Classify chain, positional and functional-group isomerism
Recognise structural isomers and distinguish changes in carbon skeleton, group position or functional group.
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 must structural isomers share?
What you will learn
- Define structural isomerism
- Identify chain and positional isomerism
- Identify functional-group isomerism
Structural isomers differ in connectivity
Structural isomers have the same molecular formula but different connectivity of atoms. Chain isomerism changes the carbon skeleton, for example straight and branched C₄H₁₀.
Positional isomerism keeps the same skeleton and functional group but changes the position of that group or multiple bond, for example but-1-ene and but-2-ene.
Functional-group isomers have different groups
Functional-group isomers share a molecular formula but belong to different functional classes, such as propanal and propanone (C₃H₆O) or propanol and methoxyethane (C₃H₈O).
Before naming the type, check formula first and connectivity second. Compounds with different molecular formulas are not isomers.
Pause and explain
How do chain isomers differ?
Worked example
Classify 1-bromopropane and 2-bromopropane, then compare propanal and propanone.
Show the worked solution
- The first pair has the same C₃ skeleton and –Br group at different positions.
- That is positional isomerism.
- Propanal and propanone have the same formula but different functional groups: aldehyde and ketone.
Answer 1-bromopropane and 2-bromopropane are positional isomers; propanal and propanone are functional-group isomers.
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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.
Classify but-1-ene and but-2-ene.
Give me a hint
Compare molecular formula, skeleton and C=C position.
Compare my reasoning
- They both have formula C₄H₈.
- The carbon chain remains straight.
- The C=C position changes, so they are positional isomers.
Positional isomers.
Look for these in your work
- Formula matched
- Skeleton matched
- Position compared
Classify straight-chain butane and 2-methylpropane.
Give me a hint
Compare C₄H₁₀ formula and carbon connectivity.
Compare my reasoning
- Both have formula C₄H₁₀.
- One has a straight chain and one a branched skeleton.
- They are chain isomers.
Chain isomers.
Look for these in your work
- Formula matched
- Skeletons distinguished
- Type stated
Propanal and propan-1-ol are proposed as functional-group isomers. Check the claim.
Give me a hint
Calculate or inspect their molecular formulas before classifying.
Compare my reasoning
- Propanal is C₃H₆O.
- Propan-1-ol is C₃H₈O.
- They have different formulas and therefore are not isomers.
The claim is false: their molecular formulas differ, so they are not isomers.
Look for these in your work
- First formula
- Second formula
- Isomer condition applied
Common mistakes
- Calling compounds isomers without checking their molecular formula
- Confusing chain branching with positional change
- Calling every same-formula pair functional-group isomers
What can you explain now?
What must structural isomers share?
Compare with the explanation
The same molecular formula
Isomers have equal molecular formula but differ structurally.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What is the first test to make before classifying a pair as structural isomers?
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.