The compound which forms one monochloro product, when treated with chlorine is

Options

A. n-pentane
B. neo-pentane
C. 2,2 dimethylpropane
D. Both (2) and (3)

Answer

To solve the problem of identifying the compound that forms only one monochloro product when treated with chlorine, we will analyze the given options step by step.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- The reaction of hydrocarbons with chlorine (Cl2) in the presence of sunlight leads to a free radical substitution reaction. This means that chlorine can replace hydrogen atoms in the hydrocarbon.

2. **Analyzing n-Pentane**:
- n-Pentane has the structure: CH3-CH2-CH2-CH2-CH3.
- When treated with Cl2, chlorine can replace hydrogen atoms at different positions:
- If Cl replaces a hydrogen at the 1st carbon: CH2Cl-CH2-CH2-CH2-CH3 (1-chloropentane)
- If Cl replaces a hydrogen at the 2nd carbon: CH3-CHCl-CH2-CH2-CH3 (2-chloropentane)
- If Cl replaces a hydrogen at the 3rd carbon: CH3-CH2-CHCl-CH2-CH3 (3-chloropentane)
- If Cl replaces a hydrogen at the 4th carbon: CH3-CH2-CH2-CHCl-CH3 (4-chloropentane)
- This results in **4 different monochloro products**. Therefore, n-pentane does not meet the criteria.

3. **Analyzing Neopentane**:
- Neopentane has the structure: (CH3)4C (or C(CH3)4).
- All four methyl groups are equivalent, meaning that chlorine can only replace one of the hydrogen atoms attached to the central carbon.
- The product formed will be the same regardless of which methyl group is substituted: CH3-C(CH3)3Cl.
- This results in **only 1 monochloro product**. Therefore, neopentane meets the criteria.

4. **Analyzing 2,2-Dimethylpropane**:
- 2,2-Dimethylpropane has the structure: CH3-C(CH3)2-CH3.
- Similar to neopentane, the two methyl groups on the second carbon are equivalent, and chlorine can only replace one of the hydrogen atoms attached to the central carbon.
- The product formed will be the same regardless of which methyl group is substituted: CH3-C(CH3)2Cl.
- This results in **only 1 monochloro product**. Therefore, 2,2-dimethylpropane also meets the criteria.

5. **Conclusion**:
- Both neopentane and 2,2-dimethylpropane form only one monochloro product when treated with chlorine. Thus, the correct answer is that both options 2 (neopentane) and 3 (2,2-dimethylpropane) are correct.

### Final Answer:
The compound which forms one monochloro product when treated with chlorine is **both 2 and 3 (neopentane and 2,2-dimethylpropane)**.