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

enter image description here

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.

  • 10
    $\begingroup$ 5-membered rings are perfectly alright, it is 3 and 4 that are most strained, and going from tertiary to secondary is bad. $\endgroup$ – orthocresol Feb 27 '19 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 Feb 28 '19 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? $\endgroup$ – Yusuf Hasan Feb 28 '19 at 4:21
  • $\begingroup$ @Zhe I agree, and that is how I reasoned the non-favourability of the expansion intermediate. Thank you for reaffirming. :) $\endgroup$ – Dan Feb 28 '19 at 7:16
  • 3
    $\begingroup$ @YUSUFHASAN Yes, if there was a hydride substituent, ring expansion would've occurred appreciably. $\endgroup$ – Dan Feb 28 '19 at 7:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.