Question

Arrange the following in the correct order of Lewis acidity.
$B{F}_3,BC{l}_3,BB{r}_3$

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

Answer 1

Answer: (C)

In BF₃, BCl₃ and BBr₃ each will show $p_{\pi }-p_{\pi }$ back bonding as follows.

Extent of overlapping is $2\text{p}-2\text{p}>2\text{p}-3\text{p}>2\text{p}-4\text{p}$ . As the difference between the size of p orbital increases, the extent of overlapping decreases the availability of vacant orbital increases and the acidic character increases.
Hence, correct order of Lewis acidity represented by option (BF₃ < BCl₃ < BBr₃).