# Reaction of ether with HCl

Most of the reactions undergo ring expansion whenever favourable. But this reaction [1, p. 514] doesn't seems to do so:

Practice Problem 11.18 Provide a mechanism for the following reaction.

Could anybody explain why?

### References

1. Solomons, T. W. G.; Fryhle, C. B.; Snyder, S. A. Organic Chemistry, 12e ed.; John Wiley & Sons, Inc: Hoboken, NJ, 2016. ISBN 978-1-118-87576-6.
• Presence of a good nucleophile Cl- that captures the stable cation faster than any ring expansion. Also 5 ring is pretty stable so expansion to 6 is not particularly favoured. Sep 10 '19 at 18:12
• But it does rearrange to a tertiary cation.....wouldn't it be better if it's a tertiary cation in a 6 membered ring? Sep 10 '19 at 18:13
• No, 6 ring is not better than 5 Sep 10 '19 at 18:14
• Ring strain slightly lower with 5 than 6 Sep 10 '19 at 18:18
• In the future, please add complete reference to the textbook you are quoting from (page number(s), edition, publisher etc.: see my edit as an example). Also, this question somewhat looks like a "covert" homework question: probably you can share with us some of your own thoughts. Other than that, it's a nice question IMO. Sep 10 '19 at 18:19

There are 2 things to consider: What would be the energy difference between before and after rearrangement from initial $$2^\circ$$-carbocation (I) and each of rearranged carbocations II and III:
It is obvious that the rearrangement of carbocation I to give carbocation II by ring expansion have minimal energy change since both initial and final carbocations are $$2^\circ$$ and ring expansion does not relieve too much energy because it is cyclopentyl to cyclohexyl. However, the rearrangement of carbocation I to give carbocation III by 1,2-hydride transfer relieves good amount of energy since initial carbocation is $$2^\circ$$ and final rearranged carbocation is $$3^\circ$$. Although the carbocation II would undergo further stabilization by 1,2-hydride transfer to relieve extra energy, I to III happens much faster than I to II transfer to give 3 as the major product.