I know that the hydroboration reaction is used to add $\ce{H2O}$ to the double bond in an anti-Markovnikov fashion.

I went to the Wikipedia page and found the following mechanism:

Hydroboration mechanism

But I could not understand as to why the addition is syn.

  • 1
    $\begingroup$ In its current form, 's not clear to me why this got downvoted. $\endgroup$
    – hBy2Py
    Nov 5, 2016 at 21:15

2 Answers 2


The addition is concerted, i.e., everything happens at the same time; there aren't multiple steps to the addition. You can expect the boron to go on one face of the double bond while the hydrogen goes on the other face. There's just no way for the hydrogen to reach over to the other side for the anti-attack. It has to be syn.


It develops a cyclic transition state between C double bond:

  CH3     H
    \    /
 H/       \H
  |         |
  |         |
  H---  ---BH2

Then replaces BH2 by OH

  • 2
    $\begingroup$ Please draw an image and upload that; it is impossible to understand what you wrote down there. $\endgroup$
    – Jan
    Nov 5, 2016 at 23:13
  • 1
    $\begingroup$ I did the best I could with it, but I would heed @Jan's sage advice here. $\endgroup$
    – jonsca
    Nov 6, 2016 at 1:06

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