Question

The order of acidic strength of boron trihalides is:
A. $B{F_3} < BC{l_3} < BB{r_3} < B{I_3}$
B. $B{I_3} < BB{r_3} < BC{l_3} < B{F_3}$
C. $BC{l_3} < BB{r_3} < {\rm{ }}B{I_3} < B{F_3}$
D. $BB{r_3} < BC{l_3} < B{F_3} < B{I_3}$

Answer 1

Boron trihalides i.e. $B{R_3}$, where R stands for any halide are examples of Lewis acid. We know that any chemical compound which can accept a pair of electrons to form a covalent bond is known as Lewis acid.

Complete answer
As we know that boron trihalides are Lewis acid because they contain six electrons in their outermost orbit. In order to complete their octets, the boron trihalides can accept two electrons i.e. one pair of electrons. The order of acidic strength of boron trihalides can be determined on the basis of back-bonding. Due to back-bonding, the electron-density is sent back to the boron atom by the filled orbitals. As a result, the Lewis acidity on boron atoms is reduced. But in case of congeners with ligand orbital 3p, 4p, etc i.e. the orbital overlap is not that much good. In such cases the extent of back-bonding decreases. Finally, the decrease in extent of back-bonding results in increase in Lewis acidity.
It can also be explained on the basis of ligand close packing model. All boron trihalides are $s{p^2}$ hybridised, but when it forms its adduct with the base then its hybridisation changes to $s{p^3}$ hybridised. Since boron fluoride is much stronger due to hydrogen bonding, so in case of boron trifluoride, it is very difficult to maintain close packing due to overcrowding of electrons. And in boron triiodide the bond length is quite higher so stable packing can take place.

**Hence, option (A) $B{F_3} < BC{l_3} < BB{r_3} < B{I_3}$, is the correct option.

Note: **
Thus, the order of acidic strength of boron trihalides i.e. $B{R_3}$ (where R is a halogen) is determined on the basis of back-bonding. Back-bonding is a type of resonance which increases stability. Back-bonding is responsible for reducing Lewis acidity on boron atoms.