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.

Halogenation:

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

F2>Cl2>Br2>I2

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. 

Mechanism:

 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.         

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.

Oxidation:

  • 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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