Question

The strongest Lewis acid among boron halides is

• A. $BB{r}_3$
• B. $BC{l}_3$
• C. $B{I}_3$
• D. $B{F}_3$

Answer 1

Answer: (C)

Generally, as the electronegativity increases, acidic character increases but in case of boron halides, $B{I}_3$ is most acidic, ie, strongest Lewis acid. This is because there is back donation of electrons from $F$ atom to boron in case of $B{F}_3$. This makes the $B{F}_3$ less electron deficient. The tendency of back donation decreases as the size of halogen atom increases (due to large energy difference). Hence, $B{I}_3$ is most electron deficient and thus, the strongest Lewis acid.