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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.

Reaction between (1‐methoxyethyl)cyclopentane and HCl

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.
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    $\begingroup$ 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. $\endgroup$ – Waylander Sep 10 at 18:12
  • $\begingroup$ But it does rearrange to a tertiary cation.....wouldn't it be better if it's a tertiary cation in a 6 membered ring? $\endgroup$ – Dheeraj Kamath Sep 10 at 18:13
  • $\begingroup$ No, 6 ring is not better than 5 $\endgroup$ – Waylander Sep 10 at 18:14
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    $\begingroup$ Ring strain slightly lower with 5 than 6 $\endgroup$ – Waylander Sep 10 at 18:18
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    $\begingroup$ 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. $\endgroup$ – andselisk Sep 10 at 18:19
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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:

Cyclopentane Rearrangement

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.

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