ester

The smell of ripe banana, pineapple, jasmine, and the fruity note of wine all come from esters. They are nature's perfumers — small esters are volatile and sweet-smelling — and they are also the molecular backbone of fats and of plastics like polyester. Few functional groups touch daily life as broadly.

An ester has the structure R-CO-O-R', where the carbonyl carbon (C=O) is bonded to an oxygen that in turn carries a carbon group (the -OR' part) rather than a hydrogen. You can think of it as a carboxylic acid in which the acidic -OH proton has been replaced by an alkyl or aryl group. That swap matters: with no acidic proton, esters are not sour and do not form the strong hydrogen-bonded networks acids do, so they tend to be pleasant-smelling liquids of lower boiling point. In the reactivity ranking of acyl derivatives, esters are moderate — less reactive than anhydrides because an alkoxide (-OR') is a poorer leaving group than a carboxylate, but more reactive than amides.

Esters are made in several ways: from a carboxylic acid plus an alcohol under acid catalysis (Fischer esterification), or far more vigorously from an acyl chloride or anhydride plus an alcohol. They can be hydrolyzed back to acid and alcohol, swapped with another alcohol (transesterification), or cleaved by base to give a carboxylate and an alcohol (saponification — literally soap-making). Biologically, the linkage between glycerol and fatty acids in fats and oils is an ester bond, and nucleic acids are held together by phosphate esters.