Alkanes and Alkenes

Chemistry· 11.4, 11.5 (2026-2028 syllabus)· 18 min read

1. Core: Structure and Classification of Alkanes and Alkenes★★☆☆☆⏱ 5 min

Alkanes and alkenes are the two simplest homologous series of hydrocarbons, which are compounds containing only carbon and hydrogen atoms.

📘 Definition

Saturated Hydrocarbon

A hydrocarbon with only single covalent bonds between all carbon atoms, so no additional atoms can be added to its structure.

Example:

Methane ( $C H_{4}$ ), ethane ( $C_{2} H_{6}$ )

Alkanes are saturated hydrocarbons with the general formula $C_{n} H_{2 n + 2}$ , where $n$ is the number of carbon atoms ( $n \geq 1$ ). Alkenes are unsaturated hydrocarbons containing one C=C double functional group, with the general formula $C_{n} H_{2 n}$ , where $n \geq 2$ (you need at least 2 carbons to form a double bond).

📐 Worked Example

Give the molecular formula of propane (alkane with 3 carbon atoms) and propene (alkene with 3 carbon atoms).

1
For propane (alkane): Use the general formula $C_{n} H_{2 n + 2}$ with $n = 3$ . Substitute: $2 (3) + 2 = 8$ , so molecular formula is $C_{3} H_{8}$ .
2
For propene (alkene): Use the general formula $C_{n} H_{2 n}$ with $n = 3$ . Substitute: $2 (3) = 6$ , so molecular formula is $C_{3} H_{6}$ .

Exam tip:

Remember alkenes have two fewer hydrogen atoms than the corresponding alkane, because the C=C double bond replaces one C-C single bond and two C-H single bonds.

2. Core: Key Reactions of Alkanes★★☆☆☆⏱ 4 min

Alkanes are relatively unreactive except for two core reactions you need to memorise: complete combustion, and substitution with halogens in ultraviolet (UV) light.

Complete combustion: Alkanes burn in excess oxygen to produce carbon dioxide and water, releasing large amounts of energy. This makes them widely used as household and transport fuels.
Substitution with halogens: In UV light (e.g. direct sunlight), alkanes react with halogens like chlorine or bromine. One hydrogen atom is replaced by a halogen atom, forming a halogenoalkane and a hydrogen halide.

📐 Worked Example

Write the word equation for the complete combustion of methane, and the word equation for the reaction between methane and chlorine in UV light.

1
Complete combustion of methane: $methane + oxygen \to carbon dioxide + water$
2
Substitution reaction of methane with chlorine: $methane + chlorine \to chloromethane + hydrogen chloride$

Exam tip:

If oxygen is limited, incomplete combustion produces carbon monoxide or soot instead of carbon dioxide, but you only need to recall complete combustion equations for Core tier questions.

3. Core: Key Reactions of Alkenes and Test for Unsaturation★★★☆☆⏱ 5 min

Alkenes are more reactive than alkanes due to the presence of the C=C double bond, which can break to form single bonds and add new atoms in a process called an addition reaction.

📘 Definition

Addition Reaction

A reaction where a small molecule adds across a double (or triple) bond, breaking one bond of the double bond to form two new single bonds, resulting in a single saturated product.

Combustion: Alkenes burn in excess oxygen to produce carbon dioxide and water, but they burn with a smokier, yellower flame than alkanes due to their higher carbon-to-hydrogen ratio.
Bromine water test for unsaturation: Orange bromine water adds across the C=C double bond of alkenes to form a colourless dibromoalkane, so the orange colour disappears when mixed with an alkene. No reaction occurs with alkanes, so the orange colour remains.

📐 Worked Example

A student adds bromine water to two test tubes, one containing hexane (alkane) and one containing hexene (alkene). Describe the observations for each test tube and explain the reaction occurring.

1
Hexane test tube: Orange bromine water remains orange. No reaction occurs, as alkanes are saturated and cannot undergo addition reactions.
2
Hexene test tube: Orange bromine water turns colourless. An addition reaction occurs, where bromine adds across the C=C double bond in hexene to form colourless dibromohexane.

Exam tip:

You must explicitly state that bromine water changes from orange to colourless for a positive alkene test. Stating it 'goes clear' is not enough for full marks, as clear solutions can still be coloured.

4. Extended Only: Additional Alkene Reactions and Cracking★★★★☆Extended only⏱ 6 min

For Extended tier, you need to memorise two additional addition reactions of alkenes, plus the industrial process of cracking used to produce alkenes from large alkane molecules.

Hydrogenation (addition of hydrogen): Alkenes react with hydrogen gas at 150°C with a nickel catalyst to form the corresponding alkane. This reaction is used to turn liquid unsaturated vegetable oils into solid margarine.
Hydration (addition of steam): Alkenes react with steam at 300°C, 60-70 atm pressure, with a phosphoric acid catalyst to form an alcohol. This is the industrial method to produce ethanol from ethene.
Cracking: Large, less useful alkane molecules from crude oil are broken down at 600-700°C with an aluminium oxide catalyst (or high temperature alone) into smaller alkanes and alkenes. Alkenes from cracking are used to make plastics and other industrial chemicals.

📐 Worked Example

Write the word equation for the hydrogenation of ethene, and the hydration of ethene. Include the conditions required for each reaction.

1
Hydrogenation of ethene: $ethene + hydrogen \to ethane$ . Conditions: 150°C, nickel catalyst.
2
Hydration of ethene: $ethene + steam \to ethanol$ . Conditions: 300°C, 60-70 atm pressure, phosphoric acid catalyst.

Exam tip:

Always include full reaction conditions for industrial Extended reactions, as they are usually awarded 1-2 separate marks in exam questions.

5. Common Pitfalls

Wrong move:

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

Why:

Confusing the general formulae of the two homologous series

Correct move:

Remember alkanes are saturated (maximum H atoms) so use $C_{n} H_{2 n + 2}$ ; alkenes have one C=C double bond so use $C_{n} H_{2 n}$

Wrong move:

Stating bromine water 'goes clear' instead of 'turns from orange to colourless' for the unsaturation test

Why:

Examiners require explicit mention of the colour change; 'clear' does not confirm a colour change has occurred

Correct move:

Always write the full colour change: orange to colourless for a positive alkene test

Wrong move:

Writing carbon monoxide or soot as products of complete combustion

Why:

Confusing complete and incomplete combustion reactions

Correct move:

Complete combustion uses excess oxygen, so only carbon dioxide and water are produced; carbon monoxide/soot are products of incomplete combustion with limited oxygen

Wrong move:

Writing substitution reactions for alkenes

Why:

Alkenes have a reactive C=C double bond and undergo addition reactions, not substitution

Correct move:

Alkanes undergo substitution reactions with halogens in UV light; alkenes undergo addition reactions with halogens with no UV light required

Wrong move:

Forgetting catalysts/conditions for Extended hydrogenation/hydration reactions

Why:

Exam questions award separate marks for correct reaction conditions

Correct move:

Memorise the exact temperature, pressure and catalyst for each industrial Extended reaction

6. Quick Reference Cheatsheet

Property	Alkanes	Alkenes
General formula	$C_{n} H_{2 n + 2}$ ( $n \geq 1$ )	$C_{n} H_{2 n}$ ( $n \geq 2$ )
Bond type	Only C-C single bonds (saturated)	One C=C double bond (unsaturated)
Reactivity	Relatively unreactive	More reactive
Combustion flame	Clean blue flame	Smoky yellow flame
Bromine water test	No change (stays orange)	Orange → colourless
Core reactions	Combustion, substitution (UV light)	Combustion, addition (bromine)
Extended reactions	Feedstock for cracking	Hydrogenation, hydration, polymerisation

7. Frequently Asked

What is the difference between a saturated and unsaturated hydrocarbon?

A saturated hydrocarbon (e.g. alkane) has only single covalent bonds between carbon atoms, so no more atoms can be added to its structure. An unsaturated hydrocarbon (e.g. alkene) has at least one C=C double covalent bond, so it can undergo addition reactions to add more atoms.

How do I distinguish between an alkane and an alkene in exam questions?

Use the bromine water test: add orange bromine water to the sample and shake. If the orange colour remains, the sample is an alkane (no reaction occurs). If the orange solution turns colourless, the sample is an alkene (addition reaction with bromine occurs).

Going deeper

syllabusCIE IGCSE Chemistry 0620 2026-2028 SyllabusRefer to sections 11.4 and 11.5

What's Next

Now that you have mastered the structure and reactions of alkanes and alkenes, you are ready to move on to the next homologous series in organic chemistry: alcohols and carboxylic acids, which build on the functional group reaction principles you have learned here. For Extended tier students, you will also apply your knowledge of alkenes to polymerisation reactions, where small alkene monomers join together to form long polymer chains used in plastics and industrial materials. Be sure to practice drawing displayed formulae for unbranched alkanes and alkenes up to 4 carbon atoms, and writing word equations for all core and extended reactions, as these are frequently tested in both Paper 3 (Core) and Paper 4 (Extended) exams. Revisit the bromine water test regularly, as it is a common 2-3 mark question across both tiers.