Which of the following groups can be replaced by a halogen atom directly?

Options

A. Hydroxyl group (–OH)
B. Amide (–`"CONH"_(2)`)
C. Nitro (`NO_(2)`)
D. Amine (- `NH_(2)` )

Answer

To determine which of the following groups can be replaced by a halogen atom directly, we need to analyze the leaving ability of each group in a nucleophilic substitution reaction.

### Step-by-Step Solution:

1. **Understanding Nucleophilic Substitution**:
- In a nucleophilic substitution reaction, a nucleophile attacks a substrate (usually an organic compound) and replaces a leaving group. The efficiency of this reaction depends on how good the leaving group is.

2. **Identifying Leaving Groups**:
- Good leaving groups are typically stable once they leave the substrate. Common good leaving groups include halides (like Cl, Br, I), water (from -OH), and tosylate.

3. **Analyzing the Hydroxyl Group (-OH)**:
- The hydroxyl group (-OH) can be converted into a better leaving group (like -Cl) using reagents such as ZnCl2. This means that -OH can be replaced by a halogen atom, making it a good candidate.

4. **Analyzing the Amine Group (-NH2)**:
- The amine group (-NH2) is a poor leaving group. When considering nucleophilic substitution, the amine does not readily leave because it is a strong base and tends to re-attack rather than leave.

5. **Analyzing the Nitro Group (-NO2)**:
- The nitro group (-NO2) is also a poor leaving group. While it can participate in nucleophilic reactions, it does not leave easily as it tends to act as a nucleophile itself, making it unsuitable for direct replacement by a halogen.

6. **Conclusion**:
- Among the groups analyzed, only the hydroxyl group (-OH) can be directly replaced by a halogen atom in a nucleophilic substitution reaction.

### Final Answer:
The only group that can be replaced by a halogen atom directly is the **hydroxyl group (-OH)**.

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