Which of the following will show Keto-enol tautomerism. Also Explain How?
Compound 1 will show keto-enol tautomerism.
Keto–enol tautomerism is a rapid chemical equilibrium between a keto and an enol form of a carbonyl compound.
The keto form is usually the more stable, but some structural features can favour the enol.
There must be an sp³ α-hydrogen next to the carbonyl carbon for tautomerism to occur.
The α-hydrogen ends up on the oxygen atom, and a C=C bond forms between the α-carbon and the original carbonyl carbon.
Compound 1 has an sp³ α-hydrogen at C-6 that can form an enol at C-1. It also has an α-hydrogen at C-5 that can form an enol at C-4. The enol formed is benzene-1,4-diol (hydroquinone):
Resonance stabilization in the benzene ring drives the position of equilibrium towards the enol form.
Compound 2 has no sp³ α-hydrogen atoms, so it cannot undergo keto-enol tautomerism.