Question: Draw a stepwise mechanism for the following reaction.

The O-H bond in alcohol is polar. Therefore, oxygen has an electronegative character.

Due to this, the oxygen atom can form a bond with protons in the solution.

The cation thus formed is removed as water from the alcohol to form alkyl carbocation. The alkyl carbocation thus formed is highly reactive and attaches to the nucleophiles.

Therefore, protons are used to activate alcohols for nucleophilic substitution reactions.

Lactones are cyclic esters formed when esterification occurs within a single molecule.

This occurs for bifunctional alkanes with a carboxylic acid group and hydroxyl group on the same carbon chain.

The lactone formation is favorable if the two groups are separated by 4 or 5 carbon atoms.

If the number of carbon atoms between the groups is small, the ring formed will be highly strained, and the lactone will be unstable.

The reaction is initiated by the proton.

The oxygen on the hydroxyl group abstracts the proton and removes it as water, forming aryl carbocation.

Abstraction of proton by oxygen

Removal of water

Electron delocalization occurs within the ring, followed by the attack of carboxylic acid on the electropositive carbon.

Lactone ring formation

A proton is removed from the compound, forming the final lactone.

Removal of proton