Question: Ignoring stereoisomers, draw the two possible enols for butan-2-one (CH3COCH2CH3), and predict which one is more stable.

The stability of enols depends on steric factor, substitution on carbon-carbon double bond and conjugation.

In keto-enol tautomerism, the keto form and enol form are in equilibrium. Usually, keto forms are more stable, but certain factors may shift the equilibrium towards enol form.

One of such factors is more substituted double bonds are more stable than less substituted double bonds.

More substituted double bonds have more stability.

The two possible enol forms of butan-2-one are:

Structure 1 is more stable as the double is a non-terminal double bond and hence, more substituted.

Keto to enol form of butan-2-one (more stable)

Structure 2 is less stable than 1 as the double bond is a terminal double bond and less substituted.

Keto to enol form of butan-2-one (less stable)