Question

The tendency of BF${}_3$, BCl${}_3$ and BBr${}_3$ to behave as Lewis acid decreases in the sequence is:

• A. BCl${}_3$ > BF${}_3$ > BBr${}_3$
• B. BBr${}_3$ > BCl${}_3$ > BF${}_3$
• C. BBr${}_3$ > BF${}_3$ > BCl${}_3$
• D. BF${}_3$ > BCl${}_3$ > BBr${}_3$

Answer 1

The correct option is B BBr${}_3$ > BCl${}_3$ > BF${}_3$

A molecule is said to be a lewis acid if it can accept a lone pair.

When the halide is more electronegative the interelectronic repulsion increases making the molecule difficult to accept a lone pair.

Size of halide also behaves in the same way. Also the degree of hydrolysis increases from fluorine to iodine,so the lewis acidic character increases in the same order.

So,the order of lewis acidity is $B{Br}_3>B{Cl}_3>B{F}_3$.

Hence, the correct option is $B$.