Question

How many structural isomers does \(\mathrm{C}_7\mathrm{H}_{16}\) have:

• A. 66
• B. 7
• C. 8
• D. 9

Hint:

For alkane isomers:
- Start with the straight chain, then add branches systematically.
- Check for unique connectivity to avoid duplicates.
- Use the formula \(\mathrm{C}_n\mathrm{H}_{2n+2}\) to confirm molecular formula.

Answer 1

The Correct Option is D

Structural isomers are compounds with the same molecular formula but different connectivity of atoms. For \(\mathrm{C}_7\mathrm{H}_{16}\), we need to find all possible isomers of heptane (an alkane, \(\mathrm{C}_n\mathrm{H}_{2n+2}\)).
Step 1: Understand alkane isomers
Heptane (\(\mathrm{C}_7\mathrm{H}_{16}\)) is a straight-chain alkane with 7 carbons, but branching creates isomers. The number of isomers increases with carbon count due to possible branching patterns.
Step 2: List isomers systematically
- n-Heptane: Straight chain (\(\mathrm{CH}_3(\mathrm{CH}_2)_5\mathrm{CH}_3\)).
- 2-Methylhexane: 6-carbon chain with a methyl group on carbon 2.
- 3-Methylhexane: 6-carbon chain with a methyl group on carbon 3.
- 2,2-Dimethylpentane: 5-carbon chain with two methyl groups on carbon 2.
- 2,3-Dimethylpentane: 5-carbon chain with methyl groups on carbons 2 and 3.
- 2,4-Dimethylpentane: 5-carbon chain with methyl groups on carbons 2 and 4.
- 3,3-Dimethylpentane: 5-carbon chain with two methyl groups on carbon 3.
- 3-Ethylpentane: 5-carbon chain with an ethyl group on carbon 3.
- 2,2,3-Trimethylbutane: 4-carbon chain with three methyl groups (two on carbon 2, one on carbon 3).
Total: 9 isomers.
Step 3: Verify
The number of structural isomers for \(\mathrm{C}_7\mathrm{H}_{16}\) is well-known to be 9, matching option (D).