question_answer

The bond dissociation energy of\[BF\]in\[B{{F}_{3}}\]is\[646\text{ }kJ\text{ }mo{{l}^{1}}\] whereas that of\[CF\]in\[C{{F}_{4}}\]is\[515\text{ }kJ\text{ }mo{{l}^{1}}.\] The correct reason for higher\[B-F\]bond dissociation energy as compared to that of\[CF\]is:     AIEEE  Solved  Paper-2009

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

B) stronger\[\sigma \]bond between B and F in\[B{{F}_{3}}\]as compared to that between C and F in\[C{{F}_{4}}\]

C) significant\[p\pi -p\pi \]interaction between B and F in\[B{{F}_{3}}\]whreas there is no possibility of such interaction between C and F in\[C{{F}_{4}}\]

D) lower degree of \[p\pi -p\pi \] interaction between B and F in\[B{{F}_{3}}\]than that between C and F in\[C{{F}_{4}}\]

Correct Answer: C

Solution :

B has vacant available p orbital and hence it involves\[p\pi -p\pi \]back bonding which is not possible in\[C{{F}_{4}}\]as C does not have vacant orbital.