Question

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

Answer 1

Acetylacetone (2,4-pentanedione) exists in two isomeric forms, shown below. The Keto form is on the left, and the Enol form is on the right.

The 2,4-pentanedione enol form shown is the more stable enol, since the lone pair on the enol oxygen can delocalize across the five atoms to the electronegative carbonyl oxygen.

H Pentan-2.4-dione (Acetylacetone) Cs 4-Hydroxy-3-penten-2-one (Enol form) د - Cs 4-Hydroxy-3-penten-2-one (Enol form) Cav

The enol form of acetylacetone is reported to be more stable and it is stabilised by an internal hydrogen bond referred to as chelated enol.  

Acetylacetone exist in a enol form to the greatest extent.

Tautomerism H-bond CH3 H3C Keto form H3C CH3 Enol from