The decreasing order of power of boron halides to act as Lewis acids is

Options

A. `BF_(3) gt BCI_(3) gt BBr_(3)`
B. `BBr_(3) gt BCI_(3) gt BF_(3)`
C. `BCI_(3) gt BF_(3) gt BBr_(3)`
D. `BCI_(3) gt BBr_(3) gt BF_(3)`

Answer

To determine the decreasing order of the power of boron halides to act as Lewis acids, we need to analyze the properties of boron trihalides (BX3) where X represents the halogens: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

### Step-by-Step Solution:

1. **Understanding Lewis Acids**:
- A Lewis acid is a species that can accept an electron pair. Boron, with its electronic configuration of 1s² 2s² 2p¹, has an incomplete octet and can accept electron pairs, making it a Lewis acid.

2. **Formation of Boron Trihalides**:
- Boron can form trihalides (BX3) with halogens. The stability and Lewis acidity of these compounds depend on the ability of the halogen to donate electron pairs to boron.

3. **Trends in Halogen Size**:
- As we move down the group in the periodic table from fluorine to iodine, the size of the halogen atoms increases. This affects their ability to donate electron pairs.

4. **Effect of Halogen Size on Lewis Acidity**:
- Smaller halogens (like fluorine) can more easily donate their electron pairs to boron compared to larger halogens (like iodine).
- Therefore, the order of stability (and hence Lewis acidity) of boron trihalides is influenced by the size of the halogen.

5. **Order of Lewis Acidity**:
- BF3 (Boron Trifluoride) is the most powerful Lewis acid because fluorine is small and can effectively donate its electron pair.
- BCl3 (Boron Trichloride) is less effective than BF3 but more effective than BBr3.
- BBr3 (Boron Tribromide) is less effective than BCl3.
- BI3 (Boron Triiodide) is the least effective Lewis acid because iodine is the largest and least able to donate its electron pair.

6. **Final Order**:
- The decreasing order of power of boron halides to act as Lewis acids is: **BF3 > BCl3 > BBr3 > BI3**.

### Conclusion:
The correct decreasing order of the power of boron halides to act as Lewis acids is **BF3 > BCl3 > BBr3 > BI3**.