Coupling reaction takes place when benzene diazonium chloride is treated with:

Options

A. Benzaldehyde
B. `alpha-` naphthol
C. N,N-dimethyl aniline
D. Phenol

Answer

To determine which compound benzene diazonium chloride can couple with, we need to analyze the options provided and their ability to activate the benzene ring for electrophilic substitution reactions.

### Step-by-Step Solution:

1. **Understanding Benzene Diazonium Chloride:**
- Benzene diazonium chloride has the structure C6H5-N2^+Cl^-. The diazonium ion (N2^+) acts as an electrophile in reactions.

2. **Identifying the Nature of the Compounds:**
- We need to identify whether the given compounds can donate electron density to the benzene ring, which would facilitate the electrophilic substitution reaction.

3. **Analyzing Each Option:**
- **Option A: Benzaldehyde (C6H5CHO)**
- The -CHO group is an electron-withdrawing group due to the electronegativity of oxygen. It decreases the electron density on the benzene ring, making it less reactive towards electrophiles. Therefore, benzaldehyde will not undergo a coupling reaction.

- **Option B: Alpha-Naphthol (C10H8O)**
- Alpha-naphthol has a hydroxyl (-OH) group that is an electron-donating group due to its +R (resonance) effect. This increases the electron density on the benzene ring, making it more reactive towards electrophiles. Thus, alpha-naphthol can couple with benzene diazonium chloride.

- **Option C: N,N-Dimethylaniline (C8H11N)**
- N,N-dimethylaniline has a nitrogen atom with a lone pair that can donate electrons to the benzene ring. This makes it an electron-donating group, which activates the ring for electrophilic substitution. Therefore, it can also couple with benzene diazonium chloride.

- **Option D: Phenol (C6H5OH)**
- Similar to alpha-naphthol, phenol has a hydroxyl group that donates electrons to the benzene ring, increasing its reactivity towards electrophiles. Hence, phenol can also couple with benzene diazonium chloride.

4. **Conclusion:**
- The compounds that can undergo coupling reactions with benzene diazonium chloride are alpha-naphthol, N,N-dimethylaniline, and phenol. Benzaldehyde cannot participate in the coupling reaction.

### Final Answer:
Benzene diazonium chloride can couple with **alpha-naphthol, N,N-dimethylaniline, and phenol**.