Identify geometric and chiral features in structures
Apply cis/trans and four-different-groups tests to displayed, skeletal and cyclic structures.
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
When testing an alkene for cis/trans isomerism, how many groups do you compare?
What you will learn
- Identify alkene carbons with different substituents
- Recognise cis/trans relationships in cyclic structures
- Locate chiral centres by comparing full attached groups
Check each alkene carbon separately
For C=C geometrical isomerism, inspect the two groups attached to each alkene carbon. Both carbons must each have two different attachments.
In a ring, the two directions around the ring may be distinct substituent paths. Cis/trans arrangements can be identified by whether the relevant substituents lie on the same or opposite sides of the ring plane.
Compare complete paths at tetrahedral centres
For a candidate chiral carbon, list its four complete attached atoms or groups and look for duplicates. In a cyclic molecule, compare the two ring paths from the centre; they count as different only when their connectivity differs.
Do not assume every tetrahedral carbon in a ring is chiral. Symmetry or identical ring paths can make two attachments equivalent.
Pause and explain
How do you test a tetrahedral carbon in a ring for chirality?
Worked example
Explain how you would test a cyclic structure for cis/trans isomerism and for a chiral centre.
Show the worked solution
- For ring stereochemistry, locate the substituents and compare whether they are on the same or opposite side of the ring plane.
- For each candidate tetrahedral carbon, list all four attached groups, including both ring directions.
- Only a carbon with four different attachments is a chiral centre.
Answer Use the cis/trans same-side/opposite-side test for ring substituents; use the four-different-groups test, including distinct ring paths, for chirality.
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Test this topic →From guided practice to a new situation
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.
A cyclic alkene has two different groups on each C=C carbon. What further feature allows distinct cis/trans forms?
Give me a hint
Consider how the ring and double bond constrain arrangements.
Compare my reasoning
- Each alkene carbon has non-identical attachments.
- The π bond restricts rotation.
- The substituents can occupy distinct same-side/opposite-side arrangements.
The different substituents and restricted rotation permit geometrical isomerism.
Look for these in your work
- Both alkene carbons checked
- Rotation restriction
- Distinct arrangements
A ring carbon has H, OH, and two ring paths. What must you compare before calling it chiral?
Give me a hint
Trace the connectivity along each direction around the ring.
Compare my reasoning
- List the four attachments.
- Compare the two ring paths atom by atom until a difference appears or symmetry shows them equivalent.
- Only four distinct groups make the carbon chiral.
Compare both complete ring paths; the carbon is chiral only if they are different from each other and from H and OH.
Look for these in your work
- All four attachments
- Ring paths compared
- Correct criterion
An alkene has two different groups on the left carbon but two identical H atoms on the right carbon. Is cis/trans isomerism possible?
Give me a hint
Both alkene carbons need two different substituents.
Compare my reasoning
- The left carbon meets the condition.
- The right carbon has two identical groups.
- The molecule lacks distinguishable cis/trans forms at this double bond.
No. One C=C carbon has identical substituents.
Look for these in your work
- First side checked
- Second side checked
- Overall test applied
Common mistakes
- Treating two ring directions as different without comparing their paths
- Forgetting to inspect both C=C atoms
- Assuming same side means cis in every unrelated context without naming the reference bond or ring plane
What can you explain now?
When testing an alkene for cis/trans isomerism, how many groups do you compare?
Compare with the explanation
The two attachments on each C of C=C
Each alkene carbon must have two different groups.
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Make it stick
Why can ring paths determine whether a substituted ring carbon is chiral?
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