4.9 Esters

General formula C_nH_2nO₂ n ≥ 3

R— C=O

|

OR’

Nomenclature

C₃H₇— C=O butanoate from butanoic acid, C₃H₇COOH

|

OCH₃ methyl from methanol, CH₃OH

The alkyl group which has come from the alcohol is named first as the ……………….yl: methyl

The carboxylic acid part comes second as the ………………oate: butanoate

Exercise 1

(a) Name the following esters.

CH₃COOCH₃ ……methylethanoate…………………………………………………………………

C₂H₅COOCH₃ ……methylpropanoate…………………………………………………………………

C₃H₇COOC₃H₇ ……propylbutanoate…………………………………………………………………

C₂H₅COOC₂H₅ ……ethylpropanoate…………………………………………………………………

C₃H₇COOC₂H₅ ……ethylbutanoate…………………………………………………………………

C₂H₅COOC₃H₇ ……propylpropanoate…………………………………………………………………

(b) Write formulae for the following;

(i) propylethanoate

(ii) phenylpropanoate

(iii) phenylbenzoate

Preparation of a liquid ester from a carboxylic acid and an alcohol. Formation from alcohols using carboxylic acids and using acyl chlorides.

Preparation of esters

1 Acid and an alcohol

(see previous work)

2 Alcohol and acyl chloride (see previous work)

Hydrolysis of esters catalysed by acid and base.

Hydrolysis of esters

Like esterification, ester hydrolysis is a slow reaction but is speeded up by an acid catalyst. The mixture must be boiled under reflux.

H⁺

ESTER + WATER ⇌ ACID + ALCOHOL

H⁺

CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

ethylethanoate ethanoic acid ethanol

The reaction is also catalysed by alkali. This removes the organic acid from the mixture as it is formed. This means that ester hydrolysis can go to completion in the presence of an alkali. The hydrolysis of an ester by an alkali is called saponification Alkaline hydrolysis of an ester yields an alcohol and the salt of acid.

CH₃COOCH₂CH₃ + NaOH CH₃COONa + CH₃CH₂OH

ethylethanoate sodium ethanoate ethanol

If a dilute mineral acid, H⁺, is added to the products the carboxylic acid is reformed due to the reaction

CH₃COONa + HCl CH₃COOH + NaCl