What is the product of the reaction of 2-methylbuta-1,3-diene (isoprene) with HBr?


What I understand is that carbon (2) will get a positive charge after protonation.

But after this, which should be the major product?

  • 3-bromo-3-methylbut-1-ene – because in this case the carbocation would be stable because of being tertiary, but the double bond would have less number of hyperconjugated structures and will be less stable


  • 1-bromo-3-methylbut-2-ene – as the carbocation would be less stable because it would be primary but the double bond would be much more stable because of 8 $\alpha$ hydrogen available



The regioselectivity of the overall reaction depends on the temperature. The carbocation intermediate can pass through the two different transition states that lead to the 1,2- and 1,4- products, respectively. If we choose a temperature low enough, then the product distribution will reflect the difference in energy between the two activation energies Ea (1,2) and Ea (1,4). So long as the reaction is not reversible, the product with the lower energy transition state will dominate which is decided by the stability of the intermediate, so in that case 3-bromo-3-methylbut-1-ene (product-1) will be more stable product.
At higher temperatures, the reaction has the potential to be reversible, this means that both the 1,2- and 1,4- products can ionize This sets up an equilibrium. The product ratio will now reflect the relative stabilities of the 1,2- and 1,4- products, not the transition states leading to their formation. In the case of 2-methylbuta-1,3-diene, the product 1-bromo-3-methylbut-2-ene (product-2) will be more stable and dominant at higher temperatures.

  • 1
    $\begingroup$ "if the temperature isn't mentioned then we usually consider the thermodynamically more stable product" - Well, there's no scientific reason why you should do that. This sounds more like a way to solve ambiguous exam questions rather than chemistry. "owing to the fact that elimination reactions occur at high temperatures as they are endothermic" - I don't see the relevance of elimination reactions to this question? $\endgroup$ – orthocresol Apr 30 at 22:50

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