Question

Maximum enol content is in:

Answer 1

In keto form alpha-hydrogen shifts to oxygen forming a hydroxyl group. The newly formed structure is known as enol. The amount of enol in keto-enol form, depends upon how stable the enol form is.

Step by step answer: The functional group having double-bonded oxygen atom and two bonds with hydrogen or alkyl group is known as the keto group and the group having hydroxyl group attached with double-bonded carbon atom is known as the enol form.
The conversion of keto to enol is known as ketone-enol tautomerism.
The shifting of alpha-hydrogen of keto to double-bonded oxygen gives enol form.
The conversion of keto to enol of each compound is shown as follows:

All the compound shows keto-enol tautomerism but the enol form of compound $(2)$ and $(4)$ is more stable due to hydrogen bonding.
The hydrogen bonding in compound $(2)$ and $(4)$ is shown as follows:

So, compounds $(2)$ and $(4)$ both enol form are stable due to the hydrogen bonding but in compound $(2)$, more amount of enol is present because, in compound $(2)$, two keto group is present whereas, in compound $(4)$, only one keto group is present.
So, the compound $(2)$ has maximum enol content.

So, options $\left( 1 \right),{\text{ }}\left( 3 \right)$ and $(4)$ are not the correct option because all have enol form but in less amount due to less stability.

Therefore, option $(2)$ is correct.

Note: The keto form is more stable than the enol form because the carbon-oxygen double bond is more stable than the carbon-carbon double bond. If enol is present in more amounts than keto form, then there will be some additional stability factor in enol form such as hydrogen bonding.