With regard to the above professor's claim, I disagree. Bent's rule, which states that atomic p-character is concentrated in orbitals directed toward more electronegative substituents, implies that the $\ce{Br-C-Br}$ bond angle is going to be smaller than 109.5 degrees since the "$\ce{sp^3}$" hybrid orbitals involved in C-Br bonding have more p-character than 75%.
Therefore, the $\ce{H-C-H}$ bond angle should be bigger than 109.5 degrees.
Questions
1) Does Bent's rule work here (i.e. am I correct?)
2) Does Bent's rule always work?
Revision
- HCH bond angle is ~110 degrees.
- HCBr bond angle is ~108 degrees.
- Br-C-Br bond angle is 112 degrees.
Rationales
- vdW repulsions play a role in Br-C-Br bond angle expansion, although the amount of p-character in the C-Br bonds might be >75%.
- HCH angle is expanded because of Bent's rule; s-character in the H-C bond exceeds 25%.
- The expansion of the Br-C-Br bond angle and the expansion of the H-C-H bond angle squeezes together the remaining H-C-Br bond angle.