Alkanes

Alkane definition

First of all, let's look at the basic definition of an alkane.

What do those terms actually mean?

• A hydrocarbon is an organic molecule that contains only hydrogen and carbon atoms.
• Saturated molecules contain only carbon-carbon (C-C) and carbon-hydrogen (C-H) single bonds.

In contrast, unsaturated hydrocarbons contain at least one carbon=carbon (C=C) double bond. Unsaturated hydrocarbons are known as alkenes, and we’ll take a quick look at them later.

Alkane functional group

You should know from "Organic Compounds" and "Functional Groups" that organic molecules have particular functional groups. These are atoms, or groups of atoms, that make them react in a certain way. The only functional group in an alkane is the C-C single bond. However, this bond is found in almost every organic compound, and so some scientists don't consider it to be a functional group. Instead, they say that alkanes are organic molecules without a functional group.

General formula of alkanes

Alkanes form a homologous series with the general formula C_nH_2n+2. Remember that a homologous series is a group of molecules that share the same chemical characteristics and general formula. In fact, they only differ in their chain length and arrangement. For example, ethane (C₂H₆ ) and propane (C₃H₈ ) are two of the simplest alkanes. Their structures are shown below. You can see that propane is very similar to ethane - it simply contains an extra -CH₂- group between the two end carbons.

Ethane, left, and propane, right. StudySmarter Originals

Alkane nomenclature

Alkanes are probably the simplest type of organic molecule to name. They follow all the basic nomenclature laws, including those involving root names and side chains (see Organic Compounds for a quick recap). Their functional group is indicated by the suffix -ane. The following alkane is a good example - see if you can have a go at naming it.

Alkane isomerism

Look at the alkane C₄H₁₀. This could represent multiple different molecules. For example, it could be either butane or 2-methylpropane:

Butane, left, and 2-methylpropane, right. StudySmarter Originals

Count the carbons and hydrogens to be sure. Both molecules have 4 carbon and 10 hydrogen atoms. These molecules are known as isomers.

Alkanes can show a type of structural isomerism called chain isomerism, as explored below.

Chain isomerism

For example, pentane and 2-methylbutane both have the same number of carbon and hydrogen atoms, but whilst pentane has a single long chain that is 5 carbons in length, 2-methylbutane has a 4-carbon chain with a methyl group side chain. Therefore, these molecules are chain isomers.

Pentane, left, and 2-methylbutane, right. StudySmarter Originals

Molecular Shape of Alkanes

Alkanes are based on a tetrahedral shape. We've used methane as an example. It looks a little something like this:

The tetrahedral shape of methane. StudySmarter Originals

The molecule is a triangular pyramid, with a hydrogen atom at each corner of the pyramid, and a carbon atom in the centre. The angle between each of the bonds is 109.5^o.

The shape of alkanes is all thanks to VSEPR theory (valence shell electron pair repulsion). VESPR theory tells us that all electron pairs repel each other, and the strength of the repulsion depends on the type of electron pair - for example, whether it is a lone pair or a bonded pair. All of the electron pairs around methane's central carbon atom are part of 4 identical single covalent bonds, and this means that they repel each other equally. Due to this repulsion, the 4 bonded electron pairs align themselves in a tetrahedral geometry, as this geometry keeps all of the bonds farthest away from each other.

How are alkanes made?

We'll now consider the sources of alkanes:

• We get many alkanes from crude oil.
• We turn some of these into shorter-chain alkanes through cracking.
• We can also synthesise alkanes by hydrogenating alkenes.

Crude oil

Alkanes are formed from dead plant and animal matter that has been squashed under high temperatures and pressures over a long, long period of time. Cast your mind back 400 million years or so, to a world completely different to Earth as we know it. The first vertebrates were only just starting to emerge on land, giant mushrooms eight metres tall were a common sight, and oceans covered the vast majority of the planet. When creatures living in these oceans died, their remains fell to the ocean floor and were buried in layers of silt and sand. Over millions of years, the layers built up higher and higher, creating a high-pressure, high-temperature anaerobic environment. This allowed the dead organisms’ remains to slowly start turning into a substance called crude oil. The process is known as carbonation.

Crude oil formation. StudySmarter Originals

Cracking

The alkanes found in crude oil are usually long-chain hydrocarbons, with chains made up of about 8 to 36 carbon atoms. These long-chain molecules aren't very useful. Instead, we break them down into smaller, more useful alkanes in a process called cracking. Large alkane molecules are heated up to around 500°C in the presence of an aluminium oxide (Al₂O₃) or silicon dioxide (SiO₂) catalyst. This breaks some of the covalent bonds within the chain, splitting it into smaller hydrocarbons.

Alkene hydrogenation

Another way of synthesising alkanes is by hydrogenating alkenes. This involves heating the alkene with hydrogen in the presence of a nickel catalyst.

Properties of alkanes

Alkanes are saturated hydrocarbons, consisting of C-C and C-H bonds only. These bonds are relatively strong, and because carbon and hydrogen have similar electronegativities, the bonds are also non-polar (see Polarity for further information). This means that the only forces between alkane molecules are van der Waal forces, which are also known as temporary or induced dipole forces.

Electrons in a molecule are constantly moving randomly, and at any one point could be anywhere in the molecule. Some might be clustered together, and some might be further apart. This creates a small dipole that is constantly changing in location and strength. Dipoles in one molecule then attract or repel neighbouring molecules, inducing dipoles in them as well, and this attraction holds the molecules together.

However, the attraction is relatively weak, giving alkanes the following properties:

Solubility

Alkanes are insoluble in water. This is because their non-polar C-C and C-H bonds cannot easily bind to polar water molecules. However, alkanes are soluble in other non-polar solvents and are good solvents themselves.

Combustibility

Alkanes are readily combustible and have high negative enthalpies of combustion, which is why we commonly use them as fuels such as petrol. They burn in excess oxygen to produce carbon dioxide and water.

Volatility

If you ever fill up your car at a petrol station, you’ll notice the stark warning signs: no lighters, no cigarettes. This is because short-chain alkanes are highly volatile and the surrounding air is likely to be saturated with their vapours. Any small spark could cause a devastating explosion. Their volatility decreases as they increase in length.

Reactivity

Alkanes are generally unreactive due to the strength of their non-polar C-H and C-C bonds. These bonds require a lot of energy to overcome, and most reactions simply can’t provide that. However, they can react with chlorine or bromine in UV light; this reaction is further explored in Chlorination. They can also be cracked to produce alkenes, and we’ll look at this in more detail in Cracking (Chemistry).

Melting and boiling points

Alkanes have relatively low melting and boiling points. This is because the only forces between alkane molecules are weak van der Waal forces, due to their C-C and C-H bonds being non-polar.

As the chain length of alkanes increases, their boiling points increase. A larger molecule has more electrons and so at any one time, its temporary dipole could be larger. It will therefore experience greater van der Waal attraction than a smaller molecule. However, as the number of branches increases, an alkane’s boiling point will decrease. This is because the molecules can’t pack together as tightly. Imagine packing strands of spaghetti in a jar - they can all fit together in neat rows in the same orientation. Now imagine if the spaghetti is branched, like twigs. The pieces can’t line up, so there are large gaps between the strands, forcing them further apart from each other and wasting space. Van der Waal forces between molecules are not very strong over long distances, and so the attraction between molecules is weaker.

Alkanes and alkenes

We know that alkanes are saturated hydrocarbons. They contain just C-C and C-H single bonds. But we can turn them into unsaturated hydrocarbons. For example, consider propene. Take off one hydrogen atom from each carbon and use the two free electrons to form another bond between these two carbons, and you should get something like the following:

Propane, left, and propene, right. StudySmarter Originals

This molecule is known as propene and is a type of alkene. We’ll explore alkenes in a later article, but for now you should know that they are unsaturated hydrocarbons that contain a C=C double bond. This bond alters their properties, making them more reactive than alkanes.