# Shouldn't the 5-membered ring expand to a six membered one in this nucleophilic addition reaction?

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

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:

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.

• 5-membered rings are perfectly alright, it is 3 and 4 that are most strained, and going from tertiary to secondary is bad. – orthocresol Feb 27 '19 at 22:58
• 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. – Zhe Feb 28 '19 at 0:09
• @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? – Yusuf Hasan Feb 28 '19 at 4:21
• @Zhe I agree, and that is how I reasoned the non-favourability of the expansion intermediate. Thank you for reaffirming. :) – Dan Feb 28 '19 at 7:16
• @YUSUFHASAN Yes, if there was a hydride substituent, ring expansion would've occurred appreciably. – Dan Feb 28 '19 at 7:18