Geometric Isomerism
A concise revision guide to stereoisomerism in alkenes, restricted rotation around C=C bonds, E-Z naming, priority groups and the link between E-Z and cis-trans isomerism.
- 13.4.2-a
- 13.4.2-b
- 13.4.3-a
- 13.4.3-b
- 13.4.5-b
What these spec points say
- 13.4.2-a describe stereoisomerism
- 13.4.2-b describe geometrical (cis/trans) isomerism as a division of stereoisomerism (use of E/Z nomenclature is acceptable but is not required)
- 13.4.3-a describe geometrical (cis/trans) isomerism in alkenes
- 13.4.3-b explain the origin of geometrical (cis/trans) isomerism in alkenes in terms of restricted rotation due to the presence of π bonds
- 13.4.5-b identify geometrical (cis/trans) isomerism in a molecule of given structural formula including cyclic compounds
Free and Restricted Rotation: Why E/Z Isomers Exist
Choose a step to watch butane turn freely about its C–C σ bond, then see how the π bond locks but-2-ene into separate Z and E isomers.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Before You Start
This page builds on alkene bonding and on structural isomers.
- An alkene contains a C=C double bond made of one σ bond and one π bond.
- Structural isomers have the same molecular formula but atoms joined in a different order.
- Atoms around each C=C carbon lie in one plane at about 120°.
- Atomic number increases across and down the Periodic Table: Cl (17) beats C (6) and H (1).
GCSE Recap: Isomers and Atomic Number
Four quick GCSE questions on isomers, alkene formulae and atomic number before you meet stereoisomers.
How Cambridge Names This Isomerism
The Cambridge syllabus divides stereoisomerism into geometrical (cis/trans) isomerism and optical isomerism. Geometrical isomerism is the type shown by alkenes, and it arises from restricted rotation about the C=C bond caused by the π bond.
Cambridge uses the cis/trans names as the standard system. The E/Z system is acceptable but not required, so you may use either in an answer.
This page teaches both, with cis/trans as the naming you should be able to use for every example.
Exam focus: Describe geometrical isomerism as a form of stereoisomerism, and explain its origin in terms of restricted rotation due to the presence of the π bond. Cambridge also expects you to identify geometrical isomerism in cyclic compounds, not only in alkenes.
Watch: why these two molecules can never be the same
Two models of 1,2-dichloroethene. They look almost the same, but no amount of twisting turns one into the other. Why not?
What Are Stereoisomers?
Stereoisomers are compounds with the same structural formula but a different three-dimensional arrangement of atoms in space.
In alkenes, this matters because the C=C double bond restricts rotation. The atoms or groups attached to the double bond cannot freely rotate into a new arrangement without breaking the pi bond.
This produces a type of stereoisomerism called E-Z stereoisomerism, also called geometric isomerism.
Key idea: E-Z isomers are not structural isomers. They have the same connectivity, but different spatial arrangement around the carbon-carbon double bond.
Quick Check: Structural Isomer or Stereoisomer?
Complete a comparison of three alkenes that are not on this page.
Why E-Z Isomerism Occurs
E-Z isomerism occurs because the carbon-carbon double bond contains a pi bond. Rotation around the C=C bond is restricted because rotating one carbon would break the sideways overlap of the p orbitals.
By contrast, a carbon-carbon single bond can rotate freely. This is why simple alkanes do not usually produce this type of fixed geometric arrangement.
Exam focus: Always link E-Z isomerism to restricted rotation around the C=C double bond.
Quick Check: Explain Two Forms Against One
Write a short explanation, then compare it with the mark points and the model answer.
Conditions for E-Z Stereoisomerism
For an alkene to show E-Z stereoisomerism, two conditions must both be met:
- There must be restricted rotation about the carbon-carbon double bond.
- Each carbon atom of the C=C bond must be attached to two different atoms or groups.
If either carbon in the C=C bond has two identical groups attached, E-Z isomerism is not possible.
E/Z isomers need restricted rotation and two different groups on each carbon of the C=C.
Quick Check: Which Alkenes Show E-Z Isomerism?
Sketch each alkene, then click every one that has E and Z isomers.
Assigning E and Z
To name an alkene with E-Z stereoisomerism, first identify the priority group on each carbon atom of the double bond.
The priority group is found by comparing the atoms directly attached to each double-bond carbon. The atom with the higher atomic number has higher priority.
| Arrangement of priority groups | Name used | Memory aid |
|---|---|---|
| Priority groups on the same side of the C=C bond | Z | Z comes from zusammen, meaning together. |
| Priority groups on opposite sides of the C=C bond | E | E comes from entgegen, meaning opposite. |
The higher atomic number on each carbon is the priority group: same side Z, opposite sides E.
Quick Check: E, Z or Neither?
Find the priority group on each carbon atom, then decide quickly.
Cis-Trans Isomerism
Cis-trans isomerism is a simpler special case of E-Z isomerism. It can be used when two of the substituent groups attached to the double bond are identical.
For but-2-ene, the two CH3 groups can be on the same side or on opposite sides of the double bond.
Z-but-2-ene
The two CH3 groups are on the same side, so this can also be called cis-but-2-ene.
E-but-2-ene
The two CH3 groups are on opposite sides, so this can also be called trans-but-2-ene.
Important: E-Z notation is the more general and accurate naming system, especially when the groups attached to the C=C bond are not identical.
Quick Check: Pick the Accurate Statement
Each round puts a common mistake next to the correct idea, in a molecule you have not met on this page.
Common Exam Mistakes
- Do not call E-Z isomers structural isomers. They have the same structural formula.
- Do not assign E or Z by only looking at the largest group visually. Use atomic number priority. For Cambridge, cis/trans names are accepted wherever two of the substituents are the same.
- Do not assume every alkene shows E-Z isomerism. Each carbon in the C=C bond must have two different groups attached.
- Do not say the molecule rotates around the double bond. The whole point is that rotation around C=C is restricted.
FAQs
These questions cover the geometrical isomerism decisions students most often get wrong.
Does but-1-ene have cis and trans isomers?
No. Carbon 1 carries two hydrogen atoms, so swapping them gives the same molecule. Each C=C carbon needs two different groups.
Do I have to use E and Z?
No. Cis/trans names are the standard here. E/Z is acceptable, but cis-but-2-ene and trans-but-2-ene are full answers.
Is Z always the same as cis?
Not always. They agree when the same group sits on each carbon, as in but-2-ene, but can differ when the priority groups are not the ones being called cis.
Are geometrical isomers structural isomers?
No. They are stereoisomers: same structural formula, different arrangement in space.
Why do alkanes not show geometrical isomerism?
A C-C single bond can rotate freely, so any arrangement turns into any other. Only the π bond of a C=C bond locks the groups in place.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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