Minimum no. of carbon atoms required for an alkane to exhibit positional isomerism is

Options

A. 4
B. 5
C. 6
D. 7

Explanation

To determine the minimum number of carbon atoms required for an alkane to exhibit positional isomerism, we can follow these steps:

### Step 1: Understand Positional Isomerism
Positional isomerism occurs when compounds have the same molecular formula but differ in the position of a functional group or substituent on the carbon chain. This means that the main carbon chain remains the same, but the location of the substituent changes.

**Hint:** Remember that positional isomerism involves changing the position of a functional group while keeping the carbon chain length constant.

### Step 2: Analyze the Carbon Chain Length
- For a carbon chain to exhibit positional isomerism, it must have enough carbon atoms to allow for the addition of a substituent or functional group.
- Start with a three-carbon chain (propane). There are no substituents to change, so it cannot exhibit positional isomerism.

**Hint:** Consider how many carbon atoms are needed to have at least one substituent that can change position.

### Step 3: Examine a Four-Carbon Chain
- A four-carbon chain (butane) can have a methyl group added, but any change in position would lead to chain isomerism rather than positional isomerism, as the main chain length changes.

**Hint:** Think about the structure of butane and whether you can change the position of a substituent without altering the main chain.

### Step 4: Consider a Five-Carbon Chain
- A five-carbon chain (pentane) can also have substituents, but similar to butane, the changes would lead to chain isomerism rather than positional isomerism.

**Hint:** Look for the possibility of adding a substituent while maintaining the same main chain length.

### Step 5: Identify the Minimum Carbon Count for Positional Isomerism
- When we reach a six-carbon chain (hexane), we can add a substituent (like a methyl group) and change its position without altering the main chain length. For example, 2-methylhexane and 3-methylhexane are positional isomers of each other.

**Hint:** Check if you can create different compounds by moving a substituent on a six-carbon chain while keeping the main chain intact.

### Conclusion
The minimum number of carbon atoms required for an alkane to exhibit positional isomerism is **6**.

**Final Answer:** 6