SEM-III, CORE COURSE-7

ORGANIC CHEMISTRY-3

TOPIC: CARBONYL AND RELATED

COMPOUNDS

Dr. Kalyan Kumar MandalAssociate Professor

St. Paul’s C. M. College Kolkata

PPT: 4

• OPPENAUER OXIDATION

• OXIDATION OF ALCOHOLS WITH PDC AND PCC

• PERIODIC ACID AND LEAD TETRAACETATE

OXIDATION OF 1,2-GLYCOLS

OPPENAUER OXIDATION

Oxidation of primary and secondary alcohols to aldehydes and ketones

respectively using aluminium tertiary butoxide in presence of a

hydride ion acceptor like acetone, cyclohexanone, benzophenone

p-benzoquinone, etc., is called Oppenauer oxidation.

Facts

• In case of oxidation of primary alcohol, non-enolisable ketones

with a relatively low reduction potential such as p-benzoquinone

is used.

• Tertiary butoxide is used as the reagent, since tertiary butanol

produced is not oxidised under these conditions.

• This reagent is particularly useful for oxidising unsaturated

secondary alcohols because it does not affect the double bond.

• Primary alcohols including the unsaturated alcohols may also be

oxidised to aldehydes if acetone is replaced by p-benzoquinone.

• In general, quinones and aromatic ketones are better hydrogen

acceptors than acetone.

• The reaction is completely reversible and therefore, to shift the

reaction towards the product-side, addition of large excess of the

hydride acceptor is used.

Mechanism

• Initially the starting alcohol reacts with aluminium t-butoxide toform a new alkoxide.

• This intermediate alkoxide then reacts with acetone or otheracceptor molecule and a hydride ion (H-) from the substrate alcoholis transferred to the carbonyl carbon atom of acetone.

• The reaction proceeds through the formation of a six-memberedcyclic transition state.

• The specific (H-) ion transfer from the carbon atom of alcoholmolecule to carbonyl carbon atom of acetone has beendemonstrated by using RCD(OH)R type labelled alcohol which ledto the formation of CH3CD(OH)CH3.

Mechanism

Oxidation of Primary Alcohols

FACTS

• The outcome of oxidation reactions of alcohols depends on the

substituents on the carbinol carbon and in order for each oxidation step to

occur, there must be at least one hydrogen atom on the carbinol carbon.

• Primary alcohols can be oxidised to aldehydes or further to carboxylic

acids.

• Oxidation of alcohols is normally carried out with Cr(VI) reagents but

these, like the Jones’ reagent (Na2Cr2O7 in H2SO4), are usually acidic.

– In aqueous media, the carboxylic acid is usually the major product.

– Some pyridine complexes of Cr(VI) compounds solve this problem by

having the pyridinium ion (pKa 5) as the only acid.

– Both PDC and PCC can convert alcohols into aldehydes and ketones,

especially in dichloromethane at room temperature and allow the

oxidation to be stopped at the intermediate aldehyde.

– PDC is less acidic than PCC and is therefore more suitable for the

oxidation of acid-sensitive substrates.

OVER-OXIDATION OF ALDEHYDES

• Aqueous methods like the Jones oxidation are no good for this,since the aldehyde that forms is further oxidized to acid via itshydrate. The oxidizing agent treats the hydrate as an alcohol andoxidizes it to the acid.

• The key thing is to avoid water—so PCC in dichloromethane worksquite well. The related reagent PDC (pyridinium dichromate) isparticularly suitable for oxidation to aldehydes

Pyridinium Chlorochromate (PCC)

Corey-Suggs Reagent

• Pyridinium chlorochromate is used as an oxidising agent in organicchemistry.

• The reagent is useful to oxidise primary and secondary alcohols tothe corresponding aldehydes and ketones respectively.

• The oxidation of primary alcohols to aldehydes is advantageous inthat sense that over oxidation to the corresponding carboxylic aciddoes not occur.

• PCC is also important for its high selectivity. For example, when anallyl alcohol is oxidized, unsaturated aldehyde is formed as soleproduct. This oxidation is known as Babler oxidation.

• Chlorochromic acid can by prepared by the dissolution of

chromium trioxide in 6 M aqueous hydrochloric acid. Addition

of pyridine gives pyridinium chlorochromate as orange crystals.

• PCC is soluble in many organic solvents, and especially

dichloromethane at room temperature has been used in most

cases, whereas DMF promotes the over-oxidation of primary

alcohols into carboxylic acids.

Preparation

Mechanism

Pyridinium Dichromate (PDC)

Pyridinium dichromate is the pyridinium salt of

dichromate that can be obtained by addition of pyridine

to a solution of chromium trioxide in water.

FACTS• Initially, PDC was used either as a solution in DMF or as a

suspension in dichloromethane. In DMF solution, PDC oxidizesprimary and secondary allylic alcohols to the corresponding α,β-unsaturated carbonyl compounds.

• Over-oxidation of primary allylic alcohols is not observed and(E,Z) isomerization does not take place.

Mechanism with PDC

OXIDATION OF 1,2-DIOLS

USING

• PERIODIC ACID (HIO4)

• LEAD TETRAACETATE [Pb(OAc)4]

• Cleavage of 1,2-diols to carbonyl compounds can be accomplished by periodic acid (HIO4) or lead tetraacetate (Pb(OAc)4]. The nature of the products depends on the nature of the substituents in the diol used.

FACTS

• Oxidation of 1,2-glycols occurs more rapidly with syn-diol than

with the corresponding anti-isomers and this fact indicates the

formation of cyclic intermediates.

• The fact that anti-isomers are oxidized suggest that the reaction

proceeds through a non-cyclic intermediate for these compounds.

This could be the non-cyclic ester formed in the first step.

• These oxidations are also subject to steric hindrance, e.g., glycol is

oxidised much faster than pinacol by periodic acid.

• The rate determining step for glycol oxidation is the fission of the

complex, whereas that for pinacol is the formation of the complex.

• The alcohol reacts to form a cyclic intermediate. The intermediate

then undergoes a rearrangement of the electrons, cleaving the C-C

bond and forming two C=O bonds.

Mechanism with HIO4

Mechanism for the ANTI-DIOL

Mechanism with Pb(OAc)4

QUESTION

Write the order of rates of oxidation of the following

1,2-diols and give reasons:

(i) HOCH2CH2OH

(ii) MeCH(OH)CH(OH)Me

(iii) Me2C(OH)C(OH)Me2

(iv) Me2C(OH)CH(OH)Me