What are alkanes in organic chemistry?

Alkanes are a class of hydrocarbons in organic chemistry that consist solely of single-bonded carbon and hydrogen atoms. They are saturated compounds, meaning they have the maximum number of hydrogen atoms attached to each carbon. Alkanes follow the general formula CnH2n+2 and are a key topic in the IB MYP Science curriculum.

How are alkanes named according to IUPAC nomenclature?

In IUPAC nomenclature, alkanes are named based on the number of carbon atoms in their longest continuous chain. The names are derived from a prefix indicating the number of carbons (like 'meth-' for one carbon, 'eth-' for two, 'prop-' for three, etc.) followed by the suffix '-ane'. For example, an alkane with four carbon atoms is called butane.

What are the physical properties of alkanes?

Alkanes are generally non-polar molecules, which makes them insoluble in water but soluble in organic solvents. They have relatively low boiling and melting points compared to other organic compounds, which increase with molecular weight. Alkanes are also less dense than water, which is why they float.

Why are alkanes considered saturated hydrocarbons?

Alkanes are considered saturated hydrocarbons because they contain only single bonds between carbon atoms, allowing them to hold the maximum number of hydrogen atoms possible. This saturation makes them relatively stable and less reactive compared to unsaturated hydrocarbons like alkenes and alkynes.

What are some common uses of alkanes?

Alkanes are widely used as fuels and lubricants. For example, methane, ethane, propane, and butane are used as heating fuels and in cooking. Higher alkanes are found in gasoline, diesel, and jet fuel. Additionally, alkanes serve as starting materials in the synthesis of various chemicals in the petrochemical industry.

Why is "Leaving an odd number of O atoms in a balanced combustion equation." a common mistake in Alkanes?

Why it happens: O₂ has an even number of atoms per molecule. How to avoid it: Count O in products first (CO₂ and H₂O), then provide that many divided by 2 as O₂. If you get a fractional coefficient, multiply the whole equation by 2 to clear fractions.

Why is "Saying alkanes decolourise bromine water." a common mistake in Alkanes?

Why it happens: Bromine water is on the syllabus. How to avoid it: Alkanes are SATURATED (no C=C) → don't react with bromine water. Only ALKENES (unsaturated) do.

Organic chemistryIB MYP Sciences (Years 1-5)

Alkanes

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Alkanes — IB MYP Sciences: the simplest hydrocarbons, their formulae and combustion

Alkanes are the simplest organic compounds — chains of carbon with all single bonds. This MYP Sciences note covers the first six alkanes, their general formula CₙH₂ₙ₊₂, why they're so unreactive (except combustion!) and the products of complete and incomplete combustion.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Recall the names and formulae of the first six alkanes.
MYP Sciences A — Use the general formula CₙH₂ₙ₊₂.
MYP Sciences A — Write balanced equations for complete and incomplete combustion of alkanes.
MYP Sciences D — Evaluate the use of alkanes as fuels (energy density vs CO₂ output).

The first six alkanes

Methane, ethane, propane, butane, pentane, hexane — CₙH₂ₙ₊₂.

The first six alkanes (memorise these):

n	Name	Formula	State at room temp	Common source / use
1	Methane	CH₄	gas	natural gas; main greenhouse gas
2	Ethane	C₂H₆	gas	in natural gas; feedstock
3	Propane	C₃H₈	gas	bottled gas (LPG)
4	Butane	C₄H₁₀	gas (compressed to liquid)	cigarette lighters, camping stoves
5	Pentane	C₅H₁₂	liquid	solvent
6	Hexane	C₆H₁₄	liquid	solvent

All follow the CₙH₂ₙ₊₂ rule:

1 C → 2(1)+2 = 4 H → CH₄.
4 C → 2(4)+2 = 10 H → C₄H₁₀.
6 C → 2(6)+2 = 14 H → C₆H₁₄.

Properties change gradually:

BP increases with chain length (CH₄ −162 °C; C₆H₁₄ +69 °C).
Viscosity (thickness) increases.
Volatility (how easily it evaporates) decreases.
Flammability decreases (longer chains burn less readily).

Memorise the first six: meth/eth/prop/but/pent/hex + -ane.
Formula: CₙH₂ₙ₊₂.
BP and viscosity increase with chain length.

Why alkanes are mostly unreactive — except for combustion

C-C and C-H bonds are strong; alkanes only react in extreme conditions.

Alkanes have very stable C-C and C-H bonds. Most reagents that easily attack alcohols, alkenes or carboxylic acids do nothing to alkanes at room temperature. So alkanes don't react with acids, bases, water, oxidising agents (in cold conditions), or bromine water.

The big EXCEPTION is combustion — burning in oxygen, an exothermic redox reaction.

Complete combustion (plenty of oxygen): $\text{C}_x\text{H}_y + (x + y/4)\text{O}_2 \rightarrow x\text{CO}_2 + (y/2)\text{H}_2\text{O}$

For methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O. For propane: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.

Incomplete combustion (limited oxygen) — produces CO (toxic carbon monoxide) AND particulates (soot). E.g. 2 CH₄ + 3 O₂ → 2 CO + 4 H₂O.

You can SEE this in a Bunsen burner:

Air hole OPEN → blue flame (complete combustion, CO₂ only). HOT but no soot.
Air hole CLOSED → yellow flame (incomplete combustion). COOLER, sooty.

This is why blocked chimneys and faulty gas appliances are so dangerous — they produce CO. A CO alarm is essential in any home with gas heating.

Generally unreactive (strong C-C and C-H bonds).
Burn well — main reaction is combustion.
Complete (lots of O₂): → CO₂ + H₂O.
Incomplete (limited O₂): → CO + H₂O + soot.

Alkanes as fuels

Energy-dense, easy to store and transport — but produce CO₂.

Alkanes are the world's biggest source of energy. Practical uses depend on chain length:

CH₄ (natural gas): piped to homes and power stations. Burns cleanly.
LPG (C₃-C₄, propane and butane): bottled gas for camping, off-grid heating.
Petrol (C₆-C₁₀): car engines.
Kerosene (C₁₀-C₁₆): jet fuel and lamp oil.
Diesel (C₁₂-C₂₀): trucks, ships, trains.
Fuel oil and bitumen (C₂₀+): ship fuel, road tar.

Why they're so widely used:

Energy density: high energy per kg.
Easy to store: liquids at modest pressure or temperature.
Easy to transport: pipelines, tankers, bottles.
Established infrastructure: filling stations everywhere.

The downside is the inevitable CO₂ from every combustion event. Burning alkanes for energy is the single biggest contributor to climate change. Modern policy is shifting toward renewable electricity, hydrogen and biofuels — but alkanes still dominate today.

MYP criterion D classic: 'Evaluate the use of alkanes as fuels.' Strong answers cover energy density (positive), CO₂ emissions (negative), comparative ease of storage vs hydrogen (positive), and air quality at point of use (incomplete combustion → CO + PM).

Natural gas → petrol → diesel → bitumen: longer chains for heavier jobs.
Energy-dense, easy to store, established infrastructure.
All burn to CO₂ — the climate problem.

How it’s examined

Criterion A asks for formulas and combustion equations. Criterion D often asks for an evaluation of alkanes vs alternative fuels.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Alkanes

Step-by-step solutions to past-paper-style questions on alkanes, written exactly the way a tutor would explain them at the board.

1Predicting an alkane formula
Getting started• formula
Question
Use the general formula to find the molecular formula of (a) octane (8 C), (b) decane (10 C).
Step-by-step solution
1. Step 1
  CₙH₂ₙ₊₂.
2. Step 2
  (a) n=8 → C₈H₁₈.
3. Step 3
  (b) n=10 → C₁₀H₂₂.
Answer
(a) C₈H₁₈, (b) C₁₀H₂₂.
2Balancing combustion of propane
Building confidence• combustion
Question
Write the balanced equation for the complete combustion of propane (C₃H₈).
Step-by-step solution
1. Step 1
  Products: CO₂ + H₂O.
2. Step 2
  3 C → 3 CO₂; 8 H → 4 H₂O.
3. Step 3
  Need 3(2) + 4 = 10 O atoms = 5 O₂. So: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.
Answer
C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.
3Yellow flame on a Bunsen
Stretch• incomplete combustion
Question
When the air hole on a Bunsen burner is closed, the flame becomes yellow and sooty. Explain in terms of complete/incomplete combustion and write a possible equation.
Step-by-step solution
1. Step 1
  Air hole closed → restricted O₂ supply → INCOMPLETE combustion.
2. Step 2
  Incomplete combustion produces CO (or even C particulates), which are responsible for the yellow colour and soot.
3. Step 3
  Example equation: 2 CH₄ + 3 O₂ → 2 CO + 4 H₂O. (Or with soot: CH₄ + O₂ → C + 2 H₂O.)
Answer
Closed air hole → less O₂ → incomplete combustion → CO + soot + H₂O. The unburnt carbon particles glowing hot give the yellow flame.

Key Formulae — Alkanes

The formulae you need to memorise for alkanes on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Alkane general formula
$CₙH₂ₙ₊₂$
$n$
number of carbons
When to use
Find molecular formula or check whether a formula is an alkane.

Key Definitions and Keywords — Alkanes

Definitions to memorise and the exact keywords mark schemes credit for alkanes answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Alkane
Examiner keyword
A saturated hydrocarbon — only single C-C bonds. General formula CₙH₂ₙ₊₂.
Hydrocarbon
Examiner keyword
A compound made ONLY of hydrogen and carbon.
Complete combustion
Examiner keyword
Reaction with plenty of O₂ to produce CO₂ + H₂O.
Incomplete combustion
Examiner keyword
Reaction with limited O₂ — produces CO (and sometimes C soot) instead of CO₂.

Common Mistakes and Misconceptions — Alkanes

The traps other students keep falling into on alkanes questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Leaving an odd number of O atoms in a balanced combustion equation.
Why it happens
O₂ has an even number of atoms per molecule.
How to avoid it
Count O in products first (CO₂ and H₂O), then provide that many divided by 2 as O₂. If you get a fractional coefficient, multiply the whole equation by 2 to clear fractions.
✕Saying alkanes decolourise bromine water.
Why it happens
Bromine water is on the syllabus.
How to avoid it
Alkanes are SATURATED (no C=C) → don't react with bromine water. Only ALKENES (unsaturated) do.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Alkanes — frequently asked questions

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Alkanes

Short Study Notes in the form of Slides

Short Study Notes — Alkanes

Detailed Notes

What you’ll learn

How it’s examined

1Predicting an alkane formula

2Balancing combustion of propane

3Yellow flame on a Bunsen

Alkane general formula

Alkane

Hydrocarbon

Complete combustion

Incomplete combustion

✕Leaving an odd number of O atoms in a balanced combustion equation.

✕Saying alkanes decolourise bromine water.

Practice questions

Past paper style quiz

Assess your understanding

Alkanes — frequently asked questions