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.
18.23:
Hydrolysis of Chlorobenzene to Phenol: Dow Process
Simple aryl halides do not react with nucleophiles under normal conditions. However, the reaction can proceed under drastic conditions involving high temperatures and high pressure to give the substituted products. For example, chlorobenzene is converted to phenol using aqueous sodium hydroxide at 350 °C under high pressure by the Dow process. The reaction follows an elimination-addition mechanism involving a benzyne intermediate. Here, the chloride ion is eliminated to generate the benzyne intermediate. The benzyne intermediate then reacts with the base, followed by an acid workup that generates phenol as the final product.