Which will have the highest enol content?

Solution

Chemistry ma’am · 3 steps

Enolise all three C=O groups of the trione and the ring becomes a benzene ring, which is aromatic.

1. What drives enolisation. A ketone normally prefers the keto form. The enol wins only when something extra stabilises it, such as conjugation, hydrogen bonding or, best of all, aromaticity.

   Option	Compound	Fully enolised form	Extra stability of enol
   (a)	cyclohexane-1,2-dione	one enol, C=C conjugated with the other C=O	conjugation only
   (b)	cyclohexane-1,3-dione	enol from the CH₂ between two C=O groups	conjugation, but the ring stays non-aromatic
   (c)	cyclohexane-1,3,5-trione	1,3,5-trihydroxybenzene (phloroglucinol)	aromatic benzene ring, 6π electrons
   (d)	cyclohexane-1,4-dione	each CH₂ sits next to only one C=O	almost none, stays keto

2. Why the trione wins. In (c) every CH₂ lies between two C=O groups. When all three enolise, the ring gets three alternating C=C bonds, giving a planar aromatic ring. Aromatic stabilisation is far larger than the gain from conjugation alone, so the enol form dominates.
3. So. Cyclohexane-1,3,5-trione, option (c), has the highest enol content because its enol is the aromatic compound phloroglucinol.

The idea to keep. To rank enol content, ask what the enol gains. Aromaticity beats conjugation and H-bonding, and an isolated ketone gains almost nothing.

Answer from the NTA final answer key. Solution by PrepWiser.