Alkanes

Summary: Alkanes are saturated hydrocarbons (CnH2n+2). First four: methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10). They undergo combustion and substitution reactions with halogens (UV light). Tags: igcse chemistry organic-chem Created: 2026-07-14 Last Updated: 2026-07-16

What are Alkanes?

Alkanes are a homologous series of saturated hydrocarbons with the general formula:

CnH2n+2

Saturated means the molecule contains only single covalent bonds (C-C bonds). Every carbon atom is bonded to the maximum possible number of hydrogen atoms — the molecule is “saturated” with hydrogen. There are no C=C double bonds.

The First Four Alkanes

Displayed formulas (showing all atoms and bonds):

• Methane (CH4): A central carbon atom with four single bonds to four H atoms (tetrahedral shape, 109.5° bond angle)
• Ethane (C2H6): Two carbon atoms joined by a single bond, each carbon bonded to three H atoms
• Propane (C3H8): Three carbon atoms in a chain, with H atoms completing four bonds per carbon
• Butane (C4H10): Four carbon atoms in a straight chain, with H atoms completing four bonds per carbon

In displayed formulas, every C-H bond must be shown. CH3-CH2-CH2-CH3 is a structural formula, not a displayed formula.

Trends in Physical Properties

As the chain length (number of carbon atoms) increases in the alkane series:

As chain length increases, intermolecular forces between molecules become stronger, so more energy is required to overcome them.

Chemical Properties of Alkanes

Alkanes are generally unreactive because:

• C-C and C-H single bonds are strong and non-polar
• The molecule is saturated — there are no C=C double bonds to react easily

Alkanes undergo two main types of reaction:

Combustion

Complete combustion (excess oxygen): Alkane + O2 (excess) → CO2 + H2O

Incomplete combustion (limited oxygen supply):

• Produces carbon monoxide (CO) and water: 2CH4 (g) + 3O2 (g) → 2CO (g) + 4H2O (l)
• With even less oxygen, carbon (soot) is produced: CH4 (g) + O2 (g) → C (s) + 2H2O (l)

As oxygen supply decreases, the products shift from CO2 → CO → C (soot).

Substitution Reaction with Halogens

Alkanes react with halogens (Cl2, Br2) in the presence of ultraviolet (UV) light. This is a photochemical reaction — UV light provides the energy to break the halogen-halogen bond.

General equation: Alkane + Halogen —(UV light)⇒ Haloalkane + Hydrogen halide

Example: Chlorination of methane

CH4 (g) + Cl2 (g) --(UV light)--> CH3Cl (g) + HCl (g)

Methane + Chlorine → Chloromethane + Hydrogen chloride

The reaction continues — each H atom can be replaced one at a time:

• CH4 + Cl2 → CH3Cl + HCl
• CH3Cl + Cl2 → CH2Cl2 + HCl
• CH2Cl2 + Cl2 → CHCl3 + HCl
• CHCl3 + Cl2 → CCl4 + HCl

Key points about substitution:

• One hydrogen atom is replaced (substituted) by one halogen atom each step
• UV light is essential — no reaction occurs in the dark
• The reaction works with chlorine (Cl2) and bromine (Br2) — but bromine reacts more slowly
• With excess halogen, all hydrogen atoms are eventually replaced

Uses of Alkanes

Environmental Concerns

Carbon monoxide (CO):

• Produced during incomplete combustion (e.g., in car engines with insufficient oxygen)
• Toxic — binds irreversibly to haemoglobin in red blood cells, preventing oxygen transport
• Symptoms: dizziness, headaches, unconsciousness, death at high concentrations
• Catalytic converters in cars oxidise CO to CO2

Carbon dioxide (CO2):

• Produced during complete combustion of all fossil fuels
• Greenhouse gas — traps infrared radiation in the atmosphere, contributing to global warming and climate change

Soot (carbon particles):

• Produced during very incomplete combustion (e.g., diesel engines)
• Causes respiratory problems and contributes to air pollution (particulates)

Key Points

• General formula: CnH2n+2
• Alkanes are saturated (only C-C single bonds)
• First four: methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10)
• Complete combustion: alkane + O2 (excess) → CO2 + H2O
• Incomplete combustion: alkane + O2 (limited) → CO + H2O (or C + H2O)
• Substitution with halogens: requires UV light; one H replaced per step
• CO is toxic — binds to haemoglobin, prevents oxygen transport
• Boiling point increases with chain length (stronger intermolecular forces)

Key Concepts from Past Papers

• Alkanes are saturated hydrocarbons with only C-C single bonds
• Substitution requires UV light
• Incomplete combustion produces CO (toxic) or C (soot)

Keywords from Past Papers

methane, ethene, point, oxygen, bonds, carbon, boiling, combustion, fractions, energy, hydrocarbons, liquid, idea, structure, water

Related Notes

• Organic Chemistry Fundamentals
• Alkenes
• Alcohols
• Carboxylic Acids

Sources

• OpenStax Chemistry 2e — [Chapter 20: Organic Chemistry (Alkanes)], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Alkanes], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 11: Organic Chemistry (Alkanes), Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 25: Alkanes], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/32 Feb/March 2017: Q33(b)(iv) (1m), Q33(c)(iii) (1m)
• 0620/32 Feb/March 2018: Q66(a)(i) (1m)
• 0620/32 Feb/March 2019: Q55(d)(iii) (1m)
• 0620/32 Feb/March 2020: Q11(a)(ii) (1m), Q11(a)(v) (1m)
• 0620/32 Feb/March 2021: Q66(c)(i) (1m)
• 0620/32 Feb/March 2022: Q22(a)(ii) (2m), Q77(b)(i) (1m)
• 0620/32 Feb/March 2023: Q66(a)(iv) (2m)
• 0620/32 May/June 2018: Q11(a)(v) (1m), Q33(a)(iv) (1m), Q33(c)(ii) (0m)
• 0620/33 May/June 2016: Q44(c)(iv) (1m), Q66(e)(i) (1m)
• 0620/33 May/June 2017: Q44(a)(ii) (1m)
• 0620/33 May/June 2019: Q11(a)(i) (2m), Q11(a)(iv) (1m), Q11(a)(vi) (1m) (+6 more)
• 0620/33 May/June 2021: Q44(a)(iii) (1m), Q44(d)(i) (1m), Q44(d)(ii) (1m) (+1 more)

Common Misconceptions