Deduce possible structural and stereoisomers from a formula
Generate valid isomer candidates by preserving formula, valency and functional-group possibilities.
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 must every proposed isomer preserve?
What you will learn
- Enumerate chain and positional alternatives
- Check functional-group isomers
- Avoid duplicates and invalid valencies
Begin with a formula and degree of unsaturation
List possible carbon skeletons and functional groups consistent with the formula. Keep the total number of each atom unchanged and use normal valencies.
For simple acyclic formulas containing C, H and a single O, compare alcohol and ether possibilities when the hydrogen count allows them. For a formula with C=C, consider positional isomers and any permitted geometrical forms.
Check identity before counting
Two drawings that differ only by rotating a single bond are not different structural isomers. Two names for the same connectivity are duplicates.
After generating candidates, check each molecular formula, functional group and connectivity. If stereoisomers are requested, count distinct geometrical or optical arrangements only when their structural conditions are satisfied.
Pause and explain
How many chain isomers are there for C₄H₁₀?
Worked example
Give the two structural isomers of C₄H₁₀ and identify their isomerism type.
Show the worked solution
- Use a four-carbon straight chain: CH₃CH₂CH₂CH₃.
- Use a three-carbon parent with one methyl branch: (CH₃)₃CH.
- Both have C₄H₁₀ but different carbon skeletons.
Answer Butane and 2-methylpropane are the two chain isomers of C₄H₁₀.
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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.
Give the structural isomers of C₃H₈O and classify one functional-group pair.
Give me a hint
Consider alcohol positions and an ether connectivity.
Compare my reasoning
- CH₃CH₂CH₂OH is propan-1-ol.
- CH₃CH(OH)CH₃ is propan-2-ol.
- CH₃OCH₂CH₃ is methoxyethane; the ether is a functional-group isomer of the alcohols.
The three structures are propan-1-ol, propan-2-ol and methoxyethane; the alcohols and ether are functional-group isomers.
Look for these in your work
- All candidates valid
- Formula conserved
- Functional types distinguished
How many positional isomers of butene have the same straight-chain skeleton?
Give me a hint
Move the double bond without redrawing duplicates from the opposite end.
Compare my reasoning
- A terminal double bond gives but-1-ene.
- An internal double bond gives but-2-ene.
- A double bond at carbon 3 is the same as but-1-ene when numbered from the nearer end.
Two: but-1-ene and but-2-ene.
Look for these in your work
- End position
- Internal position
- Duplicate removed
A proposed isomer list for C₄H₁₀ includes CH₃CH₂CH₂CH₂CH₃. Diagnose the error.
Give me a hint
Count carbon atoms before comparing structures.
Compare my reasoning
- The proposed structure contains five carbon atoms.
- C₄H₁₀ allows only four carbon atoms.
- The structure is pentane, C₅H₁₂, and must be rejected.
It has five carbons and formula C₅H₁₂, so it is not an isomer of C₄H₁₀.
Look for these in your work
- Carbon count
- Formula mismatch
- Candidate rejected
Common mistakes
- Changing the formula when branching a structure
- Counting conformations as structural isomers
- Listing an invalid carbon valency or duplicated structure
What can you explain now?
What must every proposed isomer preserve?
Compare with the explanation
The given molecular formula
Isomers must have the same molecular formula.
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Make it stick
What checks should you make before accepting an isomer as valid?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.