Alkanes: a homologous series, CnH(2n+2)
Alkanes form a homologous series — same general formula, members differing by , similar chemical properties. Their general formula is : substitute (the number of carbons) to build the molecular formula, e.g. gives propane . The first five (methane to pentane) must be named and their formulae given. Every name ends in -ane; the stem (meth-, eth-, prop-, but-, pent-) gives the carbon count.
Saturated hydrocarbon = two ideas
The phrase "saturated hydrocarbon" carries two mark-scheme ideas. Hydrocarbon — a compound of only hydrogen and carbon. Saturated — the molecule has only single carbon-carbon (C-C) bonds (no C=C double bonds), so it holds the maximum number of hydrogen atoms. Every bond in an alkane is a single covalent bond, which is why alkanes are saturated — the contrast is with unsaturated alkenes, which have a C=C double bond (C4.23).
Substitution with halogens in UV
Alkanes are generally unreactive but react with halogens (chlorine, bromine) in a substitution reaction needing UV light. In mono-substitution one hydrogen is replaced by one halogen atom, forming a haloalkane and a hydrogen halide: (chloromethane + hydrogen chloride). It is substitution because a halogen takes the place of a hydrogen; without UV the reaction does not proceed usefully.
Drawn from real examiner reports.
"Saturated hydrocarbon" needs both ideas
"Saturated hydrocarbon" is a two-part term, each part a separate marking point. State both: hydrocarbon = only hydrogen and carbon; saturated = only single C-C bonds / no C=C double bonds. Candidates give one and forget the other. The same trap appears in reverse for an unsaturated hydrocarbon (a hydrocarbon that contains a C=C double bond).
Jun23 1C Q4cii: defining an unsaturated hydrocarbon needs both ideas — hydrocarbon (only H and C) and the C=C double-bond idea — candidates define one and forget the other.
Alkane vs alkene: saturated or not
Do not confuse alkanes with alkenes. Alkanes are saturated — only single C-C bonds, general formula . Alkenes are unsaturated — they contain a C=C double bond, general formula . Only alkenes decolourise bromine water; the bromine-water test is an alkene test, not an alkane one.
UV is for substitution, not the Br test
Two linked errors. First, candidates omit the UV-light condition for an alkane reacting with a halogen — it is a marking point. Second, some wrongly attach UV light to the alkene bromine-water test, where it does not belong. Keep them apart: alkane + halogen = substitution, needs UV; alkene + bromine water = addition, no UV.
Jun23 1C Q4ciii: UV light belongs with the alkane substitution, not with the alkene bromine-water test — a confusion seen repeatedly.
Give both products: haloalkane and HX
For methane + chlorine, candidates frequently give the wrong products. Mono-substitution replaces exactly one hydrogen, so the products are chloromethane () and hydrogen chloride () — not or . Dropping the by-product is the commonest slip and unbalances the equation.
Nov24 1C Q10a: the required condition is UV light and the equation products (a haloalkane + HX) are often given incorrectly.
Use 2n+2, not 2n, for alkanes
The alkane general formula is . Using instead of gives too few hydrogens and produces an alkene formula by mistake — e.g. four carbons give (butane), not . Always add the +2.
Build any alkane from CnH(2n+2)
For an alkane with carbons the hydrogens are . E.g. pentane has , so H , giving . Use this to write or check any alkane formula quickly.
Name or formula — read the command
Read whether the command asks for a name or a formula: write "chloromethane" for a name, for a formula. Giving one when the other is required scores nothing even when the chemistry is right.
Check the substitution equation
Writing an alkane substitution: put in the halogen and the UV condition, swap one hydrogen for a halogen atom, and make sure the hydrogen halide by-product (/) appears — then check the atoms balance on both sides.
The first five alkanes (methane to pentane) — you must be able to name these and give their formulae:
| Name | Molecular formula | (C = 12, H = 1) | |
|---|---|---|---|
| Methane | 1 | 16 | |
| Ethane | 2 | 30 | |
| Propane | 3 | 44 | |
| Butane | 4 | 58 | |
| Pentane | 5 | 72 |
Molecular vs structural vs displayed formula (using ethane, ):
What is the general formula for the alkanes?
Propane is an alkane with the molecular formula .
Calculate its relative formula mass (). (Relative atomic masses: C = 12, H = 1.)
