Alkanes
Organic Chemistry
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 6% of your exam marks.
General formula, structural formulae and combustion reactions tested regularly.
What an alkane is
- Alkanes are the simplest family of organic compounds: hydrocarbons (carbon and hydrogen only) in which every carbon–carbon bond is a single bond
- A molecule with only single bonds is described as saturated — every carbon atom carries as many hydrogens as it possibly can, so the molecule cannot take on any more atoms
- Each alkane molecule consists of a chain of carbon atoms with hydrogen atoms filling all the spare bonds
- Alkanes form a homologous series — they share a single functional group (the absence of any double or triple bonds), a common general formula, and a steady gradation of physical properties as the chain grows
General formula
- The general formula of the alkane series is:
CnH2n+2
- where n is the number of carbon atoms in the molecule (n = 1, 2, 3, …)
- Plug in n to get the molecular formula:
- n = 1 → CH4 → methane
- n = 2 → C2H6 → ethane
- n = 3 → C3H8 → propane
- The +2 accounts for the extra hydrogen at each end of the chain that has no neighbouring carbon to bond to
The first five alkanes
The first five members of the series are the ones that show up most often in exam questions:
| n | Name | Molecular formula | State at room temperature |
|---|---|---|---|
| 1 | Methane | CH4 | Gas |
| 2 | Ethane | C2H6 | Gas |
| 3 | Propane | C3H8 | Gas |
| 4 | Butane | C4H10 | Gas |
| 5 | Pentane | C5H12 | Liquid |
The names follow a fixed pattern: the stem counts the carbons (meth-, eth-, prop-, but-, pent-) and the -ane ending signals the alkane family. Both pieces are needed in exam answers.
The displayed formulae of the first three members show the pattern: each carbon forms four single bonds, and every step up the series adds one more CH₂ unit.
State/give the general formula of the alkanes
What comes up: a table or short question asks for the general formula of the alkane series (often alongside the molecular formula and name).
Write: CₙH₂ₙ₊₂ (where n is the number of carbon atoms). Both the subscript format and the correct coefficients are needed for the mark.
Watch out: CₙH₂ₙ is the general formula for alkenes, not alkanes. Mixing the two is a common error that loses the mark outright.
Trends down the homologous series
- As the chain lengthens (methane → pentane → hexane → …):
- Boiling point rises steadily — bigger molecules have stronger
- falls — the heavier alkanes do not evaporate as readily
- rises — longer-chain liquid alkanes are stickier
- Flammability falls — heavier alkanes are harder to ignite, although they all still burn
Chemistry of the alkanes
- Alkanes are described as generally unreactive — their molecules have only strong C–C and C–H single bonds that do not break readily under everyday conditions
- The three reactions of alkanes worth memorising are:
- in oxygen (the basis of their use as fuels — see topic 22 section 2)
- when heated with a catalyst (the basis of converting long-chain into short-chain — see topic 22 section 4)
- Substitution by halogens in the presence of ultraviolet light — covered next in section 2
Explain why an alkane is described as saturated
What comes up: a 2-mark question asking you to explain why a named alkane (e.g. ethane) is described as a saturated compound.
Write (two marks): (1) it contains only single bonds / has no double or multiple bonds between carbon atoms; (2) so no additional atoms can be added to the molecule / it already holds the maximum number of hydrogen atoms.
Watch out: stating only that it "contains carbon and hydrogen" scores nothing — the mark requires the structural reason (single bonds only). Also accepted: "each carbon is bonded to four other atoms", or "it only undergoes substitution reactions, not addition".