Alkenes: C=C functional group, formula CnH2n
An alkene is a hydrocarbon whose functional group is the carbon–carbon double bond, C=C — the part that gives its characteristic reactions. General formula , so an alkene has two fewer hydrogens than the matching alkane. The first three are ethene , propene and butene ; there is no "methene", as a C=C needs at least two carbons. Names end in -ene, not -ane.
Alkenes are unsaturated (contain C=C)
An alkene is unsaturated because it contains a C=C double bond that can open to add atoms across it. An alkane is saturated: only single C-C bonds and the maximum hydrogens, so nothing can add. The mark-scheme definition of unsaturated is "contains a C=C double bond (so can undergo addition reactions)". Saying only "it can react more" or "it has a spare bond" scores nothing — you must name the C=C.
Alkenes react by ADDITION across the C=C
In an addition reaction the C=C opens and a small molecule adds across it, so two molecules become ONE product with nothing else formed. With bromine an alkene gives a colourless dibromoalkane: (1,2-dibromoethane). Alkenes also add hydrogen (nickel catalyst → alkane) and steam (acid catalyst → alcohol). Because every atom ends up in one product, addition can have 100% atom economy.
Drawn from real examiner reports.
Bromine-water colour change reversed
Most marks here are lost by giving the bromine-water colour change backwards. With an alkene it goes orange/yellow-brown to colourless (decolourised); with an alkane there is no change (stays orange). Do not write "colourless to orange", and do not confuse it with titration indicator colours. Give the colour before and after in the right order.
June 2024 Paper 2C Q5(b)(ii): a range of colours seen; some gave the colour change the wrong way round ("brown to colourless" reversed), possibly confusing it with titration indicators.
Defining "unsaturated" without the C=C
"Unsaturated" must be "contains a carbon–carbon double bond, C=C"; "saturated" is "only single C-C bonds / the maximum hydrogens". Vague answers — "it is more reactive", "it has spare bonds", "it is not full up" — score zero. Do not say an alkene "has a spare electron"; it has a C=C that opens in addition reactions. Always name the C=C explicitly.
June 2024 Paper 2C Q3 and Paper 1CR Q7: examiners stressed that vague statements about alkenes are not creditworthy — the C=C / addition behaviour must be stated explicitly.
Saying alkenes are used as fuels
Alkenes are used to make polymers (and other chemicals), not as fuels — the fuels are short-chain alkanes. "Alkenes are used as fuels" is not creditworthy. Cracking makes short-chain alkanes for fuel AND alkenes for polymers; asked for the products of cracking, state both, not just "smaller molecules".
June 2024 Paper 2C Q3 / Paper 1CR Q7: "alkenes are used as fuels" was explicitly not accepted — alkenes are used to make polymers.
Not showing every bond in a displayed formula
A displayed formula must show every bond as a line, including each C-H bond and the double line of the C=C. Candidates regularly drop a bond (an O-H or a C-H) and lose the mark. For ethene, all six bonds must appear: four C-H single bonds and one C=C double bond, each carbon showing four bonds in total.
June 2024 Paper 2C Q5(b)(iii) and Nov 2024 Paper 2C Q7(a)(ii): marks lost for not showing all the bonds (including the O–H) in a displayed formula.
Writing "clear" instead of "colourless"
"Decolourised" means the bromine water goes colourless, not "clear" — colourless and clear are not the same (a coloured solution can still be see-through). When asked for a colour change, give both the start colour (orange) and the final colour (colourless); the end colour alone loses a mark. Use the exact term "bromine water".
(common Edexcel chemistry error)
Confusing addition with substitution
An alkene reacts by addition: the C=C opens, two molecules give ONE product, no second product. An alkane reacts by substitution in UV light, swapping one H for a Br and always making a second product, HBr: . UV light goes with alkane substitution, not the bromine-water test.
(common Edexcel chemistry error)
A routine for addition-reaction equations
For any alkene addition: (1) find the C=C; (2) the reagent (Br2, H2 or H2O) splits, one part to each carbon; (3) two molecules give ONE product, so check it balances with no spare atoms, e.g. .
Give the test result for both substances
A chemical-test answer needs a method and a result, here for both substances: add bromine water — the alkene decolourises it (orange → colourless), the alkane shows no change. The alkene alone, or "it changes colour" without the colours, loses marks.
Explain why only the alkene reacts
To explain "why does the alkene decolourise bromine water but the alkane does not", chain it: the alkene has a C=C → it opens and bromine adds across it → the orange bromine is used up → the water decolourises; the alkane has no C=C, so it stays orange.
Alkenes are a homologous series of hydrocarbons whose functional group is the carbon–carbon double bond, . They are unsaturated, which makes their chemistry (addition reactions) very different from the saturated alkanes.
| Term | Mark-scheme definition (two elements) |
|---|---|
| Alkene | A hydrocarbon with a carbon–carbon double bond (); general formula |
| Functional group | The atom/bond in a molecule that gives the molecule its characteristic reactions (for alkenes, the ) |
| Unsaturated | Contains a carbon–carbon double bond () and so can undergo addition reactions |
| Saturated | Contains only single bonds and the maximum number of hydrogen atoms |
| Addition reaction | A reaction in which the opens and a small molecule adds across it to give a single product |
The general formula gives:
| Alkene | Formula | |
|---|---|---|
| Ethene | 28 | |
| Propene | 42 | |
| Butene | 56 |
There is no "methene" — a double bond needs at least two carbon atoms. ( = relative atomic mass; = relative formula mass.)
Define an alkene and give its general formula.
Ethene has the formula . Calculate its relative formula mass, . (: H = 1, C = 12.)
