The bond dissociation energy of B-F in `BF_(3)` is 646 kJ `mol^(-1)` whereas that of C-F in `CF_(4)` is 515 kJ `mol^(-1)`. The correct reason for higher B-F bond dissociation energy as compared to that of C-F bond is

Options

A. Lower degreee of `p pi-p pi` interaction between `B` and `F` `BF_(3)` than that between `C` and `F` in `CF_(4)`
B. Significant `p pi-p pi` interaction between `B` and `F` in `BF_(3)` whereas there is no possibility of such interaction between `C` and `F` in `CF_(4)`
C. Smaller size of `B`-atoms as compared to that of `C`-atom and the stronger bond between `B` and `F` in `BF_(3)` as compared to that between `C` and `CF_(4)`
D. Lewis acid character pf boron trihalides is as: `B Br_(3) gt BCl_(3) gt BF_(3)`

Answer

B:

Explanation

Due to `p pi-p pi` back bonding , `BF_(3)` is the strongest Lewis acid as compared to `BCl_(3)` and `B Br_(3)`. `B-F` bond is shorter in length causing it to more stronger.
In `CF_(4)`, there is no such back bonding possible as there is no vacant `p`-orbital in cation.
d. Due to strong `p pi-p pi` back bonding in `BF_(3), (2p-2p` overlap) its electron deficiency is less, so it is the weakest Lewis acid. Hence, the order of Lewis acid is
`B Br_(3) gt B Cl_(3) gt BF_(3)`
`(2p-4p)(2p-3p)(2p-2p)` overlap.