alkanes

Higher and foundation tiers

The alkanes

Image of an oil rig drilling for oil in the ocean.

Organic chemistry is probably one of the largest areas of study in chemistry. It is concerned with compounds that contain the element carbon. Hydrocarbons are organic compounds made up of molecules containing only the elements hydrogen and carbon. Hydrocarbons are found in crude oil. Crude oil is a thick, tacky "liquid" which consists of a mixture of many thousands of different hydrocarbon molecules. Crude oil is often described as the "life blood of industrial nations" because of the many valuable substances it contains. Crude oil is extracted from under the oceans and on land in many countries spread over the globe including: Venezuela, Saudi Arabia, Iran, Iraq, UAE Russia, China, Brazil, Canada as well as many others.

Crude oil is a fossil fuel formed from the remains of microscopic sea creatures called plankton. When these tiny microscopic plankton died they were buried in the sediments on the ocean floor, being buried this prevented the natural bacteria of decay to decompose these dead organisms due to the lack of oxygen present. Over millions of years the layers of sediments on top of the dead plankton built upon, this caused an increase in the temperature and pressure which effectively "slowed cooked" the remains of the plankton and formed crude oil and natural gas. Since crude oil takes millions of years to form and we are using it up faster than it can be replaced it is a finite resource and it is highly likely to run out in your lifetime.

Most of the compounds found in crude oil are hydrocarbons. There are thousands of different hydrocarbons molecules and to make it easier to name and identify them all they are placed in groups or families. The simplest family is called the alkanes. The alkanes are hydrocarbons which have single covalent bonds between the carbon and hydrogen atoms. The first 4 members of the alkane family or homologous series are methane, ethane, propane and butane. They are shown in the image below.

The alkanes

The first 4 members of the alkane homologous series are shown below: 3d models of the first four alkanes along with their displayed formula.

The alkanes shown in the image all have a backbone of carbon atoms which is surrounded by a sea of hydrogen atoms. Each atom of carbon as you would expect makes 4 covalent bonds and each atom of hydrogen atom makes one covalent bond. The alkanes are described as saturated since they have only single covalent bonds between the atoms of carbon. The alkane molecules are shown in 3d in the image above but often they are drawn out as flat molecules; generally we are not that concerned with the actual shapes of the hydrocarbon molecules. The first 5 alkanes are shown below as flat molecules.

Model of the first 6 alkane molecules.

If you study the formula for each of the alkane molecules above you will notice that in going from one alkane to the next we simply add -CH2 each time.

3d models to show the first four alkanes along with their molecular formula.

The alkanes form a homologous series; that is a series of compounds which all have the same general formula and which show trends or patterns in their chemical and physical properties. You can work out the formula for any alkane molecule using the general formula CnH2n+2. As an example decane is an alkane which has 10 carbon atoms; so what is its chemical formula? If you substitute 10 into the general formula for n, then to get the number of hydrogen atoms present in the molecule we simply multiply by 2 and then add 2: (2x10)+2=22. So the formula for the alkane molecule decane is C10H22.

Naming alkane molecules

The names for all the members of the alkane homologous series or family end in the three letters -ane. The first part of their name helps you to identify the number of carbon atoms present in the molecule. The table below gives the first part of the name of an alkane (the prefix) and the number of carbon atoms that will be present in this molecule as well as the molecular formula of the first ten alkanes.
alkane molecular formula
methane CH4
ethane C2H6
propane C3H8
butane C4H10
pentane C5H12
hexane C6H14
heptane C7H16
octane C8H16
nonane C9H20
decane C10H22

Key Points

Practice questions

Check your understanding - Questions on the alkanes and crude oil.

Check your understanding - Quick Quiz on the alkanes and crude oil

Check your understanding - Additional questions on the alkanes and crude oil.

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