I am dealing with a problem involving addition of $\ce{HCl}$ to a ring:

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According to what I think, the ring should expand to a six-membered following carbocation formation at the tertiary site, as a 6-membered ring will involve less strain than the current 5-membered one. However, the correct product is:

enter image description here

Which is what would form if the expansion is not favored. My guess is that the rearrangement is less likely to occur due to a tertiary carbocation, and that the rearranged product will then form in a minor concentration.

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    $\begingroup$ 5-membered rings are perfectly alright, it is 3 and 4 that are most strained, and going from tertiary to secondary is bad. $\endgroup$ Commented Feb 27, 2019 at 22:58
  • $\begingroup$ Note also that a ring expansion would lead to a secondary carbocation. The ring strain in a 5-ring, as mentioned is not that significant, so ring expansion would actually be energetically costly here. $\endgroup$
    – Zhe
    Commented Feb 28, 2019 at 0:09
  • $\begingroup$ @Zhe Would it be feasible,then,to expand the five membered ring to a six membered one,if the double bond had a hydrogen substituent in place of the methyl group substituent as shown for the reactant? $\endgroup$ Commented Feb 28, 2019 at 4:21
  • $\begingroup$ @Zhe I agree, and that is how I reasoned the non-favourability of the expansion intermediate. Thank you for reaffirming. :) $\endgroup$
    – ditsuke
    Commented Feb 28, 2019 at 7:16
  • 3
    $\begingroup$ @YUSUFHASAN Yes, if there was a hydride substituent, ring expansion would've occurred appreciably. $\endgroup$
    – ditsuke
    Commented Feb 28, 2019 at 7:18


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