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In the given picture, I know that the most acidic will be (a), because of potential of ring expansion. I tried a few times but wasn't able to get a satisfactory six membered ring. I think (a) is the most acidic compound among the 4 due to ring expansion only. Or if I am wrong, please correct me with a detailed solution.

The answer given is (a) but explanation is needed

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    $\begingroup$ What exactly do you call "ring opening"? $\endgroup$ Commented Mar 20, 2016 at 14:56
  • $\begingroup$ Five member ring to six member ring, like that. $\endgroup$ Commented Mar 20, 2016 at 15:07
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    $\begingroup$ I would not expect that to happen. $\endgroup$ Commented Mar 20, 2016 at 15:12
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    $\begingroup$ Definitely no ring expansion going on. $\endgroup$ Commented Mar 21, 2016 at 4:11
  • $\begingroup$ My bad. I was always removing the H from CH3. Yup no ring expansion. Realise this from Aditya's answer. $\endgroup$ Commented Mar 21, 2016 at 5:22

2 Answers 2

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Its not because of any ring expansion but because of formation of an aromatic compound.

Lets Start with (C) and (D). Both compounds after formation of carbanion are stabilized by extended conjugation:

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But (A) forms an even more stable carbanion because of aromatic nature of:

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Cyclopentadiene has a $p_{ka}$ of 16. Its high acidic strength is because of the above reason.

You can analyse (B) yourself. But it wont form a carbanion as stable as (A).

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Maybe the ring does not open. Maybe (a) is a stronger acid because removing the proton from the ring carbon that's attached to the acetyl group produces a strongly stabilized structure.

What type of strongly stabilized structure may involve a five-membered ring with a negative charge (left behind when the proton is removed)?

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    $\begingroup$ Nice shot people. I wanted not to give away the answer and this is what happens. $\endgroup$ Commented Mar 21, 2016 at 5:02
  • $\begingroup$ Haha. Keep trying. $\endgroup$ Commented Mar 21, 2016 at 5:25
  • $\begingroup$ I think you were saying right, but weren't sure if you were correct. Yes, removing Hydrogen from the ring will make it Aromatic. Otherwise your answer had been acceptable :) $\endgroup$ Commented Mar 21, 2016 at 5:26

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