The compound which forms acetaldehyde when heated with dilute NaOH, is

Options

A. 1, 1-dichloroethane
B. 1, 1, 1-trichloroethane
C. 1-chloroethane
D. 1,2-dichloroethane

Explanation

To determine which compound forms acetaldehyde when heated with dilute NaOH, we need to analyze the options provided and understand the reaction mechanism involved.

### Step-by-Step Solution:

1. **Identify the Target Compound**:
We want to form acetaldehyde (CH3CHO), which is a 2-carbon aldehyde.

2. **Understand the Reaction Mechanism**:
The reaction involves nucleophilic substitution where NaOH acts as a nucleophile. The halogen (Cl) in haloalkanes is a good leaving group, allowing the hydroxide ion (OH-) to replace it.

3. **Analyze Each Option**:
- **Option A: 1,1-Dichloroethane (CH3CCl2H)**:
- When heated with NaOH, the hydroxide ion will attack the carbon attached to the Cl, leading to the formation of an alcohol intermediate.
- The reaction will proceed to eliminate water, resulting in the formation of acetaldehyde.

- **Option B: Trichloroethane (CCl3CH3)**:
- Here, all three Cl atoms are replaced by OH groups. The resulting compound will not lead to the formation of acetaldehyde but rather a triol, which is unstable and will not yield the desired aldehyde.

- **Option C: 1-Chloroethane (C2H5Cl)**:
- This compound will also undergo substitution but will not lead to the formation of acetaldehyde directly. It will form ethanol instead.

- **Option D: 1,2-Dichloroethane (C2H4Cl2)**:
- In this case, both Cl atoms can be replaced by OH groups, leading to the formation of a diol, which again does not yield acetaldehyde.

4. **Conclusion**:
After analyzing all options, **Option A (1,1-Dichloroethane)** is the only compound that can yield acetaldehyde upon reaction with dilute NaOH.

### Final Answer:
The compound which forms acetaldehyde when heated with dilute NaOH is **Option A: 1,1-Dichloroethane**.