Only two isomeric monochloro derivatives are possible for

Options

A. `n` Butane
B. `2,4`-Dimethyl pentane
C. Benzene
D. `2`-Methyl propane

Explanation

To determine which of the given compounds has only two isomeric monochloro derivatives, we will analyze each compound step by step.

### Step 1: Analyze n-butane
1. **Structure of n-butane**:
- The molecular formula is C4H10.
- The structure can be represented as:
\[ CH_3-CH_2-CH_2-CH_3 \]

2. **Symmetry**:
- n-butane is symmetrical, meaning that substituting chlorine on one side will yield the same compound as substituting it on the other side.

3. **Monochloro derivatives**:
- Substituting chlorine at the first CH2 (from the left):
\[ CH_2Cl-CH_2-CH_2-CH_3 \quad (1) \]
- Substituting chlorine at the second CH2:
\[ CH_3-CHCl-CH_2-CH_3 \quad (2) \]
- Since the compound is symmetrical, these are the only two distinct isomers.

### Conclusion for n-butane:
- **Total isomeric monochloro derivatives**: 2

### Step 2: Analyze 2,4-dimethylpentane
1. **Structure of 2,4-dimethylpentane**:
- The molecular formula is C7H16.
- The structure can be represented as:
\[ CH_3-CH(CH_3)-CH_2-CH(CH_3)-CH_3 \]

2. **Monochloro derivatives**:
- Substituting chlorine at the terminal CH3:
\[ CH_2Cl-CH(CH_3)-CH_2-CH(CH_3)-CH_3 \quad (1) \]
- Substituting chlorine at the second CH:
\[ CH_3-CHCl-CH_2-CH(CH_3)-CH_3 \quad (2) \]
- Substituting chlorine at the fourth CH:
\[ CH_3-CH(CH_3)-CH_2-CHCl-CH_3 \quad (3) \]
- Since there are different positions for substitution, we can see that we have 3 distinct isomers.

### Conclusion for 2,4-dimethylpentane:
- **Total isomeric monochloro derivatives**: 3

### Step 3: Analyze 2-methylpropane
1. **Structure of 2-methylpropane**:
- The molecular formula is C5H12.
- The structure can be represented as:
\[ CH_3-CH(CH_3)-CH_3 \]

2. **Monochloro derivatives**:
- Substituting chlorine at the terminal CH3:
\[ CH_3-CHCl-CH_3 \quad (1) \]
- Substituting chlorine at the central CH:
\[ CH_3-CH(CH_3)-CH_2Cl \quad (2) \]
- Since the compound is symmetrical, these are the only two distinct isomers.

### Conclusion for 2-methylpropane:
- **Total isomeric monochloro derivatives**: 2

### Final Summary:
- **n-butane**: 2 isomers
- **2,4-dimethylpentane**: 3 isomers
- **2-methylpropane**: 2 isomers

### Answer:
The compounds that have only two isomeric monochloro derivatives are **n-butane** and **2-methylpropane**.