Question

The relative Lewis acid character of boron trihalides is the order

• A. \(\mathrm{BI}_3>\mathrm{BBr}_3>\mathrm{BF}_3>\mathrm{BCl}_3\)
• B. \(\mathrm{BI}_3>\mathrm{BBr}_3>\mathrm{BCl}_3>\mathrm{BF}_3\)
• C. \(\mathrm{BF}_3>\mathrm{BCl}_3>\mathrm{BBr}_3>\mathrm{BI}_3\)
• D. \(\mathrm{BCl}_3>\mathrm{BF}_3>\mathrm{BI}_3>\mathrm{BBr}_3\)

Hint:

Lewis acidity of boron trihalides follows reverse order of halide electronegativity due to back-bonding.

Answer 1

The Correct Option is B

Step 1: Underlying Concept:
Lewis acidity depends on both inductive effect and back-donation of lone pairs.
Step 2: Explanation:
BF\(_3\) is the weakest Lewis acid because F donates its lone pairs strongly to B (p\(\pi\)-p\(\pi\) back-bonding), reducing electron deficiency. As we go down the group, back-bonding decreases, so Lewis acidity increases. Order: BI\(_3\)>BBr\(_3\)>BCl\(_3\)>BF\(_3\).
Step 3: Conclusion:
\(\mathrm{BI}_3>\mathrm{BBr}_3>\mathrm{BCl}_3>\mathrm{BF}_3\)