Question

Why can we not directly form p-bromo aniline from aniline?

Answer 1

BF3 and BH3 both accept electrons at the boron atom. Fluorine is more electronegative than hydrogen, so due to inductive effects, the electron density around boron will be lower in BF3 then in BH3. Lower electron density around boron makes the BF3 more reactive toward Lewis bases than BH3.