In chemistry, esters are compounds derived by condensation of an oxoacid (one containing an oxo group, X=O, e.g., phosphoric acid (O=P(OH)₃) or a carboxylic acid (RCOOH)) with an alcohol. Many biological lipids are the fatty acid esters of glycerol derivatives. Phosphoesters form the backbone of DNA molecules. Polyesters such as poly-hydroxy butyrate are important energy storage molecules in some microbes. Cyclic esters formed from carboxylic acid and alcohol functional groups are called lactones.

Esters contain a carbonyl functionality, but unlike amides, esters are structurally flexible because there is a low energetic barrier to rotation about the X(=O)–O–C bond. Esters thus tend to be more volatile (i.e., have a lower boiling point, particularly with low molecular weight esters) than the corresponding amides. Their carbonyl group can serve as a hydrogen-bond acceptor, and this ability to engage in hydrogen bonding renders them somewhat water soluble. The...