Chlorine, free radical transfer reactions

Thermal dehydrochlorination of 1,2-dichloroethane188-190 272 273 takes place at temperatures above 450°C and at pressures about 25-30 atm. A gas-phase free-radical chain reaction with chlorine radical as the chain-transfer agent is operative. Careful purification of 1,2-dichloroethane is required to get high-purity vinyl chloride. Numerous byproducts and coke are produced in the process. The amount of these increases with increasing conversion and temperature. Conversion levels, therefore, are kept at about 50-60%. Vinyl chloride selectivities in the range of 93-96% are usually achieved. 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

Of the mononuclear complexes perhaps the most interesting are [IrCls]3 and the complex cobalt hydrides since in both cases cyclic reactions have been observed. For [IrCl6]3-, hydrogen and chlorine are formed on irradiation with 254 nm light, apparently by processes involving free radicals (equations 8-15).70 The photochemical excitation probably involves L-M charge transfer. 

Under this type of catalysis we may classify those reactions in which reversible electron transfer takes place. The electron to be transferred may be carried by an atom, and in such a case we speak about hydrogen or chlorine transfer and the like. While in the previous case the reaction was characterized by the development of a positive charge at the reactive centre, and we could speak about carbonium ions, in the present case the reaction centre is characterized by the presence of an unpaired spin, and we may speak about free radical reactions, taking into account the same precautions as previously. 

The Kochi reaction has also been used to demonstrate polar effects of remote substituents upon free-radical stereoselectivities in chlorine atom transfer. ... 

According to Scheme 11, the heteroaromatic attack overcomes the very fast competitive chlorine-transfer from the N-chloroamine [(Eq. (9)] which is a step of the free-radical chain in the Hoffman-Loeffler i3) reaction... 

Few reactions of CIO2 with hydrocarbons have been reported except under conditions very far removed from water treatment practice. Saturated alkanes and alkyl side chains appear almost inert in its presence except where unusually stable radicals may result (benzylic hydrocarbons, e. g.). In these cases, typical products of autooxidation (see Chapter 4) have been isolated, probably by attack of O2 on intermediate free radicals formed by electron transfer (Ozawa and Kwan, 1984 Rav-Acha and Choshen, 1987 Merenyi et al., 1988) or (less likely) hydrogen atom abstraction (Chen et al., 1982). A few polycyclic hydrocarbons have been shown to be partially converted to chlorinated derivatives and quinones by CIO2 (Thielemann, 1972a Taymaz et al., 1979). 

Nylon 7 and nylon 9 are part of a process developed in Russia to form polyamides for use in fibers. The process starts with telomerization of ethylene [66]. A free-radical polymerization of ethylene is conducted in the presence of chlorine compounds that act as chain-transferring agents. The reaction is... 

Free-radical Reactions.—Publications have appeared dealing with the bidirectional addition of trifluoroiodomethane and hydrogen bromide across the C=C bond in the olefin CF3 CH CHMe, peroxide-initiated addition of 1,2-dibromotetrafluoroethane to ethylene, propene, and isobutene, the addition of pentafluoroiodoethane to 3,3,4,4-tetrafluorohexa-l,5-diene (see p. 29), peroxide-initiated cyclodimerization of 3,3,4,4-tetrafluoro-4-iodobut-l-ene (see p. 29), and telomers from tribromofluoromethane or tetrabromomethane and bromotrifluoroethylene as high-density fluids for gyroscope flotation. The telomerization of chloromethanes with tetra-fluoroethylene provides a measure of the relative reactivity for both chlorine and hydrogen abstraction by the CF2 CF2 radical. The chain-transfer... 

The surface modification of polymethylsiloxane particles involved free radical chlorination. Carbon tetrachloride was employed to eliminate possible chain transfer by a solvent and small amounts of AIBN added to the reaction resulted in higher... 

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