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.
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.
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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