alkenes reactions

Chemistry only

Alkenes contain a double carbon carbon covalent bond (C=C); this is the reactive functional group present in all alkene molecules. As soon as you see this functional group in any molecule as a chemist you should think of only one thing- addition reactions! This carbon carbon double covalent bond (C=C) in an alkene molecule is vulnerable to attack by other substances which simply add across it.

Hydrogen gas for example can add to the carbon atoms in a carbon carbon double bond (C=C) present in an alkene molecule, the product of the reaction is a saturated alkane molecule. A catalyst and a temperature of around 3000C are required to carry out this hydrogenation reaction.

model, word and symbolic equations to show the hydrogenation of ethene to form ethane.

Hydrogenation of vegetable oil

Margarine is often made by hardening vegetable oils. One of the uses of the above hydrogenation reaction is in the manufacture of margarine. Margarine can be made from vegetable oils such as olive oil, however spreading vegetable oil on your toast first thing in the morning is perhaps not a good idea unless you like soggy toast! Vegetable oils are often referred to as polyunsaturated molecules, this simply means that the long chain molecules found in a typical vegetable oil contain many carbon carbon double bonds (C=C).

The presence of these double bonds lowers the melting point of the compound which is why many vegetable oils are liquids at room temperature. However by removing some or all of the sites of unsaturation in the vegetable oil its melting point can be raised. This means that the oil will now be a solid. If all the carbon carbon double bonds in the vegetable oil molecule are removed by hydrogenation then the product will likely be a hard solid fat. However by hydrogenating only a specific number of the carbon carbon double bonds (C=C) present it is possible to determine just how hard and spreadable the margarine formed will be.

Halogenation of unsaturated molecules

Addition of bromine and iodine to an alkene

Other small molecules such as bromine and iodine will also add across the carbon carbon double covalent bond (C=C) to form saturated molecules. The diagram below show the products of the addition reaction of bromine to the unsaturated alkene ethene.

model, word and symbolic equations to show the addition of bromine to an unsaturated alkene molcule, ethene in this example.

A similar addition reaction will occur if the bromine is swapped for iodine (I2) and the product of the reaction this time will be 1,2-diiodoethane. Note the numbers 1,2- in the names of the compounds mentioned above are simply used to indicate which carbon atoms the bromine or iodine atoms have added to.

Making alcohols- direct hydration

It is also possible to add steam across the C=C bond in alkenes to make alcohols. This method of making alcohols is called direct hydration. It requires a fairly high temperature of around 3000C and a pressure of about 65 atmospheres as well as a phosphoric acid catalyst. The reaction is similar to the examples above in that a small molecule; in this case steam adds across the C=C bond, this is shown below:

Direct hydration of alkenes to make alcohol.  Model, word and symbolic equations for the addition of steam or steam to the alkene ethene.

From the equation in the image above you can see that this hydration reaction is a reversible reaction. This means that the products of the reaction will be a mixture of ethanol, steam and unreacted ethene, in fact very little of the ethene and steam react when they first enter the reaction chamber. This mixture of substances formed from the hydration reaction then leaves the reaction vessel and enters a condenser, this is outlined below:

In the condenser the steam and ethanol condense and turn back into liquids while the unreacted ethene is recycled back through the reactor to react with more steam, this way the yield of the reaction can be greatly increased. The ethanol and water formed during the reaction mix freely to form a solution which leaves the condenser. The ethanol produced by hydration can be separated from this solution by fractional distillation.

Testing for Unsaturation

Cartoon image of students working in a chemistry lab

In fact the addition of bromine across a carbon carbon double covalent bond (C=C) is used as a test for the presence of unsaturation in a molecule. A few millilitres (ml) of liquid bromine is dissolved in water to form a red-brown solution called bromine water. When bromine water is added to a suspected unsaturated substance in a boiling tube and shaken then if the substance is unsaturated the bromine water will decolourise almost immediately. If the substance in the boiling tube is saturated then the bromine water will decolourise very very slowly.

You may get to try out this test for unsaturation in the lab yourself. The two alkanes and alkenes that are most commonly used for this unsaturation testing are cyclohexane and cyclohexene. These are liquid alkanes and alkenes that are available in most schools. They are both clear liquids that like most organic substances do not mix with water but instead float on top of an aqueous (watery layer) since they are less dense than water. The procedure to test these two substances for the presence of a C=C double bond is as follows:

This is shown in the drawing below: Results of  testing cyclohexane and cyclohexane with bromine water, diagram and explanation of the results.
Organic substances such as cyclohexane and cyclohexene are like oil they do not mix with water. Instead as shown in the image above they float on top of the water or any aqueous solution. However halogen molecules such as chlorine, bromine and iodine are more soluble in organic solvents such as cyclohexane or cyclohexene than they are in water. So if bromine is given an option of dissolving in water or in an organic solvent it will dissolve in the organic solvent.


In the boiling tube containing the bromine water and cyclohexane in the image above the two layers are shown coloured red orange due to the presence of dissolved bromine in each layer. However if the boiling tube is left to stand eventually any dissolved bromine in the aqueous layer will move into the organic cyclohexane layer simply because it is more soluble in the organic cyclohexane layer. This is outlined in the image below:

How to test for unsaturation in molecules using bromine water.

Key Points

Practice questions

Check your understanding - Questions on addition reactions


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