Which branched chain isomer of the hydrocarbon with molecular mass `72u` gives only one isomer of mono substituted alkyl halide ?

Options

A. Tertiary butyl chloride
B. Neopentane
C. Isohexane
D. Neohexane

Answer

To solve the problem, we need to identify the branched-chain isomer of a hydrocarbon with a molecular mass of 72 u that gives only one isomer of mono-substituted alkyl halide. Let's break it down step by step.

### Step 1: Identify Hydrocarbons with Molecular Mass of 72 u
The molecular formula of hydrocarbons can be represented as C_nH_m. The molecular mass can be calculated using the formula:
\[ \text{Molecular Mass} = 12n + m \]
where \( n \) is the number of carbon atoms and \( m \) is the number of hydrogen atoms.

To achieve a molecular mass of 72 u, we can start by testing different combinations of \( n \) and \( m \).

### Step 2: Calculate Possible Combinations
1. **If \( n = 5 \)**:
\[ 12(5) + m = 72 \implies 60 + m = 72 \implies m = 12 \]
This corresponds to **pentane (C5H12)**.

2. **If \( n = 6 \)**:
\[ 12(6) + m = 72 \implies 72 + m = 72 \implies m = 0 \]
This corresponds to **hexane (C6H0)**, which is not valid.

3. **If \( n = 4 \)**:
\[ 12(4) + m = 72 \implies 48 + m = 72 \implies m = 24 \]
This is not possible since hydrocarbons cannot have more than 2 hydrogen atoms per carbon in a saturated compound.

### Step 3: Identify Branched Isomers
Among the valid hydrocarbons, we can check for branched isomers of pentane (C5H12):
- **n-pentane** (straight chain)
- **isopentane** (branched)

The molecular mass of both isomers is 72 u, but we need to check which one gives only one isomer of mono-substituted alkyl halide.

### Step 4: Analyze Substitution Reactions
1. **n-Pentane**:
- Substitution can occur at any of the 5 hydrogen atoms, leading to multiple isomers (e.g., 1-chloropentane, 2-chloropentane, etc.).

2. **Isopentane** (or 2-methylbutane):
- All hydrogen atoms on the methyl groups are equivalent, leading to only one possible product when chlorinated (e.g., 2-chloro-2-methylbutane).

### Conclusion
The branched-chain isomer of the hydrocarbon with a molecular mass of 72 u that gives only one isomer of mono-substituted alkyl halide is **isopentane (2-methylbutane)**.

### Final Answer
The answer is **neopentane (C5H12)**.