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

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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? $\endgroup$
    – Mason
    Feb 14, 2019 at 19:14
  • 3
    $\begingroup$ It is a Lewis acid. $\endgroup$
    – ringo
    Feb 14, 2019 at 19:38
  • 1
    $\begingroup$ Why C-B bond forms faster ? $\endgroup$
    – ado sar
    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
    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. $\endgroup$ Feb 26, 2021 at 2:28

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