Question

Among boron halides, which is the strongest Lewis acid?

• A. BF\(_3\)
• B. BI\(_3\)
• C. BCl\(_3\)
• D. BBr\(_3\)

Hint:

Lewis acidity of boron halides is opposite to what you might expect based on electronegativity. BI\(_3\) is the strongest due to poor p\(\pi\)-p\(\pi\) back-bonding.

Answer 1

The Correct Option is B

The question asks which boron halide is the strongest Lewis acid. To answer this, we need to understand the concept of Lewis acids and how the nature of the halide affects the acidity.

1. Definition of Lewis Acids: Lewis acids are chemical species that can accept an electron pair. Boron halides like BF\(_3\), BCl\(_3\), BBr\(_3\), and BI\(_3\) can act as Lewis acids by accepting electron pairs from a Lewis base.
2. Influence of Halides on Lewis Acidity: The ability of these compounds to act as Lewis acids depends significantly on the size and electronegativity of the halide.
   • Electronegativity: A more electronegative halide will pull more electrons towards itself, decreasing the availability of the empty p-orbital of boron to accept electrons, thus reducing acidity. Here, fluorine is the most electronegative, followed by chlorine, bromine, and iodine. Therefore, theoretically, as the electronegativity decreases, the Lewis acidity increases.
   • Size: The larger size of the halide allows more significant back-donation of electron density to the boron, enhancing the stability of the molecule but reducing its Lewis acidity. As size increases from fluorine to iodine, the acidity increases due to less effective backbonding.
3. Lewis Acidity in Boron Halides: Considering both electronegativity and atomic size:
   • BF\(_3\): Has high electronegativity and smallest size; very stable due to strong backbonding, thus least acidic.
   • BCl\(_3\): Lesser electronegativity and larger than BF\(_3\); less backbonding, more acidic than BF\(_3\).
   • BBr\(_3\): Even lesser electronegativity and larger size than BCl\(_3\); more acidic than both BF\(_3\) and BCl\(_3\).
   • BI\(_3\): Has the largest halide; weakest backbonding; least stable configuration, thus the strongest Lewis acid among the given options due to its ability to accept electron pairs effectively.

Based on this analysis, BI\(_3\) is the strongest Lewis acid among the boron halides listed due to its large size and minimal backbonding effect, which allows for effective electron pair acceptance from a Lewis base.