Alkanes and Substitution Reactions
Alkanes are the saturated hydrocarbons — the main components of crude oil and our most important fuels. They are unreactive apart from combustion and one light-driven reaction.
1. The homologous series
Alkanes have the general formula CₙH₂ₙ₊₂ and contain only SINGLE carbon–carbon bonds.
| Alkane | Formula | Structure |
|---|---|---|
| Methane | CH₄ | CH₄ |
| Ethane | C₂H₆ | CH₃CH₃ |
| Propane | C₃H₈ | CH₃CH₂CH₃ |
| Butane | C₄H₁₀ | CH₃CH₂CH₂CH₃ |
Check the formula with the general formula: for propane, n = 3, so H = 2(3) + 2 = 8 → C₃H₈.
Alkanes are CₙH₂ₙ₊₂; alkenes are CₙH₂ₙ. The alkane has two more hydrogens for the same number of carbons.
2. Saturated structure
SATURATED means containing only SINGLE carbon–carbon bonds.
Each carbon forms four bonds; each hydrogen forms one.
“Saturated” is defined by the bonds, not by “being full of hydrogen” — though it is true that a saturated compound holds the maximum possible hydrogen.
Drawing a displayed formula: show every C–H and C–C bond, and check each carbon has exactly four bonds.
3. Why alkanes are unreactive
Alkanes are relatively UNREACTIVE because the C–C and C–H bonds are STRONG and the molecules are non-polar, with no functional group to attack.
They do not react with acids, alkalis or most reagents — and crucially:
Alkanes do NOT decolourise bromine water, because they have no C=C double bond to react with. This is the test that distinguishes them from alkenes.
4. Combustion
COMPLETE combustion (plenty of oxygen): alkane + oxygen → CARBON DIOXIDE + WATER CH₄ + 2O₂ → CO₂ + 2H₂O
Complete combustion is exothermic, which is why alkanes are used as fuels.
Balancing a combustion equation — the reliable order:
- Balance the carbons (using CO₂)
- Balance the hydrogens (using H₂O)
- Balance the oxygens last (using O₂), using a fraction if needed, then double everything
Example — ethane:
- C₂H₆ + O₂ → 2CO₂ + 3H₂O
- Oxygen needed: (2 × 2) + 3 = 7, so 3½O₂
- Double through: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
INCOMPLETE combustion (limited oxygen): produces CARBON MONOXIDE (CO) and/or carbon (soot), plus water.
Carbon monoxide is TOXIC — it binds to haemoglobin, preventing oxygen transport. It is also colourless and odourless, which is why it is so dangerous.
5. Substitution with halogens
Alkanes react with chlorine (or bromine) in the presence of ULTRAVIOLET LIGHT in a SUBSTITUTION reaction.
A hydrogen atom is REPLACED by a halogen atom, and hydrogen chloride is released.
Example:
CH₄ + Cl₂ → CH₃Cl + HCl (in UV light)
The product is chloromethane.
Further substitution can continue: CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄.
UV light is essential — the reaction does not happen in the dark. Always state the condition.
Substitution is NOT addition. In substitution an atom is swapped out and HCl is produced; in addition (alkenes) the double bond opens and nothing is lost. This is the cleanest way to tell the two families’ reactions apart.
| Alkanes | Alkenes | |
|---|---|---|
| Bonds | single only (saturated) | contains C=C (unsaturated) |
| Reaction type | SUBSTITUTION | ADDITION |
| Condition | UV light | none needed |
| Other product | HCl | none |
| Bromine water | stays orange | decolourised |
| Reactivity | unreactive | reactive |
6. Uses
Methane — natural gas, for heating and cooking Propane and butane — bottled gas (LPG) Longer alkanes — petrol, diesel, kerosene, lubricants
7. Mistakes that cost marks
Wrong general formula — CₙH₂ₙ₊₂.
Saying alkanes decolourise bromine water.
Omitting UV light from the substitution conditions.
Confusing substitution with addition.
Forgetting HCl as a product of substitution.
Unbalanced combustion equations.
Confusing the products of complete and incomplete combustion.
Carbons with the wrong number of bonds in a displayed formula.
Frequently asked questions
What is the general formula for alkanes? CₙH₂ₙ₊₂.
What does saturated mean? Containing only single carbon–carbon bonds.
Why are alkanes unreactive? Their C–C and C–H bonds are strong, and they have no functional group.
Do alkanes decolourise bromine water? No — it stays orange. That is the test.
What does complete combustion produce? Carbon dioxide and water.
What does incomplete combustion produce? Carbon monoxide and/or soot, plus water.
Why is carbon monoxide dangerous? It is toxic, binding to haemoglobin, and is colourless and odourless.
What is a substitution reaction? A hydrogen is replaced by a halogen atom, releasing HCl.
What condition does it need? Ultraviolet light.
What’s the difference between substitution and addition? Substitution swaps an atom and releases HCl; addition opens a double bond with no other product.
Quick revision checklist
- I know the general formula CₙH₂ₙ₊₂
- I can name and write formulae for the first four alkanes
- I can define saturated by the bonds
- I can draw a displayed formula with four bonds per carbon
- I can explain why alkanes are unreactive
- I know they don’t decolourise bromine water
- I can balance a combustion equation
- I know complete and incomplete combustion products
- I know why CO is dangerous
- I can write a substitution equation with HCl
- I always state UV light
- I can contrast substitution with addition
These notes cover alkanes and substitution reactions in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
Finished this topic?
Saved on this device — no account needed.