Ester

An ester is an organic compound produced by replacing the hydrogen of a carboxyl group with an alkyl or aryl group. The characteristic functional group is -COO-, linking an acyl moiety R-CO- to an alkoxy group -O-R'. The general formula is R-CO-O-R'. Names are typically alkyl alkanoates (for example ethyl acetate).

Formation and synthesis: Esters are commonly prepared by Fischer esterification, which combines a carboxylic acid with an alcohol in the presence of an acid catalyst, with removal of water to drive the equilibrium toward ester formation. Other methods include transesterification (exchange of alkoxy groups between esters) and reactions of acid chlorides with alcohols. In industry, esters are also produced via esterification of fatty acids with short-chain alcohols to furnish biodiesel precursors.

Reactions: Esters undergo hydrolysis back to carboxylic acids and alcohols under acidic or basic conditions (saponification). Reduction with suitable reagents such as lithium aluminium hydride converts esters to primary alcohols. Transesterification allows interconversion of esters by swapping the alkoxy group, a process central to biodiesel production and various polymer syntheses.

Properties and occurrence: Esters often have distinct, pleasant odors and are widely used as solvents, plasticizers, and flavor or fragrance additives. They generally exhibit lower polarity and varying solubility in water depending on chain length, with shorter esters more water-soluble. In nature, esters form fats (triglycerides) and waxes, and many fragrances and flavors are esters.