Alkanes

What you'll learn

How to use the general formula for alkanes.
Why alkanes are called saturated hydrocarbons.
How to draw and name unbranched alkanes with up to five carbon atoms.
How alkanes react with halogens in ultraviolet radiation.

Starting point: what are organic compounds?

In IGCSE Chemistry, organic chemistry is the study of compounds containing carbon. Most organic compounds also contain hydrogen, and many contain other elements such as oxygen, chlorine or bromine.

Alkanes are one of the simplest families of organic compounds. They are important because they are major components of crude oil and natural gas.

Definition

Alkane

An alkane is a hydrocarbon containing only carbon–carbon single bonds.

Hydrocarbons

A hydrocarbon is a compound that contains hydrogen and carbon only.

For example:

Methane, $CH_4$ , is a hydrocarbon.
Propane, $C_3H_8$ , is a hydrocarbon.
Ethanol, $C_2H_5OH$ , is not a hydrocarbon because it also contains oxygen.

Definition

Hydrocarbon

A hydrocarbon is a compound made from carbon and hydrogen atoms only.

The general formula for alkanes

All alkanes follow the same pattern:

C_nH_{2n+2}

Here, $n$ is the number of carbon atoms in the molecule.

So if an alkane has:

1 carbon atom, it has $2 n + 2 = 4$ hydrogen atoms, so the formula is $CH_4$ .
2 carbon atoms, it has $2 n + 2 = 6$ hydrogen atoms, so the formula is $C_2H_6$ .
5 carbon atoms, it has $2 n + 2 = 12$ hydrogen atoms, so the formula is $C_5H_{12}$ .

Key Idea

General formula

The general formula for alkanes is $C_nH_{2n+2}$ . This lets you work out the molecular formula of any alkane if you know how many carbon atoms it has.

Example

Using the alkane general formula

An alkane contains 6 carbon atoms. Work out its molecular formula.

Use the alkane general formula $C_nH_{2n+2}$ and substitute $n = 6$ because there are 6 carbon atoms.
Calculate the number of hydrogen atoms: $\left(2 \times 6\right) + 2 = 14$ .
Put the carbon and hydrogen numbers into the formula, giving $C_6H_{14}$ .

Why alkanes are saturated

Alkanes are described as saturated because their carbon atoms are joined by single covalent bonds only.

A covalent bond is a shared pair of electrons between atoms. In alkanes, the carbon atoms share electrons with other carbon atoms and hydrogen atoms.

Carbon forms 4 covalent bonds. Hydrogen forms 1 covalent bond. In an alkane, each carbon has as many hydrogen atoms attached as possible because there are no carbon–carbon double bonds.

Definition

Saturated hydrocarbon

A saturated hydrocarbon contains hydrogen and carbon only, and has only carbon–carbon single bonds.

This is why alkanes are saturated hydrocarbons:

They are hydrocarbons because they contain carbon and hydrogen only.
They are saturated because they contain only single bonds between carbon atoms.

Common Mistake

Saturated does not mean dissolved

In organic chemistry, saturated means “single carbon–carbon bonds only”. It does not mean a solution is full of solute.

Example

Checking whether a compound could be an alkane

Decide whether $C_4H_{10}$ and $C_4H_8$ could be alkanes.

For an alkane with 4 carbon atoms, substitute $n = 4$ into $C_nH_{2n+2}$ .
Calculate the expected number of hydrogen atoms: $\left(2 \times 4\right) + 2 = 10$ .
Compare each formula with the alkane pattern. $C_4H_{10}$ matches, so it could be an alkane. $C_4H_8$ does not match, so it is not an alkane.

Formulae used for alkanes

You need to recognise and draw different types of formula.

Molecular formula

The molecular formula shows the actual number of each type of atom in one molecule.

For example, propane has molecular formula $C_3H_8$ .

This tells you there are 3 carbon atoms and 8 hydrogen atoms, but it does not show how the atoms are connected.

Structural formula

The structural formula shows how the atoms are arranged, usually in a shortened form.

For example, propane can be written as:

$CH_3CH_2CH_3$

This shows a chain of 3 carbon atoms.

Displayed formula

The displayed formula shows every atom and every bond in the molecule.

For alkanes, every bond shown should be a single line because alkanes only contain single covalent bonds.

Reference diagram showing the first five unbranched alkanes with molecular formulae, shortened structural formulae and displayed formulae

Tip

Drawing displayed formulae

When drawing an alkane, make sure every carbon has 4 bonds and every hydrogen has 1 bond. This is the quickest way to spot missing or extra hydrogens.

Naming unbranched alkanes up to five carbons

An unbranched chain means the carbon atoms are joined in one continuous chain, with no side chains.

You need to know the names of the first five unbranched alkanes:

1 carbon atom: methane, $CH_4$
2 carbon atoms: ethane, $C_2H_6$
3 carbon atoms: propane, $C_3H_8$
4 carbon atoms: butane, $C_4H_{10}$
5 carbon atoms: pentane, $C_5H_{12}$

The names use a prefix to show the number of carbon atoms:

meth- means 1 carbon
eth- means 2 carbons
prop- means 3 carbons
but- means 4 carbons
pent- means 5 carbons

The ending -ane shows that the compound is an alkane.

Definition

Isomer

Isomers are compounds with the same molecular formula but different structural formulae.

At this stage, focus on naming the unbranched-chain alkanes. For example, the unbranched-chain alkane with formula $C_4H_{10}$ is butane.

Common Mistake

Molecular formula vs structural formula

$C_4H_{10}$ tells you the numbers of atoms only. $CH_3CH_2CH_2CH_3$ shows the carbon chain arrangement, so it is a structural formula.

Example

Drawing the displayed formula of butane

Draw the displayed formula of butane.

Use the name to find the number of carbon atoms. The prefix but- means 4 carbon atoms, so start with a chain of 4 carbon atoms joined by single bonds.
Add hydrogen atoms so that each carbon has 4 bonds in total. The two end carbon atoms each need 3 hydrogens, and the two middle carbon atoms each need 2 hydrogens.
Count the atoms to check the formula: there are 4 carbon atoms and 10 hydrogen atoms, so the displayed formula matches $C_4H_{10}$ .

Reactions of alkanes with halogens

Alkanes are fairly unreactive because their covalent bonds are strong and non-polar. However, they do react with halogens such as chlorine or bromine in the presence of ultraviolet radiation.

Definition

Halogen

A halogen is an element in Group 7 of the Periodic Table, such as fluorine, chlorine, bromine or iodine.

In this topic, you only need to know mono-substitution reactions. You do not need to know the detailed reaction mechanism.

Definition

Substitution reaction

A substitution reaction is a reaction in which one atom, or group of atoms, is replaced by another atom or group of atoms.

Methane and chlorine

Methane reacts with chlorine in ultraviolet radiation to form chloromethane and hydrogen chloride:

CH4(g)+Cl2(g)→CH3Cl(g)+HCl(g)CH_4\text{(g)} + Cl_2\text{(g)} \to CH_3Cl\text{(g)} + HCl\text{(g)}

One hydrogen atom in methane is replaced by one chlorine atom.

Reaction schematic showing mono-substitution of methane with chlorine under ultraviolet radiation to form chloromethane and hydrogen chloride

What mono-substitution means

Mono- means one. In mono-substitution, only one hydrogen atom in the alkane is replaced by one halogen atom.

For example:

C2H6+Cl2→C2H5Cl+HClC_2H_6 + Cl_2 \to C_2H_5Cl + HCl

Ethane reacts with chlorine to form chloroethane and hydrogen chloride.

Key Idea

Alkane halogenation

Alkanes react with halogens in ultraviolet radiation by substitution: one hydrogen atom is replaced by one halogen atom, producing a haloalkane and a hydrogen halide.

Example

Writing a mono-substitution equation

Write the equation for the mono-substitution reaction between ethane and chlorine.

Start with ethane and chlorine as the reactants: ethane is $C_2H_6$ and chlorine molecules are $Cl_2$ .
Replace one hydrogen atom in ethane with one chlorine atom. This changes $C_2H_6$ into $C_2H_5Cl$ .
The removed hydrogen atom combines with the other chlorine atom to form hydrogen chloride, $H C l$ .
Write the balanced equation: $C2H6+Cl2→C2H5Cl+HClC_2H_6 + Cl_2 \to C_2H_5Cl + HCl$ .

Common Mistake

Ultraviolet radiation is required

Alkanes do not usually react with chlorine or bromine in the dark. Ultraviolet radiation provides the energy needed to start the substitution reaction.

Naming the products

When chlorine substitutes into an alkane, the product is a chloroalkane.

For example:

Methane forms chloromethane.
Ethane forms chloroethane.

When bromine substitutes into an alkane, the product is a bromoalkane.

For example:

Methane forms bromomethane.
Ethane forms bromoethane.

The other product is a hydrogen halide:

Chlorine gives hydrogen chloride, $H C l$ .
Bromine gives hydrogen bromide, $H B r$ .

Tip

Product pattern

For mono-substitution, think: alkane + halogen → haloalkane + hydrogen halide.

Quick summary

Alkanes are saturated hydrocarbons with the general formula $C_nH_{2n+2}$ . They contain carbon and hydrogen only, and all carbon–carbon bonds are single bonds. You should be able to draw molecular, structural and displayed formulae for methane to pentane, and name the unbranched-chain alkanes. Alkanes react with halogens in ultraviolet radiation by mono-substitution, where one hydrogen atom is replaced by one halogen atom.

Exam technique

In the exam

For formula questions, substitute the number of carbon atoms into $C_nH_{2n+2}$ and check that the hydrogen number matches.
For displayed formulae, count bonds carefully: carbon must have 4 bonds and hydrogen must have 1 bond.
For halogenation equations, remember the pattern: alkane + halogen → haloalkane + hydrogen halide, and include ultraviolet radiation if conditions are asked for.

Self review

Check yourself

What is the molecular formula of the unbranched alkane with 5 carbon atoms?
Why are alkanes described as saturated hydrocarbons?
What are the products when methane reacts with chlorine in ultraviolet radiation?

What you'll learn

How to use the general formula for alkanes.
Why alkanes are called saturated hydrocarbons.
How to draw and name unbranched alkanes with up to five carbon atoms.
How alkanes react with halogens in ultraviolet radiation.

Starting point: what are organic compounds?

Alkanes are one of the simplest families of organic compounds. They are important because they are major components of crude oil and natural gas.

Definition

Alkane

An alkane is a hydrocarbon containing only carbon–carbon single bonds.

Hydrocarbons

A hydrocarbon is a compound that contains hydrogen and carbon only.

For example:

Methane, $CH_4$ , is a hydrocarbon.
Propane, $C_3H_8$ , is a hydrocarbon.
Ethanol, $C_2H_5OH$ , is not a hydrocarbon because it also contains oxygen.

Definition

Hydrocarbon

A hydrocarbon is a compound made from carbon and hydrogen atoms only.

The general formula for alkanes

All alkanes follow the same pattern:

C_nH_{2n+2}

Here, $n$ is the number of carbon atoms in the molecule.

So if an alkane has:

1 carbon atom, it has $2 n + 2 = 4$ hydrogen atoms, so the formula is $CH_4$ .
2 carbon atoms, it has $2 n + 2 = 6$ hydrogen atoms, so the formula is $C_2H_6$ .
5 carbon atoms, it has $2 n + 2 = 12$ hydrogen atoms, so the formula is $C_5H_{12}$ .

Key Idea

General formula

The general formula for alkanes is $C_nH_{2n+2}$ . This lets you work out the molecular formula of any alkane if you know how many carbon atoms it has.

Example

Using the alkane general formula

An alkane contains 6 carbon atoms. Work out its molecular formula.

Use the alkane general formula $C_nH_{2n+2}$ and substitute $n = 6$ because there are 6 carbon atoms.
Calculate the number of hydrogen atoms: $\left(2 \times 6\right) + 2 = 14$ .
Put the carbon and hydrogen numbers into the formula, giving $C_6H_{14}$ .

Why alkanes are saturated

Alkanes are described as saturated because their carbon atoms are joined by single covalent bonds only.

A covalent bond is a shared pair of electrons between atoms. In alkanes, the carbon atoms share electrons with other carbon atoms and hydrogen atoms.

Carbon forms 4 covalent bonds. Hydrogen forms 1 covalent bond. In an alkane, each carbon has as many hydrogen atoms attached as possible because there are no carbon–carbon double bonds.

Definition

Saturated hydrocarbon

A saturated hydrocarbon contains hydrogen and carbon only, and has only carbon–carbon single bonds.

This is why alkanes are saturated hydrocarbons:

They are hydrocarbons because they contain carbon and hydrogen only.
They are saturated because they contain only single bonds between carbon atoms.

Common Mistake

Saturated does not mean dissolved

In organic chemistry, saturated means “single carbon–carbon bonds only”. It does not mean a solution is full of solute.

Example

Checking whether a compound could be an alkane

Decide whether $C_4H_{10}$ and $C_4H_8$ could be alkanes.

For an alkane with 4 carbon atoms, substitute $n = 4$ into $C_nH_{2n+2}$ .
Calculate the expected number of hydrogen atoms: $\left(2 \times 4\right) + 2 = 10$ .
Compare each formula with the alkane pattern. $C_4H_{10}$ matches, so it could be an alkane. $C_4H_8$ does not match, so it is not an alkane.