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internal cyclic esters containing a —COO—group in the ring:
Lactones are classified as β-, γ-, δ-, ε-, and so forth according to the type of hydroxy acid forming the lactone, for example, β-propiolactone (I; boiling point, 155°C), γ-butyrolactone (II; boiling point, 203°-204°C), and δ-valerolactone (III; boiling point, 218°-220°C):
The principal method of lactone synthesis involves the cyclization of hydroxy acids or halogen acids [X—(CH2)nCOOH, where X = OH or a halogen].
Lactones exhibit many properties characteristic of acyclic esters, for example: (1) upon heating with acids or alkalis they hydrolyze to the corresponding hydroxy acids; (2) lactones yield amides upon heating with amines; and (3) lactone polymerization produces linear polyesters [—(CH2)nCOO—]x (the ring is opened at the C—O bond), β-lactones separate into carbon dioxide and olefins upon heating. Higher lactones undergo isomerization to form unsaturated acids at high temperatures.
Lactones are used in organic synthesis, as well as in the preparation of pharmaceuticals and aromatic substances. Some lactones are biologically active substances, for example, β-propiolactone (sterilizing agent).