Question

The number of chain isomers possible for the hydrocarbon with molecular formula \( \text{C}_5\text{H}_{12} \) is

• A. 4
• B. 3
• C. 2
• D. 1

Hint:

For alkanes, the number of structural isomers increases as the number of carbon atoms increases. It is helpful to remember the counts for smaller alkanes:
- Butane (\( \text{C}_4 \)): 2 isomers
- Pentane (\( \text{C}_5 \)): 3 isomers
- Hexane (\( \text{C}_6 \)): 5 isomers
- Heptane (\( \text{C}_7 \)): 9 isomers

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the number of chain isomers for the alkane with the molecular formula \( \text{C}_5\text{H}_{12} \), which is pentane. Chain isomers are structural isomers that differ in the arrangement of their carbon skeleton.
Step 2: Detailed Explanation:
To find all chain isomers, we systematically vary the carbon chain length and add branches:
1. n-Pentane: The five carbon atoms are arranged in a single continuous straight chain.
Structure: \( \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3 \)
2. Isopentane (2-Methylbutane): The parent chain consists of four carbon atoms, with one methyl (\( -\text{CH}_3 \)) group branched off the second carbon.
Structure: \( \text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_3 \)
3. Neopentane (2,2-Dimethylpropane): The parent chain consists of three carbon atoms, with two methyl groups branched off the central carbon atom.
Structure: \( \text{CH}_3-\text{C}(\text{CH}_3)_2-\text{CH}_3 \)
Further branching is not possible for five carbon atoms without repeating one of these structures or violating valency rules.
Step 3: Final Answer:
Thus, there are exactly three chain isomers for the molecular formula \( \text{C}_5\text{H}_{12} \), which corresponds to option (B).