Question

The strongest Lewis acid among boron halides is

• A. $BBr_{3}$
• B. $BCl_{3}$
• C. $BI_{3}$
• D. $BF_{3}$

Answer 1

Answer: (C)

$BI_{3}$

Answer 2

Generally, as the electronegativity increases, acidic character increases but in case of boron halides, $BI _{3}$ is most acidic, ie, strongest Lewis acid. This is because there is back donation of electrons from $F$ atom to boron in case of $BF _{3}$. This makes the $BF _{3}$ less electron deficient. The tendency of back donation decreases as the size of halogen atom increases (due to large energy difference). Hence, $BI _{3}$ is most electron deficient and thus, the strongest Lewis acid.