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

Step 1: Understanding the Concept:
Lewis acidity of boron halides is determined by the extent of back-donation of electrons from the halogen to the empty p-orbital of boron.

Step 2: Detailed Explanation:
In BF\(_3\), fluorine atoms can donate their lone pairs into the empty p-orbital of boron through p\(\pi\)-p\(\pi\) back-bonding. This reduces the electron deficiency of boron, making it a weaker Lewis acid. The extent of back-bonding decreases as the size of the halogen increases and the energy mismatch increases. Therefore, the order of Lewis acidity is: \[ \text{BI}_3 > \text{BBr}_3 > \text{BCl}_3 > \text{BF}_3 \] BI\(_3\) has the least back-bonding, making it the strongest Lewis acid.

Step 3: Final Answer:
BI\(_3\) is the strongest Lewis acid among boron halides, which corresponds to option (B).