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In the following reaction

Reaction between 2‐methylbut‐3‐en‐2‐ol and hydrogen bromide

shouldn't the minor product be the major one as the carbocation formed before attachment of bromine is 3°? Also, can someone tell me why the product includes only nucleophilic substitution as there is no other condition mentioned to judge whether it should show elimination or substitution.

Please mention if what favours allylic shift in such type of reactions.

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Even thought the immediate carbocation formed is tertiary, the stability of the double bond should also be taken into account. A more substituted double bond is also favored/stable. In this instance it seems that it's a competition between the double bond's stability and the carbocation's stability. There could be other situations where this is not the case, but you would need experimental data to back up stability (there's no universal ranking system that I know of to say when a double bond outweighs a positive charge).

In short, I would say that the relative stability of a tertiary double bond (compared to a tertiary carbocation) is what would justify the major product being labeled as it is.

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    $\begingroup$ There isn't a distinct primary carbocation or a tertiary carbocation. There is only one carbocation with two major resonance forms. OP's question has the same issue. $\endgroup$ – orthocresol Sep 15 at 17:04
  • $\begingroup$ Can u please elaborate what you mean? @orthocresol $\endgroup$ – Physics freak Sep 16 at 4:42

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