The Lewis acid character of boron trihalides decreases as: `B Br_(3) gt BCl_(3) gt BF_(3)`. Explain ?

Options

A. `BCl_(3) gt BF_(3) gt B Br_(3)`
B. `B Br_(3) gt BCl_(3) gt BF_(3)`
C. `B Br_(3) gt BF_(3) gt BCl_(3)`
D. `BF_(3) gt BCl_(3) gt B Br_(3)`

Answer

2

Explanation

The relative Lewis acid character of boron trihalides is found to follow the follwing order,
`B Br_(3) gtBCl_(3) gtBF_(3)`
but the expected order on the basis of electronegativity of the halogens ( electronegativity of halogens decreases from F to I ) should be ,
`BF_(3)gtBCl_(3) gtB Br_(3)`
This anomally is explained on the basis of the relative tendency of the halogen atom to back donate its unutilised electrons to vacant `p-` orbital of boron atom. If `VF_(3)` boron has a vacent `2p-` orbital and each flourine has fully filled unutilised `2p-` orbitals. Fluorine transfers two electrons to vacant `2p-` orbital of boron, thus forming `p pi-p pi` bond.
This type of bond has some double bond charater and is known as dative or back bonding. All the three bond lengths are same. It is possible when double bond is delocalized. The delocalization may be represented as `:`

The tendency to back denote decreases from F to I as energy level different between B and halogen atom increases from F to I. So, the order is `B Br_(3) gtBCl_(3)gtBF_(3)`