JEE Challenger
More from Organic Chemistry - Basic Principles
ChemEasyAdv 2007 • Paper 1

Number of Structural Isomers of Hexane

The number of structural isomers for C6H14\text{C}_6\text{H}_{14} is

Options

A

3

B

4

C

5

Correct
D

6

Topics & Concepts

Organic Chemistry - Basic PrinciplesIsomerismAlkanes, Alkenes, Alkynes

Step-by-Step Solution

To determine the total number of structural isomers for hexane (C6H14\text{C}_6\text{H}_{14}), we systematically list all possible chain lengths and variations in branching for an alkane with six carbon atoms.

1. Straight-chain alkane (6-carbon parent chain):

  • n-hexane:
    CH3−CH2−CH2−CH2−CH2−CH3\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3

2. Branched alkanes with a 5-carbon parent chain (pentane backbone):

A single methyl substituent (−CH3-\text{CH}_3) can be attached to the interior carbons of the 5-carbon chain:

  • Attachment at C-2 position gives 2-methylpentane:
    CH3−CH(CH3)−CH2−CH2−CH3\text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_2-\text{CH}_3

  • Attachment at C-3 position gives 3-methylpentane:
    CH3−CH2−CH(CH3)−CH2−CH3\text{CH}_3-\text{CH}_2-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_3

(Note: Attachment at C-4 gives 2-methylpentane due to symmetry and numbering from the opposite end).

3. Branched alkanes with a 4-carbon parent chain (butane backbone):

Two methyl substituents (−CH3-\text{CH}_3) can be added to the 4-carbon chain:

  • Both methyl groups at C-2 position give 2,2-dimethylbutane:
    CH3−C(CH3)2−CH2−CH3\text{CH}_3-\text{C}(\text{CH}_3)_2-\text{CH}_2-\text{CH}_3

  • One methyl group at C-2 and one at C-3 position give 2,3-dimethylbutane:
    CH3−CH(CH3)−CH(CH3)−CH3\text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}(\text{CH}_3)-\text{CH}_3

Conclusion:

Summing up all unique structural arrangements:

  1. n-hexanen\text{-hexane}
  2. 2-methylpentane\text{2-methylpentane}
  3. 3-methylpentane\text{3-methylpentane}
  4. 2,2-dimethylbutane\text{2,2-dimethylbutane}
  5. 2,3-dimethylbutane\text{2,3-dimethylbutane}

Thus, the total number of structural isomers for C6H14\text{C}_6\text{H}_{14} is 5.

Correct Option: (C)

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