Question

The bond dissociation energy of $B-F$ in $B{F}_3$ is 646 kJ $mo{l}^{-1}$ whereas that of $C-F$ in $C{F}_4$ is 515 kJ $mol-1$. Comment on the above statement.

• A. stronger $\sigma$ bond between $B$ and $F$ in $B{F}_3$ as compared to that between $C$ and $F$ in $C{F}_4$
• B. significant $p\pi -p\pi$ interaction between $B$ and $F$ in $B{F}_3$ whereas there is no possibility of such interaction between $B$ and $F$ in $C{F}_4$
• C. lower degree of $p\pi -p\pi$ interaction between $B$ and $F$ in $B{F}_3$ than that between $C$ and $F$ in $C{F}_4$
• D. smaller size of $B-$atom as compared to that of $C-$atom.

Answer 1

Answer: (B)

significant $p\pi -p\pi$ interaction between $B$ and $F$ in $B{F}_3$ whereas there is no possibility of such interaction between $B$ and $F$ in $C{F}_4$

$B{F}_3$ is a Lewis acid due to incomplete octet of Boron. Among $B{F}_3,BC{l}_3,BB{r}_3$ and so on, $B{F}_3$ is the weakest Lewis acid due to back bonding of lone pair electron of F to p-orbital of Boron.

B has vacant available p-orbital in $B{F}_3$ and hence it involves pπ−pπ back bonding which is not possible in $C{F}_4$ as C does not have any vacant orbital.

Hence, option $B$ is correct.