3.2.7 (HL)—Stereoisomers

Stereoisomers have the same constitution but different spatial arrangements. Cis-trans isomerism requires restricted rotation, while enantiomers are non-superimposable mirror images caused by a chiral carbon.

Check the spatial arrangement, not just connectivity, and identify whether the case is a non-cyclic alkene/cycloalkane cis-trans pair or a chiral mirror-image pair.

For alkene cis–trans isomerism, each C of the C=C must carry two different groups; restricted rotation alone is not enough. For chirality, verify four different substituents on one tetrahedral carbon and test whether the mirror-image pair can be superimposed.

In a substituted C3 or C4 cycloalkane, restricted ring geometry gives cis when relevant substituents are on the same side of the ring and trans when they are on opposite sides. For a chiral centre, a solid wedge points out of the page and a dashed wedge behind it. A pair of enantiomers rotates plane-polarized light in opposite directions; an equal racemic mixture has no net rotation. E/Z nomenclature is outside this syllabus scope.