The hydrocarbon of molecular mass 72 gives a single monochloride and two dichlorides on photochlorination is

Options

A. pentane
B. 2 - methylbutane
C. 2, 2- dimethylpropane
D. none of the above

Answer

To solve the problem, we need to identify the hydrocarbon with a molecular mass of 72 that yields a single monochloride and two dichlorides upon photochlorination. Let's break down the solution step by step:

### Step 1: Determine the Molecular Formula
The molecular mass given is 72 g/mol. We can start by determining the possible molecular formula for hydrocarbons (alkanes) that fit this mass.

- The general formula for alkanes is \( C_nH_{2n+2} \).
- We can set up the equation for molecular mass:
\[ 12n + (2n + 2) = 72 \]
Simplifying this gives:
\[ 12n + 2n + 2 = 72 \implies 14n + 2 = 72 \implies 14n = 70 \implies n = 5 \]
- Therefore, the molecular formula is \( C_5H_{12} \).

### Step 2: Identify Isomers of Pentane
The next step is to identify the structural isomers of pentane (\( C_5H_{12} \)):

1. **n-Pentane**: \( CH_3-CH_2-CH_2-CH_2-CH_3 \)
2. **Isopentane (2-methylbutane)**: \( CH_3-CH(CH_3)-CH_2-CH_3 \)
3. **Neopentane (2,2-dimethylpropane)**: \( (CH_3)_4C \)

### Step 3: Analyze Photochlorination Products
Now, we analyze the photochlorination products for each isomer:

1. **n-Pentane**:
- Monochlorination: Can produce 3 different monochlorides due to equivalent positions.
- Dichlorination: Can produce multiple dichlorides.

2. **Isopentane (2-methylbutane)**:
- Monochlorination: Can produce 4 different monochlorides.
- Dichlorination: Can produce multiple dichlorides.

3. **Neopentane (2,2-dimethylpropane)**:
- Monochlorination: All positions are equivalent, leading to only 1 monochloride.
- Dichlorination: Can produce 2 distinct dichlorides (both from the same carbon or from different carbons).

### Step 4: Conclusion
Based on the analysis:
- **Neopentane (2,2-dimethylpropane)** gives 1 monochloride and 2 dichlorides, satisfying the conditions of the problem.

Thus, the hydrocarbon of molecular mass 72 that gives a single monochloride and two dichlorides upon photochlorination is **2,2-dimethylpropane**.

### Final Answer
The hydrocarbon is **2,2-dimethylpropane**.

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