The tendency of BF3, BCl3 and BBr3 to behave as Lewis acid decreases in the sequence
- BCl3 > BF3 > BBr3
- BBr3 > BCl3 > BF3
- BBr3 > BF3 > BCl3
- BF3 > BCl3 > BBr3
Answer
(2) BBr3 > BCl3 > BF3
Solution
In BX3, the vacant 2p orbital of boron receives electron density from a filled p orbital of the halogen (p –p back bonding).
Back bonding is most effective in BF3 (2p–2p overlap, same size orbitals) and decreases as the halogen gets larger (3p, 4p).
Electron deficiency of boron: BF3 < BCl3 < BBr3
Lewis acid strength decreases as BBr3 > BCl3 > BF3
Answer: (2)