Alkanes

Edexcel International GCSE Chemistry· 4.19–4.22, Section 4(c)· 15 min read

1. Alkane General Formula & Saturated Hydrocarbon Classification★★☆☆☆⏱ 3 min

✓ Calculator OK

Alkanes are the simplest family of hydrocarbons, made exclusively of carbon and hydrogen atoms. They follow a predictable general formula that applies to all unbranched alkanes.

📘 Definition

Saturated Hydrocarbon

A hydrocarbon containing only single covalent bonds between carbon atoms, with no double or triple C-C bonds present, so the maximum number of hydrogen atoms are bonded to each carbon.

Example:

Methane ( $C H_{4}$ ) is the simplest saturated hydrocarbon.

C_nH_{2n+2}

For $n$ carbon atoms, each alkane has $2 n + 2$ hydrogen atoms. For example, if $n = 3$ (propane), the formula is $C_{3} H_{8}$ . This formula only applies to unbranched alkanes, which are the only ones you need to recall for this specification.

📐 Worked Example

Use the general alkane formula to find the molecular formula of an alkane with 5 carbon atoms.

1
Identify the value of $n$ from the question: $n = 5$
2
$Substitute $n=5$ into the general formula $C_nH_{2n+2}$$
3
$Number of H atoms = $2 \times 5 + 2 = 12$$
4
The molecular formula is $C_{5} H_{12}$ (pentane)

2. Naming Unbranched Alkanes (Up to 5 Carbon Atoms)★★☆☆☆⏱ 4 min

Unbranched alkanes have a straight chain of carbon atoms with no side groups. You are required to recall the names and formulae of the first 5 unbranched alkanes for your exam.

Number of C atoms	Name	Molecular Formula
1	Methane	$C H_{4}$
2	Ethane	$C_{2} H_{6}$
3	Propane	$C_{3} H_{8}$
4	Butane	$C_{4} H_{10}$
5	Pentane	$C_{5} H_{12}$

✓ Quick check

What is the name of the alkane with molecular formula $C_{4} H_{10}$ ?
- Propane
- Butane
- Pentane
- Ethane
Reveal answer
Butane —
$C_{4} H_{10}$ has 4 carbon atoms, so the prefix is 'but-' giving the name butane.

3. Drawing Structural & Displayed Formulae of Alkanes★★★☆☆⏱ 4 min

You may be asked to draw either structural formula (condensed, shows atom arrangement without all bonds) or displayed formula (shows every atom and every single bond) for alkanes up to 5 carbons.

📘 Definition

Displayed Formula

A structural formula that shows all atoms and all covalent bonds between them in a molecule, with each bond represented as a single line.

Example:

Displayed formula of methane shows a central C atom bonded to 4 separate H atoms with single lines for each C-H bond.

📐 Worked Example

Draw the displayed formula of propane ( $C_{3} H_{8}$ ).

1
Draw three carbon atoms in a straight line, connected by single covalent bonds.
2
Add enough hydrogen atoms to each carbon so that every carbon has 4 bonds total (carbon always forms 4 covalent bonds).
3
The two end carbons bond to 3 hydrogen atoms each, and the middle carbon bonds to 2 hydrogen atoms, giving a total of 8 hydrogen atoms.

✓ Quick check

Which of the following is a requirement for displayed formulae?
- Only show C-C bonds
- Show all C-C and C-H bonds
- Show only the carbon chain
- Use condensed groups like $C H_{3}$
Reveal answer
Show all C-C and C-H bonds —
Displayed formulae explicitly show every covalent bond in the molecule.

4. Alkane Halogenation Reactions★★★☆☆⏱ 4 min

Alkanes are generally unreactive with most common reagents, but they react with halogens (e.g. chlorine, bromine) in the presence of ultraviolet (UV) light in a substitution reaction.

📘 Definition

Mono-substitution Reaction

A reaction where one hydrogen atom in the alkane molecule is replaced by a single halogen atom, forming two products: a halogenoalkane and a hydrogen halide.

The only condition you need to recall for this reaction is UV light (e.g. sunlight). Reaction mechanisms (such as free radical steps) are explicitly excluded from the specification, so you do not need to learn these.

CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl

In this reaction, one hydrogen from methane is replaced by one chlorine atom, making chloromethane (the halogenoalkane) and hydrogen chloride (the hydrogen halide). This is mono-substitution, so only one H is replaced.

📐 Worked Example

Write the balanced symbol equation for the mono-substitution reaction between ethane ( $C_{2} H_{6}$ ) and bromine ( $B r_{2}$ ) under UV light.

1
Identify the reactants: ethane ( $C_{2} H_{6}$ ) and bromine ( $B r_{2}$ ), with UV as the condition.
2
One H atom from ethane is replaced by one Br atom, forming bromoethane ( $C_{2} H_{5} B r$ ) and hydrogen bromide ( $H B r$ ).
3
$C_2H_6 + Br_2 \xrightarrow{UV} C_2H_5Br + HBr$
4
Check the equation is balanced: 2 C, 6 H, 2 Br on both sides, so it is correct.

Exam tip:

Always include the UV light condition above the reaction arrow to gain full marks for halogenation questions.

5. Common Pitfalls

Wrong move:

Using the general formula $C_{n} H_{2 n}$ for alkanes

Why:

That formula applies to alkenes, not alkanes, so you will calculate incorrect molecular formulae

Correct move:

Memorise the alkane general formula $C_{n} H_{2 n + 2}$ , and double check H counts for each alkane

Wrong move:

Drawing displayed formula without showing all C-H bonds

Why:

Displayed formula requires every covalent bond to be visible, so missing bonds lose marks

Correct move:

Count the bonds around each carbon to ensure it has 4 bonds total, drawing every C-H and C-C bond explicitly

Wrong move:

Naming an alkane with 4 carbon atoms as pentane

Why:

Prefixes correspond to number of carbons: but- is 4, pent- is 5

Correct move:

Memorise the prefixes for 1-5 carbon atoms: meth(1), eth(2), prop(3), but(4), pent(5)

Wrong move:

Writing the halogenation reaction without UV as a condition

Why:

Alkanes do not react with halogens at room temperature without UV light, so the condition is required for full marks

Correct move:

Always write UV above the reaction arrow for alkane halogenation equations

Wrong move:

Drawing di-substituted products for alkane halogenation reactions

Why:

The specification only requires knowledge of mono-substitution (one H replaced) for this topic

Correct move:

Only write equations where one H atom is replaced by one halogen atom, forming one halogenoalkane and one hydrogen halide

6. Quick Reference Cheatsheet

Concept	Key Details to Recall
General Formula	$C_{n} H_{2 n + 2}$ for all unbranched alkanes
Saturated Hydrocarbon Definition	Only single C-C bonds, no double/triple bonds
First 5 Alkanes	Methane (1C), Ethane (2C), Propane (3C), Butane (4C), Pentane (5C)
Displayed Formula Requirement	Show all C-C and C-H covalent bonds explicitly
Halogenation Reaction	Alkane + Halogen --UV--> Halogenoalkane + Hydrogen halide (mono-substitution only)

7. Frequently Asked

Do I need to learn branched alkane names for this exam?

No, the Edexcel IGCSE 4CH1 specification only requires naming of unbranched alkanes with up to 5 carbon atoms for this sub-topic.

Is the free-radical substitution mechanism required for alkane halogenation?

No, the specification explicitly excludes reaction mechanisms for this topic; you only need to recall the mono-substitution product, reactants, and UV light condition.

What's Next

Now that you have mastered the core properties of alkanes, you can move on to related organic chemistry topics in the Edexcel IGCSE Chemistry 4CH1 specification. Alkanes are the foundational family of hydrocarbons, so understanding their structure and reactivity will help you grasp the key differences between saturated and unsaturated hydrocarbons when you study alkenes next. You will also use your knowledge of alkane properties when learning about crude oil fractionation, cracking, and the use of alkanes as fuels in earlier organic chemistry subtopics. Make sure you practice drawing displayed formulae and writing halogenation equations regularly, as these are common 1-3 mark questions in both Paper 1 and Paper 2 exams.