Question

The correct order of Lewis acid strengths of BF\(_2\)Cl, BFClBr, BF\(_2\)Br and BFBr\(_2\) is:

• A. BF\(_2\)Cl > BFClBr > BF\(_2\)Br > BFBr\(_2\)
• B. BFBr\(_2\) > BFClBr > BF\(_2\)Br > BF\(_2\)Cl
• C. BF\(_2\)Cl > BF\(_2\)Br > BFClBr > BFBr\(_2\)
• D. BFClBr > BFBr\(_2\) > BF\(_2\)Cl > BF\(_2\)Br

Hint:

The stronger the electron-withdrawing effect of the substituents on the central atom, the stronger the Lewis acid.

Answer 1

The Correct Option is B

The strength of a Lewis acid depends on the electron-withdrawing ability of substituents on the boron atom. The order of electron-withdrawing ability of halogens is as follows: F > Cl > Br. Hence, the strongest Lewis acid will be the one with the most electronegative halogen attached to boron. In this case, BFBr\(_2\) has two Br atoms, which are weaker electron-withdrawing groups compared to Cl and F, making it the strongest. The order is: \[ \text{BFBr}_2 > \text{BFClBr} > \text{BF}_2\text{Br} > \text{BF}_2\text{Cl}. \] Final Answer: (B) BFBr\(_2\) > BFClBr > BF\(_2\)Br > BF\(_2\)Cl.