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 kJ $\mathrm{mol}^{-1}$. The correct reason for higher B-F bond dissociation energy as compared to that of C - F is :

(C) significant $p \pi-p \pi$ interaction between $B$ and $F$ in $B F_{3}$ whereas there is no possibility of such interaction between C and F in $\mathrm{CF}_{4}$.

(A) smaller size of B -atom as compared to that of C - atom

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

(D) lower degree of $p \pi-p \pi$ interaction between $B$ and $F$ in $B F_{3}$ than that between $C$ and $F$ in $C F_{4}$.

JEE Main · 2009 · 12 Apr 2009 · Offline · Chemical Bonding and Molecular Structure

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The bond dissociation energy of

B - F

BF_{3}

646 kJ mol^{- 1}

whereas that of

C - F

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 $σ$ bond between B and F in $BF_{3}$ as compared to that between C and F in $CF_{4}$
(C)significant $p π - p π$ interaction between $B$ and $F$ in $B F_{3}$ whereas there is no possibility of such interaction between C and F in $CF_{4}$ .✓ Correct
(D)lower degree of $p π - p π$ interaction between $B$ and $F$ in $B F_{3}$ than that between $C$ and $F$ in $C F_{4}$ .

Answer: (C)

Solution

Boron in

B F_{3}

has a vacant

2 p

orbital that can accept a lone pair from the filled

2 p

orbitals of fluorine, setting up a

p π - p π

back-donation that adds extra double-bond character and strengthens the B–F bond. Carbon in

C F_{4}

has no such empty orbital available, so no comparable back-bonding occurs. This significant

p π - p π

interaction in

B F_{3}

, absent in

C F_{4}

, is why the B–F bond dissociation energy is the larger of the two.

JEE Main · 2009 · 12 Apr 2009 · Offline · Single correct option

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JEE Main · 2009 · 12 Apr 2009 · Offline · Chemical Bonding and Molecular Structure

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