The strength of acidic character of boron trihalides depends upon pπ−pπ back bonding. In boron trihalides, pπ−pπ -back bonding occurs due to empty orbital of boron and filled orbitals of halogens.
The pπ−pπ back bonding is shown maximum by BF3, as the size of B and F are small and comparatively same. Due to this effect tendency of accepting lone-pair of electrons of boron decreases as size of halogen increases. The order of size of halogens are F<Cl<Br<L. Thus, acidic nature is in order BF3<BCl3<BBr3<Bl3