JEE Main · 2019 · 08 Apr 2019 · Shift 2 · General Organic Chemistry

Which of the following compounds will show the maximum 'enol' content?

(A) $CH_{3} COCH_{3}$
(B) $CH_{3} COCH_{2} CONH_{2}$
(C) $CH_{3} COCH_{2} COCH_{3}$ ✓ Correct
(D) $CH_{3} COCH_{2} COOC_{2} H_{5}$

Answer: (C)

Solution

Enol content is greatest when the enol form is especially stable. In

CH X_{3} COCH X_{2} COCH X_{3}

the central methylene lies between two carbonyl groups, so its enol has a conjugated system and, crucially, an intramolecular hydrogen bond that closes a stable six-membered ring. That stabilisation by resonance and hydrogen bonding makes the enol the dominant tautomer, so this diketone shows the maximum enol content.

JEE Main · 2019 · 08 Apr 2019 · Shift 2 · Single correct option

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