How many of the following compounds undergo aldol condensation? Methanal, 2-methylpentanal, benzaldehyde, benzophenone, cyclohexanone.

Explanation

To determine how many of the given compounds undergo aldol condensation, we need to analyze each compound for the presence of alpha hydrogen atoms. Aldol condensation requires that the compound has at least one alpha hydrogen adjacent to the carbonyl group.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds we need to analyze are:
- Methanal (formaldehyde)
- 2-methylpentanal
- Benzaldehyde
- Benzophenone
- Cyclohexanone

2. **Analyze Methanal**:
- Structure: HCHO
- Methanal has no carbon atoms adjacent to the carbonyl carbon, hence it has **no alpha hydrogen**.
- Conclusion: Methanal does **not** undergo aldol condensation.

3. **Analyze 2-methylpentanal**:
- Structure: CH3-CH2-CH(CH3)-CHO
- The carbon adjacent to the carbonyl (CHO) has hydrogen atoms (specifically, the CH2 group has two alpha hydrogens).
- Conclusion: 2-methylpentanal **does** undergo aldol condensation.

4. **Analyze Benzaldehyde**:
- Structure: C6H5CHO
- The carbon adjacent to the carbonyl carbon is part of the benzene ring and has no hydrogen atoms (it is sp2 hybridized).
- Conclusion: Benzaldehyde does **not** undergo aldol condensation.

5. **Analyze Benzophenone**:
- Structure: C6H5C(O)C6H5
- The carbon adjacent to the carbonyl carbon is also part of a benzene ring and has no alpha hydrogen.
- Conclusion: Benzophenone does **not** undergo aldol condensation.

6. **Analyze Cyclohexanone**:
- Structure: C6H10O
- The carbon adjacent to the carbonyl carbon has hydrogen atoms (the cyclohexane ring provides alpha hydrogens).
- Conclusion: Cyclohexanone **does** undergo aldol condensation.

7. **Final Count**:
- Compounds that undergo aldol condensation: 2-methylpentanal and cyclohexanone.
- Total: **2 compounds undergo aldol condensation**.

### Final Answer:
**2**