Which of the following are Lewis acids?

Options

A. ` PH_3 and BCl_3`
B. ` AlCl_3 and SiCl_4`
C. ` PH_3 and SiCl_4`
D. ` BCl_3and AlCl_3`

Answer

To determine which of the given compounds are Lewis acids, we first need to understand the definition of a Lewis acid. A Lewis acid is a substance that can accept an electron pair because it has an incomplete octet or is electron-deficient.

Let's analyze each of the given compounds step by step:

1. **Ph3 (Triphenylphosphine)**:
- Phosphorus (P) has 5 valence electrons.
- In Ph3, phosphorus forms three single bonds with hydrogen atoms and has one lone pair.
- Total valence electrons around phosphorus: 3 (from H) + 2 (from lone pair) = 5.
- Since phosphorus has a complete octet (8 electrons), it is not electron-deficient.
- **Conclusion**: Ph3 is **not** a Lewis acid.

2. **BCl3 (Boron trichloride)**:
- Boron (B) has 3 valence electrons.
- In BCl3, boron forms three single bonds with chlorine atoms.
- Total valence electrons around boron: 3 (from bond pairs with Cl) = 3.
- Boron has only 6 electrons around it, which means it has an incomplete octet.
- **Conclusion**: BCl3 **is** a Lewis acid.

3. **AlCl3 (Aluminium chloride)**:
- Aluminium (Al) has 3 valence electrons.
- In AlCl3, aluminium forms three single bonds with chlorine atoms.
- Total valence electrons around aluminium: 3 (from bond pairs with Cl) = 3.
- Aluminium also has only 6 electrons around it, indicating an incomplete octet.
- **Conclusion**: AlCl3 **is** a Lewis acid.

4. **SiCl4 (Silicon tetrachloride)**:
- Silicon (Si) has 4 valence electrons.
- In SiCl4, silicon forms four single bonds with chlorine atoms.
- Total valence electrons around silicon: 4 (from bond pairs with Cl) = 4.
- Silicon has a complete octet (8 electrons).
- **Conclusion**: SiCl4 is **not** a Lewis acid.

Now, summarizing our findings:
- Ph3: Not a Lewis acid
- BCl3: Lewis acid
- AlCl3: Lewis acid
- SiCl4: Not a Lewis acid

Thus, the compounds that act as Lewis acids are **BCl3 and AlCl3**.

### Final Answer:
The Lewis acids among the given options are **BCl3 and AlCl3**.