Use homologous-series and saturation terminology
Compare members of homologous series and distinguish saturated from unsaturated organic compounds.
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
How do successive members of a homologous series differ?
What you will learn
- Define a homologous series
- Recognise its repeating –CH₂– change
- Distinguish saturated and unsaturated compounds
A homologous series has shared structural patterns
A homologous series is a family of compounds with the same functional group and general formula, where successive members differ by –CH₂–. Members have similar chemical properties and show a gradual change in physical properties.
For example, propane and butane are successive alkane members: C₃H₈ and C₄H₁₀. The added –CH₂– changes molecular size but does not change their alkane functional pattern.
Saturation depends on carbon–carbon bonds
A saturated organic compound has no carbon–carbon double or triple bonds; an unsaturated compound has at least one C=C or C≡C bond.
The term does not mean that a molecule contains no other multiple bond such as C=O. In this context, identify unsaturation in the carbon skeleton.
Pause and explain
Is but-2-ene saturated or unsaturated?
Worked example
Explain why propane and butane are in the same homologous series and classify propene as saturated or unsaturated.
Show the worked solution
- Propane and butane are alkanes with the same characteristic structural pattern.
- Their formulas differ by CH₂.
- Propene has a carbon–carbon double bond, so it is unsaturated.
Answer Propane and butane are successive alkane homologues; propene is unsaturated because it contains C=C.
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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.
State two features shared by members of a homologous series.
Give me a hint
Think about functional group, general formula and chemical behaviour.
Compare my reasoning
- They have the same functional group.
- They follow the same general formula pattern.
- They show similar chemical properties.
They share a functional group and general-formula pattern and have similar chemical properties.
Look for these in your work
- Functional group
- Formula pattern
- Chemical similarity
What is the difference in molecular formula between hexane and pentane?
Give me a hint
Compare successive alkane members.
Compare my reasoning
- Hexane is C₆H₁₄.
- Pentane is C₅H₁₂.
- Hexane has one additional CH₂ unit.
CH₂.
Look for these in your work
- Both formulas correct
- Subtraction correct
- Homologous increment named
A molecule has one C=C and one C=O bond. Is it saturated according to the carbon-skeleton definition?
Give me a hint
Look specifically for a C–C multiple bond.
Compare my reasoning
- The C=C is a carbon–carbon double bond.
- That bond makes the carbon skeleton unsaturated.
- The separate C=O does not cancel that classification.
It is unsaturated because it contains a C=C bond.
Look for these in your work
- Relevant bond identified
- Definition applied
- Carbonyl distinguished
Common mistakes
- Defining homologues only as compounds with similar physical properties
- Calling a carbonyl bond the carbon-skeleton unsaturation in this syllabus term
- Saying successive members differ by CH₃
What can you explain now?
How do successive members of a homologous series differ?
Compare with the explanation
By –CH₂–
Each successive member adds one –CH₂– unit.
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Make it stick
Which features stay similar and which can change gradually along a homologous series?
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