Esters

Esters

Methods of preparation

1. Esterification

We have already learnt that treatment of alcohols with carboxylic acids in presence of mineral acid gives esters. The reaction is carried to completion by using an excess of reactant or by removing the water from the reaction mixture.

2. Alcoholysis of Acid chloride or Acid anhydrides

Treatment of acid chloride or acid anhydride with alcohol also gives esters

Physical Properties

Esters are colour less liquids or solids with characteristic fruity smell. Flavours of some of the esters are given below.

Ester : Flavour

1. Amyl acetate: Banana

2. Ethyl butyrate: Pineapple

3. Octyl acetate: Orange

4. Isobutyl formate: Raspberry

5. Amyl butyrate: Apricot

Chemical Properties

1. Hydrolysis

We have already learnt that hydrolysis of esters gives alcohol and carboxylic acid.

2. Reaction with alcohol ( Transesterification)

Esters of an alcohol can react with another alcohol in the presence of a mineral acid to give the ester of second alcohol. The interchange of alcohol portions of the esters is termed transesterification

The reaction is generally used for the preparation of the esters of a higher alcohol from that of a lower alcohol.

3. Reaction with ammonia (Ammonolysis)

Esters react slowly with ammonia to form amides and alcohol.

4. Claisen Condensation

Esters containing at least one ∝- hydrogen atom undergo self condensation in the presence of a strong base such as sodium ethoxide to form b- keto ester.

5. Reaction with PCl₅

Esters react with PCl₅ to give a mixture of acyl and alkyl chloride

Evaluate yourself

Why is acid anhydride preferred to acyl chloride for carrying out acylation reactions ?