Question

Assertion A : Enol form of acetone \([\mathrm{CH_3COCH_3}]\) exists in < 0.1% quantity. However, the enol form of acetyl acetone \([\mathrm{CH_3COCH_2OCCH_3}]\) exists in approximately 15% quantity.

Reason R : Enol form of acetyl acetone is stabilized by intramolecular hydrogen bonding, which is not possible in enol form of acetone.

Choose the correct statement :

• A. Both A and R are true but R is not the correct explanation of A
• B. A is true but R is false
• C. A is false but R is true
• D. Both A and R are true and R is the correct explanation of A

Hint:

The enol of acetylacetone has a six-membered chelate ring from intramolecular O-H...O hydrogen bonding (plus conjugation), which acetone's enol cannot form.

Answer 1

The Correct Option is D

Acetone exists almost entirely as the keto form; its enol content is very small (below 0.1%), so the first part of Assertion A is true.
Acetylacetone (pentane-2,4-dione) has two C=O groups separated by one \(\mathrm{CH_2}\); its enol form has an OH group that hydrogen-bonds to the second carbonyl oxygen, forming a stable six-membered chelate ring with a conjugated C=C-C=O system. Hence its enol content is about 15%, so Assertion A is true.
Reason R states exactly this stabilisation by intramolecular hydrogen bonding, which cannot occur in the enol of acetone (only one carbonyl). R is true and explains A.
Answer: (4) Both A and R are true and R is the correct explanation of A