Halogenation, by free-radicals

Control of addition vs substitution by free radicals can be effected by the reaction conditions, ie, radical concentration, temperature, and phase. Using halogens as propylene reactants, high temperatures and the gas phase favor high radical concentrations and substitution reactions cold, Hquid-phase conditions favor addition reactions. 

The formation of derivatives of this type by free-radical attack has been mentioned previously (see section E above). The most common route to vinylogous halo ketones is by halogenation of dienol acetates or ethers. Both free halogen and A -halo compounds may be employed, and this approach has frequently been used to obtain 6 (axial) halo compounds ... 

Mozurkewich, M Mechanisms for the Release of Halogens from Sea-Salt Particles by Free Radical Reactions, J. Geophys. Res., 100, 14199-14207 (1995). 

Resists based on chloromethyl substitution have been extensively studied in the past few years (7-7). Halogen and halomethyl groups have been introduced by a variety of methods. Choong and Kahn (7) synthesized polychloromethylstyrene (PCMS) by free radical polymerization of chloromethylstyrene and reported a sensitivity (D 5) of 0.4 / 2 at 20 kV with a contrast of 1.5 for materials with molecular weight of about 400,000 (7). The molecular weight distribution of these polymers, all of which contained one chloromethyl group per repeat unit, was about 2. Fractionation of the polymer resulted in improved contrast as a result of narrowing the distribution. 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

The halogenation of saturated aliphatic hydrocarbons is usually achieved by free radical processes.523 Ionic halogenation of alkanes has been reported under superacid catalysis. Olah and co-workers524,525 have carried out chlorination and chlorolysis of... 

Benzyl cyanide is the product of nucleophilic substitution by cyanide ion on benzyl bromide or benzyl chloride. The benzyl halides are prepared by free-radical halogenation of the toluene side chain. 

The results we have obtained on the one-electron oxidation of sulfite and bisulfite by free radicals also are important in understanding possible chain-initiation and chain-carrying steps in S02 autoxidation. This is particularly true in systems which do not consist simply of S02 and a catalyst. For example, the production and subsequent reaction of halide free radicals could be important in any system containing halogen ions, and subsequent reactions of organic radicals with sulfite will be required to describe completely the effect of organic inhibitors. 

The a-position of an ether is susceptible to attack by free radicals and, in certain circumstances, by halogens. Ethers are slowly oxidized by the oxygen from air to form peroxides- This can be a hazard in stored bottles of ethers, particularly with the higher ethers such as di-isopropyl ether. These peroxides may be destroyed by treatment with iron(IT) sulfate. Chlorine reacts with ethers, particularly in sunlight. These a-halo ethers then decompose to the aldehyde and an alcohol. 

Mozurkewich M (1995) Mechanisms for the release of halogens from sea-salt particles by free radical reactions. J Geophys Res 100 14199-14207... 

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