Alkanes

Full Notes

Alkanes are saturated hydrocarbons, meaning they contain only single C-C and C-H bonds and have a general formula of C_nH_2n+2

They are non-polar and insoluble in water.

Cycloalkanes are also saturated hydrocarbons but have ring structures. Their general formula is C_nH_2n.

Fractional Distillation of Crude Oil

Crude oil is a complex mixture of alkanes with varying chain lengths.

It is separated into fractions using fractional distillation, which relies on differences in boiling points.

• Crude oil is vaporised and passed into a column with a temperature gradient (hot at the bottom, cool at the top).
• Short-chain alkanes condense at the top (low boiling point) and long-chain condense lower down.

Each fraction contains hydrocarbons with similar chain lengths and boiling points, all having their own use (primarily as different fuels).

Cracking and Reforming

Longer alkanes can be converted into more useful smaller molecules via cracking:

• Thermal cracking uses a high temperature and pressure and produces mainly alkenes
• Catalytic cracking uses a zeolite catalyst and lower pressure than thermal cracking and produces mainly branched and aromatic hydrocarbons

Reforming converts straight-chain alkanes into branched or cyclic ones for improved combustion (used in petrol).

Alkanes as Fuels

Alkanes make good fuels as they readily undergo combustion, releasing heat energy. Combustion can be complete or incomplete.

Combustion

Complete combustion occurs when there is enough oxygen present and carbon can be fully oxidised, forming carbon dioxide as a product (and water).

Example Equation for complete combustion:
Methane (CH₄):
CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion occurs when there is limited oxygen present and carbon can’t be fully oxidised, meaning carbon monoxide (CO) or carbon (soot) gets formed as a product (and water).

ExampleEquations for incomplete combustion:

• Carbon monoxide (CO) production:
  CH₄ + 1.5O₂ → CO + 2H₂O
• Carbon (C, soot) production:
  CH₄ + O₂ → C + 2H₂O

Pollutants from Internal Combustion Engines

The use of Internal combustion engines can produce several pollutants, released in exhaust gases

• Carbon monoxide (CO) a toxic gas that binds to hemoglobin in blood.
• Nitrogen oxides (NO_x) formed at high temperatures when N₂ and O₂ react. Specifically, NO₂ contributes to acid rain and photochemical smog.
• Carbon (C, soot) causes respiratory issues and global dimming.
• Unburned hydrocarbons contribute to smog formation.

Controlling Pollutants

Catalytic converters in car exhausts convert harmful gases to safer ones.

For example helping convert carbon monoxide (CO) and nitrogen monoxide (NO) into carbon dioxide (CO₂) and nitrogen (N₂)

Alternative Fuels

Biofuels like biodiesel (from vegetable oils) and alcohols (e.g. ethanol from fermentation) offer renewable alternatives to fossil fuels.

Pros: Renewable, potentially carbon-neutral
Cons: Land use, food crop competition, purification energy costs

Radicals and Homolytic Fission

Homolytic fission has been covered in more detail here.

A radical is a species with an unpaired electron, shown with a single dot (•).

Radicals form by homolytic fission when a bond breaks and each atom takes one electron.

Radical Substitution of Alkanes

Alkanes react with halogens (Cl₂, Br₂) under UV light to form halogenoalkanes by free radical substitution.

Example methane + chlorine

Mechanism:

Step 1: Initiation (Radicals Are Formed)

• UV light provides energy to break the Cl–Cl bond by homolytic fission.
• Each chlorine atom ends up with an unpaired electron (•), making it a radical.

Step 2: Propagation (Radicals React and Regenerate)

• Radicals react to form new radicals in a chain reaction.
• Chlorine radical reacts with methane, forming a methyl radical
• Methyl radical reacts with Cl₂, forming chloromethane and a new Cl• radical:
  The process continues, leading to further substitutions.

Step 3: Termination (Radicals Are Removed)

• Radicals combine to form stable (non-radical) molecules, stopping the reaction.
• There are several possible termination reactions:
  Termination stops the chain reaction.

Matt’s exam tip

Be aware that further substitution can occur, forming CH₂Cl₂, CHCl₃, and CCl₄ and remember that UV light is required to initiate the reaction by homolytic fission.
This process can lead to a mixture of products (mono-, di-, tri-substituted etc.) and is not selective, making it unsuitable for controlled synthesis.

Summary

• Alkanes are saturated hydrocarbons with the general formula C_nH_2n+2.
• Fractional distillation separates crude oil into useful fractions by boiling point.
• Cracking produces smaller, more useful molecules and reforming improves fuel quality.
• Combustion can be complete or incomplete and incomplete combustion forms CO or soot.
• Exhaust pollutants include CO, NO_x, SO₂, particulates and unburned hydrocarbons.
• Catalytic converters reduce harmful emissions and biofuels offer renewable alternatives.
• Free radical substitution of alkanes proceeds via initiation, propagation and termination.