# What is the most stable structure amongst the keto-enol tautomers of pentane-2,4-dione?

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

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).

• 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. – Martin - マーチン Aug 12 '18 at 10:21
• 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. – Martin - マーチン Aug 15 '18 at 14:40