Alkanes are organic compounds made up of carbon and hydrogen atoms. They are the simplest form of hydrocarbons, meaning they contain only single bonds and no other functional groups. Alkanes have the general formula CnH2n+2, and can be divided into three categories: linear straight-chain alkanes, branched alkanes, and cycloalkanes.

Nomenclature is the way we name alkanes. The simplest alkane is methane, which has the molecular formula CH4. The next alkane is ethane, with the molecular formula C2H6. As the number of carbons in the chain increases, the names of the alkanes become more complex. For example, a six-carbon alkane is called hexane.

Isomerism is when two molecules have the same molecular formula but different structures. Alkanes can have structural isomers, meaning they have the same molecular formula but different structures. For example, butane (C4H10) has two structural isomers, n-butane and isobutane.

Alkanes have a tetrahedral geometry, meaning that each carbon atom is bonded to four other atoms. The bond angles are 109.5 degrees.Alkanes can be made through a process called cracking. Cracking is a process in which large molecules are broken down into smaller molecules. This process is used to make alkanes from other hydrocarbons.Alkanes have several general properties. They are nonpolar, meaning they do not have a positive or negative charge. They are also insoluble in water, but soluble in organic solvents. Alkanes are different from alkenes, which are hydrocarbons with double bonds. Alkenes have the general formula CnH2n. Alkenes are more reactive than alkanes due to the presence of the double bonds.

Alkane definition

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?

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.

Alkane general formula

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.

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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.

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.

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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 name1meth-2eth-3prop-4but-5pent-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.

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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.

Alkane isomerism

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

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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.

Chain isomerism

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.

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To find out more about the other types of isomerism, take a look at "Isomerism".

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 molecule described in the query is ethane, C2H6. The shape of ethane is determined by VSEPR theory, which states that electron pairs around a central atom will repel each other, and the shape of the molecule is determined by the arrangement of these electron pairs that minimizes repulsion. In the case of ethane, the four electron pairs around the central carbon atom form a tetrahedral shape, with a hydrogen atom at each corner of the pyramid and the carbon atom in the center. The angle between each of the bonds is 109.5o.

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 a result of millions of years of organic matter being exposed to high temperatures and pressures. The process of forming alkanes is known as carbonation. About 400 million years ago, when the Earth was a completely different place, with giant mushrooms towering eight metres tall and the first vertebrates starting to emerge on land, most of the planet was covered in oceans. When plants and animals living in these oceans died, their remains fell to the ocean floor and were buried in layers of sand and silt. Over time, these layers built up, creating a high-pressure, high-temperature, and oxygen-free environment. Under these conditions, the remains of these creatures slowly began to turn into crude oil, which contains alkanes.

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.

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. 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.

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 have several important properties that make them useful in a variety of applications. They are insoluble in water due to the non-polar nature of their C-C and C-H bonds, but they are soluble in other non-polar solvents and are good solvents themselves. They are highly flammable and are commonly used as fuels like petrol. Short-chain alkanes are highly volatile and can easily ignite, which is why smoking or using lighters near petrol stations is strictly prohibited. Alkanes are generally unreactive due to the strength of their non-polar bonds, but they can react with chlorine or bromine in UV light and can be cracked to produce alkenes. Their melting and boiling points are relatively low due to the weak van der Waal forces between alkane molecules, but as the chain length of alkanes increases, their boiling points also increase due to the greater van der Waal attraction between molecules. However, as the number of branches increases, an alkane’s boiling point decreases because the molecules can't pack together as tightly.

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:

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Propene is an example of an alkene, which is an unsaturated hydrocarbon that contains a C=C double bond. Alkenes are more reactive than alkanes due to the presence of the double bond.

To recap, alkanes are saturated hydrocarbons with the general formula CnH2n+2. They are named using standard nomenclature rules and the suffix -ane. Long-chain alkanes are found in crude oil, and short-chain alkanes can be produced by cracking longer molecules or by hydrogenating alkenes. The non-polar C-C and C-H bonds within alkanes are relatively strong, making them insoluble in water, readily combustible, and giving them low melting and boiling points. In contrast, alkenes have one or more C=C double bond, which makes them more reactive than alkanes.

Alkanes

What is the general formula for alkanes?

Alkanes have n carbon atoms, and 2n+2 hydrogen atoms.

What are alkanes?

Alkanes are saturated hydrocarbons.

Are alkanes saturated or unsaturated?

Alkanes are saturated, as they only have C-C and C-H single bonds.

What is the difference between alkanes and alkenes?

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.

Why are alkenes more reactive than alkanes?

Alkenes are more reactive than alkanes due to their C=C double bond.

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