Inchemistry,halogenation is achemical reaction which introduces one or morehalogens into achemical compound.Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production ofpolymers,drugs.[1] This kind of conversion is in fact so common that a comprehensive overview is challenging. This article mainly deals with halogenation using elemental halogens (F2,Cl2,Br2,I2). Halides are also commonly introduced using salts of the halides and halogen acids.[clarification needed] Many specializedreagents exist for and introducing halogens into diversesubstrates, e.g.thionyl chloride.
Several pathways exist for the halogenation of organic compounds, includingfree radical halogenation,ketone halogenation,electrophilic halogenation, andhalogen addition reaction. The nature of thesubstrate determines the pathway. The facility of halogenation is influenced by the halogen.Fluorine andchlorine are moreelectrophilic and are more aggressive halogenating agents.Bromine is a weaker halogenating agent than both fluorine and chlorine, whileiodine is the least reactive of them all. The facility ofdehydrohalogenation follows the reverse trend: iodine is most easily removed from organic compounds, andorganofluorine compounds are highly stable.
Halogenation ofsaturated hydrocarbons is asubstitution reaction. The reaction typically involvesfree radical pathways. Theregiochemistry of the halogenation ofalkanes is largely determined by the relative weakness of theC–H bonds. This trend is reflected by the faster reaction attertiary andsecondary positions.
Free radical chlorination is used for the industrial production of somesolvents:[2]
Naturally-occurringorganobromine compounds are usually produced by free radical pathwaycatalyzed by theenzymebromoperoxidase. The reaction requiresbromide in combination withoxygen as anoxidant. Theoceans are estimated to release 1–2million tons ofbromoform and 56,000 tons[which?] ofbromomethane annually.[3]
Theiodoform reaction, which involves degradation ofmethyl ketones, proceeds by the free radical iodination.
Because of its extreme reactivity, fluorine (F2) represents a special category with respect to halogenation. Most organic compounds, saturated or otherwise, burn upon contact withF2, ultimately yieldingcarbon tetrafluoride. By contrast, the heavier halogens are far less reactive toward saturated hydrocarbons.
Highly specialised conditions and apparatus are required for fluorinations with elementalfluorine. Commonly, fluorination reagents are employed instead ofF2. Such reagents includecobalt trifluoride,chlorine trifluoride, andiodine pentafluoride.[4]
The methodelectrochemical fluorination is used commercially for the production ofperfluorinated compounds. It generates small amounts of elemental fluorinein situ fromhydrogen fluoride. The method avoids the hazards of handling fluorine gas. Many commercially importantorganic compounds are fluorinated using this technology.
Unsaturated compounds, especiallyalkenes andalkynes,add halogens:
Inoxychlorination, the combination ofhydrogen chloride andoxygen serves as the equivalent ofchlorine, as illustrated by this route to1,2-dichloroethane:
The addition of halogens to alkenes proceeds viaintermediatehalonium ions. In special cases, such intermediates have been isolated.[5]
Bromination is moreselective than chlorination because the reaction is lessexothermic. Illustrative of the bromination of an alkene is the route to theanesthetichalothane fromtrichloroethylene:[6]
Iodination and bromination can be effected by the addition ofiodine andbromine to alkenes. The reaction, which conveniently proceeds with the discharge of the color ofI2 and Br2, is the basis of theanalytical method. Theiodine number andbromine number are measures of thedegree of unsaturation forfats and other organic compounds.
Aromatic compounds are subject toelectrophilic halogenation:
This kind of reaction typically works well forchlorine andbromine. Often aLewis acidiccatalyst is used, such asferric chloride.[7] Many detailed procedures are available.[8][9]Becausefluorine is soreactive, other methods, such as theBalz–Schiemann reaction, are used to prepare fluorinated aromatic compounds.
In theHunsdiecker reaction,carboxylic acids are converted toorganic halide, whosecarbon chain is shortened by onecarbon atom with respect to the carbon chain of the particular carboxylic acid. The carboxylic acid is first converted to itssilver salt, which is then oxidized withhalogen:
Manyorganometallic compounds react with halogens to give the organic halide:
Allelements aside fromargon,neon, andhelium formfluorides by direct reaction withfluorine.Chlorine is slightly more selective, but still reacts with mostmetals and heaviernonmetals. Following the usual trend,bromine is lessreactive andiodine least of all. Of the many reactions possible, illustrative is the formation ofgold(III) chloride by the chlorination ofgold. The chlorination of metals is usually not very important industrially since thechlorides are more easily made from theoxides andhydrogen chloride. Where chlorination ofinorganic compounds is practiced on a relatively large scale is for the production ofphosphorus trichloride anddisulfur dichloride.[10]
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