éster

An ester is a chemical compound derived from a carboxylic acid and an alcohol, in which the hydrogen of the carboxyl group is replaced by an alkyl or aryl group to form the functional group RCOOR'. They are commonly represented by the general formula RCOOR' and are typically formed by condensation, releasing water.

Esters are mainly prepared by esterification, a reaction between a carboxylic acid and an alcohol, usually catalyzed by an acid such as sulfuric acid; the reaction is reversible and water removal drives the process. Esters can also form via transesterification, exchanging alkoxy groups between esters, or by reacting acid chlorides with alcohols. Hydrolysis of esters reverses their formation: under acid or base catalysis, ester hydrolysis yields a carboxylic acid (or carboxylate salt) and an alcohol; saponification refers specifically to base-catalyzed hydrolysis.

Esters occur widely in nature and industry. Fats and oils are triglycerides, esters of glycerol with fatty acids. Many natural fragrances and flavors are esters, such as ethyl acetate and methyl salicylate, which are also common solvents. Polyesters are important polymers, notably polyethylene terephthalate (PET). In biology, ester linkages connect fatty acids to glycerol in lipids, and phosphoester bonds form the backbone of nucleic acids.

Common examples include ethyl acetate, methyl benzoate, and ethyl butyrate (a fruity aroma). Esters are typically polar and often volatile, with physical properties varying widely by substituents. They are generally flammable and can be irritating; standard handling precautions apply in laboratory and industrial settings.