The order of acidic strenght boron trihalides is:

Options

A. `BF_(3) lt BCl_(3) lt BBr_(3) lt BI_(3)`
B. `BI_(3) lt BBr_(3) lt BCl_(3) lt BF_(3)`
C. `BCl_(3) lt BBr_(3) lt BI_(3) lt BF_(3)`
D. `BBr_(3) lt BCl_(3) lt BF_(3) lt BI_(3)`

Answer

A

Explanation

The strength of acidic character of boron trihalides depends upon `ppi -p pi ` back bonding.
In boron trihalides, `p pi-p pi-` back bonding occurs due to empty orbital of boron and filled orbitals of halogens.

The pp-pp 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 lt Cl lt Br lt I`. Thus, acidic nature is in order
`BF_(3) lt BCI_(3) lt BBr_(3) lt BI_(3)`