All the boron trihalides except `BI_(3)` may be prepared by direction between the elements. Boron trihalides consist of trigonal-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 `BCl_(3)` are gases, `BBr_(3)` is a volatile liquid and `BI_(3)` is a solid. Boron trihalides are Lewis acids because they form simple Lewis complexes with suitable bases, as in the reaction:
`BF_(3)(g)+NH_(3)(g)toF_(3)B-NH_(3)(s)`
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_(2)O(l)toB(OH)_(3)(aq)+3HCl (aq)`
It is supposed that the first step in the above reaction is the formation of the complex `Cl_(3)B larr OH_(2)` which then eliminates `HCl` and reacts with water.
Which of the follwoing is the best order of Lewis acid strength of `BF_(3),BCl_(3)` and `BBr_(3)` ?
A. `BF gt BCl_(3) gt BBr_(3)`
B. `BF = BCl_(3) = BBr_(3)`
C. `BF lt BCl_(3) lt BBr_(3)`
D. `BBr_(3) lt BF_(3) lt BCl_(3)`
