Hydrocarbons are compounds that contain only carbon and hydrogen:
- Carbon atoms make four bonds each.
- Hydrogen atoms can make one bond each
To make hydrocarbon, hydrogen atoms ract with carbon atoms to form covalent bonds . when this happens, carbon atoms share a pair of electrons with hydrogen atoms to make a covalent bond.
when a hyddrocarbon chain has one or more double carbon-carbon (C=C) covalent bonds, it's called an Alkene. Double bonds have two shared pairs of electrons. The name of an alkene always ends in -ene.
Alkenes have at least one double covalent bond, so carbon atoom isn't bonded to the maximum number of hydrogen atoms. Alkenes are described as being un-saturated.
test for Alkenes
A simple test to distinguish between alkenes and Alkanes is to add bromine water:
- Alkenes decoulourise bromine water. (the unsaturrated alkene reacts with it)
- Alkanes have no effect on bromine water, i.e. the bromine water stays orange. (The saturated alkane cant react with it.)
This reaction is a test for unsaturation. It is an addition reaction between bromine water ans the C=C to make a colourless dibromo compound.
The alkenes made by cracking are small molecules which can be used as monomers. The double bonds in alkenes are easily broken, so monomers can be joined together to make polymers (large, long-chain molecules.) molecules in plastic are called polymers.
When the alkenes join together to make a polymer, the reaction is called polymerisation. This process needs high pressure and a catalyst. you could use displayed formulae to show polymerisation reaction for example, ethene monomers making poly(ethene).