The reaction conditions leading to the best yield of `C_2H_5Cl` are

Options

A. `C_2H_6("excess") + Cl_2 overset("UV light")to`
B. `C_2H_6+Cl_2 underset("temp.")overset("dark room")to`
C. `C_2H_6 + Cl_2 ("excess") underset("light")overset(UV)to`
D. `C_2H_6 + Cl_2 overset("UV light")to`

Explanation

To determine the reaction conditions leading to the best yield of C2H5Cl (monochloroethane), we will analyze the four reactions provided:

### Step 1: Analyze Reaction Conditions
1. **First Reaction**: Ethene (C2H4) reacts with Cl2 under halogenation conditions.
- If ethane (C2H6) is in excess, the major product formed will be C2H5Cl (ethyl chloride) along with HCl. This is a favorable reaction for producing monochloroethane.

2. **Second Reaction**: C2H6 reacts with Cl2 in the dark.
- No reaction occurs in the dark because there is insufficient energy to break Cl2 into free radicals. Therefore, no C2H5Cl is produced.

3. **Third Reaction**: C2H6 reacts with Cl2 in excess.
- This reaction can lead to multiple chlorination products (C2H4Cl, C2H3Cl2, C2H2Cl3, etc.), resulting in a mixture of products rather than a significant yield of C2H5Cl.

4. **Fourth Reaction**: C2H6 reacts with Cl2 in the presence of UV light.
- This reaction generates Cl radicals, leading to a free radical substitution mechanism. While C2H5Cl can be formed, the presence of UV light and excess Cl2 can lead to over-chlorination, producing multiple chlorinated products and reducing the yield of monochloroethane.

### Step 2: Conclusion
Based on the analysis, the best yield of C2H5Cl is obtained from the **first reaction**, where ethane is in excess and reacts under halogenation conditions. This reaction specifically favors the formation of monochloroethane as the major product.

### Final Answer
The reaction conditions leading to the best yield of C2H5Cl are those of the **first reaction** (halogenation of ethene with excess ethane).

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