Question

Which one of the following compounds would exist in an ENOL form to the greatest extent? H H H H

Answer 1

In most keto-enol tautomerisms, the equilibrium lies by far toward the keto form, indicating that the keto form is usually much more stable than the enol form, which means that a carbon-oxygen double bond is stronger than a carbon-carbon double bond.

In general keto form is thermodynamically more stable than enol form but in the following cases enol form becomes more stable:

If enol form is aromatic.

If enol form contains extended pi conjugation.

If enol form is derived from most acidic hydrogen and forms intra molecular H-bonding.

Here, only option 1 is the compound which has extended pi conjugation in its enol form.

Compound in option 4 will not show extended pi conjugation because it doesn't has alpha-hydrogen (which is acidic). So ,it can't exist in enol form.

Hence, the correct answer is option 1.