All the boron trihalides except `Bl_3`, may be prepared by direct reaction between the elements. Boron trihalides consistoftrigonal-planar `BX_3` molecules. Unlike the halides of the other elements in the group they are monomeric in the gas, liquid and solid states, `BF_3 and BCI_3` are gases, `BBr_3` is a volatile liquid and `BI_3` is a solid. Boron trihalides are Lewis acids because they form Lewis complexes with suitable bases.
`BF_(3(g)) + : NH_(3(g)) to F_3B - NH_(3(g))`
However, boron chlorides, bromides and iodides are susceptible (sensitive) to protolysis by mild proton sources such as water, alcohols and even amines for example `BCl_3` undergoes rapid hydrolysis.
`BCl_(3(g)) + 3H_2O((l)) to3(OH)_(3(aq.)) + 3HCl_((aq.))`
It is supposed that the first step in the above reaction is the formation of the complex `Cl_3B larr OH_2` which then eliminates HCl and reacts further with water.
Which of the following is the best order of Lewis acid strength of `BF_3 BCl_3 and BBr_3` ?

Options

A. `BF_3 gt BCl_3 gt BBr_3`
B. `BF_3 = BCl_3 = BBr_3`
C. `BF_3 lt BCl_3 gt BBr_3`
D. `BBr_3 gt BF_3 gt BCl_3`

Answer

C

Explanation

Due to more predominant back bonding in `BF_3` it is weakest base.