Alkanes, which are the major constituents of natural gas, petroleum and coal, are much more abundant in nature than alkenes. However the lack of reactivity of alkanes limits their use as starting materials for the synthesis of other types of organic substances. Alkenes, on the other hand, are much more reactive than alkanes due to the presence of the C=C double bond. The reactivity of alkenes means they are much more useful as building blocks for the petrochemical industry. Ethene in particular is an extremely versatile starting material for the production of many different organic compounds. A summary of the industrially important reactions of ethene is given.
Ethene, the first member of the alkene series, undergoes reactions typical of this group. The chemistry of ethene, as with all alkenes, is determined largely by its reactive double bond. The characteristic reaction of alkenes is the addition reaction. In addition reactions two new atoms or groups of atoms are ‘added’ across the double bond, one to each carbon atom linked by the double bond. This converts the carbon–carbon double bond to a single bond so that an unsaturated compound is converted to a saturated one. Ethene may undergo a large number of addition reactions to produce many useful products. Some of the more important reactions are outlined below.
Addition of hydrogen to ethene (hydrogenation):
Ethene is converted to ethane by heating it with hydrogen in the presence of a metal catalyst such as nickel, platinum or palladium.
Addition of halogens to ethene (halogenation):
When a halogen such as chlorine or bromine reacts with ethene, the double bond opens out and an addition reaction takes place. These halogenation reactions are useful tests for distinguishing between saturated hydrocarbons such as alkanes and unsaturated hydrocarbons including alkenes. A solution of bromine in a non-polar, organic solvent such as carbon tetrachloride has a distinctive red-brown colour. When a non-aqueous solution of bromine is added to an alkene, it loses its colour as bromine atoms add across the double bond in the alkene.
When a non-aqueous solution of bromine is added to a saturated hydrocarbon, no reaction takes place and the solution retains its red-brown colour.
An aqueous solution of bromine, known as bromine water, can also be used to dis¬tinguish between saturated and unsaturated hydrocarbons. Unsaturated hydrocarbons decolourise the yellow-brown solution whereas saturated hydrocarbons do not. The products of this reaction vary but may include a bromoalkanol and hydrogen bromide.
The addition of halogens to ethene produces some important products. For example 1,2-dibromoethane, which is produced by the reaction of ethene with bromine, is used with tetraethyllead as an additive in petrol to improve its performance. 1,2-dichloroethane, which is produced by the reaction of ethene with chlorine, is used to manufacture chloroethene which in turn is used to produce the plastic, poly(vinyl chloride), commonly known as PVC.
Addition of hydrogen halides to ethene (hydrohalogenation):
Hydrogen halides such as hydrogen chloride also react with alkenes.
Addition of water to ethene (hydration):
One of the most important industrial uses of ethene is the production of ethanol. Ethanol is prepared industrially from ethene by the addition of water in the presence of a sulfuric acid or phosphoric acid catalyst.
Oxidation of ethene:
The mild oxidation of ethene produces 1,2-ethanediol (ethylene glycol). This mild oxidation can be achieved by reacting ethene with cold, dilute potassium permanganate or alternatively with oxygen/water.
1,2-ethanediol is commonly used as 'antifreeze' in the cooling systems of cars as it has a lower freezing point and higher boiling point than water and does not cause corrosion.
It is also used in the manufacture of magnetic tapes and photographic film as well as for making synthetic fibres.
Other reactions of ethene
Ethene is also used to make many intermediate compounds, which are in turn used to make other products. For example, ethene reacts with benzene under appropriate conditions to form styrene which can then be used to make poly(styrene). Ethene is also used in the production of chloroethene (vinyl chloride) which can then be used to make polyvinyl chloride) or PVC.
The main use of ethene however, is to make the polymer poly(ethene) or polyethylene. In fact about 60% of the ethene produced in Australia is used in the manufacture of this important polymer. Substitution reactions of alkanes: Alkanes are less reactive and undergo substitution reactions. A substitution reaction of alkane with bromine water is very slow and only occurs in the presence of light.