Alkanes
Saturated hydrocarbons, general formula CnH2n+2, and substitution reactions.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 11.4.1State the general formula of the alkanes and describe them as saturated hydrocarbons.
- 11.4.2Describe the substitution reaction of alkanes with chlorine in the presence of ultraviolet light.Supplement
5 minute read
Alkanes
Alkanes are the "boring but reliable" homologous series — the baseline every other organic family gets compared against, precisely because they are saturated: every carbon–carbon bond is a single bond, with no C=C double bond to react across.
General formula
The general formula of the alkanes is CₙH₂ₙ₊₂. Substituting n = 1, 2, 3, 4 gives methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀).
Why "saturated" matters
Being saturated means alkanes are relatively unreactive: they do not react with bromine water (unlike alkenes), and they burn cleanly in excess oxygen (complete combustion) to give carbon dioxide and water. Their main reactions are combustion (as fuels) and, under specific conditions, substitution.
Substitution with chlorine
In the presence of ultraviolet light, alkanes react with chlorine in a substitution reaction — a hydrogen atom is replaced (substituted) by a chlorine atom, e.g. CH₄ + Cl₂ → CH₃Cl + HCl. This is fundamentally different from how alkenes react with bromine: alkanes cannot do addition because they have no double bond to open up, so the only way to fit chlorine into the molecule is by kicking a hydrogen out.
Think of it like this
Substitution in an alkane is like swapping one passenger for another in a full car — the car (molecule) is already "full" (saturated) so a new passenger can only get in if an existing one gets out.
Worked examples
Method, step by step
Write a symbol equation for the reaction between ethane and chlorine in ultraviolet light, forming one substitution product.
- 1Ethane, C₂H₆, has a hydrogen replaced by chlorine.
- 2One product is chloroethane, C₂H₅Cl.
- 3The other product is hydrogen chloride, HCl, from the displaced hydrogen.
C₂H₆ + Cl₂ → C₂H₅Cl + HCl
Common misconceptions
- Thinking alkanes react with bromine water — they do not; the bromine-water test is specifically used to distinguish alkanes (no change) from alkenes (decolourised) because alkanes lack a C=C bond.
- Confusing substitution (alkanes + halogens, one atom swapped for another) with addition (alkenes + halogens, atoms added across a broken double bond) — these are different mechanisms and exams test that you know which applies to which series.
In the exam
- If a question mentions "ultraviolet light" and an alkane, it is almost always testing the substitution reaction with a halogen — write the word "substitution" explicitly, not just "reaction".
- Complete combustion of an alkane needs "excess oxygen" specified in your answer — with limited oxygen you get incomplete combustion (carbon monoxide/soot) instead.