Question

Arrange the following in the correct order of Lewis acidity.
$BF_{3},BCl_{3},BBr_{3}$

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

Answer 1

Answer: (C)

In $BF_3, BCl_3$ and $BBr_3$ 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 $(BF_3 < BCl_3 < BBr_3)$.