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.
- 5.2-a
- 5.2-b
- 5.3-a
- 5.3-b
What these spec points say
- 5.2-a be able to explain geometric isomerism in terms of restricted rotation around a C=C double bond
- 5.2-b be able to explain geometric isomerism in terms of the nature of the substituents on the carbon atoms
- 5.3-a understand the E-Z naming system for geometric isomers
- 5.3-b understand why the E-Z naming system is necessary when the cis-trans naming system breaks down
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 GCSE work on isomers, displayed formulae and the C=C bond in alkenes.
- Isomers have the same molecular formula but a different arrangement of atoms.
- Alkenes contain a C=C double bond and have the general formula CnH2n.
- A displayed formula shows every atom and every bond in a molecule.
- The atomic number of an element is the number of protons in the nucleus of its atom.
GCSE Recap: Isomers and Atomic Number
Four quick GCSE questions on isomers, alkene formulae and atomic number before you meet stereoisomers.
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.
Two checks decide E/Z isomerism: restricted rotation about C=C, and two different groups on each carbon of the double bond.
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. |
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.
- 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.
Z means the priority groups are on the same side and E on opposite sides; cis and trans are used only when the groups match.
FAQs
These questions secure the decisions that cost marks when you identify and name E-Z isomers.
Does but-1-ene have E and Z isomers?
No. Carbon 1 of the C=C bond carries two hydrogen atoms, so swapping the groups on that carbon gives the same molecule. E-Z isomerism needs two different groups on each carbon of the double bond.
Is the Z isomer always the cis isomer?
Not always. Cis and trans compare identical groups, while E and Z compare priority groups. In 2-bromobut-2-ene, the isomer with both CH3 groups on the same side is cis, but it is named E because Br outranks CH3.
Do I decide priority by the size of a group or by atomic number?
By the atomic number of the atom directly attached to the double-bond carbon. Size alone does not count: Cl outranks a CH2CH3 group because chlorine has atomic number 17 and carbon has 6.
Do I compare groups on the same carbon or across the double bond?
Both, in that order. First choose the higher-priority group on each carbon separately. Then compare where those two priority groups sit: on the same side is Z, on opposite sides is E.
Are the E and Z isomers really different compounds?
Yes. They cannot turn into each other at room temperature because that would mean breaking the π bond. They can be separated and have different physical properties, such as their boiling temperatures.
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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