Phenols are aromatic compounds that have an -OH attached directly to the aromatic ring. They are acids and not alcohols because the proton dissociates form the -OH group. The parent compound, phenol or carbolic acid, dissociates to produce the phenolate, or phenoxide ion (right).

As its dissociation constant, or , indicates, phenol is a weak acid, comparable with HCN, but it is acidic nonetheless. Why should aromatic -OH compounds behave like acids, whereas aliphatic or open chain -OH compounds are alcohols instead?

The answer is essentially the same as for nitric and sulphuric acids, which were discussed in Chapters 5 and 6. Electrons in the negative ion are more delocalized than in the undissociated acid molecule, thereby making the ion more stable than it would be without delocalization. This encourages dissociation and makes the molecule a stronger acid than it would be otherwise. In Chapter 9 we saw that the extent of delocalization in an aromatic molecule could be determined by writing all possible resonance structures for the molecule, and noting the atoms that were connected by double bonds in at least one resonance form. This approach treats each resonance model as a partial or incomplete picture of the actual delocalized-electron bond structure.