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.
- 4.1.3(a)-ii
- 4.1.3(c)(i)-a
- 4.1.3(c)(i)-b
- 4.1.3(c)(i)-c
- 4.1.3(c)(ii)
- 4.1.3(d)-i
- 4.1.3(d)-ii
What these spec points say
- 4.1.3(a)-ii restricted rotation of the π-bond
- 4.1.3(c)(i)-a explanation of the term stereoisomers (compounds with the same structural formula but with a different arrangement in space)
- 4.1.3(c)(i)-b explanation of the term E/Z isomerism (an example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group)
- 4.1.3(c)(i)-c explanation of the term cis–trans isomerism (a special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same)
- 4.1.3(c)(ii) use of Cahn–Ingold–Prelog (CIP) priority rules to identify the E and Z stereoisomers
- 4.1.3(d)-i determination of possible E/Z stereoisomers of an organic molecule, given its structural formula
- 4.1.3(d)-ii determination of possible cis–trans stereoisomers of an organic molecule, given its structural formula
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.
Cahn–Ingold–Prelog Priority Rules
When the groups attached to a double-bond carbon are not single atoms, priority is decided using the Cahn–Ingold–Prelog (CIP) rules. OCR requires these rules to identify E and Z stereoisomers.
Rule 1: compare the atoms directly attached
Compare the atomic numbers of the atoms bonded directly to each carbon of the C=C bond. The atom with the higher atomic number has the higher priority. For example, Br (35) outranks Cl (17), and Cl outranks C (6), which outranks H (1).
Rule 2: if the first atoms tie, move outwards
If the directly attached atoms are the same, list the atoms bonded to each of them in order of decreasing atomic number and compare the lists atom by atom. The first point of difference decides the priority.
For example, an ethyl group, CH2CH3, outranks a methyl group, CH3. Both attach through carbon, but the ethyl carbon carries (C, H, H) while the methyl carbon carries (H, H, H).
Carbon outranks hydrogen at the first difference, so ethyl has the higher priority.
| Groups being compared | First atom attached | Decision |
|---|---|---|
| Br and Cl | Br (35) and Cl (17) | Br has priority by Rule 1 |
| Cl and CH3 | Cl (17) and C (6) | Cl has priority by Rule 1 |
| CH2CH3 and CH3 | C and C (tie) | Ethyl has priority by Rule 2: (C,H,H) beats (H,H,H) |
| CH(CH3)2 and CH2CH3 | C and C (tie) | Isopropyl has priority by Rule 2: (C,C,H) beats (C,H,H) |
Once the priority group on each carbon is known, the isomer is Z if the two priority groups are on the same side of the C=C bond and E if they are on opposite sides.
Exam focus: State the priority on each carbon separately, then compare the two sides. Do not compare a group on one carbon with a group on the other carbon.
Assign the priority group on each carbon separately, then compare the two sides of the double bond.
Quick Check: When the First Atoms Tie
Use Rule 2 on a pair of groups that the table does not show.
Quick Check: Assign E or Z
Find the priority group on each carbon atom of the isomer shown, then compare sides.
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 and, where the first atoms tie, the Cahn–Ingold–Prelog rules.
- 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 points.
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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