Question

Though fluorine is more electronegative than chlorine yet BF₃is a weaker lewis acid than BCl_3.Give reason.

Answer 1

In boron halides, there exists certain degree of pi bonding involving halogen lone pairs and empty 2p orbital of boron.This bonding is strongest in case of BF₃ and when a boron halide accepts electrons from a donor molecule, this bonding is lost. Due to this, BF₃ resists this change most strongly followed by other halides and thus the acceptor strength increases in the order BF₃ < BCl₃ < BBr₃ < BI₃

Due to this, BI₃ a stronger Lewis acid than BCl₃ and BF₃