Reaction of Alkanes with Halogens
Alkanes react with halogens through radical mechanisms. This reaction involves the substitution of one or several hydrogens of the alkane with halogens.
Mechanism of Radical Halogenation
The mechanism of radical halogenation consists of three stages: initiation, propagation, and termination. In initiation, the halogen molecule breaks homolytically generating radicals. In the propagation stage, the substitution of the alkane's hydrogens with halogens occurs. When the reactants are exhausted, the radicals in the medium combine, resulting in the termination stage.
Reactivity of Halogens
The first stage of propagation determines the rate of the reaction. For fluorine, this stage has a low activation energy, making fluorine the most reactive halogen. In the case of iodine, the activation energy is very high, and the reaction does not occur. Order of reactivity in radical reactions: F2>Cl2>Br2>I2. In summary, iodine is not reactive in radical halogenation, and fluorine reacts violently.
Polyhalogenations
The halogenation reaction is difficult to stop, as the halogenated product is more reactive than the original alkane. To avoid this problem, known as polyhalogenations, an excess of the alkane is used.
Stability of Radicals
The mechanism of these reactions proceeds with the formation of an intermediate called a radical whose stability depends on the number of substituents surrounding the carbon that contains the lone electron. Radicals formed during the propagation stage are stabilized by hyperconjugation. The order of stability of the radicals is as follows: tertiary > secondary > primary.
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