Describe stereoisomerism and cis-trans isomerism
Distinguish stereoisomerism from structural isomerism and explain cis/trans arrangements around restricted bonds.
Section 13 · An introduction to AS Level organic chemistry
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the published functional-group scope and nomenclature limits; draw formula and mechanism evidence with correct connectivity, electron movement and stereochemistry. Written practice is self-checked
Before you begin
- Use covalent structures, valency, molecular formulas, lone pairs, and single, double and triple bonds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is unchanged between a pair of stereoisomers?
What you will learn
- Define stereoisomerism
- Distinguish geometrical and optical isomerism
- Explain cis/trans isomerism in alkenes
Stereoisomers keep the same connectivity
Stereoisomers have the same molecular formula and atom connectivity but different three-dimensional arrangements. Geometrical (cis/trans) and optical isomerism are two types.
In an alkene, each carbon of the C=C must be attached to two different groups for cis/trans isomerism to be possible. The groups can then occupy corresponding same-side or opposite-side arrangements.
Restricted rotation permits distinct arrangements
A π bond prevents free rotation around C=C because rotation would disrupt sideways overlap. The two arrangements therefore do not rapidly interconvert by simple rotation.
For but-2-ene, cis places the two CH₃ groups on the same side of C=C; trans places them on opposite sides. But-1-ene does not show cis/trans isomerism because one alkene carbon has two identical H atoms.
Pause and explain
What condition is needed at both C=C carbons for alkene cis/trans isomerism?
Worked example
Explain why but-2-ene has cis/trans isomers but but-1-ene does not.
Show the worked solution
- Both carbons of but-2-ene have two different attached groups.
- Its π bond restricts rotation, preserving distinct arrangements.
- One but-1-ene carbon has two identical H atoms, so the required condition fails.
Answer But-2-ene can have cis/trans isomers because each C=C carbon has two different groups and rotation is restricted; but-1-ene has two identical H atoms on one alkene carbon.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define stereoisomerism in a sentence.
Give me a hint
State what stays the same and what changes.
Compare my reasoning
- The molecular formula is the same.
- The connectivity is the same.
- The three-dimensional arrangement differs.
Stereoisomers have the same molecular formula and connectivity but differ in spatial arrangement.
Look for these in your work
- Formula
- Connectivity
- Spatial difference
Describe cis and trans but-2-ene using the methyl groups.
Give me a hint
Compare the position of the two CH₃ groups relative to the C=C bond.
Compare my reasoning
- In cis-but-2-ene the CH₃ groups are on the same side.
- In trans-but-2-ene they are on opposite sides.
- Restricted rotation keeps the arrangements distinct.
Cis: methyl groups on the same side; trans: methyl groups on opposite sides of C=C.
Look for these in your work
- Cis arrangement
- Trans arrangement
- Rotation condition
Assess whether CH₂=CHCH₂CH₃ can have cis/trans isomers.
Give me a hint
Check whether both alkene carbons have two different attached groups.
Compare my reasoning
- The terminal carbon has two H atoms.
- The groups on that C are identical.
- Therefore the required cis/trans distinction is absent.
No. One C of the double bond has two identical hydrogen atoms.
Look for these in your work
- Carbon checked
- Identical groups recognized
- Conclusion stated
Common mistakes
- Applying cis/trans to an alkene carbon with two identical groups
- Confusing structural and stereoisomers
- Saying the sigma bond causes restricted rotation
What can you explain now?
What is unchanged between a pair of stereoisomers?
Compare with the explanation
Atom connectivity
Stereoisomers differ in spatial arrangement, not connectivity.
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Make it stick
Why does a C=C bond permit geometrical isomerism while a freely rotating single bond does not?
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