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Khadri et al. / Pharma Science Monitor 5(3), Jul-Sep 2014, 220-245

Pharma Science Monitor 5(3), Jul-Sep 2014

STUDY OF KINETICS OF SOME REACTIONS: A REVIEW

Khadri Shafeulla*1 and Dr. Sayyed Hussen2 1Research Scholar, Dept. of Chemistry, JJT University, Jhunjhunu-333001. 2Asst. Professor, P. G. Dept. of Chemistry, Sir Sayyed College, Aurangabad.

ABSTRACT Kinetic Reaction vitality deals with rates of blend techniques. Any substance methodology may be isolated into progression of one or more single step strategies alluded to their as simple systems, fundamental reactions, or essential steps. Fundamental reactions for most part incorporate their singular open effect between two molecules, which we imply as bimolecular step, or division/ isomerization of lone reactant molecule, which we insinuate as unimolecular step. Once in while, under conditions of two extraordinary degree high weight, termolecular step may happen, which incorporates synchronous effect of three reactant iotas. Basic point to see is that various reactions that are made as lone. This will end up being basic as we take in additional about theory of substance reaction rates. The main objectives to write this review articles are to provides information into how instrument is thought about, to provides understanding that can used to arrange designed pathway to new blends and to provides understanding that can to use to enhance execution of catalysts. KEYWORDS: Kinetic Reaction, Isomerization, Reactant molecule and Reaction rates.

INTRODUCTION

Chemical Kinetic of substance is not exactly how brisk reactants get changed over into thing

furthermore garing of all physical and compound strategies which happen in midst obviously of

reaction. Part of reaction gives clear photograph of sanctioned complex. It is said that

"framework is to science as semantic use is to lingo". Examination of vitality is truly isolated

into two areas. Principle part addresses rates of reactions what reaction rate suggests, how to

choose reaction rate by driving examinations and how components, for instance, meetings of

reactants and temperature effect rates. Second part is concerned with effect speculation of

reactions and with mechanisms, point by point pathway taken by particles and molecules as

reaction proceeds. Every mixture reaction completed in single step or in different step, to ponder

complete reaction. Examinations of endeavours of reaction instrument are fundamental. We will

dissect instruments of both substitution and redox structures encountering noteworthy change

metal buildings.

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AN INTERNATIONAL JOURNAL OF PHARMACEUTICAL SCIENCES

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Hydrolysis of ester in dilute acid solution:

An ester, such as methyl acetate, is hydrolysed by water to give acid alcohol.

Response is moderate in unadulterated water, however in vicinity of weaken mineral corrosive,

response goes for all intents and purposes to culmination at sensible rate.

Rate of hydrolysis of methyl acetic acid derivation is trailed by measuring measure of acidic

corrosive framed at clear time interims. This is finished by pulling back examples of response

blend at unequivocal time interims. Proposals tests are quickly cooled by filling super cold carafe

and titrated against standard soluble base. Cooling is important to keep any further response in

example pulled back amid titration. This procedure is known as solidifying response. Measure of

soluble base expended whenever relates to amount of corrosive created and subsequently,

amount of ester disintegrated up to that time.

This responses takes after first request energy, albeit two atoms partake in change. Dynamic

mass of water does not change obviously in weaken arrangement, in light of fact that water is

available in huge abundance. Particular rate steady may n by computed as takes after;

Where is initial titre value, is titre value at any time t, and is final titre value, when

reaction has gone to completion. Value is, generally, taken after expiry of 24 hours.

Saponification of ethyl acetate by alkali, say NaOH, is represented as follows:

As response continues, NaOH is devoured, subsequently advancement of response can be trailed

by deciding measure of lingering NaOH in response blend at diverse interims of time.

A known volume of weaken arrangement of ethyl acetic acid derivation of known fixation

(generally 100 mL of M/40 concn.) in cup, which is kept up in indoor regulator at positive

temperature. At distinct interims of time (say 10 minutes), 5 mL of response blend is pulled back

and instantly filled known volume of overabundance of standard corrosive arrangement, say 5

mL of M/40 HCl (to check response all things considered). Overabundance of corrosive is n

titrated against standard soluble base arrangement (M/40 NaOH arrangement) utilizing

phenolphthalein as marker. Convergence of lingering soluble base (a–x) around n in blend is

figured straightforwardly from titre esteem.

3 3 2 3 3CH COOCH H O CH COOH CH OH

02.303 2.303log log

( ) t

T Tak

t a x t T T

0T tT T

T

3 2 5 3 2 5CH COOC H NaOH CH COONa C H OH



Trial information hence got is n substituted in rate consistent statements of to begin with, second,

third request turn by turn, and estimation of k figured for every situation. Comparison which

gives concordant (steady) estimation of k relates on request of response. Outcomes have

demonstrated that this response take after second order kinetics, i.e.

Inversion of Cane-sugar is represented as:

Cane-sugar Glucose Fructose

(Dextro-rotattheory) (Laevo-rotattheory)

Laevo-rotattheory

This change can be effortlessly trailed by taking note of turn of plane of polarization of light in

polarimeter, on grounds that change from sucrose (dextro-rotattheory) to glucose-fructose blend

(laevo-rotattheory) realizes adjustment in revolution, which is corresponding to measure of

genuine sweetener hydrolyzed.

Let be angles of rotation (with correct signs, positive for dextro-rotattheory)

and negative for laevo-rotattheory) at beginning and end of experiment. Let be rotation after

time t. This means that is directly proportional to initial concentration of sucrose, i.e.,

‘a’ in kinetic equation and is directly proportional to amount of sucrose remaining after

time t, i.e., (a–x).

And

Substituting se values of and in first order kinetic equation, we get:

Or

From this equation, specific rate constant k can be evaluated by direct calculations (or

graphically).

Alcohols

( )

xk

a t a x

12 22 11 2C H O H O 6 12 6 6 12 6C H O C H O

0 and r r

1 r

0r r

tr r

0a r r

ta x r r

a x

2.303

loga

kt a x

02.303log

t

r rk

t r r



Alcohols are compound of general equation R – OH, where R is any alkyl, or substituted alkyl

bunch. In this way, alcohols are subordinates of alkanes in which one or more H–atoms are

supplanted by – OH bunches. At end of day, every single natural compound in which one

hydroxyl bunch (–OH) is joined to soaked carbon particle are termed as liquor, i.e., liquor may

be portrayed as take:

Alcohols are very important industrial chemicals. Ethanol is widely used as

antiseptic in form of rectified spirit, it is main component of all alcoholic beverages, and is used

as solvent for lacquers and varnishes.

Some alcohols occur in nature and are used for manufacture of perfumes and flavours due to ir

pleasant odour. For example,

Electronic Structure of Hydroxyl group in alcohols

Hydroxyl group, – OH, in alcohols contains one hydrogen atom and one oxygen atom.

Oxygen atom has six electrons in its outermost shell. s and p orbits of valence

shell are hybridized to form four hybrid orbitals oriented tetrahydrally around oxygen

atom. Two of hybrid orbitals are singly occupied, while or two are occupied by two electrons

each.

Or two hybrid orbitals of oxygen atom are occupied by two lone pair of electrons, one in

each.

Fig. 1: Electronic structure of – OH groups in alcohols.

Classification of Alcohols

Alcohols can be classified on following basis:

A. On basis of number of –OH groups present in molecule of alcohol.

2 5( )C H OH

2 2 4(1 2 2 )s s p

3sp 3sp

3sp



B. On basis of nature of carbon atom bonded to –OH group.

A. Classification of Alcohols on basis of Number of –OH groups

Alcohols are classified on basis of number of hydroxyl groups present in molecule of alcohol.

i. Alcohols containing one –OH group in ir molecules are called monohydric alcohols. For

example, are monohydric alcohols

ii. Alcohols containing two –OH groups in ir molecules are called dihydric alcohols. For

example, is dihydric alcohol.

iii. Alcohols containing three – OH groups in ir molecule are called trihydric alcohols. For

example,

is trihydric alcohol.

iv. Alcohols containing four or more –OH groups in ir molecules are termed polyhydric

alcohols.

It should be noted that in molecules of alcohols having more than one –OH groups, each –OH

group is attached to different carbon atom.

i. Monohydric Alcohols

Alcohols which contain only one –OH group in ir molecules are called monohydric alcohols.

Monohydric alcohols can be represented by general formula . Monohydric

alcohols may be primary, secondary tertiary alcohol.

a. Classification of Monohydric Alcohols

Monohydric alcohols can be further classified on basis of nature of hybridization of carbon atom

attached to hydroxyl (–OH) group.

3 2 5,CH OH C H OH

2

2

CH OH

CH OH

2

2

CH OH

CHOH

CH OH

2 1 or n nC H OH ROH



Monohydric Alcohols containing Bond

In se alcohols, –OH group is attached to hybridized carbon atom of alkyl group. These are

further classified as follows;

Primary, Secondary and Tertiary alcohols.

When carbon atom having –OH group is attached to only one carbon atom, alcohol is

termed as primary (1 ) alcohol.

Example: alcohol having formula R – CHOH is primary alcohol

When carbon atom having –OH group is attached to two carbon atoms, alcohols is

termed as secondary (2 ) alcohol.

Example: alcohol having formula R2 – CHOH is secondary alcohol.

When carbon atom having –OH group is attached to three carbon atoms, alcohols is

termed as tertiary (3 ) alcohol.

Example: alcohol having formula R3 – CHOH is secondary alcohol.

Structural formulae of primary, secondary and tertiary alcohols are given below:

In structure II and III, R is any alkyl group, but not hydrogen.

Structural units in se alcohols are shown below:

Allylic alcohols

In allylic alcohols, –OH group is attached to hybridized carbon next to carbon-carbon

double bond, that is to allylic carbon. For example

Benzylic alcohols

In se alcohols, –OH group is attached to hybridized carbon atom bonded to aromatic ring.

3spC OH

3sp

3sp

3sp



Allylic and benzylic alcohols may be primary, secondary or tertiary.

Monohydric Alcohols Containing Bond

In se alcohols, –OH group is attached to hybridized carbon, i.e., to carbon that is bonded to

carbon atom by double bond.

Such carbon atom is called vinylic carbon, and alcohols in which –OH group is attached

to vinylic carbon are called vinylic alcohols.

Compound in which –OH group is attached to hybridized carbon of aryl ring is

called phenol.

Formulae of typical vinylic alcohols and phenols are given below;

PHYSICAL PROPERTIES:

Solubility:

i. First three members of alcohols are highly soluble in water but solubility decreases

with increasing molecular weight.

ii. Solubility of alcohol in water is due to formation of hydrogen bonding between

alcohols and water molecules. Extensive hydrogen bonding liberates large amount of

energy and stabilize systems. Energy of hydrogen bonding is 21 kJ/mole.

iii. Solubility of higher members in water is less due to, alcohols possess both hydrophilic

(water loving) polar –OH group and hydrophobic (water hating) nonpolar alkyl group.

Thus increases molecular mass of alcohol hydrophobic character increases. This

decreases ir water solubility. In isomeric alcohols, solubility increases with increases

2spC OH

2sp

2sp

H R H R

O H O H O H O H



in branching because branching results decreases in relative volume and hydrophobic

portion.

iv. Solubility of alcohols also depends upon tendency to form hydrogen bond with water.

More tendency to form hydrogen bond with water more solubility of alcohols.

When molecular mass and molecular size increases tendency to form hydrogen bond with water

decreases, hence solubility decreases.

Boiling points and melting points:

i. Lower members of alcohols have low boiling point, it increases with increasing

molecular weight. This is because of increases in Vender Waal’s force.

ii. In isomeric alcohols branched alcohols have lower boiling point than normal alcohols.

Because branching results compactness in molecule, decrease surface are and decrease

Vender Waal’s force. Hence boiling point decreases.

iii. It may be noted that alcohols have higher boiling point, alkyl halides, and alkanes of

comparable molecular mass. This is due to presence of intermolecular hydrogen

bonding in alcohols.

Toxicity (Narcotic action):

All alcohols are toxic in nature. Methyl alcohol is highly toxic and is not good for drinking

purpose. Ethyl alcohol is less toxic and have been used for drinking purpose.

Density:

These are lighter than water. Although density increases as molecular weight increases.

State:

Lower alcohols are liquids having pleasant smell and burning taste. Middle members C4 to C12

are oily liquids. While higher alcohols having more than 12 carbon atoms are wax like

colourless tasteless solids.

Nature:

Alcohols are neutral but they act as Lewis bases, because of presence of lone pair of electrons on

oxygen. These are neutral towards litmus. Ethanol is highly hygroscopic.

Oxidation of Primary, Secondary, and tertiary alcohols

Distinction of Primary, Secondary, and tertiary alcohols by oxidation reaction: Oxidation of

alcohols involves breaking of O–H and C–H bonds and formation of C–O bond.

R R R

O H O H O H



Hence oxidation reactions are also known as degradation reaction (loss of hydrogen atom from

alcohol) product of oxidation reaction depends upon types of alcohols i.e., ( ) and also

nature of oxidizing agents.

Primary alcohols: Primary alcohol is easily oxidized at room temperature to form carboxylic

acid. Carboxylic Acid formed contain same number of carbon atoms as starting alcohols. When

methanol is reacted with , gives methanal and n formic acid containing

same number of carbon atom.

i) .

ii)

iii)

iv)

1 2 3

2 2 7 2 4.K Cr O dil H SO

3 2

.2 2 7 2 4

methanol methanal

formic acid

PCC

K Cr O dil H SO

CH OH O HCHO H O

HCHO O HCOOH

2 5 3 2

.2 2 7 2 43 3

ethanol ethanal

acetic acid

PCC

K Cr O dil H SO

C H OH O CH CHO H O

CH CHO O CH COOH

3 2 2 3 2 2

.2 2 7 2 43 2 3 2

1-Propanol propanal

PCC

K Cr O dil H SO

CH CH CH OH O CH CH CHO H O

CH CH CHO O CH CH COOH

Propionic acid

3 2 2 2 3 2 2 2

.2 2 7 2 43 2 2 3 2 2

1-butanol butanal

PCC

K Cr O dil H SO

CH CH CH CH OH O CH CH CH CHO H O

CH CH CH CHO O CH CH CH COOH

butyric acid

3 2 3 22 2

.2 2 7 2 43 32 2

iso-butyl alcohol iso-butyraldehyde

iso-butyric acid

PCC

K Cr O dil H SO

CH CHCH OH O CH CHCHO H O

CH CH CHO O CH CH COOH



Secondary alcohols: These are oxidized by or PCC to form ketones.

Ketones may be further oxidised under strong (drastic) condition by breaking of C–C bond and

form mixture of acids.

While ketone contains same number of carbon atom at starting alcohol, acid formed contain

lesser number of carbon atoms.

In case of unsymmetrical ketones, breaking of C–C bond occurs in such way that keto group

retained on smaller alkyl group. This is known as Popff’s rule.

e.g.

i. When isopropyl alcohol is oxidised , gives acetone and n acetic

acid contains one carbon atom less than alcohol.

ii. When 2-butanol on oxidation gives ethyl methyl ketone. In oxidation of ethyl methyl

ketone major mode of bond breaking is according to Popoff’s rule in which methyl group

retained with keto group and produces acetic acid and formic acid as major product.

Major mode of C–C bond breaking is according to Popoff’s rule.



3 3

.2 2 7 2 43

K Cr O dil H SO

CH CH

CH CH OH O

3 2

2 Propanol acetone

CH C O H O

3

.2 2 7 2 43 drastic o

4K Cr O dil H SO

CH

CH C O O

3 2 2xidation

acetic acid

CH COOH CO H O

2 5 2 5

2 2 73

K Cr O d

C H C H

CH CH OH O

. 2 4

3 2

2-butanol ethyl methyl ketone

il H SOCH C O H O



Tertiary alcohols: Tertiary alcohols are stable towards oxidizing agents, because of absence of

atom but when heated with strong oxidizing agent like they easily

get dehydrated.

Tertiary alcohols are oxidized by , first gives alkene and n ketone which on

further oxidation gives acid, both containing lesser number of carbon atoms.

e.g.

Actually, formic acid is obtained along with acetic acid but it readily undergoes further oxidation

gives and water.

Oxidation Reaction of Some Organic Compound

Reagents for oxidation are:

Aluminium tert. Butoxide, in acetone oxidises 1 and 2 alcohols

(Particularly) into aldehydes and ketones (Oppenauer oxidation). N-Bromosuccinimide (NBS)

also oxidises primary and secondary alcohols to aldehydes and ketones.

1. Pyridinium chlorochromate (PCC) in (Sarret reagent) oxidises primary

alcohols to aldehydes, [PCC is equimolar mixture of , HCl and pyridine i.e.,

or Corey’s reagent].

H .2 2 7 2 4K Cr O dil H SO

.2 2 7 2 4K Cr O dil H SO

3 3

.2 2 7 2 43 2strong heat

K Cr O dil H SO

CH CH

CH C OH CH

2 2

3 3

C H O CO

CH CH

iso butylene

3 3

.2 2 7 2 42 drastic oxidation

4K Cr O dil H SO

CH CH

CH C O

3 2 2

3

acetone

CH C O H O CO

CH

3

.2 2 7 2 42 3 2 2drastic oxidation

4

acetic ac

K Cr O dil H SO

CH

CH C O O CH COOH H O CO

id

2CO

3 3 3CH CO Al

2 2CH Cl

3CrO

3 5 5 5 5 3 or CrO C H N HCl C H NH ClCrO



2. Pyridinium dichromate (PDC) is and it oxidises primary alcohols to

aldehyde and secondary alcohol to ketone (in excellent yield) without affecting carbon-carbon

multiple bonds.

DMSO in (COCl)2 and also oxidises primary alcohols to aldehyde without affecting

carbon-carbon multiple bonds (Swern reaction).

Tosyle derivative of alcohols is also oxidised by DMSO.

3. MnO2 selectively oxidises –OH group of allylic and benzylic 1 and 2 alcohols to

aldehydes and ketones respectively and so is Collins reagent (one mole of and 2 mole of

pyridine in ).

4. Jones reagent ( , aqueous and acetone as solvent) oxidises alcohol without

affecting carbon-carbon multiple bonds, allylic or benzylic C–H bonds and or acid sensitive

groups. This reaction is carried out at 273 K.

At higher temperature (above 298 K), aldehydes convert into carboxylic acids.

5. Oxidation of Glycerol

It gives different oxidation products depending on nature of oxidizing agent. Following products

may be visualized during oxidation of glycerol.

25 5 2 7

2C H NH Cr O

/ 2 26 5 2 6 5

PDC CH ClC H CH CHCH OH C H CH CHCHO

CHO

O

CH2OH

OH

2 5 3C H N

( )2 ( ) / 3

i TsCl

ii DMSO NaHCOR CH OH R CHO

3CrO

2 2CH Cl

22 2 2(acetone)

26 5 3 6 5 3(CCl )4

MnO

MnO

H C CH CH OH H C CH CHO

OH O

C H CH CH C H C CH

3CrO 2 4H SO

3 2 4

2 2 23 22

Allyl alcohol Acrolein

CrO H SO

CH CO H OH C CH CH OH H C CH CHO

, acetone, aq. 3 2 42 273 K

CrO H SOHC C CH CH CH OH HC C CH CH CHO



Reaction due to ehteral Oxygen

Oxidation and peroxide formation: Add up atmospheric oxygen or ozonised oxygen through

coordination of one of lone pair of oxygen to form peroxides in presence of sunlight or

ultraviolet light.

Peroxide of diethyl er is heavy, pungent oily liquid unstable compound and decomposes

violently on heating. There is concept of formation of peroxide in long contact with air and light

according to following reaction.

All that have been exposed to atmosphere for any length of time invariably contain peroxides.

Boiling point of peroxides is higher. It is left as resides in distillation and may cause explosion.

For this reason, there should never be evaporated to dryness. It is always essential to remove

peroxide by shaking with ferrous salt solution or distillation with conc. Traces of water and

alcohol. This is called Absolute and can be prepared by distillation of ordinary from

concentrated and subsequent storing over metallic sodium.

Note:

1. Formation of peroxide can be prevented by adding small amount of Cu2O.

2. With strong oxidizing agents like acid dichromate are oxidised to aldehydes and n to

acetic acid.

2 5 2 5 2 5 2 5 2 5 2: or

: :

C H OC H O C H OC H C H O O

O

3 2 2 3 2 3 2 3

Diethyl ether

Light

OOH

CH CH O CH CH O CH CH O CH CH

Peroxide of diethyle ether

(1-Ethoxy ethyl hydroperoxide)

2 4H SO



Oppenauer Oxidation: Secondary alcohols can be oxidised to ketones by aluminum tert.

Butoxide, . Secondary alcohol is refluxed with reagent and n acetone (or

Cyclohexanone or p-benzoquinone, etc) is added.

Secondary alcohol oxidises to ketone and acetone reduces to isopropyl alcohol.

Unsaturated sec. alcohol can be oxidised to unsaturated ketones (without affecting

double bond) by this reagent.

By oxidation of 1, 2-glycols with lead tetra-acetate or periodic acid (HIO4)

1, 2- Glycols undergo cleavage to form carbonyl compounds. This reaction is not observed with

non-vicinal glycols.

23 2 2 3 3 2

3

2

Diethyl ether Acetaldehyde

2 2

OCH CH OCH CH CH CHO H O

O CH COOH

3 3 3CH CO Al

3

R

R

3 3 3

R

CHOH CH CO Al

R

3 3

3

CHO Al CH C OH

R

R

3

33

3

CH

CHO Al

CH

R

C O

R

3

3

H C

C O

H C

3

CHO Al

'

R

R

3

3

H C

CHOH

H C

3 3 3

'

CH CO AlR

C O

R

3

3

H C

C O

H C

sec. alcohol Acetone (excess) Ketone Isopropyl alcohol

(2 )

CHOH

C C

OH O

CH

Aluminium tert. butoxide3 2 2 3 2 2cyclohexanone

Pent-4-en-2-ol Pent-4-en-2-one

CH CH CH CH CH C CH CH CH

HO O

Al[OCH(CH3)2]3

Acetone

Cyclohex-2-en-ol Cyclohex-4-en-1-one



Oxidation by selenium dioxide (SeO2): Selenium dioxide oxidises reactive methylene group

adjacent to carbonyl group at –carbon to form dicarbonyl compounds. For example,

A reactive methylene group adjacent to group at –carbon forming part of ring also

oxidises to ketonic group, for example,

OH OH

R

3 3 34 2' ' 2

Aldhehydes

CH CH R Pb OOCCH RCHO R CHO CH COOH CH COO Pb

R

R

C C

OH OH

'

'

OR R

R R

'

'

R

C O

R

Ketones

C O

2 2 2

Acetaldehyde

H CH CHO SeO H C C H Se H O

O O

Glyoxal

2 3 2 3 2

Acetone

O

HCH C CH SeO H C C CH Se H O

O O

Methyl glyoxal

3 2

O O O O O

CH C CH

3 2 3 3 2 3 2

Ethyl methyl ketone Butan-2, 3-dione Ethyl glycoxal

CH SeO H C C CH H C C CH CH Se H O

(Dimethyl glyoxal) (Minor)

(Major)

C O

SeO2/H2O

O O

O

Cyclohexanone Cyclohexa-1,2-dione



Oxidation of isopropyl benzene (Cumene):

Cumene is oxidised with oxygen or air into cumene hydroperoxide in presence of catalyst. This

is decomposed by dilute sulphuric acid into phenol and acetone.

Reaction Mechanism:

By mechanism of chemical reaction, we mean actual series of discrete steps which are involved

in transformation of reactants into products. Now since organic compounds are covalent,

organic reactions involve:

A. breaking of old covalent bond, and

B. Making of new covalent bond.

First step, i.e. breaking of covalent bond between two atoms can take place mainly in two

alternative ways, viz homolytic or heterolytic fission depending upon relative electronegativity

of two concerned atoms, e.g.

(1) Homolytic fission takes place when two atoms (say and B) are usually of similar

electronegativity.

(2) Heterolytic fission takes place when two atoms (A and B) are of different

electronegativities. It may again take place in two different ways.

(a) Where is more electronegative than B :

(b) When B is more electronegative than A:

+ CH3CH2CH2Clor (CH3CH=CH2)

AlCl3, 423 K

or H3PO4

(F.C. alkylation)Benzene

CH CH3

CH3

Isopropylbenzene (Cumene)

Catalyst

at 373 KCumene

CH

H3C CH3

O2

353 K

Cumene

C(CH3)2

H2O/H+

Hydroperoxide

O OH

Phenol

OH

+ (CH3)2 CO

Acetone

A : B A + B

Free radicals



Different Intermediate formation during oxidation reaction

Free Radicals:

A free radical may be defined as atom or group of atom having odd or unpaired electron. These

results on account of hemolytic fission of covalent bond and are denoted by putting dot ( )

against symbol of atom or group of atoms.

Formation of free radical is initialed by heat, light or catalyst.

Characteristics of free radicals:

i. Free radicals are formed by hemolytic bond fission.

ii. They are short lived reaction intermediates.

iii. They show paramagnetic character due to presence of unpaired electrons.

iv. They show following three types of reactions:

(a) Mutual combination of free radical forms neutral molecules.

(b) Reaction between free radical and neutral molecule gives new radical.

2 ( : )Cl Cl Cl Cl Cl

3 3 3 3 3 3 ( : )CH CH CH CH H C CH

4 3CH Cl CH HCl



(c) A free radical can lose neutral molecules to form new radical.

Relative Stabilities of free radicals:

Tiertiary alkyl free radicals are most stable and methyl free radical is least stable, i.e.

free radical formed easily has greater stability.

Stability of carbon (alkyl) free radical is not influenced by inductive effect because y have no

charge (difference from carbonium ions and carbanions). However, y are stabilized by hyper

conjugation (no-bond resonance). Relative order of stabilities of some common free radicals is

given below:

Triphenylmethyl > benzyl > allyl > tertiary > secondary > primary > methyl > vinyl

Extra stability of aromatic acid allyl radicals is due to resonance. Relative stability order of ter.,

sec. and pri-free radicals is explained on basis of hyper conjugation.

Structure of alkyl free radicals:

Carbon atom of alkyl free radicals which is bonded to only three atoms or groups of

atom is hybridized. Thus, free radicals have planar structure with odd electron situated in

unused p-orbital at right angle to plane of hybrid orbitals.

Heterolytic fission or heterolysis:

3 3 2

Acetate radical Methyl radical

CH COO CH CO

2sp



This involves breaking of covalent bond in such way that both electrons of shared pair

are carried away by one of atoms.

This type of fission occurs when two atoms differ considerably in ir electronegativities.

Electron pair is carried away by atom which is more electronegative in comparison to or.

Heterolytic fission leads to formation of charged or ionic species, one having positive

charge and or negative charge. This type of fission occurs most readily with polar compounds in

polar solvents like water or alcohol and is influenced by presence of ions due to acid and base

catalyst. Quite often ionic species formed by heterolytic fission bear positive or negative charge

on carbon atom. Such ionic species are known as carbonium ions or carbanions according to

charge which carbon atom carries (positive or negative).

1. Carbonium ions (carbocations):

When covalent bond, in which carbon is linked to more electronegative atom or group, breaks up

by heterolytic fission, more electronegative atom takes away electrons pair while carbon loses its

electron and thus acquires positive charge.

Such organic ions carrying positive charge atom are known as carbonium ions or carbocations.

Carbonium ions are named by adding words ‘Carbonium ion’ to parent alkyl group. These are

also termed as primary, secondary, tertiary, depending upon nature of carbon atom bearing

positive charge.

Formation of carbonium ions

(i) By Heterolysis: y are formed by heterolysis of halogen compounds.

(ii) By protonation of alkenes or alcohols:

3 33 3H C C Cl CH C Cl



(iii) By decomposition of Diazo compounds:

Characteristics of carbonium ion:

i. It formed by heterolytic bond fission.

ii. Carbon atom carrying positive charge has six electrons in its valence shell, i.e. 2 electrons

less n octet.

iii. It is short lived reaction intermediates.

iv. Carbonium ion is diamagnetic in nature.

v. Due to electron deficiency, it behave as Lewis acid.

Structure of carbonium ion:

Positively charged carbon atom in carbonium ion is in state of hybridization three

hybridized orbits which lie in same plane or involved in formation of three bonds with or atoms

while unhybridized p-orbitals remains vacant. Carbonium ion has planar structure.

Stability of carbonium ion:

Stability of carbonium ions is influenced by both resonance and inductive effects. Alkyl group

has electron releasing inductive effect. Alkyl group attached to positively charged carbon of

carbonium ion tends to release electrons towards that carbon. In doing so it reduces positive

charge on carbon. In or words, positive charge gets dispersed as alkyl group becomes somewhat

positively charged itself. This dispersal of charge stabilities carbon ion.

More number of alkyl groups, greater dispersal of positive charge, and more stability of

carbonium ion is observed.

2 2 2 3

22 2

H

H H O

CH CH CH CH

R O H R O H R H O

26 5 2 6 5 2 6 5 2

Cl NC H N Cl C H N C H N

2sp



Stability decreases as +I decreases (dispersal of positive charge decreases)

Stability decreases as molar mass decreases or +I effect decreases.

Allyl and benzyl carbonium ions are much more stable as se stabilized by resonance.

Group like –NO2 and –Br which have –I effect reduce stability of carbonium ions.

2. Carbanions:

When covalent bond, in which carbon is attached to lesser electronegative atom, breaks up by

heterolysis atom leaves without taking away bonding pair of electrons and thus carbon atom

acquires negative charge due to extra electron.

Such organic ions which contains negatively charged carbon atom are called carbanions. These

are named after parent alkyl group and adding word carbanions.

These are also termed as primary, secondary and tertiary depending on nature of carbon atom

bearing negative charge.

Organic compounds which possess labile or acidic hydrogen have tendency to produce

carbanions as in case of reactive methylene compounds which lose proton in presenece of

sodium ethoxide ( )

3 2 2 2 3 2 2 3 2 3CH CH CH CH CH CH CH CH CH CH

2 2 2 2 (Allyl)CH CH CH CH CH CH

2 5C H ONa



Characteristics of carbanions:

i. It is product of heterolytic bond fission.

ii. It is short lived reaction intermediates.

iii. Carbon atom carrying negative charge has eight electrons in its valence shell.

iv. It behaves as Lewis base.

Structure of carbanions:

Negatively charged carbon is in state of hybridization. Hybrid orbitals are directed towards

corners of tetrahedron. Three of hybrid orbitals are involved in formation of single covalent

bonds with or atoms while fourth hybrid contains lone pair of electrons. Thus, carbanions have

pyramidal structure similar to NH3 molecules.

Stability of Carbanions: stability of carbanions is influenced by resonance, inductive effect and

s-character of orbitals. Group having +I effects decrease stability while groups having –I effect

increases stability of carbanions.

Stability decreases as +I effect increases (Methyl > 1 > 2 > 3 carbanions). Alkyl and

benzyl carbanions are stabilized due to resonance.

Stability of carbanions increases with increase in s-character of orbitals.

2CH2 5

2 5

2 5

COOC H

C H O CH

COOC H

2 5

2 5

2 5

COOC H

C H OH

COOC H

3sp



Reagents: Most of attacking reagent carry positive or negative charge. Positively charged

reagents attack regions of high electron density in substrate molecule while negatively charged

reagents will attack regions of low electron density in substrate molecules. Fission of substrate

molecule to create centre of high or low electron density is influenced by attacking reagents.

Most of attacking reagents can be classified into two main groups.

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For Correspondence Khadri Shafeulla Email: [email protected]

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