Alkenes and Alkynes
Senior 3 · Chemistry
Alkenes and Alkynes
Lesson 3 of 5
35 min
Pour tomato juice into a tall tube and trickle bromine water on top. Within minutes the red juice turns into bands of colour. The red pigment of tomatoes, lycopene, is a hydrocarbon chain with many C=C double bonds, and bromine adds across them one by one; each step changes the colour. The same kind of reaction, much faster, is the standard laboratory test for an alkene.
Ethene is made by removing water from ethanol (dehydration). In a school laboratory, ethanol vapour is passed over heated aluminium oxide, which acts as a catalyst:
Heat the aluminium oxide strongly and warm the ethanol-soaked wool only gently, so that its vapour passes over the hot catalyst. Ethene is almost insoluble in water and is collected over water (Figure 3.1). Another method heats ethanol with excess concentrated sulfuric acid at about 170 °C. As with methane, lift the delivery tube out of the water before you stop heating, to prevent suck-back.
Alkenes are like the alkanes with the same number of carbon atoms: ethene, propene and the butenes are colourless gases, pent-1-ene and longer alkenes are liquids, and all are insoluble in water, less dense than water and non-conductors. Ethene has a faint sweet smell.
Alkenes burn in plenty of air to give carbon dioxide and water:
Their flames are yellower and smokier than those of alkanes, because a larger share of their mass is carbon: ethene is carbon, ethane only . Some carbon is left unburnt as glowing soot.
Definition
Addition reaction: a reaction in which two molecules join to form a single product; in an alkene, one bond of the C=C opens and an atom or group adds to each carbon. Example: ethene + bromine → 1,2-dibromoethane.
Test for unsaturation: orange-brown bromine water becomes colourless when shaken with an alkene, quickly and without light. Example: bromine water shaken with ethene is decolourised; with ethane it stays orange.
The C=C bond is the reactive part of an alkene. One of its two bonds breaks easily, and a new atom joins each carbon. The product is saturated.
No light is needed and no hydrogen bromide is formed. Compare an alkane: it reacts with bromine only in light, slowly, by substitution, and gives fumes of HBr. Chlorine adds in the same way to give 1,2-dichloroethane.
A nickel catalyst at about 150 °C is needed. The same reaction turns liquid vegetable oils, which contain C=C bonds, into solid fats for margarine.
The product is bromoethane. Hydrogen chloride gives chloroethane.
Industry makes ethanol this way, at about 300 °C and 60 to 70 atmospheres, with a phosphoric acid catalyst.
Shaken with an alkene, the purple solution turns colourless. The alkene is oxidised: ethene gives ethane-1,2-diol, . Alkanes leave the solution purple, so this is a second test for unsaturation.
Worked example
Problem. Write equations and name the organic products when propene, , reacts with (a) bromine; (b) hydrogen.
Solution. One atom adds to each carbon of the double bond, carbons 1 and 2.
(a) 1,2-dibromopropane.
(b) propane, with a nickel catalyst.
Worked example
Problem. What mass of bromine reacts with 0.70 g of ethene? (C = 12, H = 1, Br = 80)
Solution. , so mol. One mole of ethene adds one mole of : 0.025 mol. , so mass g.
Definition
Monomer: a small molecule that can join with many others like it to form a polymer. Example: ethene.
Polymer: a very large molecule made of many repeating units joined in a chain. Example: poly(ethene).
Addition polymerisation: the joining of many alkene molecules, with no other product, as their double bonds open. Example: n ethene → poly(ethene).
Under high pressure with a catalyst, ethene molecules add to each other. Each double bond opens and the molecules join into chains thousands of carbon atoms long:
Propene gives poly(propene) and chloroethene gives poly(chloroethene), PVC, in the same way. These plastics are light, strong and resist water and decay, which also makes them last for centuries as litter. Rwanda prohibits plastic carry bags and single-use plastic items by Law N° 17 of 2019.
Table 3.1: Uses of alkenes and their products
| Alkene | Made into | Used for |
|---|---|---|
| ethene | poly(ethene) | bags, bottles, water tanks, pipes and plastic sheeting |
| ethene | ethanol (by adding steam) | solvent, hand sanitiser, fuel |
| ethene | ethane-1,2-diol | antifreeze; making polyester fibres |
| ethene (itself) | — | ripening fruit in ripening rooms |
| propene | poly(propene) | ropes, crates, plastic chairs, food containers |
| chloroethene | poly(chloroethene), PVC | water pipes, cable insulation, floor covering |
| vegetable oils (contain C=C) | hydrogenated fats | margarine |
Warning
Safety and common mistakes. Bromine and its vapour are toxic and corrosive: use only dilute bromine water, a few drops at a time, with eye protection and good ventilation. Concentrated sulfuric acid causes severe burns. In answers: (1) bromine water is decolourised; it does not "turn clear", since a coloured solution can be clear. (2) An addition reaction gives one product; substitution gives two. (3) In a polymer, the repeat unit has single bonds only: the double bond has gone.
FuseSchool: Testing Alkenes With Bromine Water. Watch what bromine water is, why it loses its colour with an alkene, and why an alkane gives no quick change.
Practice
Sources and credits
