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Hydrocarbons are compounds made from carbon and hydrogen atoms joined by covalent bonds. Alkanes are saturated - they have only single bonds. Alkenes have a double bond - they are unsaturated. Alkenes react with brown bromine water and decolourise it, but alkanes do not.
Alkenes can act as monomers. Under high pressure and in the presence of a catalyst many monomer molecules join together to make polymer molecules. These polymer molecules are saturated.
Hydrocarbons and alkanes
Hydrocarbons
Most of the compounds in crude oil are hydrocarbons. This means they only contain hydrogen and carbon atoms, joined together by covalent bonds. Remember that a covalent bond is a shared pair of electrons.
Alkanes
The number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, the molecular formula of methane is CH4. For ethane, it is C2H6.
Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H), and the covalent bonds between them by a straight line.
Structure of alkanes
| alkane | formula | chemical structure | ball-and-stick model |
|---|---|---|---|
| methane | CH4 |  |  |
| ethane | C2H6 |  |  |
| propane | C3H8 |  |  |
| butane | C4H10 |  |  |
Notice that the molecular models on the right show that the bonds are not really at 90º.
Alkanes are saturated hydrocarbons. This means their carbon atoms are joined to each other by single covalent bonds.
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