18.23 : Hydrolysis of Chlorobenzene to Phenol: Dow Process

Recall that unactivated halobenzenes do not react with nucleophiles under ordinary reaction conditions.

However, if extreme temperature and pressure conditions are employed, the reaction occurs to give the substituted product.

For example, consider the conversion of chlorobenzene to phenol.

The hydroxyl group displaces the chloro group through the Dow process, which uses dilute aqueous sodium hydroxide at 350 °C and under high pressure.

The reaction most likely proceeds via the elimination–addition mechanism involving the benzyne intermediate.

The elimination step is initiated by the hydroxide ion that abstracts the proton adjacent to the leaving group to generate a carbanion.

This facilitates the elimination of the chloride ion to generate the highly reactive benzyne intermediate.

Benzyne's interaction with the aqueous base generates sodium phenoxide, which on subsequent acidification gives phenol as the final product.