Question

Why are phenols and anilines especially reactive in aromatic electrophilic substitution reactions?

Answer 1

Phenols are in fact potent nucleophiles, but their electron density is NOT localized to the oxygen centre.

Explanation:

Phenols are said to be more acidic than aliphatic alcohols in that oxygen electron density is conceived to be delocalized around the ring. This approach may be a bit simplistic and wrong-headed (reduced charge density of the phenoxide seems to be the determinant); nevertheless, it does some give some clue to explain the REACTIVITY of phenols.

Phenols are considered to be more reactive than benzene in circumstances of aromatic electrophilic substitution by several orders of magnitude. Benzene, when it reacts with an electrophile *by acting as a NUCLEOPHILE, usually requires some Lewis acid catalysis to effect the reaction.

In contrast, phenols and anilines (whose reactivity is enhanced by lone pairs on the oxygen or the nitrogen) are many more times reactive towards electrophiles. With respect to benzene, both aniline and phenol are activated nucleophiles, and often do not require Lewis acid catalysis. The lone pairs on phenol donate electron density to the entire ring, and the phenyl ring becomes MORE nucleophilic.

I would consult a text on the enhanced reactivity of phenols and anilines in the section of aromatic chemistry.