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 is not a Lewis acid, we need to understand the definition of a Lewis acid. A Lewis acid is a species that can accept a lone pair of electrons. This typically requires the presence of a vacant orbital to accommodate the incoming electron pair.

Let's analyze each option provided in the question:

1. **SiF4 (Silicon Tetrafluoride)**:
- Silicon (Si) has the electronic configuration of Ne 3s² 3p². In SiF4, silicon forms four bonds with fluorine atoms. The hybridization of silicon in SiF4 is sp³, and it has a tetrahedral geometry.
- Although silicon has a full octet in this compound, it can still accept a lone pair due to the presence of empty 3d orbitals. Therefore, SiF4 can act as a Lewis acid.

2. **C2H4 (Ethylene)**:
- Ethylene has the structure H2C=CH2. Each carbon atom in ethylene is sp² hybridized, forming three sigma bonds and one pi bond.
- Importantly, both carbon atoms in ethylene do not have vacant orbitals to accept electron pairs. They are fully involved in bonding and do not have the capacity to act as Lewis acids. Therefore, C2H4 is not a Lewis acid.

3. **BF3 (Boron Trifluoride)**:
- Boron (B) has the electronic configuration of 1s² 2s² 2p¹. In BF3, boron forms three bonds with fluorine atoms and has an incomplete octet (only six electrons around it).
- Boron has an empty p-orbital and can accept a lone pair of electrons, making BF3 a Lewis acid.

4. **FeCl3 (Iron(III) Chloride)**:
- Iron (Fe) has the electronic configuration of [Ar] 4s² 3d⁶. In FeCl3, iron loses three electrons to form Fe³⁺, which has the configuration [Ar] 3d⁵.
- The 4s orbital is empty, and the 4p and 4d orbitals are also available for accepting electrons, allowing FeCl3 to act as a Lewis acid.

Based on this analysis, the answer to the question "Which of the following is not a Lewis acid?" is **C2H4 (Ethylene)**.

### Step-by-Step Solution:
1. **Understand the definition of a Lewis acid**: A Lewis acid is a species that can accept a lone pair of electrons.
2. **Analyze SiF4**: Silicon has empty orbitals and can accept electrons, thus it is a Lewis acid.
3. **Analyze C2H4**: Ethylene has no vacant orbitals to accept electrons, thus it is not a Lewis acid.
4. **Analyze BF3**: Boron has an empty p-orbital and can accept electrons, making it a Lewis acid.
5. **Analyze FeCl3**: Iron can accept electrons due to empty orbitals, thus it is a Lewis acid.
6. **Conclude**: The only species that does not act as a Lewis acid is C2H4.