Final answer:
In phenol, tautomerism can be observed due to the resonance structure that allows for the presence of an alpha hydrogen atom, even though the alpha carbon is sp2 hybridized. This allows phenol to exist in both the keto and enol forms.
Step-by-step explanation:
In order for a molecule to show tautomerism, it must have at least one alpha carbon that is sp3 hybridized and has at least one alpha hydrogen atom.
Tautomerism is the rapid interconversion of constitutional isomers, called tautomers, through the migration of a hydrogen atom and a double bond. In the case of phenol, the keto form (phenol) can tautomerize to its enol form (catechol) through the migration of a hydrogen atom and a double bond.
The alpha carbon in phenol, which is the carbon bonded to the hydroxyl group (-OH), is sp2 hybridized and does not have an alpha hydrogen atom.
However, if we consider the resonance structure of phenol, we can see that the lone pair of electrons on the oxygen atom can participate in resonance with the pi system of the benzene ring, effectively creating an alpha hydrogen atom. This allows phenol to show tautomerism and exist in both the keto and enol forms.