15.2    Reactions of Alkanes

Reactivity of alkanes

• Alkanes are saturated and generally unreactive because they are non-polar, hence they are unattractive towards nucleophiles and electrophiles
• Alkanes will only react with non-polar reagents in the presence of heat or ultraviolet light

Combustion

• Alkanes undergo complete combustion under excess oxygen gas to give carbon dioxide and For example:
  CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)        ΔH° = -890 kJ mol⁻¹

• In general,

• Larger alkanes are more difficult to ignite. This is because alkanes only burn in the gaseous state and larger alkanes have stronger van der Waals’ forces that hold them Therefore more energy is required to vaporise it.
• When the supply of oxygen is limited, alkanes undergo incomplete combustion. The possible products are carbon monoxide, carbon and water
  For example,
  2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(g)      or CH4(g) + O2(g) → C(s) + 2H2O(g)

Halogenation

• Alkanes undergo halogenation via free-radical substitution to give a range of products. In free-radical substitution, hydrogen atoms in the molecule are gradually substituted by halogen atoms
• Reagent   : Chlorine gas, Cl2 or bromine gas, Br2
  Condition : The presence of ultraviolet(UV) light or sunlight
  Product :  Halogenoalkanes  (alkanes  with halogens)
• Free-radical substitution proceeds via a chain reaction. During a chain reaction, for every reactive species you start off with, a new one is generated at the end, and this keeps the process going
  • When methane reacts with chlorine in sunlight, the greenish-yellow colour fades and steamy acidic fumes of hydrogen chloride can be observed.
  • However, this is a chain reaction and does not end here. More hydrogen atoms will be substituted as long as there are sufficient chlorine atoms.

• The mechanism of free-radical substitution:
  i) Step 1: Initiation step
  The reaction is initiated by the homolytic ftssion of chlorine molecule in the presence of  UV light, forming chlorine free radicals.
  Step 2: Propagation step
  The free radicals then attack the methane molecules, leading to a chain reaction.
  Step 3: Termination step
  The reaction ends with the free radicals joining up with each other.
  

• To produce mainly tetrachloromethane, an excess of chlorine gas is used so that all the hydrogen atoms eventually get substituted

• Bromine gas reacts in a similar manner but the reaction is slower