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

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 ${\text{BC}}{{\text{l}}_3}$.