Chemical Properties of Alkanes | Halogenation and Oxidation in alkanes

Chemical Properties of Alkanes::

Alkanes being saturated compounds mostly show substitution reactions. Their oxidation is very difficult thus even strong oxidizing agents like KMnO4, K2Cr2O7 etc cannot oxidize them.


Halogens replace hydrogen from alkanes and it is called halogenation of alkanes. The order of reactivity of halogens with alkanes is


Fluorine shows a reaction to explosive violence. The reaction of iodine is slow and reversible.

Halogenation is a substitution reaction and it proceeds through the free radical mechanism.

  • In dark no reaction occurs CH4   +   Cl2 -> No reaction
  • In direct sunlight reaction occurs with explosive violence and carbon soot is produced
  • CH4   +   2Cl2  —> C   +   4 HCl
  • In diffused sunlight a chain reaction occurs.

CH4        +    Cl2 —> CH3Cl   +   HCl

CH3Cl    +     Cl2 —> CH2Cl2  +     HCl

CH2Cl2  +       Cl2 —> CHCl3    +     HCl

CHCl3    +     Cl2 —> CCl4       +    HCl

If Cl2 is taken in excess then tetra chloromethane will be major product. If methane is taken in excess then chloromethane will be the major product. 


 It is a free radical substitution reaction. Its steps are divided into three categories.

Step-I  Initiation Reaction:

   Reaction Initiating Step: The step in which molecule splits and free radical is generated and due to which then reaction starts is called initiation step.         

chlorine free radical formation

Step-II   Reaction Propagation Step:

The step in which one free-radical gives birth to another free radical is called propagation. Following are reaction propagation steps.

Step-III  Reaction Terminating Step

The step in which two free radicals combine to give a molecule is called the termination step.


  • Combustion:

Alkanes on heating strongly in air start burning. A combustion reaction is an oxidation reaction.

Alkanes on burning in sufficient supply of air produce carbon dioxide and water vapours and heat is liberated.

CH4        +     2O2        →    CO2        +    2H2O     + 891kJ

But in limited supply of air carbon monoxide and water vapours is produced and less heat is liberated.

   2CH4      +     3O2        →    2CO       +    4H2O    + Heat

And in very little supply of air carbon soot and water are produced and least heat is liberated.

    CH4      +    O2      →      C    +   2H2O     + Heat

              Ethane or burning also produce same products.

2C2H6    +    7O2     →    4CO2      +    6H2O   ΔH = – 3081 KJ   or  -1540.5 KJ/mol

  • Catalytic Oxidation:

Lower alkanes on heating in air in presence of copper catalyst and applying pressure, oxidize to mixture of useful products.

Alkanes first oxidize to corresponding alcohols

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