Aldehydes that do not undergo aldol condensation are
(1) propanal (2) trichloroethanal (3) 2- phenylethanal (4) ethanal (5) benzaldehyde

Options

A. 3 and 4 only
B. 2 and 5 only
C. 1, 2 and 3 only
D. 2, 3 and 5 only

Explanation

To determine which aldehydes do not undergo aldol condensation, we need to identify those that lack alpha hydrogen atoms. Aldol condensation requires at least one alpha hydrogen on the carbon adjacent to the carbonyl group (the functional group).

### Step-by-Step Solution:

1. **Understanding Aldol Condensation**:
- Aldol condensation occurs when aldehydes or ketones with alpha hydrogen atoms react to form β-hydroxy aldehydes or ketones, which can further dehydrate to form α,β-unsaturated carbonyl compounds.

2. **Identifying Alpha Hydrogen**:
- Alpha hydrogen is the hydrogen atom attached to the carbon atom adjacent to the carbonyl group (C=O). If there are no alpha hydrogens, aldol condensation cannot occur.

3. **Analyzing Each Aldehyde**:
- **(1) Propanal (CH3CH2CHO)**:
- The alpha carbon is CH2, which has 2 alpha hydrogens.
- **Conclusion**: Can undergo aldol condensation.

- **(2) Trichloroethanal (CCl3CHO)**:
- The alpha carbon is CHCl3, which has no hydrogen atoms (all replaced by Cl).
- **Conclusion**: Cannot undergo aldol condensation.

- **(3) 2-Phenylethanal (C6H5CH2CHO)**:
- The alpha carbon is CH2, which has 2 alpha hydrogens.
- **Conclusion**: Can undergo aldol condensation.

- **(4) Ethanal (CH3CHO)**:
- The alpha carbon is CH3, which has 3 alpha hydrogens.
- **Conclusion**: Can undergo aldol condensation.

- **(5) Benzaldehyde (C6H5CHO)**:
- The alpha carbon is directly attached to the benzene ring, which has no hydrogen atoms (the carbon is fully bonded to other carbons).
- **Conclusion**: Cannot undergo aldol condensation.

4. **Final Conclusion**:
- The aldehydes that do not undergo aldol condensation are **trichloroethanal (2)** and **benzaldehyde (5)**.

### Answer:
The aldehydes that do not undergo aldol condensation are **(2) trichloroethanal and (5) benzaldehyde**.

---