13.4.1—Structural isomerism and its division

Structural isomers have the same molecular formula but different atom connectivity. Chain isomerism changes the carbon skeleton, positional isomerism changes a group or multiple bond position, and functional-group isomerism changes the functional group.

Draw the connectivity before comparing names. The atoms and molecular formula stay constant, but the arrangement changes reactivity and often physical properties.

C₄H₁₀ gives butane and 2-methylpropane (chain isomers). C₃H₈O gives propan-1-ol and propan-2-ol (positional isomers), and also methoxyethane (functional-group isomer).

Different conformations from rotation around a single bond are not automatically different structural isomers.