addition reactions

Reactions of alkenes

Unsaturated molecules such as alkenes really only undergo one type of reaction - addition reactions. You may recall from your gcse science that small molecules can "add" across the carbon carbon double bond (C=C) to form new saturated molecules. For example bromine and chlorine will add across the C=C bond in unsaturated molecules such as ethene to form a saturated halogenalkane, this is outlined in the image below:

3d models to show the addition of bromine and chlorine to ethene.  the word and symbolic equations are also shown.

When fluorine (F2) and iodine (I2) react with alkenes the reactions are not particularly useful. Fluorine generally undergoes explosive reactions with alkenes while iodine will undergo electrophilic addition to an alkene but the 1,2-diiodo product is generally unstable and decomposes to reform the reactants; the alkene and iodine.

Mechanism of addition reactions

The addition of small molecules to a carbon carbon double bonds (C=C) proceeds by electrophilic addition. An electrophile is an electron deficient species, that is to say it is looking to gain electrons and so will be attracted to areas in another molecule where there is high electron density, such as the pi(π) covalent bond in an alkene. You may recall that a carbon carbon double bond contains one sigma and one pi bond. The pi bond is formed by the sideways or partial overlap of p-orbitals on the two atoms involved in the covalent bond. A pi bond consists of two molecular orbitals or lobes of electron density above and below the two nuclei of the carbon atoms forming the covalent bond, this is shown below. These two molecular orbitals contain two electrons and will be a large target for any electrophile.

Sigma and pi bonding in alkenes

Explanation of how pi bonds are formed in alkenes by the partial overlap of p-orbitals to form new molecular orbitals with lobes of electron density above and below the plane of the carbon atoms.

The mechanism of addition reactions

The diagram below gives a step by step account of how a bromine molecule adds to the carbon carbon double bond (C=C). The reaction is very easy to carry out. The alkene can simply be bubbled through liquid bromine or a solution of bromine in an organic solvent.
One thing to be aware of is the position of the curly arrows. The curly arrows are used to show the movement of a pair of electrons and they give an exact indication of where any new bonds will be formed. Since a covalent bond requires two electrons and curly arrow show the movement of a pair of electrons the connection between the two should be obvious. I would also caution you to take care with where you start and end your curly arrows, as any examiner will swiftly deduct marks if you get it wrong. I would ensure that:

detailed notes on the mechanism of electrophilic addition using the addition of bromine to ethene as an example.

Testing for Unsaturation

Details of the bromine test for unsaturation using cyclohexane and cyclohexene as examples.

In fact the addition of bromine across a carbon carbon double bond (C=C) is used as a test for unsaturation. Here 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 slowly. In the image opposite the left hand test tube contains cyclohexane (C6H12); a saturated hydrocarbon while the right-hand test tube contains cyclohexene (C6H10); an unsaturated hydrocarbon.

If a small amount of bromine water is added to each test tube and then both test tubes are given a quick shake to mix the contents together and allow any reactions to take place. The results are shown in the image. The bromine water is instantly decolourised in the test tube containing the unsaturated cyclohexene while the bromine water stays orange brown in the test tube containing the saturated cyclohexane. It is worth mentioning that the orange colour of the bromine will switch layers in the test tube containing the saturated cyclohexane. Halogens such as bromine are more soluble in organic solvents than in water, so when the contents of the test tube are shaken up, although no chemical reaction will occur the bromine will move into the top less dense organic layer of cyclohexane while the aqueous layer which initially contained the bromine dissolved in it will turn clear as the bromine dissolves in the cyclohexane layer, this is shown in the left hand test tube in the image.

Reaction of bromine and ethene

The main product of the reaction of bromine water with the alkene ethene is a colourless solution of 2-bromoethanol. You may have expected the product to be 1,2-dibromoethane as was seen above when ethene was bubbled through bromine or a solution of bromine in an organic solvent. However if ethene is bubbled through an aqueous solution then the hydroxide ions (OH-) present in water take part in the reaction to form the substituted alcohol 2-bromoethanol. However 1,2-dibromoethane is also present in the solution but as a minor product. Equations for the reactions taking place are shown below: Word and symbolic equations for the reaction of bromine water with ethene.

Key Points

Practice questions

Check your understanding - Questions on addition reactions