10.3 - Alkenes
10.3.1: Describe, using equations, the reactions of alkenes with hydrogen and halogens.
Alkenes have a double bond which acts as a focus for attack by species that are attracted by electrons (electrophiles). This allows alkenes to react with both hydrogen and halogens.
Reaction with hydrogen
Hydrogen gas will react with alkenes in the presence of a nickel catalyst at 150ºC.
This reaction is used in industry to convert vegetable oils into margarine. The vegetable oils are poly-unsaturated, i.e. they have more than one double bond. Hydrogenation converts these double bonds (or some of them) to saturated single bonds.
This increases the melting point of the vegetable oil until it becomes a waxy solid (margarine).
Reaction with halogens
Halogens react with alkenes at room temperature.
This reaction is utilised to test for the presence of double bonds (unsaturation). Bromine changes colour from the red/brown of the element to a colourless brominated halgenoalkane. The bromine can be in the form of a solution in water, bromine water, making it easier to handle.
Iodine does react with double bonds, but only slowly and with difficulty. However, iodine can be reacted using Wijs reagent, a compound of chlorine triiodide.
10.3.2: Describe, using equations, the reactions of symmetrical alkenes with hydrogen halides and water
Reaction with hydrogen halides
Hydrogen halides react with alkenes at room temperature.
The product is a halogenoalkane.
Reaction with water
Water can react with alkenes at elevated temperatures in the presence of a phosphoric acid catalyst.
The product is an alcohol.
10.3.3: Distinguish between alkanes and alkenes using bromine water.
Bromine water (a solution of bromine in water) has a red/orange colour. This colour gets discharged by reaction with alkenes, while alkanes do not react with bromine in the dark.
This can be used as a test for double bonds.
10.3.4: Outline the polymerisation of alkenes
Alkenes can be made to add together in the presence of a catalyst to make molecules consisting of many of the original molecules joined in a string. This is called polymerisation.
poly = many
mer = part
These polymers are effectively giant alkane molecules, bonded to one another by strong van der Waals forces.
This is called addition polymerisation
10.3.5: Outline the economic importance of the reactions of alkenes
Hydrogenation of vegetable oils in the manufacture of margarine
Hydration of ethene in ethanol manufacture
Polymerisation in the manufacture of polymers and plastics