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Esters are a class of organic compounds derived from carboxylic acids and alcohols. They feature the functional group consisting of a carbonyl bonded to an alkoxy group, often represented as R-CO-O-R'. Esters are typically formed by esterification, a condensation reaction in which a carboxylic acid reacts with an alcohol with removal of water, usually in the presence of an acid catalyst. They can also be produced from acyl chlorides with alcohols or by transesterification between esters.

Nomenclature of esters follows two common systems. In common usage, esters are named as the alkyl group from the alcohol followed by the name of the carboxylate, as in ethyl acetate. In IUPAC nomenclature, they are named as alkyl (or aryl), alkanoate, for example ethyl ethanoate. The choice of alkyl and acyl groups determines the physical and chemical properties of the ester.

Properties and reactions: esters are often liquids with pleasant odors and relatively low acidity. They are generally less polar than carboxylic acids, with varying water solubility. Esters undergo hydrolysis to give the corresponding carboxylic acid and alcohol, under acidic or basic conditions (saponification in base). They can be reduced to primary alcohols, or participate in transesterification to exchange alkoxy groups. Esters are also key monomers and intermediates in polymer chemistry, including polyesters such as PET.

Uses and occurrence: esters are widely used as solvents, flavors, and fragrances. Fatty acid esters form fats and oils in biology, and fatty acid methyl esters are central to biodiesel production.

Examples include ethyl acetate, methyl salicylate, and isoamyl acetate.