Preparation of phenols

PREPARATION OF PHENOLS

Key Notes

Incorporation

A phenol group can be incorporated into an aromatic ring by sulfonation, followed by conversion of the sulfonic acid group into the phenol by heat-ing in strong base. The reaction is limited to alkyl substituted phenols. Alternatively, the aromatic ring can be nitrated and the nitro group reduced to an aniline, which is then converted to a diazonium salt and hydrolyzed. Although longer, the method is more general and a wider variety of substituents is tolerated.

Functional group transformations

Phenyl esters can be hydrolyzed to their constituent carboxylic acid and phenol. Aryl ethers are cleaved by heating with HI or HBr to give an alkyl halide and a phenol.

Incorporation

Phenol groups can be incorporated into an aromatic ring by sulfonation of the aromatic ring followed by melting the product with sodium hydroxide to convert the sulfonic acid group to a phenol (Fig. 1). The reaction conditions are harsh and only alkyl-substituted phenols can be prepared by this method.

A more general method of synthesizing phenols is to hydrolyze a diazonium salt, prepared from an aniline group (NH₂).

Functional group transformation

Various functional groups can be converted to phenols. Sulfonic acids and amino groups have already been mentioned. Phenyl esters can be hydrolyzed. Aryl ethers can be cleaved. The bond between the alkyl group and oxygen is specifically cleaved since the Ar–OH bond is too strong to be cleaved.