Which one shows the best formation of adduct ?

Options

A. `O(CH_(3))_(2)+BF_(3)`
B. `O(SiH_(3))_(2)+BF_(3)`
C. `O(SiH_(3))_(2)+BBr_(3)`
D. `O(CH_(3))_(2)+BBr_(3)`

Answer

To determine which compound shows the best formation of an adduct, we need to analyze the electron donor and acceptor properties of the given options. An adduct is formed when an electron pair donor (Lewis base) interacts with an electron pair acceptor (Lewis acid).

### Step-by-Step Solution:

1. **Understanding Adduct Formation**:
- An adduct is formed when an electron pair donor (A) donates a pair of electrons to an electron pair acceptor (B).
- For effective adduct formation, A should be a strong electron donor, and B should be a strong electron acceptor.

2. **Identifying Electron Donors and Acceptors**:
- In the given options, we have compounds containing OCH3 (methoxy) and SiH3 (silyl) groups, as well as BF3 (boron trifluoride) and BBr3 (boron tribromide).
- The oxygen atom in OCH3 has lone pairs and can act as a good electron donor.
- Silicon in SiH3 has empty orbitals that can accommodate lone pairs, which may reduce its ability to act as a strong electron donor.

3. **Evaluating OCH3 and SiH3**:
- OCH3 (methoxy) can donate electrons effectively due to the electron-donating +I effect of the methyl groups, which increases electron density on oxygen.
- SiH3 (silyl) has a vacant 3d orbital, which can accept electron pairs, thus reducing its ability to donate electrons effectively.

4. **Comparing BF3 and BBr3**:
- BF3 is a well-known Lewis acid and can accept electron pairs, but it has a smaller atomic size and higher electronegativity compared to BBr3.
- BBr3, with larger bromine atoms, has a more spread-out electron density, making it a better electron acceptor than BF3.

5. **Final Comparison**:
- Since OCH3 is a strong electron donor and BBr3 is a strong electron acceptor, the combination of OCH3 and BBr3 will yield the best adduct formation.
- Therefore, the correct answer is the option that includes OCH3 and BBr3.

### Conclusion:
The best formation of an adduct is shown by the combination of OCH3 and BBr3.