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I am confused with this particular problem.

I began the problem with a mechanism as following. In the beginning $\ce{H+} $ protonates the alcohol and leaves as $\ce{H_2O}$. Then everything is about carbocation rearrangement.

But then my professor gave out a different answer saying that ring expansion doesn't happen.

Why doesn't ring expansion take place? Isn't a six-membered ring more stable than a five-membered ring? I am not sure where I am making the mistake.

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    $\begingroup$ With your proposed ring expansion you are going from a tertiary cation to a secondary - ain't going to happen $\endgroup$ – Waylander May 13 '17 at 16:32
  • $\begingroup$ @Waylander. Generally which of the two products are stable? $\endgroup$ – Suraj S May 13 '17 at 16:37
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    $\begingroup$ There's also no real driving force for ring expansion here - many of the common examples involve 3- or 4-membered rings which expand in order to reduce the strain present. $\endgroup$ – orthocresol May 13 '17 at 16:38
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    $\begingroup$ @orthocresol but isn't the 6-membered ring a tad more stable than a 5-membered one? $\endgroup$ – Pritt Balagopal May 14 '17 at 3:20
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    $\begingroup$ @PrittBalagopal Just a tad, yes. Not quite the same as 4-ring to 5-ring, though. $\endgroup$ – orthocresol May 14 '17 at 12:16
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Your proposed mechanism involves a ring expansion from cyclopentane to cyclohexane. This gives very little driving force in terms of relieving strain in the system. Cyclopentane, in a twisted or "envelope" conformation, has some angle strain ($102-106^o$ vs $109.5^o$ $^{ref~1}$) and it does also has some torsional strain down some of its bonds. https://edurev.in/studytube/Configurational-and-Conformational-isomerism-in-cy/1cf8478a-dacb-4d32-b7e4-3b26ce6bcf7d_t This is mostly alleviated by the move to cyclohexane, if it is in a full chair conformation, but the stabilization of the system is minimal ($25-29kJmol^{-1} $ $^{ref~2}$). Here is a link to a question that deals with ring strain and the calculation of it from $\Delta H$ of combustion.

Along with this, your mechanism proposes this ring opening involving an alkyl shift that moves the carbocation from tertiary to secondary. This involves a destabilizing of the cation due to the reduced hyperconjugation (and inductive effect). Studies have shown that a secondary carbocation is around $67-75kJmol^{-1} $less stable than a tertiary, for simple substituents (and in the gas phase$^{ref~3}$).

In total, the ring opening from a five to six-membered ring in your mechanism does not result in an overall increase in stability. Hence why the second mechanism is suggested to happen instead.

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  • $\begingroup$ There seems to be something wrong with your ref 2 hyperlink. I can't seem to reach the page it links to. $\endgroup$ – Tan Yong Boon May 13 at 22:54
  • $\begingroup$ Sorry. I have fixed the link. It is a secondary source: masterorganicchemistry.com/2014/04/18/… I will try to get more primary data. $\endgroup$ – Withnail May 13 at 23:19
  • $\begingroup$ I also edited the post. It was meant to read chair conformation, not boat, for cyclohexane. $\endgroup$ – Withnail May 13 at 23:21
  • $\begingroup$ Ref 2 updated to umn.edu notes. The data for the overall ring strain of cyclopentane comes from heats of combustion relative to cyclohexane $\endgroup$ – Withnail May 14 at 12:04

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