The second bond in a C=C is a weak, exposed π bond. It makes alkenes far more reactive than alkanes, and the reactions it allows, addition and oxidation, are the starting point for polymers, alcohols and margarine.
Alkenes are unsaturated, CnH2n, with a C=C made of a σ and a π bond. The π electrons are a region of high electron density above and below the plane, so alkenes undergo addition: with H2 (Ni catalyst) to alkanes, with steam (H3PO4 catalyst) to alcohols, with HX to halogenoalkanes and with X2 to dihalogenoalkanes. Bromine water turns from orange to colourless. Cold, dilute acidified KMnO4 gives a diol; hot, concentrated acidified KMnO4 splits the C=C, and the products reveal where the double bond was.
Look at the π bond in the simulator, then try the three ways of putting a double bond into a molecule.
| Method | Conditions | Example |
|---|---|---|
| cracking | heat, zeolite catalyst | C10H22 → C8H18 + C2H4 |
| dehydration of an alcohol | heated Al2O3, or hot concentrated H2SO4 or H3PO4 | C2H5OH → C2H4 + H2O |
| elimination from a halogenoalkane | NaOH in ethanol, heat | C2H5Br + NaOH → C2H4 + NaBr + H2O |
Run each reaction in the simulator and note the product and the conditions.
| Reagent | Conditions | Product from ethene |
|---|---|---|
| H2 | Ni catalyst, about 150 °C | ethane (hydrogenation, used to make margarine) |
| H2O(g) | H3PO4 catalyst, 300 °C, 6 MPa | ethanol |
| HBr(g) | room temperature | bromoethane |
| Br2 | room temperature | 1,2-dibromoethane; bromine water decolourised |
| cold, dilute acidified KMnO4 | room temperature | ethane-1,2-diol; purple to colourless |
| hot, concentrated acidified KMnO4 | heat | C=C split (Fig. 1); ethene gives CO2 |
Deducing structure: but-2-ene with hot KMnO4 gives two ethanoic acid molecules; but-1-ene gives propanoic acid and CO2; 2-methylpropene gives propanone and CO2.
Cold, dilute KMnO4 adds two OH groups (a diol); hot, concentrated KMnO4 breaks the C=C. Do not mix them up. In the cleavage products, each carbon of the original C=C becomes a C=O carbon: =CH2 goes all the way to CO2.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is seen when ethene is bubbled through bromine water?
Which catalyst is used to add hydrogen to an alkene?
What is the product when ethene reacts with steam over a phosphoric acid catalyst?
Which reagent converts ethanol into ethene?
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
An alkene is oxidised by hot, concentrated acidified KMnO4 to give propanone and ethanoic acid only. What is the alkene?
What is the product of the reaction of propene with cold, dilute, acidified KMnO4?
Which reagent and conditions convert 2-bromopropane to propene?
Which compound gives only one organic product, ethanoic acid, with hot concentrated acidified KMnO4?
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