The homologous series of alkanes

The homologous series of alkanes is a family of saturated hydrocarbons with general formula CnH2n + 2.

Hydrocarbons are compounds containing carbon and hydrogen only. The main source of hydrocarbons is crude oil which is a mixture of fuels such as petrol, diesel, kerosene and natural gas. These fuels are separated from crude oil by fractional distillation since they have different boiling points.

Besides their use as fuels, hydrocarbons are also used as raw materials to manufacture many new compounds, such as plastics.

Crude oil as a source of alkanes and other hydrocarbons

Crude oil is the world’s main source of organic chemicals such as alkanes. It is called a fossil fuel because it was formed millions of years ago by the breakdown of plant and animal remains at the high pressures and temperatures deep below the Earth’s surface. Due to the fact that crude oil took millions of years to be formed, it is definitely a nonrenewable resource.

Crude oil is a complex mixture of hydrocarbons, mostly alkanes, cycloalkanes and aromatic compounds. Since crude oils is a mixture, its composition is not definite. Instead, crude oils from different sources have different compositions, typical containing variable proportions of

  • petroleum gases
  • petrol
  • naphtha
  • kerosene
  • gas oil
  • fuel oil
  • wax

In addition to the above-mentioned components, crude oil contains small amounts of other compounds dissolved in it. These other compounds come from other elements, such as sulphur, in the original plants and animals the oil was formed.

Fractional distillation of crude oil

Crude oil is a mixture of compounds of different boiling points and can therefore be separated by fractional distillation. This is done by heating it and condensing the fractions that boil over different ranges of temperatures. Each fraction is a mixture of hydrocarbons of similar chain length and therefore of similar properties.

The process proceeds as follows:

  • crude oil is first heated in a furnace.
  • the crude oil vapour passes into a fractionating tower that is cooler at the top than at the bottom.
  • crude oil vapour pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool enough to condense a certain fraction. The fraction whose boiling point matches the temperature of the tray condenses to liquid and is piped off.
  • shorter chain hydrocarbons condense in the trays nearer to the top of the tower, where it is cooler, because they have lower boiling points.
  • tar and bitumen collects as a thick residue at the base of the tower.

Fractional distillation of crude oil

Fractional distillation is a physical process, and therefore no covalent bonds within the hydrocarbon molecules are broken. Only the van der Waals forces between the molecules that are broken and reformed during the process.


The general formula of alkanes

Alkanes have a general formula CnH2n + 2, in which n is the number of carbon atoms in the molecule. For example, an alkane with 4 carbon atoms has the molecular C4H2(4) + 2 which is C4H10.

The table below shows a list of the first 10 alkanes and their molecular formula.

Number of carbon atomsPrefixName of alkaneFormula
1meth-methaneCH4
2eth-ethaneC2H6
3prop-propaneC3H8
4but-butaneC4H10
5pent-pentaneC5H12
6hex-hexaneC6H14
7hept-heptaneC7H16
8oct-octaneC8H18
9non-nonaneC9H20
10dec-decaneC10H22

All the carbon-carbon bonds in alkanes are single covalent bonds. The carbon atoms in alkanes have the maximum number of hydrogen atoms in their molecules and therefore therefore alkanes are classified as saturated hydrocarbons.

Formula of alkanes

Besides molecular formula there are many other ways of representing organic molecules, such as:

  • displayed formula
  • structural formula
  • skeletal formula

Example – Structural formula of butane

CH3CH2CH2CH3

Example – Displayed formula of butane

The homologous series of alkanes

Example – Skeletal formula of butane

Skeletal formula of butane

Physical properties of alkanes

Polarity

Alkanes are non-polar because the electronegativities of carbon and hydrogen are almost so similar and as a result, the only intermolecular forces between alkane molecules are weak van der Waals forces.


Boiling points

Boiling points of alkanes increase as the length of the carbon chain increase due to the increasing intermolecular forces.

  • Short chain alkanes, such as methane and ethane are gases at room temperature.
  • Pentane which has five carbon atoms, is a liquid with a low boiling point (about 36°C).
  • At a chain length of about 18 carbons, the alkanes become waxy solids at room temperature.
  • Alkanes with branched chains have lower melting points than straight chain alkanes for the same number of carbon atoms. This is due to the fact that their molecules cannot pack together as closely as unbranched chains to make the van der Waals forces that effective.

The table below shows the boiling and melting points of selected alkanes

alkanemolecular formulamelting point /°Cboiling point/°C
ethaneC2H6–183–89
butaneC4H10–1380
hexaneC6H14–9569
decaneC10H22–30174
dodecaneC12H26–10216

Solubility

Alkanes are insoluble in water because they are non-polar and also because the hydrogen bonds which hold the water molecules are much stronger than the van der Waal’s forces that act between alkane molecules. However, alkanes do mix with other non-polar liquids.