Which of the following alkyl halide is used as ethylating agent ?

Options

A. `CH_3I`
B. `C_2H_5`
C. `C_2H_4Br_2`
D. `C_2H_5OH`

Explanation

To determine which alkyl halide is used as an ethylating agent, we need to analyze the options provided and understand the concept of ethylation.

### Step-by-Step Solution:

1. **Understanding Ethylating Agent**:
- An ethylating agent is a compound that introduces an ethyl group (C2H5) into another molecule during a nucleophilic substitution reaction. The ethyl group must be transferred to a nucleophile.

2. **Analyzing the Options**:
- We have four options to consider for their ability to act as ethylating agents:
- A) CH3I (Methyl iodide)
- B) C2H5Cl (Ethyl chloride)
- C) C2H4Br2 (Dibromoethane)
- D) C2H5OH (Ethanol)

3. **Option A: CH3I**:
- CH3I can react with a nucleophile, but it introduces a methyl group (CH3) instead of an ethyl group (C2H5). Therefore, it is a methylating agent, not an ethylating agent.
- **Conclusion**: Not an ethylating agent.

4. **Option B: C2H5Cl**:
- C2H5Cl (ethyl chloride) contains the ethyl group (C2H5). When a nucleophile attacks, it can replace the chlorine atom (Cl-) with the ethyl group, effectively introducing the ethyl group to the nucleophile.
- **Conclusion**: This is an ethylating agent.

5. **Option C: C2H4Br2**:
- C2H4Br2 (dibromoethane) has two bromine atoms. When a nucleophile attacks, it can only introduce a CH2 group or a CH group, not a complete ethyl group (C2H5). Thus, it does not serve as an ethylating agent.
- **Conclusion**: Not an ethylating agent.

6. **Option D: C2H5OH**:
- C2H5OH (ethanol) has a hydroxyl group (OH) that can also act as a nucleophile. However, it cannot act as an ethylating agent because it does not transfer an ethyl group; instead, it retains its own ethyl group.
- **Conclusion**: Not an ethylating agent.

### Final Answer:
The alkyl halide that is used as an ethylating agent is **B) C2H5Cl** (ethyl chloride).