Question

The bond dissociation energy of B – F in BF₃ is 646 kJ mol–1
whereas that of C – F in CF₄ is 515 kJ mol⁻¹. The correct
reason for higher B – F bond dissociation energy as compared
to that of C – F is
(a) stronger σ bond between B and F in BF₃ as compared
to that between C and F in CF₄.
(b) significant pp – pp interaction between B and F in BF₃
whereas there is no possibility of such interaction
between C and F in CF₄.
(c) lower degree of pp – pp interaction between B and F in
BF₃ than that between C and F in CF₄.
(d) smaller size of B– atom as compared to that of C– atom.

Answer 1

The bond dissociation energy of B – F in BF₃ is 646 kJ mol–1 whereas that of C – F in CF₄ is 515 kJ mol⁻¹. The correct reason for higher B – F bond dissociation energy as compared to that of C – F is :

• The higher B-F hond dissociation energy in BF3 as compared to that of C-F in CF4 is due to the significant pπ−pπ interaction between B and F due to back bonding of a lone pair of electrons from filled 2p-orbital of F to the empty 2p-orbital of B atom. Such interaction does not occur between C and F in CF4.

• Hence option b i.e significant pπ−pπ interaction between B and F in BF₃ whereas there is no possibility of such interaction

between C and F in CF₄.