lactone
Summary
A lactone is a cyclic ester: the carbonyl carbon and the ester's single-bonded oxygen are both ring atoms (the C=O oxygen is outside the ring). It forms when a hydroxy acid esterifies intramolecularly, and five- and six-membered (γ and δ) rings form most readily; hydrolysis opens the ring.
Detail
Meaning: A lactone is an ester in which the carbonyl carbon and the alkoxy oxygen are both ring atoms; the C=O oxygen is outside the ring. It forms by intramolecular esterification: the –OH and –COOH of a hydroxy acid react under acid catalysis, losing H₂O. Five-membered (γ) and six-membered (δ) rings form most readily because they have little ring strain. Counting from the carbon next to the carbonyl (α), the γ carbon carries the ring-closing oxygen. Aqueous acid or base opens the ring; base gives the hydroxy carboxylate.
On the MCAT: You may be asked to predict the product when a γ- or δ-hydroxy acid is heated with acid, to spot a lactone in a drawn natural product, or to give the product of ring-opening hydrolysis. A sugar acid (an oxidized aldose) can also close into a lactone. In IR, the lactone C=O resembles an ester C=O, and strained small rings absorb at somewhat higher wavenumber.
Common confusion: Students mix up a lactone with a lactam (cyclic amide, N in the ring) and with a cyclic hemiacetal such as glucopyranose. A lactone has a C=O; a cyclic hemiacetal has none, and its anomeric carbon bears an OH.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.