Halogenation

In chemistry, halogenation is a chemical reaction that replaces a hydrogen atom with a halogen atom. More specific words exist that specify which halogen: fluorination, chlorination, bromination, and iodination.

Free radical halogenation

This reaction is typical of alkanes and alkyl-substituted aromatics. We use as an example the chlorination of methane, a reaction which has some industrial significance. We first need an initation step to generate halogen atoms:

Cl₂ --> 2 Cl^.

Once the halogen atoms are formed, the chain reaction can begin:

CH₄ + Cl^. --> CH₃^. + HCl

CH₃^. + Cl₂ --> CH₃Cl + Cl^.

Notice how each step generates a reactive intermediate (CH3^. or Cl^.) which is used in the other step. In this way the chain reaction, once begun by the initiation step, is kept going. The net reaction is:

CH₄ + Cl₂ --> CH₃Cl + HCl.

In most hydrocarbons there are more than one possible product, depending on which hydrogen is replaced. Butane (CH₃-CH₂-CH₂-CH₃), for example, can be chlorinated at the "1" position to give 1-chlorobutane (CH₃-CH₂-CH₂-CH₂Cl) or at the "2" position to give 2-chlorobutane (CH₃-CH₂-CHCl-CH₃). The product distribution depends on relative reaction rates: in this case the "2" position of butane reacts faster and 2-chlorobutane is the major product. Free radical halogenation generally proceeds in the following order:

• Fastest
• Carbons with one or more aryl substituents (benzylic positions)
• Carbons with three alkyl substituents (tertiary positions)
• Carbons with two alkyl substituents (secondary positions)
• Carbons with one or zero substituents (primary positions)
• Slowest

Chlorination is generally less selective than bromination. Fluorination is not only even less selective than chlorination, but also highly exothermic and care must be taken to prevent an explosion or a runaway reaction. Free radical iodination is usually not possible because iodine is less reactive than the other halogens.

Halogenation of ketones

The positions next to the carbonyl group are easily halogenated, due to their ability to form enolates in basis solution. A similar mechanism occurs in acid solution where the ketone can be enolized. An example is the bromination of acetone in basic solution:

CH₃-CO-CH₃ + OH^- --> CH₃-CO-CH₂^- + H₂O

CH₃-CO-CH₂^- + Br₂ --> CH₃-CO-CH₂Br + Br^-

This reaction has two steps, but is not a chain reaction. Multiple bromination can occur giving CH₃-CO-CHBr₂ etc.

Electrophilic halogenation

This reaction is typical of aromatic compounds. An example is the chlorination of benzene:

(Discusstion of this reaction will require structural formulas) Overall: C6H6 + Cl2 --> C6H5Cl + HCl

A few types of aromatic compounds, such as phenol, will react without a catalyst, but for typical benzene derivatives a Lewis acid catalyst is required. Typical Lewis acid catalysts include AlCl₃, FeCl₃, FeBr₃, and ZnCl₂. These work by forming a highly electrophilic complex, such as AlCl₄-Cl, which attacks the benzene ring.

See also

Halogenoalkanes Halogenoarenes Electrophilic substitution