Question

Which of the following has the least Lewis acidic character?

• A. $B{F}_3$
• B. $BC{l}_3$
• C. $BB{r}_3$
• D. All have the same acidic character

Answer 1

The correct option is A $B{F}_3$

Lewis acids are electron pair acceptors which accept the electron pairs in vacant p or d orbital.

a) In $B{F}_3$, $F$ donates the electron density in a vacant $p$ orbital via strong back bonding $(2p\pi -2p\pi )$. Hence, $p$ orbital is not available to accept electrons.

b) In $BC{l}_3$, $Cl$ donates the electron density in a vacant $p$ orbital via poor back bonding $(2p\pi -3p\pi )$. Hence, there is less availability to accept electrons.

c) In $BB{r}_3$, $Br$ donates the electron density in a vacant $p$ orbital via very poor back bonding $(2p\pi -4p\pi )$. Hence, $p$ orbital is almost completely available to accept electrons.

Hence, $B{F}_3$ has the least acidic character.