Question

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 $C - F$ is

• A. Smaller size of B atom as compared to that of C atom
• B. Stronger $\sigma$ bond between $B$ and $F$ in $BF_3$ as Compared to that between C and F in $CF_4$
• C. Lower degree of $p \pi-p \pi$ interaction between $B$ and $F$ in $BF _{3}$ than that between $C$ and $F$ in $CF _{4}$
• D. 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 $CF _{4}$

Answer 1

Answer: (D)

In $BF _{3},, B$ is $sp ^{2}$ hybridised and has a vacant $2 p$-orbital which overlaps laterally with a filled $2 p$-orbital of $F$ forming strong $p \pi - p \pi$ bond. However in $CF _{4}, C$ does not have any vacant $p$-orbitals to undergo $p$-bonding. Thus $BE _{ B - F }> BE _{ C - F }$.