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enter image description here

While attempting this problem I thought that P1 would simply be Bromine getting substituted by OH through nucleophilic substitution.

P2 would give a product formed through ring expansion as a 4 membered ring would tend to rearrange to a 5 membered ring due to strain factors.(SN1 reaction(acidic conditions))

This is the mechanism I thought of:

enter image description here

This pointed towards option (2) but what's given is option (3).
Why does this happen?

It could be that the four-membered ring being strained rearranges, as Waylander suggests but if it would rearrange, why would it rearrange into a 3 membered ring?
Usually, this happens only when there is a stabilising factor and the only stabilising factor related to a 3 membered ring I can think of, is sigma bond resonance(Dancing resonance)

I would appreciate an explanation of what's happening here.

Waylander
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Prajwal Tiwari
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    Cyclobutanes are very highly strained, more strained than cyclopropanes. If they can rearrange they will. – Waylander Jun 17 '21 at 12:38
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    Even into three membered rings? I thought three membered are even more strained... Does dancing resonance(sigma resonance) operate here? – Prajwal Tiwari Jun 17 '21 at 13:03
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    Can the 'close' voters give some feedback about the post? I have edited it to make the post more clear but I would still appreciate some constructive criticcism so that I can improve. – Prajwal Tiwari Jun 17 '21 at 15:48
  • @ https://www.sciencedirect.com/science/article/abs/pii/S0040403900703820 This might be helpful.(Or not it is behind the paywall :/)(There is a first page preview though.) – Rishi Jun 17 '21 at 15:53
  • @PrajwalTiwari If you wish you can add the mechanism which you think should be followed regarding the question in focus. :=) – Rishi Jun 17 '21 at 15:58
  • @Rishi Done! Thanks for the suggestion – Prajwal Tiwari Jun 17 '21 at 16:37
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    @PrajwalTiwari dancing resonance is not an officially accepted term for cyclopropyl methyl carbocation, stick to sigma resonance in the future. – napstablook Jun 18 '21 at 03:02
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    @napstablook Alright! Sorry for that, I'm a student so don't have much idea about all that and during the lectures, my teacher usually uses these terms without worrying about 'accepted' terms. I will take care in the future :) – Prajwal Tiwari Jun 18 '21 at 09:08
  • What software/website do you use to draw chemical structures and reactions and upload them here as an image? Would be of immense help to me. Thanks! – TRC Jun 18 '21 at 14:57
  • @TRC I have heard of chemdraw/ Chemdoodle, etc. The other more experienced members might be able to help you out with that. Here, I just used the 'preview' utility that MacOS provides since I didn't have access to any other chem structure tool. – Prajwal Tiwari Jun 19 '21 at 13:11
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    Why did this get closed though? I provided all my approaches, thoughts, even added a mechanism that I used, what else should I have provided to make this a better post? – Prajwal Tiwari Jun 19 '21 at 13:13

1 Answers1

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This reaction should be leaning towards SN1 because the approach of $\ce{H2O}$ for a back attack to the ABMO of the $\ce{C-Br}$ is heavily hindered since the neighborhood is congested due to the small ring of cyclobutane.

Once cation formation occurs the reaction will interestingly, result in a ring contraction because the carbocation formed would be very stable due to this kind of resonance.

After the ring contraction the reaction would proceed in a fairly standard nucleophilic attack by $\ce{H2O}$ resulting in the $P_1$ your answer key alludes to. The overall reaction should proceed in the manner shown below:-

enter image description here

napstablook
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