Isomers: same formula, different properties

One formula, several molecules

Two molecules can have exactly the same molecular formula (the same number of each atom) without being identical. These are called isomers.

A simple example: C4H10

The molecular formula C4H10 corresponds to two different molecules:

• butane: CH3-CH2-CH2-CH3 (linear chain);
• methylpropane (isobutane): CH₃-CH(CH₃)-CH₃ (branched chain).

These are chain isomers: same molecular formula, different carbon backbone.

Other types of isomerism

• Positional isomerism: the characteristic group changes position on the same chain. Example: butan-1-ol (CH₃-CH₂-CH₂-CH₂-OH) and butan-2-ol (CH₃-CH₂-CH(OH)-CH₃), both with the formula C₄H₁₀O.
• Functional isomerism: the nature of the characteristic group changes. Example: ethanol CH₃-CH₂-OH (an alcohol) and methoxymethane CH₃-O-CH₃ (an ether), both with the formula C₂H₆O.

Why the properties differ

Two isomers have the same molar mass, but often different physical and chemical properties. The spatial arrangement of the atoms changes the way the molecules interact with one another and react with other species.

Common mistake

Don’t assume that ‘same molecular formula’ means ‘same molecule’: in organic chemistry, it is precisely the opposite that is of interest. Always check the structural or semi-structural formula before concluding that two molecules are identical.