3.1.5Physical Chemistry
Stereoisomerism — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 68 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Defining Stereoisomers
Stereoisomers have the same structural formula but a different arrangement of atoms in space.
Same connectivity
- Same connectivity Atoms are joined in the same order in each stereoisomer, so the structural formula is identical.
Different spatial arrangement
- Different spatial arrangement The difference lies only in how groups are positioned in three dimensions around a fixed centre or bond.
Two types at this level
- Two types at this level At AS level the stereoisomerism studied is E/Z isomerism in alkenes, which includes cis–trans isomerism as a special case.
Different from structural isomers
- Different from structural isomers Structural isomers differ in which atoms are bonded together,
- while stereoisomers differ only in spatial orientation.
Part 2 of 4Why E/Z Isomers Exist
The key idea is restricted rotation about the carbon–carbon double bond.
1
A C=C double bond contains a π bond formed by sideways overlap of p orbitals above and below the plane of the molecule.
2
Rotating one carbon relative to the other would break the π overlap, so the double bond cannot rotate freely.
3
Groups attached to each carbon of the double bond are therefore locked on one side or the other.
4
Two stereoisomers arise only if each carbon of the double bond carries two different groups.
5
If a carbon carries two identical groups, as in propene or but-1-ene, no E/Z isomerism is possible.
Part 3 of 4Cis–Trans and E/Z Naming
Cis–Trans and E/Z Naming
Z isomer
- Z isomer From the German "zusammen" meaning together: the higher-priority groups lie on the same side.
E isomer
- E isomer From "entgegen" meaning opposite: the higher-priority groups lie on opposite sides of the double bond.
Cis and trans
- Cis and trans When each carbon bears the same pair of groups, cis means the two identical groups are on the same side, and trans means opposite sides.
Physical differences
- Physical differences The isomers differ in properties such as melting and boiling point because their shapes and polarity differ.
| Isomer | Higher-priority groups on... | Example |
|---|---|---|
| Z (cis) | Same side of the double bond | (Z)-but-2-ene |
| E (trans) | Opposite sides of the double bond | (E)-but-2-ene |
Part 4 of 4Exam Checklist & Tips
3.1.5 – Key Points for the Exam
- Stereoisomers Compounds with the same structural formula but a different spatial arrangement of atoms.
- E/Z isomerism A type of stereoisomerism in alkenes caused by restricted rotation about the C=C bond.
- Restricted rotation The inability of a double bond to rotate because the π bond would be broken.
- Cis–trans isomerism A special case of E/Z isomerism where each double-bond carbon carries one identical group.
Common Mistakes
- Claiming stereoisomers have different structural formulae; they have the same one.
- Thinking any alkene shows E/Z isomerism, when each carbon needs two different groups.
- Using the word "cis" for a molecule where the identical groups are on opposite sides.
- Forgetting to mention restricted rotation when asked to explain why the isomers exist.
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