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

  • Option 1)

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

  • Option 2)

    stronger \sigma bond between B \: and\: F in B F_{3} as compared to that between  C \: and\: F inC F_{4}

  • Option 3)

    significant p\pi -p\pi interaction between B \: and\: Fin B F_{3} whereas there is no possibility of such interaction between C \: and\: F inC F_{4}

  • Option 4)

    lower degree of  p\pi -p\pi interaction between B \: and\: Fin B F_{3} then that between C \: and\: F inC F_{4}

 

Answers (1)

As we learnt in

Atomic radius of boron family -

Increases down the group except aluminium

- wherein

Abrupt increase in Al radius is due to greater screening effect of Al than in B

 

 B has vacant p orbitals and so it involves p\pi -p\pi back bonding which is not possible in CF as C does not have any vacant orbital.

 

Therefore, B - F bond is stronger than C - F bond.


Option 1)

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

This option is incorrect.

Option 2)

stronger \sigma bond between B \: and\: F in B F_{3} as compared to that between  C \: and\: F inC F_{4}

This option is incorrect.

Option 3)

significant p\pi -p\pi interaction between B \: and\: Fin B F_{3} whereas there is no possibility of such interaction between C \: and\: F inC F_{4}

This option is correct.

Option 4)

lower degree of  p\pi -p\pi interaction between B \: and\: Fin B F_{3} then that between C \: and\: F inC F_{4}

This option is incorrect.

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