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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

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.

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.