been suggested for the electrochemical oxidation of aromatic

compounds.

a. Direct electron transfer to form a cationic species (cation radical

or dication) 170,171. In such cases it is presumed that the electron

would have been lost from the highest occupied molecular orbital

to the anode. The radical cations generated in this have been



characterized by their UV spectra, ESR spectra and pulse

radiolysis 172-174.

b. Reaction of the aromatic compound with an electrogenerated

oxidizing agent such as hydroxy radicals, anode oxides,

supporting electrolyte radical, etc.

c. Dehydrogenative chemisorption in which the adsorbed aromatic

dissociates into chemisorbed hydrogen atoms and organic

residues.

1.8.2 Characteristics of Anodic oxidations

The actual oxidation route was observed to be depended upon

a. solvent – supporting electrolyte combination

b. stability of the electrogenerated cation radical / radical / cation

c. role of the anion

d. role of the anode material

e. specific adsorption of various species and

f. The electrode potential

The marked effect of the solvent-supporting electrolyte system is well

illustrated in the anodic reactions of 1,4-Dimethoxybenzene 175.

If the reaction of the cation radical with the nucleophile is

considered, the impact of polar effects on the stability of subsequent

intermediates formed controls the sequence of the reaction. The position

of the nucleophilic attack will largely governed by the spin density

distribution of positive charge in the cation radical 176,177. Anodic



Cyanation, methoxylation and hydroxylation reactions apparently

proceed according to this explanation; but other factors, including the

dielectric constant of the medium, steric requirements, ion salvation and

so on 178 are certainly important in anodic nucleophilic substitutions

reactions. Only in recent years have the mechanisms of reaction ion

radicals been examined and electrochemical studies have provided a

significant portion of the available information on these interesting

species. Parker and Adams suggest that it is the stabilities of the

corresponding cation radicals and cations that govern the differences in

electrochemical behaviors 179.

The cyclic voltammogram obtained in many instances generalizes

the fact that cations from alkyl aromatic compounds that show 1-electron

oxidation in non-aqueous solvents are selective and those from

compounds that undergo 2-electron oxidation are non selective.

Parker and Burgert have studied the electrochemical oxidation of

toluene in CH3CN-H2O at sufficiently positive potentials that the anion of

the supporting electrolyte may be discharged 180. On the other hand

Eberson and Olofsson, reexamined the anodic oxidation of toluene and

other methyl substituted alkyl benzenes. On the basis of current-voltage

data they proposed direct oxidation of these compounds at the anode to

cation radical intermediates 181. They observed that electrolyte derived

radicals are not involved, quoting the comparative discharge potentials

of the supporting electrolytes in the solvents used and the substrates 182.

Eberson and Olofsson also examined the effect of water concentration



on the distribution of reaction products in the anodic oxidation of methyl

substituted alkyl benzenes. As water is much stronger nucleophile than

acetonitrile, it would be expected that less stable cations should be less

selective toward a mixture of acetonitrile and water than more stable

cations. The latter should react with water preferentially. In fact, opposite

behavior was observed electrochemically and it appeared that the more

stable cations were more selective toward the weaker nucleophile.

Parker established the effect of anode potential on the course of the

electrolysis and the generation of cation radical which apparently either

undergo dimerization or further oxidation at a higher potential to a

dication. These reaction schemes were well substantiated with cyclic

voltammograms, the esr spectrum and product isolation 183.

Among the anodic reactions of aromatic / heterocyclic

compounds, the following are the well recognized categories on the

basis of the product formation, proposal of mechanisms and their

comparison with chemical reactions

1. Hydroxylation – oxidation in aqueous media

2. Alkoxylation

3. Acyloxylation

4. Cyanation

5. Formation of C- N bonds

6. Electrooxidative Coupling, etc.



1.8.3 Electro cross coupling

There has been no systematic study of cross coupling reactions

brought about by electrolysis to date. However, some authors, to explain

gelation and electrode coating, have postulated cross-linking 184. The

phenomenon of cross coupling is invariably associated with

electrochemical dimerization in which more than one substrate is

present in the electrolytic solutions. Electrochemical dimerization of

butadiene and cross coupling of butadiene with styrene in methanol -

sodium perchlorate – graphite has been studied in detail 185-187. Cross

coupling has been reported in electrochemical environment with

conjugated phenols and allyl phenols in methanol – chloroform

containing lithium perchlorate on the potential controlled glassy carbon

anode 188,189. An examination of the number of papers published in the

last five years compared with the previous period indicates quite clearly

the growing and continuing interest in the electro initiation of cross

couplings. Of particular notice is the increased number of patents and

the increased industrial activity of which this is evidence 190-192.

This growth and interest in the subject parallels the recent

advances in the much broader field of electrochemistry.

1.9 ANODIC ALKOXYLATION

The electrochemical oxidation of aromatic compounds in alcoholic

media leads to ethers or acetals resulting from addition or substitution by

alkoxy groups. The Clauson-Kass alkoxylation of substituted furans is



the most thoroughly studied reaction 193, 194 and finds a logical alternative

to the chemical method of preparation of alkoxy furan derivatives 195,196.

The oxidations of furans in methanolic solution at platinum anode with

various electrolytes are performed 197-199. The reaction scheme involves

discharge of methanol to methoxy radicals which undergo 1,4-addition to

the furan ring 200. The successful methoxylation of negatively substituted

furans apparently requires sulfuric acid as electrolyte in order to extend

the useful anodic limit of the solvent to a potential at which the furan may

be discharged 201.

As far as the mechanism of electrochemical alkoxylation is

concerned, the proposal of Eberson is worth mentioning. According to

him, anodic methoxylation and acetoxylation of anisole appear to be

similar reactions 202. A concerted mechanism involving a 2e- transfer

from anisole with the formation of a C-O bond has been suggested for

acetoxylation, which very much holds good for methoxylation too. This

scheme is similar to that proposed for electrophilic aromatic substitution

in homogeneous solution 203.

Alkyl substituted aromatics are reported to be electrochemically

alkoxylated in - position, resulting in parallel free radical reactions with

peroxy, tertiary butoxy and methyl radicals 204-206. On the other hand,

chemically generated methoxy radicals have been reported to react with

aromatic hydrocarbons only to provide high yields of methanol and

benzylic dimers and not methoxylated products 207. The role of

electrolyte radicals in alkoxylation has been supported by several pieces



of experimental evidences 208,209. Attempted methoxylation of some

heterocyclics, leading to ring opening has some relevance in the

application of alkoxylations 210. These methods gain importance in the

light of their wide application in pharmaceutical industry in particular and

pilot chemical industries in general 58,211-212.

1. 10. DEVELOPING TRENDS IN ELECTROORGANIC CHEMISTRY

Some obvious developments in the field of electroorganic

synthetic processes are bound to come up in the new era.

Even for the commercially established processes, new cell

designs may be developed.

Attempts to develop some processes that have shown

promises at preparative scale to commercial level would

proceed.

Attempt to learn and utilize non-aqueous solvents in industrial

electroorganic processes would continue.

Continuous efforts would be made to replace or regenerate

costly redox reagents by electrochemical means.

New reagents such as superoxides, may find synthetic

applications.

1.10.1 Paired Electro Synthesis

Paired electro synthesis where both cathodic and anodic

processes are utilized for the preparation of electrochemicals 213, is



receiving renewed attention 214-216. One approach is to produce useful

products at both the electrodes. For example, on oxidation of sugar,

while calcium gluconate is produced at the anode, glucose itself is

reduced to sorbitol, on the Raney Nickel cathode.

1.10.2 Modified Electrodes

A wide range of chemicals is now being prepared on a few metal,

metal oxide and carbon electrodes only. However, attempts are now

directed towards synthesizing specific electrode materials attached with

inorganic, organic and organometallic electrocatalysts by means of

covalent linkage or electro adsorption. These electrodes can specifically

catalyze the desired electrochemical processes alone. These types of

‘catalyst – bound – electrodes’ are termed as ‘Chemically Modified

Electrodes’ 217. In the near future, one may expect the use of these

electrodes in electrosyntheses of stereospecific and optically active

compounds 218.

1.10.3 Solid Polymer Electrolytes

Electroorganic synthesis using solid Polymer Electrolyte [SPE]

cell is another direction which promises highly energy efficient, with

practically no iR drop, electrochemical route 219-222.

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