The bond dissociation energy of $\mathrm{B}-\mathrm{F}$ in $\mathrm{BF}_{3}$ is $646 \mathrm{~kJ}…

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. stronger $\sigma$ bond between $\mathrm{B}$ and $\mathrm{F}$ in $\mathrm{BF}_{3}$ as compared to that between $\mathrm{C}$ and $\mathrm{F}$ in $\mathrm{CF}_{4}$.
  2. significant $p \pi-p \pi$ interaction between $\mathrm{B}$ and $\mathrm{F}$ in $\mathrm{BF}_{3}$ whereas there is no possibility of such interaction between $\mathrm{C}$ and $\mathrm{F}$ in $\mathrm{CF}_{4}$.
  3. lower degree of $p \pi-p \pi$ interaction between $\mathrm{B}$ and $\mathrm{F}$ in $\mathrm{BF}_{3}$ than that between $\mathrm{C}$ and $\mathrm{F}$ in $\mathrm{CF}_{4}$.
  4. smaller size of B-atom as compared to that of $\mathrm{C}$ -atom.

Solution

The delocalised $\mathrm{p} \pi-\mathrm{p} \pi$ bonding between filled $p$ -orbital of $\mathrm{F}$ and vacant $p$ -orbital of $\mathrm{B}$ leads to shortening of $\mathrm{B}-\mathrm{F}$ bond length which results in higher bond dissociation energy of the B-F bond.

Asked in: JEE-TOPICTESTS-CHEMISTRY

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