Alkenes — unsaturated hydrocarbons, functional group C=C
Alkene = an unsaturated hydrocarbon. Hydrocarbon means only hydrogen and carbon; unsaturated means it holds a carbon-carbon double bond (C=C), so not the maximum number of hydrogens. The functional group — the part driving the family reactions — is that C=C bond. Alkenes form a homologous series with general formula . The series starts at ethene (): you need two carbons to make a C=C, so there is no methene.
Naming and formulae — ethene, propene, butene
Names end in -ene (alkanes end in -ane); the stem gives the carbon count (eth- 2, prop- 3, but- 4). Apply the general formula : ethene (), propene (), butene (). The displayed formula shows every atom and bond, C=C as two lines. Ethene is : each carbon carries two hydrogens plus the shared double bond — four bonds each.
Bromine addition and the bromine-water test
Addition: the C=C double bond opens and one bromine atom joins each carbon, with no by-product — (dibromoethane). No UV light is needed. This is the basis of the bromine-water test for unsaturation: an alkene decolourises bromine water (orange/yellow-brown to colourless) as bromine adds across the C=C; an alkane gives no change, as it has no C=C to react with.
Drawn from real examiner reports.
Unsaturated hydrocarbon needs both ideas
"Unsaturated hydrocarbon" is a two-part term and each part is a separate mark. State BOTH: hydrocarbon = a compound of only hydrogen and carbon, AND unsaturated = contains a carbon-carbon double bond (C=C). Candidates give one and forget the other — "it has a double bond" but not "only H and C", or the reverse. Always supply both elements of the definition.
Jun23 1C Q4cii
Alkene formula is CnH2n, not CnH2n+2
The alkene general formula is , NOT (that is the alkane). The C=C means an alkene has two fewer hydrogens than the alkane with the same carbons: ethene is , not (which is ethane). Quick check: for an alkene, the number of hydrogens is exactly double the number of carbons.
Jun23 1C Q4ai
Say "colourless", not "clear"
In the bromine-water test the alkene result must be described as "colourless", not "clear". A solution can be clear (transparent) yet still coloured, so "goes clear" can miss the mark. The correct word is decolourised: the bromine water changes from orange/yellow-brown to colourless as the bromine adds across the C=C double bond. Always write colourless explicitly.
Jun23 1C Q4ciii
State both the start and end colour
A colour change needs BOTH the starting and the finishing colour. For an alkene the mark-scheme answer is "bromine water changes from orange/yellow-brown to colourless". Writing only "it goes colourless" or "the colour changes" can drop the mark because the starting colour is missing. Always state the full change: orange/yellow-brown to colourless.
Nov24 1C Q10aiii
Addition (no UV) vs substitution
Keep the two reactions apart. Alkene + bromine water is addition: the bromine adds across the C=C, there is no by-product, and NO UV light is needed. Alkane + halogen is substitution: a hydrogen is swapped out, or is released, and UV light IS needed (that reaction is on C4.19). Attaching "UV light" to the alkene bromine-water test is a costly error.
Jun23 1C Q4ciii
Answer a "distinguish" question in full
To distinguish an alkene from an alkane, name the reagent (bromine water) and give the observation for BOTH substances: the alkene decolourises it (orange/yellow-brown to colourless); the alkane gives no change. Give a method and a result for each substance.
Build any alkene formula from CnH2n
For an alkene with carbons the hydrogens are . Butene: , so H , giving . Then find by adding every atom: . If you accidentally use you have built an alkane, not an alkene.
Write the addition equation and balance it
In the alkene + bromine addition, the C=C opens and a bromine atom joins each carbon, so add with no by-product: . Do not release an HBr by-product — that is substitution.
The first alkenes (ethene to butene) — you must be able to name these and give their formulae:
| Name | Molecular formula | (C = 12, H = 1) | |
|---|---|---|---|
| Ethene | 2 | 28 | |
| Propene | 3 | 42 | |
| Butene | 4 | 56 |
Molecular vs structural vs displayed formula (using ethene, ):
What is meant by an "unsaturated hydrocarbon"?
Ethene is an alkene with the molecular formula .
Calculate its relative formula mass (). (Relative atomic masses: C = 12, H = 1.)
