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Alkanes are hydrocarbon compounds containing the functional group – C = C –.
The simplest alkene is ethene with a molecular formula of C2H4. The next alkene is propene with a molecular formula of C3H6. From these two formulae we can see that for alkenes the number of hydrogen atoms is double the number of carbon atoms. The general formula for alkenes can therefore be represented as CnH2n where n is the number of carbon atoms.
Using this general formula, we can predict the molecular formula of any alkene. The general formulae and structural formulae for alkenes with up to four carbon atoms are shown in the table below:
Alkenes are classed as unsaturated hydrocarbons as they contain at least one double bond between two carbon atoms. This double bond is made by the carbon atoms sharing two pairs of electrons and is represented in the displayed formula using the two lines. The displayed formulae for the first four alkenes are shown below:
Chemical reactivity
The presence of the double bond in alkenes means that they are more reactive than alkanes. Alkenes will also react with halogens but, unlike alkanes, UV radiation is not needed for the reaction to occur. Alkenes react with bromine (Br2) to produce dibromoalkanes at room temperature.
For example, ethene reacts with bromine to produce dibromoethane. The double bond between the two carbon atoms breaks and the bond between the bromine atoms in the bromine molecule breaks. The two bromine atoms separate and the carbon atoms in the alkene are able to form single covalent bonds, with each bromine atom producing the new compound dibromoethane.
This is known as an addition reaction because the two bromine atoms are added into the original structure of the alkene to produce one single product.
Distinguishing between alkanes and alkenes
Alkanes and alkenes are both colourless liquids. As we know that alkenes will react with halogens at room temperature, but alkanes will not, we can use this reaction to distinguish between them. Bromine water (Br2(aq)) is an orange coloured solution containing bromine liquid dissolved in water.
To test whether a substance is an alkane or an alkene we would add a small amount of the substance in a test tube and add a small volume of bromine water. The test tube is then sealed with a bung and the contents gently shaken.
When bromine water is added to an alkane, no colour change is seen. The bromine cannot be added into the alkane structure so remains in the solution. The orange colour of the solution is due to the presence of bromine. Therefore, if bromine is still present, the orange colour of the solution will not change.
When bromine water is added to an alkene, the bromine is added into the structure. Therefore, the bromine is removed from the solution and the orange colour disappears. When bromine water is added to an alkene, a colour change from orange to colourless is observed.
