Question

The bond dissociation energy of \[{\text{B}} - {\text{F in B}}{{\text{F}}_3}\] is \[646{\text{ kJ/mol}}\] whereas that of \[{\text{C}} - {\text{F in C}}{{\text{F}}_4}\] is \[515{\text{ kJ/mol}}\]. Comment on the above statement.
A ) stronger sigma bond between \[{\text{B and F in B}}{{\text{F}}_3}\] as compared to that between \[{\text{C and F in C}}{{\text{F}}_4}\].
B ) significant \[p\pi - p\pi \] interaction between \[{\text{B}} - {\text{F in B}}{{\text{F}}_3}\] whereas there is no possibility of such interaction between \[{\text{C and F in C}}{{\text{F}}_4}\].
C ) lower degree of \[p\pi - p\pi \] interaction between \[{\text{B}} - {\text{F in B}}{{\text{F}}_3}\] whereas there is no possibility of such interaction between \[{\text{C and F in C}}{{\text{F}}_4}\].
D ) smaller size of boron atom as compared to that of carbon atom.

Answer 1

Hint: Boron atom forms an incomplete octet. Due to this, back-bonding is possible in boron trifluoride. Carbon atom has completed its octet. Due to this back-bonding is not possible in carbon tetrafluoride.
Complete step by step answer:
The atomic number of boron is 5. It has 3 valence electrons. In \[{\text{B}}{{\text{F}}_3}\] the central boron atom has six valence electrons. These six valence electrons are present as three \[{\text{B}} - {\text{F}}\] bond pairs of electrons. Thus, boron atoms form an incomplete octet. The vacant p orbital of boron can accept an electron pair from fluorine. This gives rise to \[p\pi - p\pi \] interaction. This interaction is the back-bonding interaction. The atomic number of carbon is 6. It has 4 valence electrons. In \[{\text{C}}{{\text{F}}_4}\] the central carbon atom has eight valence electrons. These eight valence electrons are present as four \[{\text{C}} - {\text{F}}\] bond pairs of electrons. Thus, carbon atoms form a complete octet. There is no vacant p orbital on a carbon atom. Hence, the carbon atom cannot accept an electron pair from fluorine. There is no possibility of \[p\pi - p\pi \] interaction in carbon tetrafluoride.
Hence, the correct option is the option B ) significant \[p\pi - p\pi \] interaction between \[{\text{B}} - {\text{F in B}}{{\text{F}}_3}\] whereas there is no possibility of such interaction between \[{\text{C and F in C}}{{\text{F}}_4}\].

So, the correct answer is “Option B”.

Note: In \[{\text{B}}{{\text{F}}_3}\] \[p\pi - p\pi \] interaction (back-bonding interaction) is present. This increases electron density on the central boron atom. Due to this, \[{\text{B}}{{\text{F}}_3}\] is much weaker lewis acid than \[{\text{BB}}{{\text{r}}_3}\] or \[{\text{BC}}{{\text{l}}_3}\].