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Structural Isomers of Alkanes

Key Concepts

Structural isomers are molecules with:
  • same molecular formula

  • different structural formulae

Structural isomers of alkanes have:

  • same number of atoms of carbon and hydrogen

  • different arrangement of carbon and hydrogen atoms

Animated Tutorial

Examples

  1. Butane has the molecular formula C4H10.
    All possible structural isomers of butane must also have the molecular formula C4H10.

    name structural formula molecular formula
    butane
     H
    |
     H
    |
     H
    |
     H
    |
     
    H-C-C-C-C-H
     |
    H
     |
    H
     |
    H
     |
    H
     
    C4H10

    2-methylpropane
       H
    |
       
     H
    |
     H-C-H
    |
     H
    |
     
    H-C-C-C-H
     |
    H
     |
    H
     |
    H
     
    C4H10


    Butane and 2-methylpropane are structural isomers.
    They have the same molecular formula, C4H10, but different structural formulae.

  2. Pentane has the molecular formula C5H12.
    All possible structural isomers of pentane must also have the molecular formula C5H12.

    name structural formula molecular formula
    pentane
     H
    |
     H
    |
     H
    |
     H
    |
     H
    |
     
    H-C-C-C-C-C-H
     |
    H
     |
    H
     |
    H
     |
    H
     |
    H
     
    C5H12

    2-methylbutane
       H
    |
        
     H
    |
     H-C-H
    |
     H
    |
     H
    |
     
    H-C-C-C-C-H
     |
    H
     |
    H
     |
    H
     |
    H
     
    C5H12

    2,2-dimethylpropane
       H
    |
       
     H
    |
     H-C-H
    |
     H
    |
     
    H-C-C-C-H
     |
    H
     |
    H-C-H
     |
    H
     
       |
    H
       
    C5H12


    Pentane, 2-methylbutane and 2,2-dimethylpropane are structural isomers.
    They have the same molecular formula, C5H12, but different structural formulae.


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