Halogenation is a chemical reaction that involves the reaction of a compound, usually an organic compound, with halogen. Dehalogenation is the reverse, the removal of halogen from a molecule.[1] The pathway and stoichiometry of halogenation depends on the structural features and functional groups of the organic substrate as well as the halogen. Inorganic compounds, for example. metals, also undergo halogenation.
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There are several processes for the halogenation of organic compounds, including free radical halogenation, ketone halogenation, electrophilic halogenation, and halogen addition reaction. The determining factors are the functional groups. Saturated hydrocarbons typically do not add halogens but undergo free radical halogenation, involving substitution of hydrogen atoms by halogen. Unsaturated compounds, especially alkenes and alkyne]s, add halogens. Aromatic compounds compounds are subject to electrophilic halogenation.[2]
The facility of halogenation is influenced by the halogen. Fluorine and chlorine are more electronegative and are more aggressive halogenating agents. Bromine is a weaker halogenating agent than both fluorine and chlorine, while iodine is least reactive of them all. The facility of dehydrohalogenation follows the reverse trend: iodine is most easily removed from organic compounds and organofluorine compounds are highly stable. Bromination and iodination are more likely to substitute at the beta carbon.[3]
Organic compounds, saturated and unsaturated alike, react readily, usually explosively, with fluorine. This process requires highly specialized conditions. In practice, organic compounds are fluorinated electrochemically. Reactions occur at an anode using hydrogen fluoride is used as the source of fluorine. The method is called electrochemical fluorination. Aside from F2 and its electrochemically generated equivalent, a variety of flourinating reagents are known such as xenon difluoride and cobalt(III) fluoride.
Chlorination is generally highly exothermic. Both saturated and unsaturated compounds react directly with chlorine, the former usually requiring UV light to initiate homolysis of chlorine. Chlorination is conducted on a large scale industrially, major process include routes to 1,2-dichloroethane (precursor to PVC) and various chlorinated ethanes as solvents. Competitive with direct chlorination (use of Cl2) is oxychlorination which uses hydrogen chloride in combination with oxygen.[4]
Bromination is more selective than chlorination because the reaction is less exothermic. Most commonly bromination is conducted by the addition of Br2 to alkenes. Bromination of saturated hydrocarbons and aromatic substrates is common in nature, giving rise to a host of organobromine compounds. The usual catalyst is the bromoperoxidase which utilizes bromide in combination with oxygen as an oxidant. An example of bromination can be found in the organic synthesis of the anesthetic halothane from trichloroethylene:[5]
Iodine is the least reactive halogen and is reluctant to react with most organic compounds. The addition of iodine to alkenes is the basis of the analytical method called the iodine number, a measure of the degree of unsaturation for fats. The iodoform reaction involves degradation of methyl ketones.
The addition of halogens to alkenes is usually thought to proceed via intermediate salts of halonium ions. In special cases, such intermediates have been isolated.[6]
Alkanes react via free-radical routes, and the regiochemistry of these reactions is usually determined by the relative weakness of the available C-H bonds. The preference for reaction at tertiary and secondary positions results from greater stability of the corresponding free radicals and the transition state leading to them. The bond-dissociation enthalpy to form a free radical by a breaking a bond between a hydrogen atom and a carbon atom is greatest for a methyl carbon, and it decreases for a primary carbon, a secondary carbon, and a tertiary carbon. The more highly substituted the carbon, the less energy is required to form the free radical. .
Specific halogenation methods are the Hunsdiecker reaction (from carboxylic acids) and the Sandmeyer reaction (arylhalides).
All elements aside from argon, neon, and helium form fluorides by direct reaction with fluorine. Chlorine is slightly more selective, but still reacts with most metals and heavier nonmetals. Following the usual trend, bromine is less reactive and iodine least of all. Of the many reactions possible, illustrative is the formation of gold(III) chloride by the chlorination of gold. The chlorination of metals is usually not very important industrially since the chlorides are more easily made from the oxides and the hydrogen halide. Where chlorination of inorganic compounds is practiced on a relatively large scale is for the production of phosphorus trichloride and sulfur monochloride.[7]