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Which of the following compound has highest enol content?

a: acetylacetone; b: 1‐phenyl‐1,3‐butanedione; c: dibenzoylmethane; d: 3‐methylpentane‐2,4‐dione

The answer is an option (c) and I think the reason behind it is that there are two phenyl groups attached to it, hence there will be a good conjugation. Also, since phenyl is an electron-withdrawing group, it will provide stability to the structure (as the $\ce{C-C}$ double bond formed after tautomerism will have π-electrons). But I am not quite sure about my logic and I don't know the reasons to eliminate other options.

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2 Answers 2

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The extent of enol content can be explained on the basis of acidic hydrogen, intramolecular hydrogen bonding.

  • The stability of the carbanion formed is dependent on its stability after it is formed. In compound $\ce{(c)}$, the carbanion is stabilized by extended conjugation due to presence of phenyl group. Such stabilization does not occur in $\ce{(d)}$. In $\ce{(d)}$, methyl substituent at carbanion increases negative charge via inductive polarization. The resulting carbanion is unstable.

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  • After enolization , enol is stabilized via intramolecular hydrogen bonding, which is a common feature in all examples provided.

  • The enol in compound $\ce{(c)}$ is stabilized via extended conjugation.

Taking the above factors into account, $\ce{(c)}$ has highest enol content.

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Well, to comment on the enol content we have to check the stability of the carbanion formed during the mechanism process and also look at factors like hydrogen bonding, extended conjugation, aromaticty formation. So, here we can comment the answer is c because the intermediate carbanion is stabilized by resonance with the carbonyl and phenyl.

Note: the intermediated carbanion will be formed at the site of the most acidic H. Here it's connecting $\ce{CH2}$ between the two carbonyls.

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