In the hydroboration of a typical alkene, the boron fragment ends up attached to the less substituted carbon:

Hydroboration of an alkene

What are the electronic or steric factors that lead to this regioselectivity?


1 Answer 1


For the hydroboration of isobutene

enter image description here

there is a steric aspect:


And though sterics may play some part, it doesn't explain near 100% regioselectivity. More important in determining regiochemistry, I would say, are the electronics of the transition state:


Boron is electrophilic, and as such will draw the electron density of the alkene π-bond towards itself. This C–B bond formation occurs earlier than the C–H bond formation, which in turn imparts a partial positive charge to the other carbon of the double bond in the transition state of the reaction.

This somewhat resembles a carbocation, which you might remember are stabilized by additional substituents via hyperconjugation. Thus, hydroboration results in an anti-Markovnikov product for mostly the same reason that acid catalyzed hydration results in a Markovnikov product — stabilization of positive charge.

  • 1
    $\begingroup$ How boron will draw the electron density of the alkene π-bond towards itself while carbon is more electronegative than it? $\endgroup$
    – Muhammad
    Commented Feb 14, 2019 at 19:14
  • 3
    $\begingroup$ It is a Lewis acid. $\endgroup$
    – ringo
    Commented Feb 14, 2019 at 19:38
  • 1
    $\begingroup$ Why C-B bond forms faster ? $\endgroup$
    – ado sar
    Commented May 20, 2019 at 15:36
  • $\begingroup$ How can hyperconjugation explain stability of partial charges.? There is no p orbital on carbon to hyperconjugate $\endgroup$
    – user226375
    Commented May 21, 2019 at 3:51
  • 2
    $\begingroup$ Hyperconjugation doesn't need to go into a pure p orbital, it works with any kind of orbital that isn't full, as long as there is sufficient overlap. $\endgroup$ Commented Feb 26, 2021 at 2:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.