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PRACTICALS
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1. Combustion
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Alkenes tend to burn with sooty, luminous (yellow) flames. This is because the presence of the double bond reduces the amount of hydrogen in the molecule. This means that it will contain a greater proportion of carbon compared to a saturated molecule. It is difficult to get enough oxygen to the flame to ensure complete combustion, so carbon appears in the flame which is seen as black specks. It also gives the flame its characteristic yellow colour. If a compound burns with a yellow flame it provides good evidence that the compound is unsaturated. |
Complete combustion would give carbon dioxide and water, although complete combustion clearly does not occur in this case:
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C6H10(l) + 8½O2(g) ⇒ 6CO2(g) + 5H2O(l) |
2. Oxidation
Alkenes rapidly decolourise acidified potassium manganate(VII), and this is often used as a test for an alkene. With insufficient acid, a brown precipitate may form rather than decolourisation taking place. The alkene is oxidized to a 1,2-diol. The equation for the reaction, including the acidified manganate(VII) ion, is complicated. We often use a simplified system in which [O] represents oxygen from the oxidizing agent:
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3. Action of bromine
This is the standard test for unsaturation (presence of double/triple bonds). Bromine is rapidly decolourised by alkenes, producing a 1,2-dibromo compound (in this case 1,2-dibromocyclohexane):
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With bromine water the bromine and water reach equilibrium as follows:
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Br2(l) + H2O(l) ⇒ HOBr(aq) + HBr(aq) |
This leads to two alternative products - bromocyclohexane and:
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You are expected to give this last type of equation in the exam for the reaction between an alkene and bromine water.
4. Polymerization
You will have seen that the original liquid monomer quickly became more viscous, becoming a solid in about twenty minutes. The equation for Perspex polymer formation is:
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You should know about the polymerization of ethene to produce poly(ethene).
