We know that halogenation of alkane take place via free radical substitution reaction mechanism and stability of carbocation follow as 3°> 2° > 1° and rate of reaction of Halogens with alkanes, follows the order: F2 > Cl2 > Br2 > I2
(1) Iodination is very slow and a reversible and carried out in the pressure of oxidizing agents such as HIO3 or HNO3 to oxidised HI to I2.
(2) Chlorination of most alkanes whose molecules contain more than two carbon atoms gives a mixture of isomeric monochloro products. The percentages given are based on the total amount of monochloro products formed in each reaction. The numbers of mono-halogenated products formed are equal to the number of different type of H-atoms.
The ratios of products that we obtain in chlorination reactions of higher alkanes are not identical with what we would except if all the hydrogen atoms of the alkane were equally reactive.
We find that there is a correlation between reactivity of different hydrogen atoms and the type of hydrogen atom (1 ° 2 ° or 3 ° ) being replaced. The tertiary hydrogen atoms of an alkane are most reactive, secondary hydrogen atoms are next most reactive and primary hydrogen atoms are the least reactive.
(3) Bromine is less reactive toward alkanes in general than chlorine, but bromine is highly selective means the difference between relative rate of bromination is large. Bromine shows a much greater ability to discriminate among the different types of hydrogen atoms. The reaction of isobutane and bromine, for example, gives almost exclusive replacement of the tertiary hydrogen atom.
(5) Fluorination is too violent to be controlled, Fluorine being much more reactive than chlorine is even less selective than chlorine. Because the energy of activation for the abstraction of any type of hydrogen by a fluorine atom is low, there is very little difference in the rate at which a 1°, 2° or 3° hydrogen reacts with fluorine. Reactions of alkanes with fluorine give (almost) the distribution of products that we would expect if all of the hydrogens of the alkane were equally reactive.
(5) Relative rate of substitution of H-atoms of alkane in free radical halogenation is:
Illustrative Example:
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