Why is the direction of addition of diborane to a double bond the opposite to what is observed for other HX reagents?
While it is not appropriate in this activity to go into the detailed pathways for these reactions, the difference between diborane and the other HX reagents that is responsible for the different direction of addition is shown by the electron density maps for these compounds given below
The electrostatic potential maps show that the hydrogens in H2SO4, HBr and HCl are MUCH more positive than the hydrogens in diborane.
In fact, the electronegativity of H is slightly higher than the electronegativity of borane so that the bond is polarised δ+B–Hδ-.
This difference in the direction of polarisation of the E-H bond in the reagent is the reason for the difference in the direction of addition.
While it is not appropriate in this activity to go into the detailed pathways for these reactions, the difference between diborane and the other HX reagents that is responsible for the different direction of addition is shown by the electron density maps for these compounds given below
Note that electrostatic potenial maps are red in more negative regions and blue in more positive regions of the electron cloud.
 |  |
 |  |
| Cl–H |  |
| Br–H |  |
The electrostatic potential maps show that the hydrogens in H2SO4, HBr and HCl are MUCH more positive than the hydrogens in diborane.
In fact, the electronegativity of H is slightly higher than the electronegativity of borane so that the bond is polarised δ+B–Hδ-.
This difference in the direction of polarisation of the E-H bond in the reagent is the reason for the difference in the direction of addition.