Ester bond is formed when the carboxyl group of fatty acid combine with the hydroxyl group of glycerol. Esters are formed by the reaction of alcohol and carboxylic acid inthe presence of a sulphuric acid as a catalyst. The reaction is known as esterification.

Esters are composed of one carboylic acid and one alkohol. R1-COOH + HO-R2 => R1-CO-O-R2 +H20 Ester hydrolysis can separate the two again.

The above is a carboxylate ester.

Esters contain a carbonly center, which gives rise to 120 ° C–C–O and O–C–O angles. Unlike amides, esters are structurally flexible functional groups because rotation about the C–O–C bonds has a low barrier. Their flexibility and low polarity is manifested in their physical properties; they tend to be less rigid (lower melting point) and more volatile (lower boiling point) than the corresponding amides. The pka of the alpha-hydrogens on esters is around 25.