Question

In which of the following compounds, keto form is more dominating than enol form?

• A.
• B.
• C.
• D.

Answer 1

Answer: (A), (B), (C)

In Keto-enol tautomerism, delocalisation takes place between a >C=O group and an alpha H where the equilibrium lies between a keto form and an enol form.

In (a), the keto form is more stable than enol form because the enol form has 4-$\pi$ electrons which obeys 4n rule and thus it is an antiaromatic so it is less stable. So Keto form dominates over enol.

In (b), the keto form is more stable because in enol form double occurs at bridgehead carbon so by Bredt's rule it is less stable. Hence keto form dominates enol form.

In (c), the Keto form is more stable because in enol form due to steric hindrance of bulky tertiary alkyl group the H-bonding is less favoured, hence enol form becomes less stable.

In (d), enol form dominates over keto form because the enol form is stabilised by conjugated double bond and by the formation of 6 membered cyclic ring by hydrogen bonding. Thus, enol group dominates over keto form.

Therefore, in (a), (b) and (c) keto form is dominating.