Meeting the Alkanes

Study the structure, naming, combustion and substitution reactions of alkanes.

About 2 min read
  • Use the general formula of alkanes
  • Name straight-chain alkanes
  • Write equations for complete combustion
  • Describe substitution by halogens
Before you start

You know that carbon forms four covalent bonds, and that a hydrocarbon contains only carbon and hydrogen.

A saturated family

Alkanes are a homologous series of saturated hydrocarbons. Every carbon-carbon bond is a single bond, so each carbon has as many hydrogen atoms as possible. As chains grow longer, the boiling point rises because forces between molecules increase.

Definition: homologous series

A family of compounds with the same general formula and similar chemical properties.

Definition: saturated

Containing only single bonds between carbon atoms.

General formula of alkanes

CnH2n+2

Naming alkanes

The first four alkanes are methane, ethane, propane and butane. Their names show the number of carbon atoms: meth is 1, eth is 2, prop is 3 and but is 4.

Reactions of alkanes

Alkanes are generally unreactive, but they burn well. Complete combustion in plenty of oxygen forms carbon dioxide and water. Limited oxygen can produce carbon monoxide, which is toxic. In ultraviolet light, a halogen can replace a hydrogen atom in a substitution reaction.

Complete combustion of propane

Write a balanced equation for the complete combustion of propane.

Maths
C₃H₈ + O₂ → CO₂ + H₂O
balance carbon: 3CO₂
balance hydrogen: 4H₂O
balance oxygen: 5O₂
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

UV light is required

Alkanes react with chlorine by substitution only in the presence of ultraviolet light.

Exam focus
Say it like this“Alkanes are saturated because they contain only single carbon-carbon bonds.”
Not like this“Alkanes are saturated because they are full.” — Refer to the type of bonds in the molecule.
Common mistakeWriting an addition reaction for alkanes. Alkanes undergo substitution with halogens.

Practice

Give the formula of butane.
C₄H₁₀.
Find the formula of an alkane with 8 carbon atoms.
Using CnH2n+2 gives C₈H₁₈.
Name the toxic gas formed by incomplete combustion.
Carbon monoxide, CO.
Going further
Write the equation for the reaction of methane with chlorine in UV light and name the organic product.
CH₄ + Cl₂ → CH₃Cl + HCl. The organic product is chloromethane.

Quick check

Alkanes quick check

What is the general formula of alkanes?

Each carbon chain carries two hydrogens per carbon plus one at each end.

Which compound is an alkane?

With three carbons, the general formula requires eight hydrogens.

What condition lets chlorine react with methane?

This energy splits chlorine molecules so that substitution can begin.
Key takeaways
  • Alkanes are saturated hydrocarbons with the formula CnH2n+2.
  • They are named by chain length.
  • Complete combustion forms carbon dioxide and water.
  • Halogens substitute in ultraviolet light.
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