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 Concept:
Isomers are distinct chemical compounds that share the exact same molecular formula but have different structural arrangements. Chain isomers specifically differ in the branching arrangement of their carbon skeleton.
Step 2: Key Formula or Approach:
Systematically draw and name the alkane skeletons for 5 carbons, starting from a straight chain and systematically shortening it to add branches.
Step 3: Detailed Explanation:
For the alkane pentane ($C_{5}H_{12}$), the following unique carbon skeletons can be constructed:
1. n-Pentane: A completely unbranched straight chain of 5 carbons.
($CH_{3}-CH_{2}-CH_{2}-CH_{2}-CH_{3}$)

2. Isopentane (IUPAC: 2-methylbutane): A 4-carbon main chain with a single methyl branch attached to the second carbon.
($CH_{3}-CH(CH_{3})-CH_{2}-CH_{3}$)

3. Neopentane (IUPAC: 2,2-dimethylpropane): A 3-carbon main chain with two methyl branches attached to the central carbon.
($CH_{3}-C(CH_{3})_{2}-CH_{3}$)
Any other attempt to branch (like 3-methylbutane) is just a reverse numbering of an already listed structure.
Step 4: Final Answer:
There are 3 unique chain isomers for pentane.