How do you synthesise a lactone from a hydroxy acid?

A lactone can be synthesised from a hydroxy acid through a process called intramolecular esterification or cyclisation.

Intramolecular esterification, also known as cyclisation, is a process where a hydroxy acid is converted into a lactone. This process involves the formation of an ester linkage between the carboxylic acid group and the hydroxyl group within the same molecule, leading to the formation of a cyclic ester, or lactone.

The first step in this process is to heat the hydroxy acid. This causes the hydroxyl group (-OH) to lose a hydrogen ion (H+), forming a negatively charged oxygen ion. At the same time, the carboxylic acid group (-COOH) loses a hydroxyl group (OH-), forming a positively charged carbocation. These ions are attracted to each other, and they form a bond, creating an ester linkage and a water molecule.

This reaction is a type of condensation reaction, specifically a dehydration reaction, as a water molecule is produced as a by-product. The reaction is also an example of nucleophilic acyl substitution, where the hydroxyl group acts as a nucleophile, donating a pair of electrons to form a new bond with the carbocation.

The resulting molecule is a lactone, a cyclic ester. The size of the ring in the lactone depends on the position of the hydroxyl group in the original hydroxy acid. If the hydroxyl group is on the carbon adjacent to the carboxylic acid group, a three-membered ring (gamma-lactone) is formed. If the hydroxyl group is two carbons away, a four-membered ring (delta-lactone) is formed, and so on.

The formation of lactones from hydroxy acids is a common reaction in organic chemistry and is used in the synthesis of many natural and synthetic compounds. It's important to note that this reaction requires heat and may also require a catalyst to proceed at a reasonable rate. The specific conditions needed can vary depending on the particular hydroxy acid being used.