Define hydrocarbons and recognise alkanes
Classify hydrocarbons by their elements and recognise alkanes as saturated hydrocarbons without a functional group.
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
Which description defines a hydrocarbon?
What you will learn
- Define a hydrocarbon
- Recognise an alkane as a simple hydrocarbon
- Distinguish a carbon skeleton from a functional group
Hydrocarbons contain only carbon and hydrogen
A hydrocarbon is a compound made up of carbon and hydrogen atoms only. If another element is present, the compound is not a hydrocarbon.
An alkane is a simple saturated hydrocarbon with only C–C single bonds. In this syllabus, alkanes have no functional group; their C–H and C–C framework is the carbon skeleton.
Formula patterns support classification
A straight-chain open alkane has general formula CₙH₂ₙ₊₂. The formula pattern is a quick check for an acyclic saturated alkane, but a molecular formula alone may allow more than one structure.
Hydrocarbons include other classes such as alkenes; the defining test is still that only C and H are present.
Pause and explain
Why is chloroethane not a hydrocarbon?
Worked example
Classify C₄H₁₀ and C₂H₅Cl, and explain whether either is a hydrocarbon alkane.
Show the worked solution
- C₄H₁₀ contains only C and H and matches CₙH₂ₙ₊₂ for n = 4.
- It is an alkane hydrocarbon with no functional group.
- C₂H₅Cl contains chlorine, so it is not a hydrocarbon.
Answer C₄H₁₀ is an alkane hydrocarbon with no functional group; C₂H₅Cl is not a hydrocarbon because it contains chlorine.
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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.
Define a hydrocarbon and classify CH₄.
Give me a hint
Check which elements appear in the formula.
Compare my reasoning
- CH₄ contains carbon.
- It contains hydrogen and no other element.
- It is a hydrocarbon and the simplest alkane.
A hydrocarbon contains carbon and hydrogen only; CH₄ is a hydrocarbon alkane.
Look for these in your work
- Definition states only C and H
- Formula checked
- Class stated
Explain why propan-1-ol is not a hydrocarbon.
Give me a hint
Look for elements beyond carbon and hydrogen.
Compare my reasoning
- Its formula contains oxygen.
- A hydrocarbon can contain only C and H.
- Therefore propan-1-ol is not a hydrocarbon.
Propan-1-ol contains oxygen, so it is not a hydrocarbon.
Look for these in your work
- Extra element identified
- Definition applied
- Conclusion stated
A learner says but-1-ene is not a hydrocarbon because it is unsaturated. Correct the claim.
Give me a hint
Separate the definition of hydrocarbon from the alkane class.
Compare my reasoning
- But-1-ene contains only C and H.
- A hydrocarbon may be saturated or unsaturated.
- But-1-ene is an unsaturated hydrocarbon, but not an alkane.
But-1-ene is a hydrocarbon because it contains only C and H; its C=C bond makes it an alkene rather than an alkane.
Look for these in your work
- Hydrocarbon definition applied
- Saturation distinguished
- Classified correctly
Common mistakes
- Defining a hydrocarbon as any carbon-containing compound
- Calling a halogenoalkane a hydrocarbon
- Calling an alkane's C–C single bonds a functional group
What can you explain now?
Which description defines a hydrocarbon?
Compare with the explanation
A compound containing carbon and hydrogen only
Other elements are excluded by the definition.
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Make it stick
What two tests distinguish a hydrocarbon from an alkane?
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.