Hydrocarbons that have one double bond between carbon atoms in a chain are called alkenes, and their general formula is \mathrm{C}_n\mathrm{H}_{2n}. The simplest alkene is ethene, \mathrm{C_2H_4}, which is also called ethylene.
The 6 atoms of ethene all lie in one plane, and the angles between the bonds are about 120°. The two carbon atoms joined by the double bond cannot rotate about it.
Ethene is a colourless gas with a faint smell. It is made in large amounts from petroleum and is the starting material for many plastics. Plants also produce it, and it makes fruit ripen.
When ethene is bubbled into bromine water, the orange-brown colour of the bromine disappears.
\mathrm{CH_2{=}CH_2} + \mathrm{Br_2} \longrightarrow \mathrm{CH_2BrCH_2Br}
One of the two bonds of the double bond breaks, and a bromine atom joins each carbon atom, forming 1,2-dibromoethane. A reaction like this, in which atoms join the carbon atoms of a double or triple bond, is called an addition reaction. Alkanes do not react in this way, so bromine water is used to test for double bonds.
With a catalyst such as nickel or platinum, hydrogen adds to ethene to give ethane.
\mathrm{CH_2{=}CH_2} + \mathrm{H_2} \longrightarrow \mathrm{CH_3CH_3}
Water also adds to ethene at high temperature and pressure with a phosphoric acid catalyst, giving ethanol. Some of the ethanol used in industry is made in this way.
\mathrm{CH_2{=}CH_2} + \mathrm{H_2O} \longrightarrow \mathrm{CH_3CH_2OH}
When ethene is heated under pressure with a suitable catalyst or initiator, the double bonds open, and a great many molecules join one after another into a long chain. The product is polyethylene, which is also called poly(ethene).
A reaction in which small molecules join together into a large molecule is called polymerization, and polymerization by addition is called addition polymerization. The small molecules are called monomers, and the large molecule is called a polymer.
Polyethylene is used for plastic bags and containers. In the same way, propene gives polypropylene, and chloroethene (vinyl chloride) gives poly(vinyl chloride).
In the laboratory, ethene is made by heating a mixture of ethanol and concentrated sulfuric acid to about 160 to 170 °C. The concentrated sulfuric acid takes a molecule of water out of each ethanol molecule.
\mathrm{CH_3CH_2OH} \longrightarrow \mathrm{CH_2{=}CH_2} + \mathrm{H_2O}
Ethene hardly dissolves in water, so it is collected over water. At a lower temperature, about 130 to 140 °C, diethyl ether forms instead, so the thermometer is placed in the liquid to check its temperature.