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enter image description here

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"? $\endgroup$ – Ivan Neretin Mar 20 '16 at 14:56
  • $\begingroup$ Five member ring to six member ring, like that. $\endgroup$ – Reeshabh Ranjan Mar 20 '16 at 15:07
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    $\begingroup$ I would not expect that to happen. $\endgroup$ – Ivan Neretin Mar 20 '16 at 15:12
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    $\begingroup$ Definitely no ring expansion going on. $\endgroup$ – SendersReagent Mar 21 '16 at 4:11
  • $\begingroup$ My bad. I was always removing the H from CH3. Yup no ring expansion. Realise this from Aditya's answer. $\endgroup$ – Reeshabh Ranjan Mar 21 '16 at 5:22
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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:

enter image description here

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

enter image description here

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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    $\begingroup$ Oh! I was removing the H from Ch3. Thanks a lot! $\endgroup$ – Reeshabh Ranjan Mar 21 '16 at 5:07
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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. $\endgroup$ – Oscar Lanzi Mar 21 '16 at 5:02
  • $\begingroup$ Haha. Keep trying. $\endgroup$ – Reeshabh Ranjan Mar 21 '16 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 :) $\endgroup$ – Reeshabh Ranjan Mar 21 '16 at 5:26

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