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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