If in a reaction the carbocation is formed outside the ring then why does the ring expansion does not happen as follows:

Oxygen migration after carbocation formation

carbon migration after carbocation formation

  • $\begingroup$ Is there any reaction that could create a carbocation at that place? $\endgroup$
    – Kenny Lau
    Commented May 27, 2016 at 14:03
  • $\begingroup$ @KennyLau i have now posted that reaction $\endgroup$
    – akbar
    Commented May 27, 2016 at 14:11

1 Answer 1


To understand which rearrangement pathway is preferred, we must consider the relative stabilities of the intermediates. The rearrangement that puts the carbocation one carbon away from the the oxygen does not have any stabilizing resonance structures:

$\hspace{5.4cm}$enter image description here

The rearrangement that places the carbocation adjacent to the oxygen delocalizes the positive charge to two atoms, and is much more stable:

$\hspace{3.7cm}$enter image description here

  • $\begingroup$ +1 That's how dihydropyrane can be obtained from tetrahydrofurfuryl alcohol via catalytic dehydration on alumina at elevated temperatures. $\endgroup$ Commented May 27, 2016 at 15:30

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