# The bond dissociation energy

Question:

The bond dissociation energy of $\mathrm{B}-\mathrm{F}$ in $\mathrm{BF}_{3}$ is $646 \mathrm{~kJ} \mathrm{~mol}^{-1}$ whereas that of $\mathrm{C}-\mathrm{F}$ in $\mathrm{CF}_{4}$ is $515 \mathrm{~kJ} \mathrm{~mol}^{-1}$. The correct reason for higher $\mathrm{B}-\mathrm{F}$ bond dissociation energy as compared to that of $\mathrm{C}-\mathrm{F}$ is :-

1. Significant $\mathrm{p} \pi-\mathrm{p} \pi$ interaction between $\mathrm{B}$ and $\mathrm{F}$ in $\mathrm{BF}_{3}$ whereas there is not possibility of such interaction between $\mathrm{C}$ and $\mathrm{F}$ in $\mathrm{CF}_{4}$.

2. Lower degree of $\mathrm{p} \pi-\mathrm{p} \pi$ interaction between $\mathrm{B}$ and $\mathrm{F}$ in $\mathrm{BF}_{3}$ than that between $\mathrm{C}$ and $\mathrm{F}$ in $\mathrm{CF}_{4}$

3. Smaller size of B-atom as compared to that of C-atom

4. Stronger $\sigma$ bond between $\mathrm{B}$ and $\mathrm{F}$ in $\mathrm{BF}_{3}$ as compared to that between $\mathrm{C}$ and $\mathrm{F}$ in $\mathrm{CF}_{4}$

Correct Option: 1

Solution:

Due to back bonding in $\mathrm{BF}_{3}$ molecule all $\mathrm{B}-\mathrm{F}$ bond having partial double bond character.