Topic 10.3: Alkenes

10.3.1 Describe, using equations, the reactions of alkenes with hydrogen and halogens

With hydrogen

Hydrogen reacts with alkenes to form alkanes in the presence of a nickel catalyst at about 150 degree celcius.

With halogens

Halogens react with alkenes to produce dihalogeno compounds. These reactions happen quickly at room temperature and are accompanied by the loss of colour of the reacting halogen.

10.3.2 Describe, using equations, the reactions of symmetrical alkenes with hydrogen halides and water

With hydrogen halides

Hydrogen halides react with alkenes to produce halogenoalkanes. These reactions take place rapidly in solution at room temperature

With water

The reaction with water is known as hydration and converts the alkene into an alcohol. It can be achieved by using concentrated sulphuric acid as a catalyst. The reaction involves an intermediate in which both hydrogen ion and hydrogen sulphide  ions are added across the double bond. This is quickly followed by a hydrolysis with a replacement of the hydrogen sulphide ion by a hydroxide ion.

10.3.3 Distinguish between alkanes and alkenes using bromine water

We can use the fact that alkenes readily undergo addition reactions, whereas alkanes will not (and will only undergo substituion reactions in UV light), as the basis of tests to distinguish between members of the two homologous series.

Alkanes have no reaction with bromine water, thus the solutions remain brown. Alkenes react with bromine water causing it to become colour because bromoalkanes are colourless.


10.3.4 Outline the polymerization of alkenes

Due to the fact that alkenes undergo addition reactions by breaking their double bonds, they can be joined together to produce long chain known as polymers. The alkene used in this reaction is known as the monomer and its chemical nature will determine the properties of the polymer.

Ethene polymerizes to form polyethene.

10.3.5 Outline the economic importance of the reactions of alkenes

Polychloroethene is also known as PVC and is widely used in all forms of construction materials, packaging, electrical cable sheathing and so on. It is one of the world's most important plastics. Its widespread use is, however, somewhat controversial as its synthesis is associated with some toxic-by products