Two compounds can have the same molecular formula yet differ in how their atoms are connected. Such compounds are constitutional isomers.

For example, the formula $\mathrm{C_4H_{10}}$ can represent either the straight-chain structure

$$\mathrm{CH_3CH_2CH_2CH_3}$$

or the branched structure

$$\mathrm{(CH_3)_3CH}.$$

These are different compounds because their carbon-carbon connectivity differs, even though they contain the same numbers of carbon and hydrogen atoms.

Constitutional isomerism can arise in several ways:

• skeletal isomerism: the carbon framework is connected differently;
• positional isomerism: the same functional group or multiple bond appears at a different position on the same basic skeleton;
• functional-group isomerism: the atoms are connected so that different functional groups result.

For example, ethanol and dimethyl ether both have formula $\mathrm{C_2H_6O}$:

$$\mathrm{CH_3CH_2OH}$$

and

$$\mathrm{CH_3OCH_3}.$$

The first is an alcohol and the second an ether, so they are functional-group constitutional isomers.

A molecular formula therefore does not uniquely specify a molecular structure. To decide whether two drawings represent constitutional isomers, compare which atom is bonded to which, not merely the overall formula or the visual shape of the drawing.

Different drawings can also represent the same connectivity. Rotating a skeletal drawing on the page, choosing a different zig-zag orientation, or writing a condensed formula from the opposite end does not create a new constitutional isomer.

Constitutional isomers often have different physical and chemical properties because changing connectivity changes functional groups, branching, polarity, intermolecular contacts and the local environments of atoms.

This distinction prepares a second major category of isomerism: stereoisomers have the same connectivity but differ in the three-dimensional arrangement of their atoms.