Which one of the following has highest Lewis acid strength ?

Options

A. `BI_(3)`
B. `BBr_(3)`
C. `B_(3)`
D. `BCl_(3)`

Explanation

To determine which compound has the highest Lewis acid strength among boron halides, we can analyze the Lewis acid strength of the following compounds: BF3, BCl3, and BI3.

### Step-by-Step Solution:

1. **Understanding Lewis Acids**:
- Lewis acids are substances that can accept an electron pair. The strength of a Lewis acid is influenced by its ability to accept electrons, which is affected by factors such as back bonding and the size of the halogen atom.

2. **Analyzing Boron Halides**:
- The boron halides we will consider are BF3, BCl3, and BI3.
- BF3 has strong back bonding due to the small size of fluorine, which allows for effective overlap between the p-orbitals of boron and fluorine. This back bonding reduces the Lewis acidity of BF3.
- BCl3 has moderate back bonding because chlorine is larger than fluorine, leading to less effective overlap. Therefore, BCl3 is a stronger Lewis acid than BF3 but still has some back bonding.

3. **Examining BI3**:
- In BI3, the size of iodine is much larger than that of fluorine and chlorine. This leads to minimal back bonding between boron and iodine.
- The vacant p-orbital on boron in BI3 does not participate in back bonding effectively because of the large size of iodine, making BI3 a very strong Lewis acid.

4. **Comparing Lewis Acid Strengths**:
- Based on the analysis:
- BF3 < BCl3 < BI3
- Therefore, BI3 has the highest Lewis acid strength among the three compounds.

5. **Conclusion**:
- The compound with the highest Lewis acid strength is **BI3**.