Esters are the functional group isomers of carboxylic acids.
Esters are formed when a molecule of an organic acid reacts with an alcohol.
The above reaction is a condensation reaction, as two small molecules have joined together to form a larger molecule with the elimination of a smaller molecule (water in the reaction above).
An acid catalyst is usually present in the above reaction, as the esterfication process would be too slow without it.
Esters have strong, sweet smells. They are used widely in glues, as solvents, and in fragrances.
Esters are named from the alcohols and acids from which they are derived. For example,
Ethanol and Ethanoic acid react to produce Ethyl Ethanoate
Phenol and Benzoic acid react to produce Phenyl Benzoate
There are three methods for making esters:
2) Acylation using an acid chloride:
The reaction of an acid and an alcohol to produce the ester and water. Requires a concentrated strong acid catalyst, such as 1 mol dm-3 H2SO4, and needs to be heated under reflux to ensure the reaction reaches equilibrium:
CH3COOH(aq) + CH3OH(aq) CH3COOCH3(aq) + H2O(l)
3) Acylation using an acid anhydride:
Acid chlorides are very reactive and fume in moist air forming hydrogen chloride. They react at room temperatures with alcohols/phenols to form esters.
CH3COCl(aq) + CH3CH2CH2OH (aq) CH3COOCH2CH2CH3(aq) + HCl (aq)
Less reactive than using an acid chloride (but more reactive than using carboxylic acids) and are easier to handle. They react, when warmed, with alcohols/phenols to produce esters.
(CH3CO)2O(aq) + CH3CH2CH2OH (aq) CH3COOCH2CH2CH3(aq) + CH3COOH (aq)
Hydrolysis of Esters
The reverse reaction of an esterfication reaction is a hydrolysis reaction.
Esters are reasonably unreactive compounds (used as solvents for this reason). They can be hydrolysed using acidic or alkaline conditions.
The conditions for this reaction are a moderately concentrated strong acid (catalyst) mixed with the ester, heated under reflux. After equilibrium is reached, no further hydrolysis takes place:
CH3COOCH3(aq) + H2O (l) CH3COOH (aq) + CH3OH (aq)
Sometimes referred to as saponification, this reaction uses dilute alkaline solution and reflux:
CH3COOC6H5(aq) + NaOH (aq) CH3COO-Na+ + C6H5OH (aq)
Useful books for revision
Revise A2 Chemistry for Salters (OCR A Level Chemistry B)
Salters (OCR) Revise A2 Chemistry