Halogenation

Halogenation is the replacement of one or more hydrogen atoms in an organic compound by a halogen (fluorine, chlorine, bromine or iodine). Unlike the complex transformations of combustion, the halogenation of an alkane appears to be a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed. The chlorination of methane, shown below, provides a simple example of this reaction.

CH₄   +   Cl₂   +   energy   →   CH₃Cl   +   HCl

Since only two covalent bonds are broken (C-H & Cl-Cl) and two covalent bonds are formed (C-Cl & H-Cl), this reaction seems to be an ideal case for mechanistic investigation and speculation. However, one complication is that all the hydrogen atoms of an alkane may undergo substitution, resulting in a mixture of products, as shown in the following unbalanced equation. The relative amounts of the various products depend on the proportion of the two reactants used. In the case of methane, a large excess of the hydrocarbon favors formation of methyl chloride as the chief product; whereas, an excess of chlorine favors formation of chloroform and carbon tetrachloride.

CH₄   +   Cl₂   +   energy    →   CH₃Cl   +   CH₂Cl₂   +   CHCl₃   +   CCl₄   +   HCl

The following facts must be accomodated by any reasonable mechanism for the halogenation reaction.

1. The reactivity of the halogens decreases in the following order: F₂ > Cl₂ > Br₂ > I₂.
2. We shall confine our attention to chlorine and bromine, since fluorine is so explosively reactive it is difficult to control, and iodine is generally unreactive.
3. Chlorinations and brominations are normally exothermic.
4. Energy input in the form of heat or light is necessary to initiate these halogenations.
5. If light is used to initiate halogenation, thousands of molecules react for each photon of light absorbed.
6. Halogenation reactions may be conducted in either the gaseous or liquid phase.
7. In gas phase chlorinations the presence of oxygen (a radical trap) inhibits the reaction.
8. In liquid phase halogenations radical initiators such as peroxides facilitate the reaction.

The most plausible mechanism for halogenation is a chain reaction involving neutral intermediates such as free radicals or atoms. The weakest covalent bond in the reactants is the halogen-halogen bond (Cl-Cl = 58 kcal/mole; Br-Br = 46 kcal/mole) so the initiating step is the homolytic cleavage of this bond by heat or light, note that chlorine and bromine both absorb visible light (they are colored). A chain reaction mechanism for the chlorination of methane has been described.
Bromination of alkanes occurs by a similar mechanism, but is slower and more selective because a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as reflected by the higher bond energy of H-Cl than H-Br.

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