lactones

Lactones are cyclic esters that form when a hydroxyl group and a carboxyl group within the same molecule react intramolecularly, releasing water. They can also arise from ring-closing reactions of linear esters or dehydration of hydroxy acids under appropriate conditions. The most common lactones are five- and six-membered rings, known as gamma- and delta-lactones, respectively. Four-membered beta-lactones are more strained and less common, while larger rings such as epsilon-lactones also occur in nature and synthesis.

Formation typically involves intramolecular esterification of hydroxycarboxylic acids upon heating or under acid or base catalysis. Natural lactones are widespread in essential oils and fruits, where they contribute characteristic fragrances and flavors. Industrially important lactones include caprolactone, the monomer for polycaprolactone, and gamma-butyrolactone (GBL), a versatile solvent and chemical building block.

Chemically, lactones exhibit typical ester behavior: hydrolysis regenerates the corresponding hydroxy acid, and nucleophilic ring-opening, often with accompanying transesterification, yields open-chain products. Ring strain influences reactivity: smaller rings such as beta-lactones are highly reactive but less stable, while larger rings are more common but less strained. Lactones can participate in polymerization to form polyesters, as in polylactide and polycaprolactone, or undergo various transformations in organic synthesis.

Nomenclature links the ring size to the Greek letters (beta-, gamma-, delta-, etc.). Spectroscopically, the carbonyl stretch of lactones appears in the ester region around 1730–1745 cm^-1, with additional features influenced by substituents and ring size.