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An ester has a carbonyl group bonded to an alkyl or aryl group on one side and an oxygen bound to another alkyl or aryl group on the other side, represented by the general formula RCOOR'. The R and R' groups determine the ester's properties, including its smell and flavor. Simple esters with small functional groups often have fruity or flowery aromas.

Fischer esterification combines a carboxylic acid and alcohol in the presence of an acid catalyst, typically sulfuric acid, to form an ester and water. The catalyst protonates the carbonyl oxygen, activating it for nucleophilic attack by the alcohol. The reaction is reversible and proceeds through an intermediate before water is eliminated and the ester is deprotonated to neutrality.

Ester properties depend heavily on their R and R' alkyl or aryl groups. Even adding a single carbon atom or changing one functional group significantly alters the molecule's scent. For example, propyl acetate smells like pears, while butyl acetate, with one additional carbon, smells like apples. Changing a hydroxyl group to an amine shifts the scent from wintergreen to grape.

A 1:1 molar ratio of carboxylic acid to alcohol reaches equilibrium with only about 70% ester yield. By increasing the concentration of one reactant—typically using excess alcohol—the equilibrium shifts to counteract this change, producing more ester. Additionally, sulfuric acid acts as a dehydrating agent, removing water and further driving the reaction toward product formation.

Triglycerides form when three fatty acids, which are carboxylic acids with long alkyl chains, react with glycerol. Each reaction creates an ester group, resulting in a molecule with three ester linkages and very high molecular weight. Triglycerides serve as biological energy storage and are commonly used industrially in soap and detergent production.

Simple esters have low molecular weight and small functional groups, producing fruity or flowery aromas found in fruits and flowers. Long-chain esters, called fatty acids, have extended alkyl chains and high molecular weight. Fatty acids are essential biological compounds for energy storage and cellular processes, whereas simple esters are primarily valued for their distinctive fragrances.

Sulfuric acid serves as both an acid catalyst and a dehydrating agent in Fischer esterification. It protonates the carbonyl oxygen of the carboxylic acid, activating it for nucleophilic attack and increasing the reaction rate. By sequestering water molecules away from the reaction, sulfuric acid removes a product and shifts equilibrium toward ester formation, significantly improving yield.