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Chemical Properties of Alkanes

Alkanes are quite unreactive. They do not react with many of the usual reagents. Alkanes are inert (or less reactive). Inertness of alkanes is due to the presence of strong carbon-carbon and carbon-hydrogen bonds that are difficult to break. However, other atoms or radicals under drastic conditions of reaction, such as high temperature, presence of ultraviolet light or a catalyst, etc, can substitute hydrogen. Alkanes undergo a few reactions, the most important being halogenations, oxidation and thermal decomposition (cracking).

Alkanes also undergo halogenations if a mixture of alkane and halogen is heated to 600 K. For example, when a mixture of methane and chlorine is exposed to diffused sunlight or ultraviolet light, or heated to 600 K, the hydrogen atoms are replaced by chlorine atoms, one after another.
CH4 + Cl2                CH3Cl              +       HCl
                      or heated to 600 K  methyl chloride
CH3Cl   +    Cl2             CH2Cl2         +        HCl
                                          methylene chloride 
CH2Cl2  +    Cl2             CHCl3       +            HCl

CHCl3   +    Cl2            CCl4               +               HCl
                                      carbon tetrachloride

The nature of products formed depends upon the amount of chlorine. If excess of chlorine is used, then the product contains larger amounts of carbon tetrachloride. Smaller amounts of chlorine give more of less-chlorinated products (such as methyl chloride) in this reaction.

In case the reaction is allowed to take place for a smaller duration of time, then the less chlorinated products are formed in larger amounts. In case the duration of the reactions is larger, then highly chlorinated products are formed in larger amounts.

The order of reactivity of halogens with alkanes is F2 > Cl2 > Br2 > I2. Fluorine reacts with alkanes with a violent explosion. The reaction with chlorine is less vigorous than fluorine and with bromine less vigorous than chlorine. The reaction with iodine is slow and reversible
CH4 + I2CH3I + HI
The ease of substitution of a hydrogen atom by a halogen atom is tertiary > secondary > primary.

Mechanism of Halogenation of Alkanes

  1. Initiation Step
    In this process free radicals are formed. Free radicals are highly reactive species and are electrically neutral. The breaking up of a covalent bond in the chlorine molecule forms chlorine-free radicals.
Cl - Cl     Cl    +     Clchlorine               chlorine free
molecule                   radical
Thus, the chlorine molecule undergoes homolytic fission. Homolytic fission is the breaking of a covalent bond so that the bonded atoms retain one electron out of the shared pair of electrons. The step of homolytic fission of chlorine requires a large amount of energy (243 KJ mol-1) that is supplied by heat or ultraviolet light.
  1. Propagation step
    Free radicals have an unpaired electron. Therefore, they have a tendency to pair up with other electrons to complete their octet. This accounts for the highly reactive nature of free radicals.

    The chlorine-free radicals, being reactive, remove a hydrogen atom from ethane to generate a methyl-free radical ( CH3).
    CH4 + Cl CH3      +   HCl
    In this step, one free radical of Cl is used up but another free radical is also produced which can react to generate more free radicals. Such a reaction is called a chain reaction or the propagation step.
    CH3 + Cl2CH3Cl + Cl
    Cl + CH3Cl  CH2Cl + HCl
    CH2Cl + Cl2 CHCl2 + HCl
    and so on.
  2. Termination step
    The chain reaction comes to end when two free radicals combine with each other and do not form new free radicals.
    Cl +  CH3CH3Cl
     +  CH2Cl CH2Cl2
    CH3 +  CH3CH3-CH3
    The termination step generally occurs when most of the chlorine and methane have already reacted.


Alkanes are quite unreactive, but they can, however, be oxidized. The nature of products formed depends upon the oxidizing agent and the conditions of the reaction.
  1. Burning in excess of oxygen
    Alkanes burn with a blue flame in air or oxygen. In this process alkanes get oxidized to carbon dioxide and water.
    CH4 + 2O2  CO2 + 2H2O
  2. Burning in a limited supply of oxygen
    Alkanes burn with a sooty flame (smoky flame). Incomplete oxidation of alkanes gives carbon black (a variety of carbon is used in the manufacture of printing inks and tyres).

CH4   + O2  C        + 2H2O

  1. Controlled oxidation
    On controlled oxidation alkanes give alcohols which are further oxidized to aldehydes (or ketones), acids and finally to carbon dioxide and water.

methane       methyl                   formal                formic
                     alcohol                  dehyde               acid


Higher alkanes when heated to a high temperature (700-870 K) and under pressure undergo thermal decomposition to give several smaller alkane molecules and other products. Cracking can also be done by heating alkanes under pressure in the presence of a suitable catalyst. This reaction follows a free-radical me

The nature of the product formed on cracking of alkanes depends upon several factors, such as the structure of the alkane, pressure, temperature, nature of catalyst, etc.

Cracking is used for converting higher alkanes into lower alkanes that are more useful and in great demand.
C10H22   C5H12 + C4H10  + CH4 + H2 +  C4H8 + .....  

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