Halogenation with radicals

Mechanism and selectivity of free-radical halogenation with cyclic N-haloimides 99MI19. 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

Adhikari, S., Kapoor, S., Chattopadhyay, S., and Mukheijee, T. 2000. Pulse radiolytic oxidation of P-carotene with halogenated alkylperoxyl radicals in a quaternary microemulsion Formation of retinal. Biophys. Chem. 88 111-117. 

Bromination of an optically active form of the corresponding chloro compound (l-chloro-2-methylbutane) also results in an optically active product, and retention of configuration. It may be that an actual bridged radical is formed, but a somewhat less concrete interaction seems more likely, as halogenation with the more reactive chlorine is found to lead wholly to racemisation. 

Hubig, S. M., Jung, W. and Kochi, J.K. (1994). J. Org. Chem. 59, 6233. Note the competition between ion-pair and radical-pair collapse in halogenation with iodine monochloride on p. 279 is exactly analogous to that in nitration with tetranitro-methane in eqns 82/83 (vide infra)... 

A key factor in the work with halogenated alkyl radicals appears to have been the wide variation in relative rates of trapping of different radicals when different traps are employed. In particular, a-halogenoalkyl radicals are apparently not trapped with MNB (Gasanov et al., 1975b see also, e.g. Gasanov et al., 1978). This phenomenon has yet to be given a sound quantitative foundation. 

Several reactions of halogen-substituted carbon-centered radicals with silanes have been studied, but limited kinetic information is available for reactions of halogen-substituted radicals with tin hydrides. A rate constant for reaction of the perfluorooctyl radical with Bu3SnH was determined by competition against addition of this radical to styrenes, reactions that were calibrated directly by LFP methods.93 At ambient temperature, the n-C8F17 radical reacts with tin hydride two orders of magnitude faster than does an alkyl radical, consistent with the electron-deficient nature of the perflu-oroalkyl radical and the electron-rich character of the tin hydride. Similar behavior was noted previously for reactions of silanes with perhaloalkyl radicals. 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

The initial reaction of OH with HFC-134a, CH2FCF3, produces a halogenated alkyl radical that... 

One important prerequisite to the application of this reaction in hydrocarbon synthesis is the selective monochlorination of methane. Usual radical chlorination of methane is not selective, and high CH4 CI2 ratios are needed to minimize formation of higher chlorinated methanes (see Section 10.2.5). In contrast with radical halogenation, electrophilic halogenation of methane was shown to be a highly selective process.412... 

It is significant that no other hydrogen halides add to alkenes, contrary to the Markovnikov rule, even in the presence of free radicals. This appears to be due to the difficulty with which other halogen atoms, particularly fluorine and chlorine, are produced by reaction of the hydrogen halides with radicals ... 

The reaction of aryl and hetaryl halides with the nitrile-stabilized carbanions (RC -CN) leads to derivatives of the type ArCH(R)CN. Sometimes, however, dimeric products of the type ArCH(R)CH(R)Ar are formed (Moon et al. 1983). As observed, 1-naphthyl, 2-pyridyl, and 2-quinolyl halides give the nitrile-substituted products, while phenyl halides, as a rule, form dimers. The reason consists of the manner of a surplus electron localization in the anion radical that arises upon replacing halogen with the nitrile-containing carban-ion. If the resultant anion radical contains an unpaired electron within LUMO covering mainly the aromatic ring, such an anion radical is stable, with no inclination to split up. It is oxidized by the initial substrate and gives the final product in the neutral form, Scheme 1-7 ... 

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