Question

The correct Lewis acid order for boron halides is

• A. $\text{BBr}_{\text{3}} \, \text{>} \, \text{BCl}_{\text{3}} \, \text{>} \, \text{Bl}_{\text{3}} \, \text{>} \, \text{BF}_{\text{3}}$
• B. $\text{BI}_{\text{3}} \, \text{>} \, \text{BF}_{\text{3}} \, \text{>} \, \text{BBr}_{\text{3}} \, \text{>} \, \text{BCl}_{\text{3}}$
• C. $\text{BF}_{\text{3}} \, \text{>} \, \text{BCl}_{\text{3}} \, \text{>} \, \text{BBr}_{\text{3}} \, \text{>} \, \text{Bl}_{\text{3}}$
• D. $\text{BI}_{\text{3}} \, \text{>} \, \text{BBr}_{\text{3}} \, \text{>} \, \text{BCl}_{\text{3}} \, \text{>} \, \text{BF}_{\text{3}}$

Answer 1

Answer: (D)

(i) On the basis of back bonding, smaller be the halide atom, more effective be the back bonding and show less tendency to accept pair of electrons

(ii) Size of halides increases down the group hence ability to accept pair of electrons increases down the group.

Hence, correct order of Lewis acid for boron-halides is

$\text{Bl}_{\text{3}} \, \text{>} \, \text{BBr}_{\text{3}} \, \text{>} \, \text{BCl}_{\text{3}} \, \text{>} \, \text{BF}_{\text{3}}$