Only two resonance structures can be drawn for the undissociated
phenol molecule, corresponding to the two
structures for benzene (right). The oxygen atom has two lone
electron pairs and a bond each to H and C. None of its electrons
participate in the ring delocalization.
In contrast, once the electrons of the O-H bond are liberated by
dissociation of H,
they can be shifted into a double bond to the ring carbon atom.
In addition to the two -like
resonance structures with the negative charge on the oxygen, other
structures can be drawn with a C=O double bond, and with the negative
charge transferred to the ring. Three more resonance-bond models
can be drawn for the phenolate ion than for the phenol.
A good practical rule of thumb is that the more resonance structures
that can be drawn for a molecule or ion, the greater its delocalization,
and the more stable it is.
The delocalized system in the phenolate ion is enlarged by one oxygen
atom and by two more electrons than in the undissociated phenol.
The equilibrium between phenol and its ion thus is shifted enough
in favour of the ion to make phenol an acid instead of an alcohol.