Alkanes

Full Notes

Alkanes

• Alkanes are saturated hydrocarbons, meaning they contain only single C–C and C–H bonds.
• General formula: C_nH_2n+2
• They are non-polar and insoluble in water.

Alkanes are generally unreactive

• C–C and C–H bonds are strong and non-polar.
• Molecules have low polarity, so they don't attract polar reagents.
• Require high energy to initiate reactions (e.g., UV for halogenation).

Production of Alkanes – Hydrogenation of Alkenes

Alkenes react with hydrogen gas in the presence of a nickel or platinum catalyst and heat to form alkanes.

Conditions:

• Catalyst: Ni or Pt
• Temp: 150–200°C

This is also an example of a reduction reaction.

Cracking of Long-Chain Alkanes

Cracking breaks large hydrocarbon molecules into smaller, more useful ones.

Shorter chain hydrocarbons are in greater demand for use of fuels (they ignite more easily and are less likely to undergo incomplete combustion).

• Catalyst: Al₂O₃ or zeolite
• Heat: 450–750°C

Example Cracking reaction

C₁₀H₂₂ → C₈H₁₈ + C₂H₄

Combustion of Alkanes

Heat energy is released when alkanes undergo combustion as the process is exothermic. This released heat energy can be harnessed and used for other things - such as generating electricity. This makes alkanes useful as fuels.

Combustion describes the reaction that occurs when an alkane rapidly reacts with oxygen, at a high temperature.

Complete vs. 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 of methane:

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).

Example Equations for incomplete combustion:

• CH₄ + 1.5O₂ → CO + 2H₂O
• CH₄ + O₂ → C + 2H₂O

Free-Radical Substitution

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

Example Methane + Chlorine

C₂H₆ + Cl₂ → C₂H₅Cl + HCl

Mechanism:

• Step 1: Initiation
  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 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.
• Step 3: Termination
  Radicals combine to form stable (non-radical) molecules, stopping the 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.

Environmental Issues and Pollutants

For more detail see nitrogen and sulfur.

Internal Combustion Engines produce:

• Carbon Monoxide, CO
  binds to haemoglobin and is toxic
• Nitrogen Oxides, NO and NO₂
  formed when N₂ and O₂ react at high temps; causes acid rain and smog
• Carbon, C (soot)
  causes respiratory problems
• Unburnt hydrocarbons
  contribute to photochemical smog

Catalytic Converters are used in vehicles to remove harmful gases:

Summary

• Alkanes are saturated hydrocarbons with strong C–C and C–H bonds, making them generally unreactive.
• They can be produced by hydrogenation of alkenes.
• Cracking of longer chain alkanes can produce shorter chain alkanes (in higher demand for use as fuels)
• Combustion of alkanes can be complete (producing CO₂ + H₂O) or incomplete (producing CO, soot, and hydrocarbons).
• They undergo halogenation by free-radical substitution under UV light.
• Combustion of alkanes contributes to pollution; catalytic converters reduce harmful emissions.