I was going through a reference book when I stumbled upon this question

Is that even possible? enter image description here

  • $\begingroup$ In ethanol with heating? Seems highly unlikely. $\endgroup$
    – Zhe
    Jan 3 '17 at 18:24
  • $\begingroup$ @Zhe So it's fine for me to consider this as a false answer? $\endgroup$ Jan 3 '17 at 19:13
  • $\begingroup$ The question is: ‘can the product be observed experimentally?’ If the answer to that is ‘yes’, then the mechanism the book proposes is practically the only plausible mechanism, unless you call a concerted rearrangement and dehalogenation mechanism a different one. (Meaning the primary carbocation is quenched before it is formed.) $\endgroup$
    – Jan
    Jan 3 '17 at 19:44
  • 2
    $\begingroup$ FYI, there is an error in the direction of the curved arrow for the last step. The electrons go toward oxygen, not hydrogen. $\endgroup$
    – jerepierre
    Jan 3 '17 at 19:45
  • 1
    $\begingroup$ Personally, I would never, ever propose a primary cation for these kinds of mild conditions, but I don't have access to the literature any more, so I can't do a search to confirm. $\endgroup$
    – Zhe
    Jan 3 '17 at 20:08

The cation is produced when a methyl group migrate from C-2 to C-1 as the C-Br bond is broken. The simultaneous migration of alkyl group and departure of leaving group to form a tertiary cation is therefore faster and more favourable than formation of primary cation due to simple loss of leaving group.

The products observed are those that result from further reaction of the so formed tertiary cation. The mechanism mentioned in the pic you've attached is incorrect, 1° carbocation intermediate is not formed at all.

Several comments correctly mention that formation of primary carbocation is not favourable. So, it is important to note that methyl shift and departure of leaving group in this case are simultaneous processes - primary carbocation is not formed at all!

P.S. Also, I'd like to quote some evidence I found for a very similar reaction of 1-bromo 2,2-dimethyl propane:

enter image description here

Above is the solvolysis reaction of 1-bromo 2,2-dimethyl propane.

Source: Reactive Intermediates in Organic Chemistry: Structure, Mechanism, and Reactions - By Maya Shankar Singh (Wiley Publishers)


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