Question

The Lewis acidity of $BF_3$ is less than $BCl_3$ even though fluorine is more electronegative than chlorine. It is due to

• A. stronger $2p(B)-2p (F) \sigma$ - bonding
• B. stronger $2p(B)-2p(F) \pi$ - bonding
• C. stronger $1 p( B )-3 p( Cl ) \sigma$ -bonding
• D. stronger $2p(B)-3p(Cl) \pi$ - bonding

Answer 1

Answer: (B)

stronger $2p(B)-2p(F) \pi$ - bonding

Answer 2

Boron and fluorine do not have $d$ -orbitals. Hence, both of these participate in strong $2 p(B)-2 p(F)$ back $\pi$ -bonding. On the other hand, due to large size and availability of vacant $d$ -orbitals, Cl does not participate in such type of back $\pi$ -bonding. Hence, $BF _{3}$ is less acidic than $BCl _{3}$.