Alkanes
Alkanes: saturated hydrocarbons—definition, general formula, naming and isomerism, plus key reactions (combustion and substitution).
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The core idea
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Learning objectives
- describe the alkanes as a homologous series of saturated hydrocarbons with the general formula CnH2n+2
- draw the structures of branched and unbranched alkanes, C1 to C4, and name the unbranched alkanes methane to butane
- define isomerism and identify isomers
- describe alkanes (exemplified by methane) as being generally unreactive except in terms of combustion and substitution by chlorine
Build this topic in a simple order: recognise the saturated carbon chain, use the general formula, compare physical trends, identify isomers, then apply combustion and UV substitution reactions.
1. Definition
A. Alkane
An alkane is a saturated hydrocarbon with only carbon-carbon single bonds, C-C. Alkanes form a homologous series with general formula CₙH₂ₙ₊₂.
B. Saturated hydrocarbon
A saturated hydrocarbon is a hydrocarbon with only C-C single bonds (no C = C).
2. Key Ideas
- General formula: CₙH₂ₙ₊₂ (alkanes).
- In a homologous series, successive members differ by CH₂.
- As n increases, boiling point and viscosity generally increase.
- Isomers: same molecular formula, different structural formulae.
- Required alkane reaction: substitution by chlorine in UV light.
- Combustion: complete → CO₂ + H₂O; incomplete → CO and/or soot.
Homologous series basics: Introduction to Organic Chemistry
Unsaturated hydrocarbons + bromine water test: Alkenes
Combustion + air pollutants: Air pollutants
3. Detailed Explanations
- Alkanes are saturated hydrocarbons with only C-C single bonds; general formula CₙH₂ₙ₊₂.
- Bromine water test: alkanes do not decolourise bromine water under normal conditions.
- Key reaction tested: substitution of methane by chlorine under UV light: CH₄ + Cl₂ → [UV] CH₃Cl + HCl.
- Combustion: complete → CO₂ + H₂O; incomplete → CO and/or soot (C). CO is toxic.
- Isomers: same molecular formula, different structural formula (common with C₄H₁₀ and above).
A. General formula and the first few alkanes
The general formula for alkanes is CₙH₂ₙ₊₂.
| Name | Molecular formula | One-line structural formula | State (at r.t.p.) |
|---|---|---|---|
| methane | CH₄ | CH₄ | gas |
| ethane | C₂H₆ | CH₃-CH₃ | gas |
| propane | C₃H₈ | CH₃-CH₂-CH₃ | gas |
Required C4 alkane
| Name | Molecular formula | One-line structural formula | State (at r.t.p.) |
|---|---|---|---|
| butane | C₄H₁₀ | CH₃-CH₂-CH₂-CH₃ | gas |
B. Gradation in physical properties (do not overclaim)
As n increases (molecules get larger):
- boiling point generally increases because intermolecular forces (forces between molecules) are stronger,
- viscosity generally increases (liquids flow less easily).
Boiling Point Trend from Methane to Propane (Approx.)
Approximate boiling points for methane through propane plotted against carbon number, showing boiling point increases as chain length increases.
Scroll across the graph to read all labels.
View figure data
| Number of carbon atoms (unitless) | Alkanes |
|---|---|
| 1 | -162 |
| 2 | -89 |
| 3 | -42 |
C. Isomerism (what it is and why it matters)
Isomers have the same molecular formula but different structural formulae.
Example: C₄H₁₀ has two isomers:
- butane (straight-chain): CH₃-CH₂-CH₂-CH₃
- methylpropane (branched): CH₃-CH(CH₃)-CH₃
Recognise that these are different structures with the same molecular formula. You only need to recall the unbranched names methane to butane.
D. Combustion (complete vs incomplete)
Complete combustion happens in a plentiful supply of oxygen: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
Incomplete combustion happens when oxygen is limited and can produce carbon monoxide and/or soot: 2C₂H₆(g) + 5O₂(g) → 4CO(g) + 6H₂O(l)
CO is colourless and odourless. Write: “forms carboxyhaemoglobin, so blood carries less oxygen”.
E. Substitution by chlorine (the key alkane reaction)
Methane is generally unreactive, but in UV light it reacts with chlorine. One hydrogen atom is replaced by a chlorine atom, so this is a substitution reaction.
Example (methane + chlorine): CH₄(g) + Cl₂(g) → [UV] CH₃Cl(g) + HCl(g)
4. Common Mistakes
- Saying alkanes decolourise bromine water (they do not; that is alkenes).
- Writing “addition reaction” for Cl₂ with alkanes (alkanes do substitution).
- Forgetting the condition: UV light (in the dark, no reaction).
- Writing incomplete combustion products as “only CO₂” (limited oxygen gives CO and/or soot).
- Using “always” for physical-property trends (write “generally”).
5. Exam Tips
Write: “substitution reaction”, “UV light”, and “a hydrogen atom is replaced by a chlorine atom”.
Complete combustion: “CO₂ and H₂O”. Incomplete combustion: “CO and/or soot due to limited oxygen”.
6. Worked Examples
Modelled example 1
Identify an alkane from formula
Problem
Study the worked solution
Substitute the carbon number
Method
Set n = 4 in CₙH₂ₙ₊₂.Reason
The subscript on carbon gives the value of n.Working
2n + 2 = 2(4) + 2 = 10.Compare the formula
Method
Compare the predicted and given hydrogen subscripts.Reason
A matching molecular formula is consistent with the alkane homologous series.Working
C₄H₁₀ fits CₙH₂ₙ₊₂; therefore it is an alkane.
Guided practice 2
Complete combustion equation (balancing)
Problem
Balance C, then H, then O
Hints
Hint 1: products
Hint 2: order
View solution step by step
Balance carbon and hydrogen
Method
Use three CO₂ and four H₂O.Reason
These coefficients account for three C atoms and eight H atoms in propane.Working
C₃H₈ + O₂ → 3CO₂ + 4H₂O.Balance oxygen
Method
Use five oxygen molecules.Reason
The products contain 3(2) + 4(1) = 10 O atoms.Working
C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l).
Common misconception 3
Correct an isomer claim
Learner claim
Compare molecular and structural formulae
View solution step by step
Compare atom counts
Method
Show that both compounds have molecular formula C₄H₁₀.Reason
Isomers must contain the same numbers of each type of atom.Working
Butane: C₄H₁₀; methylpropane: C₄H₁₀.Compare connectivity
Method
State that their structural formulae are different.Reason
Butane is unbranched while methylpropane has a branched carbon skeleton.Working
Same molecular formula + different structural formulae → structural isomers.
Examiner practice 4
Substitution (condition + product)
Examination question
Give the condition, products and balanced equation
View solution step by step
State the condition
1 markMethod
State UV light.Reason
UV light initiates the alkane–halogen substitution reaction.Working
Condition: UV light.Form the substituted products
1 markMethod
Replace one H atom in methane with Cl and pair the removed H with the other Cl.Reason
A substitution replaces a hydrogen atom; it does not add chlorine across a double bond.Working
Products: CH₃Cl and HCl.Write the balanced equation
1 markMethod
Use a 1:1:1:1 coefficient ratio.Reason
The equation then conserves C, H and Br atoms.Working
CH₄(g) + Cl₂(g) → [UV] CH₃Cl(g) + HCl(g).
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the condition, products and balanced equation.
Challenge 5
Draw the two C4 alkane isomers
Structure transfer
Keep the formula while changing the carbon skeleton
Hints
Hint 1: straight then branched
Hint 2: count atoms
View solution step by step
Draw and name the straight chain
Method
Place all four carbon atoms in one continuous chain.Reason
The required unbranched name for C₄H₁₀ is butane.Working
butane: CH₃-CH₂-CH₂-CH₃.Draw the branched structure
Method
Use a three-carbon chain with the fourth carbon attached to the middle carbon.Reason
The atoms are connected differently while the molecular formula remains C₄H₁₀.Working
CH₃-CH(CH₃)-CH₃; this is the branched isomer.
7. Mind Stretchers
Mind stretcher 1: Spot the wrong testExtension
Question: A student says: “Propane decolourises bromine water because it is a hydrocarbon.” Explain why this is wrong.
Show Answer
Decolourising bromine water is a test for a C = C double bond (unsaturation).
Propane is an alkane and has only C-C single bonds (saturated), so it does not decolourise bromine water under normal conditions.
Final: Bromine water tests for C = C; propane has none.
Mind stretcher 2: CO vs CO2 (exam trap)Extension
Question: A heater produces fumes. A student writes: “The dangerous gas is CO₂ from incomplete combustion.” Correct the gas and give the mark-scheme reason it is dangerous.
Show Answer
The dangerous gas is carbon monoxide, CO, produced in incomplete combustion.
Reason: CO forms carboxyhaemoglobin, so blood carries less oxygen.
Final: CO; reduces oxygen transport in blood (carboxyhaemoglobin).
8. Quiz
Test general formula checks, isomerism, trends, combustion products, and UV substitution.