Reactions of Alkanes
Quick Notes
- Alkanes are chemically unreactive.
- High bond enthalpy and non-polar σ-bonds make them resistant to attack by most reagents.
- Alkanes are used as fuels because they release energy when burned.
- Combustion:
- Complete: produces CO2 and H2O (carbon fully oxidised).
- Incomplete: produces CO and/or C (soot) when oxygen is limited.
- Reaction with Cl2 or Br2 in UV light occurs via free radical substitution.
- Three steps: initiation, propagation, termination.
- Limitations: further substitution and substitution at multiple positions form mixtures of products.
Full Notes
Reactivity of Alkanes
Alkanes are generally unreactive due to:
- High bond enthalpies of C–C and C–H bonds (require large amounts of energy to break).
- Very low polarity of C–C and C–H bonds (don’t react with polar reagents).
As a result, alkanes do not easily undergo reactions like addition or substitution under normal conditions.
Combustion of Alkanes
Heat energy is released when alkanes undergo combustion (an exothermic process), making them useful as fuels.
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
Radical Substitution Mechanism
When alkanes are exposed to ultraviolet light in the presence of Cl2 or Br2, a substitution reaction occurs via free radicals.
The mechanism has three steps: initiation, propagation, and termination.
For Example: Methane reacts with chlorine under UV light to form chloromethane.
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.
Be aware that further substitution can occur (see below), 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.
Limitations of Radical Substitution
Radical substitution of alkanes is difficult to control because:
- Further substitution: multiple H atoms can be substituted, giving CH3Cl, CH2Cl2, CHCl3, CCl4.
- Different positions: in larger alkanes, substitution can occur at different chain positions, producing structural isomers.
Summary
- Alkanes are unreactive due to strong, non-polar bonds.
- Combustion can be complete (producing CO2 + H2O) or incomplete (producing CO or C).
- Alkanes react with halogens in UV light by radical substitution.
- The mechanism involves initiation, propagation, and termination steps.
- Radical substitution produces mixtures due to further substitution and different chain positions.