Chemistry - Complete Pblock- Question with Solution | TestHub

The strength of the acidic character of boron trihalides depends upon $\mathrm{\pi }-\mathrm{\pi }$ back bonding. In boron trihalides, $\mathrm{\pi }-\mathrm{\pi }$ -back bonding occurs due to empty orbital of boron and filled orbitals of halogens.

The $\mathrm{\pi }-\mathrm{\pi }$ back bonding is shown maximum by${\mathrm{BF}}_3$, 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 the size of halogen increases. The order of size of halogens are $\mathrm{F} < \mathrm{Cl} < \mathrm{Br} < \mathrm{I}$ . Thus, acidic nature is in order ${\mathrm{BF}}_3<{\mathrm{BCl}}_3<{\mathrm{BBr}}_3<{\mathrm{Bl}}_3$.