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Recently I have came across this question.

enter image description here

I went with the 3rd option as ring expansion cannot occur due to the formed stable 3 degree carbocation. But the answer key claims 4th option. How can product through the ring expansion be the major product here ? As the 3 degree carbocation is stable, and if ring expansion happens, a 3 degree carbocation to 2 degree carbocation transition should occur (although in the end 3 degree carbocation would form) as I have written in the mechanism for this pathway here:

enter image description here

But this pathway has to undergo activation barrier for the 3 degree to 2 degree carbocation transition. So how would this product be major product ? If the starting compound is a 4 membered or 3 membered ring, then the ring stability may be more dominating factor than the stability of the carbocation, but in this case it is a quite stable 5 membered ring.

Links:

Ring expansion in cyclo compounds

Carbocation rearrangement with ring expansion

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    $\begingroup$ All steps are reversible until you get the thermodynamically most stable product. $\endgroup$
    – Mathew Mahindaratne
    Commented Nov 11, 2023 at 21:11
  • $\begingroup$ @MathewMahindaratne Yes. But isn't the product through ring expansion a minor product ? $\endgroup$
    – Kavin Ishwaran
    Commented Nov 12, 2023 at 2:35
  • $\begingroup$ If it's minor would depend on conditions of reaction. $\endgroup$
    – Mithoron
    Commented Nov 16, 2023 at 17:52

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