In the presence of an acid or a base, esters undergo hydrolysis reactions to yield the corresponding acids or alcohols.
Under acidic conditions, the hydrolysis of esters is incomplete; conversely, under basic conditions, ester hydrolysis tends to proceed to completion. This is because, in a basic environment, hydroxide ions (OH⁻)-rather than water molecules-directly participate in the reaction.
Esters are neutral substances. Esters derived from lower-molecular-weight monocarboxylic acids undergo slow hydrolysis in water to form carboxylic acids and alcohols. Ester hydrolysis is more difficult than that of acyl chlorides or acid anhydrides, and thus requires catalysis by an acid or a base. Many naturally occurring fats, oils, and waxes can be hydrolyzed to produce their corresponding carboxylic acids; specifically, the sodium salts of higher fatty acids-generated through the alkaline hydrolysis of fats and oils-constitute what is known as soap. Alcoholysis of esters is a reaction in which the alkoxy group of an ester is displaced by the alkoxy group of another alcohol; this reaction must be carried out under acid or base catalysis and is frequently employed to convert one type of ester into another. Esters can be catalytically reduced to yield two molecules of alcohol; the most widely used catalyst for this process is copper chromite, and the reaction is conducted under conditions of high temperature and high pressure. If the ester molecule contains a carbon-carbon double bond, that bond may also be simultaneously reduced during the process. The reaction of esters with Grignard reagents allows for the synthesis of tertiary alcohols bearing two identical substituents.
The products of ester hydrolysis differ depending on whether the reaction takes place in an acidic or a basic environment. Under acidic conditions, the products are an alcohol (or phenol) and an acid. Under basic conditions, the products are an alcohol (or phenol) and a carboxylate salt.
Esters undergo a decomposition reaction-known as ammonolysis (or aminolysis)-upon reaction with ammonia (NH₃), primary amines (RNH₂), or secondary amines (R₂NH); this process generates an amide while releasing an alcohol or a phenol. Because ammonia and amines are basic in nature and possess strong nucleophilic character, the ammonolysis (or aminolysis) of esters proceeds more readily than does their hydrolysis or alcoholysis.
Ester condensation is a type of reaction involving the formation of a carbon-carbon bond.
