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I was assigned the following question and I cannot decide which is the correct answer.

Which is the most stable structure?

Which is the most stable structure?
(a) pentane-2,4-dione
(b) 4-hydroxypent-4-en-2-one
(c) (Z)-4-hydroxypent-3-en-2-one
(d) (Z)-penta-1,3-diene-2,4-diol

I am confused between options (c) and (d). Both seem to be stable due to resonance, but I think (d) should have more resonance as the oxygen's lone pair in the $\ce{-OH}$ bond will also participate in resonance making it more stable.

But the answer is given as (c).

Could you please explain why (c) is more stable than (d).

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  • $\begingroup$ The correct answer is (a). Andy other structure is much less stable, even though you can measure the existence of (c) to a significant degree. Your book's solution is wrong. $\endgroup$
    – Martin - マーチン ♦
    Aug 12, 2018 at 10:21
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    $\begingroup$ My previous comment is somewhat wrong. The equilibrium is very solvent dependent. In water it is (a), in less polar solvents the answer is (c). There is a small article on Wikipedia about it, and some references within, which are quite interesting. I think this question deserves reopening and a more elaborate answer. $\endgroup$
    – Martin - マーチン ♦
    Aug 15, 2018 at 14:40

2 Answers 2

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The compound in option (c) is highly stable because it forms a 6 membered ring by tautomerism and that ring is highly stable because all bonds in that ring have developed partial double character. Whereas in option (d) there is ring formation but only two bonds of that ring have partial double character. This makes (c) more stable than (d).

enter image description here tautomers of pentan-2,4-dione

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The C structure will have the highest stability. Despite the keto form of tautomer having the highest stability due to strong C=O bond it would not occur so in the special cases of intra-molecular hydrogen bonding The Intra molecular hydrogen bonding would stabilize structures b,c and d However b tends to be less stable than c due to lesser alkene substitution which owes to the inability to form the maximum number of hyperconjugation structures such as c itself The Structures c and d seem to be controversial but however the more electronegative oxygen would exist in the case of c due to resonance with the intervening double bond between the two functional group containing carbons and hence would have more electron density localized to oxygen, a highly electronegative element and hence would have higher stability. However it is also correct to think of the structure d as something with more number of hydrogen bonding but it doesn't compensate the former reasoning.

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