E/Z Nomenclature & Geometric Isomerism

CompleteFundamentals of Chemistry

Geometric isomers differ by the arrangement of groups about a double bond (cis/trans, E/Z).

Full notes

Definition: Geometric (cis-trans) isomerism arises when restricted rotation (a C=C bond or a ring) holds groups in fixed positions, and the isomers interconvert only by breaking the double bond.

1. Cis / trans

Cis: same priority groups on the same side; trans: on opposite sides. 2-butene: cis and trans forms have different physical properties (cis b.p. 3.7°C, trans b.p. 0.9°C; trans is more stable).

2. E/Z (CIP-based) nomenclature

When cis/trans is ambiguous, use E/Z: compare the two groups on each carbon by CIP priority. If the two higher-priority groups are on the same side → Z (zusammen, together); opposite sides → E (entgegen, opposite).

3. Geometric isomerism in oximes

C=N of oximes allows syn/anti (geometrical) isomerism. In aldoximes, syn-aldoxime (OH same side as H) and anti-aldoxime. This property lets Beckmann rearrangement proceed (anti group migrates).

4. In alicyclic rings

Rings restrict rotation too: 1,2-dimethylcyclopropane exists as cis/trans isomers. Note: terms cis/trans in rings reflect the up/down (same/opposite face) arrangement of substituents.

Example: 1-bromo-1-chloropropene: two different pairs on the double bond; decide E or Z by comparing Br vs Cl and H vs CH₃.

Note: Exam point: unchanged 'cis/trans' is only valid for identical pairs; otherwise use E/Z and justify with CIP priorities.

Exam notes

cis/trans = same/opposite side. E/Z from CIP. Oximes syn/anti. Rings too. Trans more stable generally.

Important points

Geometric isomerism requires restricted rotation; diastereomers of alkenes are cis/trans pairs.

Common mistakes

Using cis/trans when the substituents are not identical pairs.

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