Chemical reactions of alkanes

• Alkanes are unreactive due to:

  • C-C and C-H sigma bonds are strong- have high bond enthalpies
  • C-C bonds are non-polar
  • the electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar
• In a plentiful supply of oxygen, alkanes burn completely to produce carbon dioxide and water.
• When oxygen is limited during combustion, the hydrogen atoms in the alkene are always oxidized to water but combustion of the carbon may be incomplete, this can form the toxic gas carbon monoxide or carbon itself as soot.
• Reactions with halogens

  In the presence of sunlight, alkanes react with the halogens. The high-energy ultraviolet radiation present in sunlight provides an initial energy for a reaction to take place.
• The mechanism for the halogenation of an alkane is radical substitution.
• The mechanism for radical substitution takes place in three stages:

  1. initiation
  2. propagation
  3. termination
• In the initiation stage, the reaction is started when the covalent bond in a halogen molecule is broken by homolytic fission. Each halogen atom takes one electron from the pair forming two highly reactive halogen Radicals the energy for this bond fission is provided by UV radiation.
• A radical is a very reactive species with an unpaired electron..
• In the propagation stage, the reaction propagates through two propagation steps a chain reaction.

  In the first propagation step a halogen radical reacts with a C-H Bond in the alkane forming an alkane radical and a hydrogen halide molecule.

  In the second propagation step, each alkane radical reacts with another halogen molecule formula forming a new organic product and a new radical.
• In the termination stage, two radicals collide, forming a molecule with all electrons paired.
• Limitations of radical substitution in organic synthesis:

  • Further substitution can take place on the alkene when moving all the hydrogens from the molecule
  • Substitution at different positions in the carbon chain results in a mixture of substituted isomers as different hydrogens can be substituted.