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In 1-phenylbutane-1,3-dione (1), which enol tautomer will be predominant?

Tautomerism in 1-phenylbutane-1,3-dione

Both carbonyl groups possess α-hydrogens, so can undergo keto-enol tautomerism. In particular, the carbonyl group on the right can enolise in two ways since there are two unique sets of α-hydrogens.

The answer is supposed to be enol 2. Why can't it be either 3 or 4?

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    $\begingroup$ You need to retain a conjugated carbonyl group which is more stable than two enols. [Acetone exists as a ketone with little enol.] The real question is why the given answer without considering the tautomeric form of the second structure. $\endgroup$ – user55119 Sep 23 '18 at 14:50
  • $\begingroup$ The question editor has modified the question, and maybe assumes that he has thought of every possible enol structure, but he didn't. $\endgroup$ – mykhal Sep 24 '18 at 10:02
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enolic form

The compound is highly stabilised by higher Conjugation and is thus favored over the other enolic form. Another factor is intramolecular hydrogen bonding which stabilises the enolic form even more.

It can also be explained on the basis of acidity of H in keto from.

Keto form

Due to presence of 2 Electron withdrawing groups, the H atom is highly acidic and will favor the enolic form.

The reason (3) is not formed is because after the formation of (2), the H marked in the 2nd picture will not be acidic anymore, making the equillibrim highly towards (2) form. Also, formation of the allene leads to leads to loss of comjugation, since the pi orbitals are now perpendicular to each other, which makes the equillibrim even backwards (towards 2nd form).

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  • $\begingroup$ I think this is not really answering the question. The diagrams in the question were not very clear, but I have edited it. There is a more fundamental reason why the allene 3 does not form, namely the loss of conjugation (as the pi orbitals are orthogonal). And you also don't explain why the terminal methyl group doesn't enolise, e.g. in 4. To me, it feels like you are just explaining why 1,3-dicarbonyls tend to enolise, without explicitly considering the alternative structures which OP asked about. $\endgroup$ – orthocresol Sep 23 '18 at 21:25
  • $\begingroup$ @orthocresol I have added explanation for (3). If the answer still doesn't answer the question, please inform me and i will delete it because i cannot answer the question any further than that. $\endgroup$ – Groverkss Sep 24 '18 at 4:18

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