13.4.6—The possible isomers for an organic molecule

To deduce isomers from a molecular formula, first enumerate different carbon skeletons, then place functional groups or multiple bonds, and finally check for geometrical or optical stereoisomers.

Reject duplicates by drawing the same connectivity in a different orientation. Check valency, the required degree of unsaturation and the syllabus limits before counting a structure.

For C₄H₈, consider butenes and methylpropene, then ask whether but-2-ene has cis/trans forms. A cyclic structure may also satisfy the formula if rings are in scope.

Rotating or redrawing one molecule does not create a new isomer. Count distinct connectivity and spatial arrangements only.