Esters

Esters and their Classification

Definition

Esters are regarded as the alkyl derivative of carboxylic acids having characteristic ester group (- COOR). Esters are a class of organic compounds that are formed when the hydroxylic hydrogen atom of the carbonyl group is replaced by an alkyl group.

Type formula or Class formula

R - COO - R

 

General Formula

They form the homologous series with following general formulae:

a).        C_nH_2n+1---COOR          [n = 1, 2, 3, ------].

c).        C_nH_2n+2CO₂                 [n = 1, 2, 3, -----].

b).        C_nH_2nO₂                       [n = 2, 3, 4, -----].

 

Some important esters

1.   Methyl formate      (HCOO-CH₃)                         

2.   Methyl acetate      (CH₃COO-CH₃)

3.   Ethyl acetate         (CH₃COO-C₂H₅)                    

4.   Methyl propionate (CH₃CH₂COO-CH₃)

 

Functionally Isomeric with

Carboxylic acid

Type of Isomerism shown

1. Metamerism

2. Functional Isomerism

Physical properties of Ethyl acetate

1. It is a colourless fragrant liquid with pleasant fruity smell.

2. It boils at 77°C.

3. Its specific gravity is 0.9 (0.89).

4. It is less soluble in water but readily soluble in alcohol and ether.

5. Its freezing point is –83.6°C.

Uses of Ethyl acetate

1. It is used as a good solvent for paints, varnishes, oils, gums, etc.

2. It is used as a Plasticizer (to reduce brittleness and improve elasticity of plastic).

3. It is used as a synthetic fruit essence & for the preparation of artificial food flavours. Peculiar odours of different esters are listed below:

a) Apple ----- isoamyl valerate

b) Banana ----- isoamyl acetate

c) Orange ----- octyl acetate

d) Pineapple ----- methyl butyrate

e) Respberry ----- isobutyl formate

General Methods of Preparation of Esters

1. By treating alkyl halide with sodium carboxylate

2. By Esterification between acid and alcohol

3. By the action of acid halide on alcohol or sodium alkoxide

1. By treating alkyl halide with sodium carboxylate

Alkyl halide undergoes SN reaction with sodium carboxylate in which halide ion is replaced by carboxylate ion to form esters.

2. By Esterification between acid and alcohol

The reversible reaction of carboxylic acids with alcohol catalyzed by conc. H2SO4 to yield ester is called Esterification. In this reaction, alcohol loses hydrogen and carboxylic acid loses –OH group.

3. By the action of acid halide on alcohol or sodium alkoxide

Acid halide reacts with alcohol or sodium alkoxide to give ester.

Chemical Properties of Esters

1. Hydrolysis in acidic medium

Esters are hydrolyzed with water in acidic medium to form acid and alcohol. This is the reverse of Esterification.

2. Hydrolysis in basic medium

Esters are hydrolyzed with 10% NaOH in basic medium to form salt of acid (sodium carboxylate) and alcohol.

This reaction is used for the preparation of sodium salts of higher fatty acids called soaps and glycerol from naturally occurring esters commonly known as Fats and Oils. Oils and fats are trimesters of trihydric alcohol (glycerol) with long chain carboxylic acids called Fatty acids which on hydrolysis with 10% NaOH yield glycerol and sodium salt of higher fatty acids called soaps. This hydrolysis is called as Saponification.