# E/Z isomerism

Rotation about a C=C double bond is blocked, so groups on each side stay fixed. Cis-trans (E/Z) isomers differ in which side groups sit. Rank the two groups on each carbon by priority: if the higher ones are on the same side it is Z, opposite sides is E.

Chemistry (university) · Part 09: Organic structure and stereochemistry · Card 6 of 9 · Last updated 2026-10-19

## Analogy

Two people holding a rope between them cannot swap sides without letting go.

## Example

**But-2-ene**

Same-side CH₃ groups: (Z) / cis
Opposite-side CH₃ groups: (E) / trans
Boiling points: cis 3.7 °C,
trans 0.9 °C

## In practice

Trans fats are unsaturated fats with trans double bonds, which pack together straight and behave more like saturated fats.

## Practice

### Flashcards

- **What does Z mean in an alkene name?** The higher-priority groups are on the same side of the double bond.

## Questions people ask

### Why can't double bonds rotate?

A double bond has a sigma bond and a pi bond. The pi bond is made by sideways overlap of p orbitals, and twisting would pull them apart and break it. That costs a large amount of energy, so at room temperature the groups stay fixed in place.

### What is the difference between cis-trans and E/Z?

Cis-trans names work when each carbon has one identical or obvious group, such as the two methyl groups in but-2-ene. E/Z uses priority rules for any alkene and always gives an answer. When both can be used, cis usually matches Z and trans matches E.

## Related concepts

- [Constitutional isomers](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-3)

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