An alkane of mol. Weight 72 gives on monochlorination only one product. Name the alkane:

Options

A. 2 - methylbutane
B. n - pentane
C. 2, 2 - dimethylpropane
D. none of these

Answer

To solve the problem of identifying the alkane with a molecular weight of 72 that gives only one product upon monochlorination, we can follow these steps:

### Step 1: Determine the molecular formula of the alkane
The molecular weight of the alkane is given as 72 g/mol. The general formula for alkanes is \( C_nH_{2n+2} \).

### Step 2: Calculate the number of carbons (n)
To find \( n \), we can set up the equation based on the molecular weight:
- The molecular weight can be expressed as:
\[ 12n + (2n + 2) = 72 \]
where 12 is the atomic weight of carbon and 1 is the atomic weight of hydrogen.

- Simplifying the equation:
\[ 12n + 2n + 2 = 72 \]
\[ 14n + 2 = 72 \]
\[ 14n = 70 \]
\[ n = 5 \]

### Step 3: Write the molecular formula
With \( n = 5 \), the molecular formula of the alkane is \( C_5H_{12} \).

### Step 4: Identify the possible alkanes
The possible alkanes with the formula \( C_5H_{12} \) are:
- n-Pentane
- Isopentane (2-methylbutane)
- Neopentane (2,2-dimethylpropane)

### Step 5: Analyze monochlorination
Monochlorination refers to the substitution of one hydrogen atom with a chlorine atom.

- **n-Pentane**: This compound has multiple positions for chlorination, resulting in different products.
- **Isopentane (2-methylbutane)**: This compound also has multiple positions for chlorination, leading to different products.
- **Neopentane (2,2-dimethylpropane)**: This compound is symmetrical, and any chlorination will yield the same product regardless of where the chlorine is substituted.

### Step 6: Conclusion
Since only neopentane gives one unique product upon monochlorination, the alkane with a molecular weight of 72 that gives only one product upon monochlorination is **neopentane (2,2-dimethylpropane)**.

### Final Answer
The alkane is **neopentane (2,2-dimethylpropane)**.

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