Isomers are compounds that have the same molecular formulas but different structural arrangements of atoms. They fall into two categories: constitutional isomers and stereoisomers. Constitutional isomers are isomers that have different atomic connectivities. Examples of constitutional isomers include butane and isobutane (both have the molecular formula C₄H₁₀, but different structures; Figure 1) and ethanol and dimethyl ether (both have the formula C₂H₆O, but again the two differ structurally; Figure 2). Stereoisomers are isomers whose constituent atoms are connected in the same sequence, but in different spatial patterns. Examples of stereoisomers include the cis and trans isomers of the alkenes (Figure 3).

Stereoisomers can be further subdivided into: enantiomers and diastereomers. Enantiomers are stereoisomers whose molecules are nonsuperposable mirror-images of each other. Diastereomers are stereoisomers whose molecules are not mirror images of each other. The atoms of diastereomers are joined together in the same order but differ in the...