Halogenation

Halogenation is a chemical reaction that involves the reaction of a compound with a halogen and results in the halogen being added to the compound. Organic compounds undergo halogenation much more readily than inorganic compounds. Dehalogenation is the reverse of halogenation and results in the removal of a 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 on the specific halogen. Inorganic compounds such as metals also undergo halogenation.

Organic chemistry

Halogenation by reaction type

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.

Free radical halogenation

Saturated hydrocarbons typically do not add halogens but undergo free radical halogenation, involving substitution of hydrogen atoms by halogen. The regiochemistry of the halogenation of alkanes 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. Free radical halogenation is used for the industrial production of chlorinated methanes:^[2]

CH₄ + Cl₂ → CH₃Cl + HCl

When halogenation takes place rearrangement may take place in order to stabilise the free radical. This creates a stable compound.

Addition of halogens to alkenes and alkynes

Unsaturated compounds, especially alkenes and alkynes, add halogens:

RCH=CHR' + X₂ → RCHX-CHXR'

The addition of halogens to alkenes proceeds via intermediate halonium ions. In special cases, such intermediates have been isolated.^[3]

Structure of a bromonium ion

Halogenation of aromatic compounds

Aromatic compounds are subject to electrophilic halogenation:^[4]

RC₆H₅ + X₂ → HX + RC₆H₄X

The facility of halogenation is influenced by the halogen. Fluorine and chlorine are more electrophilic 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 hydrogenolysis follows the reverse trend: iodine is most easily removed from organic compounds and organofluorine compounds are highly stable.

Other halogenation methods

In the Hunsdiecker reaction, from carboxylic acids are converted to the chain-shortened halide. The carboxylic acid is first converted to its silver salt, which is then oxidized with halogen:

RCO₂Ag + Br₂ → RBr + CO₂ + AgBr

The Sandmeyer reaction is used to give aryl halides from diazonium salts, which are obtained from anilines.

In the Hell–Volhard–Zelinsky halogenation, carboxylic acids are alpha-halogenated.

Halogenation by halogen type

Fluorination

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 as the source of fluorine. The method is called electrochemical fluorination. Aside from F₂ and its electrochemically generated equivalent, a variety of fluorinating reagents are known such as xenon difluoride and cobalt(III) fluoride.

Chlorination

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 processes include routes to 1,2-dichloroethane (a precursor to PVC), as well as various chlorinated ethanes, as solvents. Competitive with direct chlorination (use of Cl₂) is oxychlorination, which uses hydrogen chloride in combination with oxygen.^[2]

Oxychlorination is the process of chlorinating hydrocarbons, using a mixture of hydrogen chloride (HCl) and oxygen (O₂). This process is attractive industrially, because hydrogen chloride is less expensive than chlorine.^[5] The most common substrate for this reaction is ethylene:

CH₂=CH₂ + 2 HCl + ½ O₂ → ClCH₂CH₂Cl + H₂O

The reaction is initiated by copper(II) chloride (CuCl₂), which is the most common catalyst in the production of 1,2-dichloroethane. In some cases, CuCl₂ is supported on silica in presence of KCl, LaCl₃, or AlCl₃ as cocatalysts. Aside from silica, a variety of supports have also been used including various types of alumina, diatomaceous earth, or pumice. Because this reaction is highly exothermic (238 kJ/mol), the temperature is monitored, to guard against thermal degradation of the catalyst. Since the CuCl2 molecule is the donor of the chloride atom, this catalyst has an important role in the formation of the hydrocarbon double bond of chlorination. The reaction is as follows:

CH₂=CH₂ + 2 CuCl₂ → 2 CuCl + ClH₂C-CH₂Cl

The copper(II) chloride is regenerated by sequential reactions of the cuprous chloride with oxygen and then hydrogen chloride:

½ O₂ + 2 CuCl → CuOCuCl₂

2 HCl + CuOCuCl₂ → 2 CuCl₂ + H₂O

Oxychlorination is of special importance in the making of 1,2-dichloroethane, which is then converted into vinyl chloride. As can be seen in the following reaction, 1,2-dichloroethane is cracked:

ClCH₂CH₂Cl → CH₂=CHCl + HCl

2 HCl + CH₂=CH₂ + ½ O₂ → ClCH₂CH₂Cl + H₂O

The HCl from this cracking process is recycled by oxychlorination. The fact that the reaction is self-supplied is one of the reasons that industry uses oxychlorination instead of direct chlorination.^[6]

Bromination

Bromination is more selective than chlorination because the reaction is less exothermic. Most commonly bromination is conducted by the addition of Br₂ to alkenes. An example of bromination can is the organic synthesis of the anesthetic halothane from trichloroethylene:^[7]

Organobromine compounds are the most common organohalides in nature. Their formation is catalyzed by the enzyme bromoperoxidase which utilizes bromide in combination with oxygen as an oxidant. The oceans are estimated to release 1–2 million tons of bromoform and 56,000 tons of bromomethane annually.^[8]

Iodination

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.

Inorganic chemistry

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.^[9]

Chemical dehalogenation

Chemical dehalogenation is a treatment to remove halogens from harmful chemicals or contaminated areas by making them less toxic. There are two types of dehalogenation: glycolate dehalogenation and base-catalyzed decomposition ^[10]

See also

Wikiquote has quotations related to: Halogenation

• Haloalkane (Alkyl halide)
• Halogenoarene (Aryl halide)
• Free radical halogenation
• Haloketone
• Electrophilic substitution

References