Alkanes and Free Radical Substitution

Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They are relatively unreactive due to strong, non-polar C–C and C–H bonds. Their most important reactions are combustion and free radical substitution. This lesson covers alkane properties, combustion, and a thorough treatment of the free radical substitution mechanism.

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Properties of Alkanes

Bonding and Shape

• All C atoms in alkanes are sp³ hybridised, giving a tetrahedral arrangement around each carbon with bond angles of approximately 109.5°.
• C–C bond enthalpy: approximately 348 kJ mol⁻¹. C–H bond enthalpy: approximately 412 kJ mol⁻¹. Both are relatively strong, contributing to alkane stability.
• C–H bonds have very low polarity (electronegativity: C = 2.5, H = 2.1), so alkanes are essentially non-polar.

Physical Properties

Property	Trend	Explanation
Boiling point	Increases with chain length	More electrons → stronger London dispersion forces
Boiling point	Decreases with branching	More compact shape → smaller surface area → weaker London forces
Solubility in water	Insoluble	Cannot form hydrogen bonds with water
Density	Less dense than water	All alkanes float on water

Key Definition: London dispersion forces (also called induced dipole–induced dipole forces) are temporary intermolecular forces arising from uneven electron distribution. They are the only intermolecular forces present between non-polar molecules.

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Combustion of Alkanes

Alkanes are used extensively as fuels. Combustion is their most economically important reaction.

Complete Combustion

In excess oxygen, an alkane burns to produce carbon dioxide and water:

CₙH₂ₙ₊₂ + (3n + 1)/2 O₂ → nCO₂ + (n + 1)H₂O

Specific examples:

CH₄ + 2O₂ → CO₂ + 2H₂O

C₂H₆ + 3½O₂ → 2CO₂ + 3H₂O (or multiply through: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O)

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O (or: 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O)

Incomplete Combustion

In limited oxygen, incomplete combustion produces carbon monoxide (CO) and/or carbon (soot, C) along with water:

CH₄ + 1½O₂ → CO + 2H₂O

CH₄ + O₂ → C + 2H₂O

Why this matters:

• CO is a toxic, colourless, odourless gas. It binds irreversibly to haemoglobin (forming carboxyhaemoglobin), preventing oxygen transport. This can be fatal.
• Soot (particulates) contributes to respiratory disease and climate change (black carbon absorbs radiation).

Pollutants from Combustion

Pollutant	How it forms	Environmental effect
CO₂	Complete combustion	Greenhouse gas → global warming
CO	Incomplete combustion	Toxic; forms carboxyhaemoglobin
Particulates (C)	Incomplete combustion	Respiratory problems; global dimming
NO and NO₂ (NOₓ)	N₂ + O₂ at high temperatures in engines	Acid rain (HNO₃); photochemical smog; respiratory irritant
SO₂	Combustion of sulfur impurities in fuels	Acid rain (H₂SO₃ / H₂SO₄); respiratory irritant

Exam Tip: When writing combustion equations, balance C first, then H, then O last. If you get a fractional coefficient for O₂, multiply the entire equation through by 2.

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Free Radical Substitution — The Mechanism

Alkanes react with halogens (Cl₂ or Br₂) in the presence of ultraviolet (UV) light via free radical substitution. This is the only organic reaction mechanism at A-Level involving radicals.

Conditions and Reagents

	Detail
Reagents	Alkane + halogen (Cl₂ or Br₂)
Conditions	UV light (provides energy for homolytic fission)
Type of mechanism	Free radical substitution
Type of bond fission	Homolytic (each atom gets one electron)

Step 1: Initiation

UV light provides sufficient energy to break the Cl–Cl bond by homolytic fission, producing two chlorine radicals:

Cl₂ → 2Cl•