Alkanes are everywhere. Take a quick look around - you are sure to find many products containing or derived from alkanes. The surface of that road outside your house was made from long-chain alkanes, and it’s highly likely that the fuel you put in your car is made from alkanes too. The plastic in your toothbrush is a type of polymer made up of chains of alkanes, and alkanes form the base of many chemicals, such as your toothpaste and soap. But what actually are they?
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Jetzt kostenlos anmeldenAlkanes are everywhere. Take a quick look around - you are sure to find many products containing or derived from alkanes. The surface of that road outside your house was made from long-chain alkanes, and it’s highly likely that the fuel you put in your car is made from alkanes too. The plastic in your toothbrush is a type of polymer made up of chains of alkanes, and alkanes form the base of many chemicals, such as your toothpaste and soap. But what actually are they?
First of all, let's look at the basic definition of an alkane.
An alkane is a saturated hydrocarbon.
What do those terms actually mean?
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.
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.
Alkanes form a homologous series with the general formula CnH2n+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
The alkane butane has four carbon atoms. Calculate its number of hydrogen atoms.
Alkanes are represented by the general formula CnH2n+2. The question tells us that butane has four carbon atoms, and so here, n = 4. We can see from the formula that alkanes h (2n + 2) hydrogen atoms. Substituting our value for n into this expression, we find that butane has (2(4) + 2) = 10 hydrogen atoms.
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.
Name the following alkane:
An unknown alkane. StudySmarter Originals
First, identify the longest carbon chain in the molecule. Sometimes this chain is hard to spot as it could be part of what looks like a side branch. Here, the longest chain is 5 carbon atoms long. If we take a look at the table of root names, shown below, we know that this molecule must be based on pent-. Because it is an alkane, it has the suffix -ane.
Number of carbons in longest chain | Root name |
1 | meth- |
2 | eth- |
3 | prop- |
4 | but- |
5 | pent- |
Next look at the side chains. There are 2 methyl groups (-CH3) attached to 2 of the carbons, and so the prefix dimethyl- will be used. But which carbons are they joined to? To find out, number the carbons from both ends of the chain. We've shown this down below.
The unknown alkane with its carbon chain numbered. StudySmarter Originals
The methyl groups are either attached to carbons 3 and 4 if you count from the right, or 2 and 3 if you count from the left. However, as you know from Organic Compounds, the numbers of the carbons with the extra side chains and functional groups must add up to the lowest total possible. Therefore, in this molecule we count the carbons from the left. This gives us the overall name of 2,3-dimethylpentane.
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.
Isomers are molecules with the same molecular formula but different arrangements of atoms.
Alkanes can show a type of structural isomerism called chain isomerism, as explored below.
Structural isomers are molecules that have the same molecular formula but different structural formulas. Specifically, chain isomers differ in their arrangement of the carbon chain.
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
To find out more about the other types of isomerism, take a look at "Isomerism".
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 109.5o.
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.
We'll now consider the sources of alkanes:
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
When mined from the sea bed, crude oil is our primary source of alkanes. However, because the process takes so long, crude oil is seen as an unsustainable resource, and it is linked to many environmental issues. Take a look at Combustion for more.
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.
Another way of synthesising alkanes is by hydrogenating alkenes. This involves heating the alkene with hydrogen in the presence of a nickel catalyst.
Hydrogenating certain alkenes produces trans fats, found in margarines and lots of ultra-processed foods. Many health organisations associate these molecules with an increased risk of coronary heart disease, and as a result, trans fats are banned in some nations - including the US. Check out Reactions of Alkenes for a closer look at hydrogenation, or head over to Cardiovascular Disease to learn more about other factors that affect the health of your heart.
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:
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.
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.
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.
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).
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.
See Intermolecular Forces for a further explanation on van der Waal forces.
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.
Alkanes have n carbon atoms, and 2n+2 hydrogen atoms.
Alkanes are saturated hydrocarbons.
Alkanes are saturated, as they only have C-C and C-H single bonds.
Alkenes are unsaturated, meaning they contain at least one C=C double bond, whereas alkanes are saturated and contain only C-C and C-H single bonds.
Alkenes are more reactive than alkanes due to their C=C double bond.
What is an alkane?
A saturated hydrocarbon.
Define saturated.
Containing only C-H and C-C single bonds.
Give three properties of a homologous series.
How do alkanes differ from alkenes?
Hexane has 6 carbon atoms. How many hydrogen atoms does it have?
14
Where do alkanes come from?
Crude oil reserves in sea beds formed from dead plant and animal matter over millions of years.
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