addition reactions

Reactions of alkenes

Unsaturated molecules such as alkenes really only undergo one 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 iodine will add across the C=C bond in unsaturated molecules such as ethene to form a saturated halogenalkane, this is shown below:

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

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 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:

Testing for Unsaturation

test for unsaturation

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. This is shown in the image opposite.

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:

Key Points

Practice questions

Check your understanding - Questions on addition reactions