Final answer:
Borane adds to the sp² carbon bonded to the most hydrogens because of the sp² hybridization of boron in the compound.
Step-by-step explanation:
The observed structure of the borane molecule, BH3, suggests sp² hybridization for boron in this compound. The molecule is trigonal planar, and the boron atom is involved in three bonds to hydrogen atoms.
We can illustrate the comparison of orbitals and electron distribution in an isolated boron atom and in the bonded atom in BH3. We redistribute the three valence electrons of the boron atom in the three sp² hybrid orbitals, and each boron electron pairs with a hydrogen electron when B-H bonds form.