In the following reaction, why does ring expansion not take place?

Hydroclorination of alkene

Mechanism that was the correct one:

Hydroclorination mechanism

In the second step when there is a protonated carbon the ring should expand to become 6 membered as that is more stable. This should be followed by a methyl shift from Quaternary to tertiary and then the $\ce{Cl}$ should get attached. But this doesn't happen. Why is that? According to my textbook's theory, what I described should occur but the mock test says otherwise. Are there some more criteria-besides.

  1. protonated carbon next to ring,

  2. forming of a more stable 6 number ring

for ring expansion that my text book is missing?

Any help would be appreciated.

  • 1
    $\begingroup$ 5- and 6-member rings are fairly close in terms of ring strain. That's probably the main issue here. $\endgroup$
    – Zhe
    Commented Jul 4, 2020 at 18:58

1 Answer 1


Simple answer is consider the stability of carbocations. You already got the tertiary carbocation in hand that is much stable than secondary carbocation, which would be resulted by ring expantion. Sure, a methide migration would give you another tertiary carbocation, but before you get that, the reaction progress has to climb another actvation barrier, which would be steep ($3^\circ \rightarrow 2^\circ$). Bottom line is rate of clorination of carbocation is faster than rate of this rearrangement followed by methide migration ($E_\mathrm{a2} \lt E_\mathrm{a3}$):

Energy Diagram

  • $\begingroup$ Using which software did you draw these colorful reaction profile diagrams? $\endgroup$ Commented Jul 4, 2020 at 17:59
  • $\begingroup$ @Chem-Learner: No software. Just being creative with what available in internet. $\endgroup$ Commented Jul 4, 2020 at 18:04
  • $\begingroup$ this clears up a lot for me. Are there anymore such reactions where ring expansion could happen and it doesnt? Or does this apply only to halogenation ? $\endgroup$ Commented Jul 6, 2020 at 6:03
  • 1
    $\begingroup$ To my knowledge, it is situational. Mostly carbocation forming reactions such as $\mathrm{S_N1}$ and $\mathrm{E1}$ reactions. $\endgroup$ Commented Jul 6, 2020 at 7:21
  • $\begingroup$ @MathewMahindaratne i came across another such reaction except the cyclo hexane ring wasn't there. Instead of HCl, HI was used. This time ring expansion mechanism was followed. Why is that so ? $\endgroup$ Commented Jul 26, 2020 at 12:59

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