What is the major product of the following reaction?

$\hspace{40 mm}$Reaction

$\hspace{20 mm}$Products

It looks to me that the $\ce{Cl}$ group leaves then there's an alkyl shift to where the $\ce{Cl}$ group leaves in order to form a tertiary carbocation. Then $\ce{EtOH}$ attacks the beta hydrogen and forms a double bond to give product B:

$\hspace{35 mm}$Mechanism

However, the answer I came up with was incorrect. Any ideas why?

  • 1
    $\begingroup$ It's a good attempt, but as I said there are quite a number of alkyl groups you could migrate - you might want to try to list all of them and see why each of them may or may not be so favoured. $\endgroup$ Nov 20, 2015 at 21:12
  • 1
    $\begingroup$ Before solving this problem, you should know the migrating aptitude of groups. en.wikipedia.org/wiki/Migratory_aptitude $\endgroup$
    – Aditya Dev
    Nov 22, 2015 at 1:32
  • $\begingroup$ I've thought of another one I guess. I'm pretty confident nobody actually mentioned it as something that happens in my ochem class but could the ring expand with an alkyl shift? So it would create product A. Problem is why would A be favored over B? Both involve a tertiary carbocation... is the carbocation in a ring more stable or something? $\endgroup$
    – John
    Nov 22, 2015 at 6:16
  • $\begingroup$ I would say it is because there is less ring strain in A than in B. $\endgroup$ Nov 22, 2015 at 12:12

1 Answer 1


You are correct that an alkyl shift will take place but there are two possible options for which group moves. Your option is possible but there is another option which involves a ring expansion to generate product A.

enter image description here

This mechanism is favoured because it relieves the ring strain in the four membered ring. Since the reaction is run under reversible conditions, the thermodynamic product (with the least ring strain) will dominate.

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    $\begingroup$ The migration of the primary alkyl group is also favoured over the methyl group since it can stabilise the positive charge in the TS better. $\endgroup$ May 27, 2016 at 21:39

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