Which of the following is not a Lewis acid ?

Options

A. `SiF_4`
B. `C_2H_4`
C. `BF_3`
D. `FeCI_3`

Answer

To determine which of the following compounds is not a Lewis acid, we need to analyze each option based on the definition of a Lewis acid. A Lewis acid is defined as an electron-deficient species that can accept an electron pair.

### Step-by-Step Solution:

1. **Understanding Lewis Acids**:
- A Lewis acid is a substance that can accept an electron pair. This typically means it is electron deficient or has an incomplete octet.

2. **Analyzing the Options**:
- **Option 1: SiF4 (Silicon Tetrafluoride)**:
- Silicon has four valence electrons and forms four bonds with fluorine.
- It has a tetrahedral structure and possesses empty d-orbitals.
- Since it can accept electron pairs, SiF4 can act as a Lewis acid.

- **Option 2: C2H4 (Ethene)**:
- Ethene consists of two carbon atoms, each forming four bonds (two with hydrogen and one with the other carbon).
- Each carbon has a complete octet (8 electrons) and there are no empty d-orbitals.
- Since it has a complete octet and does not have the ability to accept electron pairs, C2H4 is not a Lewis acid.

- **Option 3: BF3 (Boron Trifluoride)**:
- Boron has three valence electrons and forms three bonds with fluorine.
- Boron has an incomplete octet (only 6 electrons around it).
- Due to this incomplete octet, BF3 can accept electron pairs and acts as a Lewis acid.

- **Option 4: FeCl3 (Iron Trichloride)**:
- Iron has multiple oxidation states and in FeCl3, it forms three bonds with chlorine.
- Iron also has an incomplete octet in this case, making it electron deficient.
- Therefore, FeCl3 can accept electron pairs, acting as a Lewis acid.

3. **Conclusion**:
- Among the given options, C2H4 (Ethene) is the only species that does not act as a Lewis acid because it has a complete octet and lacks the ability to accept electron pairs.

### Final Answer:
The compound that is not a Lewis acid is **C2H4**.

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