The step in the mechanism for the formation of an ester from an acid and an alcohol are the reverse of the steps for the acid-catalyzed hydrolysis of an ester. The reaction can go in either direction depending on the conditions used. A carboxylic acid does not react with an alcohol unless a strong acid is used as a catalyst, protonation makes the carbonyl group more electrophilic and enables it to react with the alcohol, which is a weak nucleophile.

An alcohol is capable of displacing another alcohol from an ester. This alcoholysis (cleavage by an alcohol) of an ester is called transesterification. Transesterification is catalysed by acid (`H_2SO_4` or dry `HCl`) or base (usually alkoxide ion). Transesterification is an equilibrium reaction. To shift the equilibrium to the right, it is necessary to use a large excess of the alcohol. See fig.1.

The difference in the boiling points of the alcohols allows the equilibrium to be shifted toward the higher molecular weight ester by distilling the methanol out of the reaction mixture. Esters are hydrolysed by acids or alkalis : See fig.2.

Acidic hydrolysis is reversible and hence the mechanism for hydrolysis is also taken in the opposite direction of the mechanism for esterification. When hydrolysis is carried out with alkali the carboxylic acid is obtained as its salt. This reaction is essentially irreversible, since a resonance stabilized carboxylate anion show little tendency to react with an alcohol. See fig.3.

Since the alkali salts of the higher acids are soaps, alkaline hydrolysis is known as saponification; Saponification is far more rapid than acid hydrolysis. If an ester is hydrolyzed in a known amount of base (taken in excess), the amount of base used up can be measured and used to give the saponification equivalent; the equivalent weight of the ester, which is similar to the neutralization equivalent of an acid.

i) Catalytic hydrogenation : See fig.1.

ii) Chemical reduction is carried out by use of sodium metal and alcohol, or more usually by use of lithium aluminium hydride. See fig.2.