Topic 4.5
COMPOUNDS CONTAINING THE CARBONYL GROUP
Aldehydes and Ketones
Carboxylic Acids and their Salts
Esters
Acyl Chlorides and Acid Anhydrides
ALDEHYDES AND KETONES
1. Introduction
Aldehydes and ketones are collectively known as carbonyls and represented by the general formula CnH2nO. They have the following functional group:
In aldehydes one of the groups attached to the carbon is a hydrogen atom and the other is
a hydrogen atom or an alkyl groups. In ketones both of the groups attached to the carbon are alkyl groups:
ethanal methanal butanal
butanone pentan-2-one
2. Redox reactions
The redox reactions involving carbonyls have been discussed at AS level and are summarized by the following diagram:
a) oxidation reactions
Carbonyls are formed by the oxidation of alcohols.
Partial oxidation of a primary alcohol results in the formation of an aldehyde.
Oxidation of a secondary alcohol results in the formation of a ketone.
Primary alcohols, secondary alcohols and aldehydes are oxidized using potassium dichromate (K2Cr2O7) in sulphuric acid (H2SO4). Gentle warming is necessary to start the reaction.
When making an aldehyde, care must be taken to prevent further oxidation to the carboxylic acid. This is achieved by using distillation apparatus so that the aldehyde can be distilled off as soon as it is formed.
The orange Cr2O72- is reduced to the green Cr3+.
Simplified equations for these oxidation reactions can be written using the symbol [O] as the oxidant:
Primary alcohol aldehyde
R-CH2OH + [O] R-CHO + H2O
Secondary alcohol ketone
R1-CH(OH)-R2 + [O] R1-CO-R2 + H2O
Aldehyde carboxylic acid
R-CHO + [O] R-COOH
The most important difference in the reactions of aldehydes and ketones is the fact that aldehydes can be readily oxidized to carboxylic acids but ketones are not readily oxidized....