Alkenes carry a C=C double bond, and that one feature makes them far more reactive than alkanes. Small molecules add straight across the double bond, which gives a simple test for unsaturation and routes to margarine, ethanol and plastics.
Alkenes are unsaturated hydrocarbons with the general formula CnH2n and a C=C double bond as the functional group. To tell an alkene from an alkane, shake it with aqueous bromine: an alkene turns it from orange to colourless, an alkane leaves it orange. (Extended) Alkenes undergo addition reactions, in which the double bond opens and two atoms or groups add on, giving a single product: with bromine (ethene → 1,2-dibromoethane), with hydrogen using a nickel catalyst (ethene → ethane), and with steam using an acid catalyst (ethene → ethanol).
Add bromine to ethene in the simulator and watch the double bond open. You need the reactants, products and colour change at IGCSE, not the step-by-step mechanism shown in the lab.
| Alkane (e.g. hexane) | Alkene (e.g. hexene) | |
|---|---|---|
| bonds between carbons | single only (saturated) | contains C=C (unsaturated) |
| general formula | CnH2n+2 | CnH2n |
| shake with aqueous bromine | stays orange | orange to colourless |
Examples: ethene CH2=CH2 (C2H4), propene CH3CH=CH2 (C3H6), but-1-ene CH2=CHCH2CH3 (C4H8). Extended. The bromine adds across the double bond: CH2=CH2 + Br2 → CH2BrCH2Br (1,2-dibromoethane), which is colourless.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the functional group of the alkenes?
What is seen when ethene is bubbled through aqueous bromine?
Which compound is an alkene?
Why are alkenes described as unsaturated?
Extended. Try each reagent on ethene in the simulator. Every time, the C=C becomes C−C and the two new atoms or groups end up on the two carbons that were double bonded.
| Reagent | Conditions | Equation (ethene) | Product |
|---|---|---|---|
| bromine (or aqueous bromine) | room temperature | C2H4 + Br2 → C2H4Br2 | 1,2-dibromoethane |
| hydrogen | nickel catalyst, heated (about 150 °C) | C2H4 + H2 → C2H6 | ethane |
| steam | acid catalyst (phosphoric acid, H3PO4), 300 °C, 6000 kPa | C2H4 + H2O → C2H5OH | ethanol |
With propene, the same reagents give 1,2-dibromopropane CH3CHBrCH2Br, propane CH3CH2CH3, and propanol C3H7OH. Adding hydrogen to unsaturated vegetable oils (hydrogenation) is used to make margarine.
An addition reaction gives one product only: there is no HBr or H2O left over, unlike substitution. Learn the catalysts as pairs: hydrogen with nickel, steam with an acid (phosphoric acid). And the bromine test result is orange to colourless, never "clear": clear means transparent, and orange bromine water is already clear.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
(Extended) Which catalyst is used when ethene reacts with hydrogen?
(Extended) Which conditions are used to make ethanol from ethene and steam?
(Extended) What is the product when propene reacts with bromine?
A hydrocarbon X has formula C4H10. What happens when X is shaken with aqueous bromine?
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