Molecular Radicals Halogen Oxide

The work by Ashmore and co-workers on the nitric oxide-chlorine system has already been mentioned in the context of reactions of nitric oxide with molecular halogens. It was first suggested in 1953 by Ashmore and Chanmugam10 that the second-order molecular mechanism for the thermal decomposition of nitrosyl chloride might be accompanied by a radical mechanism at higher temperatures, which they envisioned as... 

Coal and many coal-derived liquids contain polycyclic aromatic structures, whose molecular equivalents form radical cations at anodes and radical anions at cathodes. ESR-electrolysis experiments support this (14). Chemically, radical cations form by action of H2SO4 (15,19), acidic media containing oxidizing agents (15,20,21,22), Lewis acid media (18,23-35) halogens (36), iodine and AgC104 (37,38), and metal salts (39,40). They also form by photoionization (41,42,43) and on such solid catalytic surfaces as gamma-alumina (44), silica-alumina (45), and zeolites (46). Radical anions form in the presence of active metals (76). 

The oxidation of halide ions to molecular halogen is relatively easy consequently, the mechanism of anodic halogenation, brought about by oxidation of organic substrates in the presence of halides is not always clear [249-254]. In many cases it may involve halogenation by anodically generated halogen. In other cases, where the substrate is easily oxidized, the halide has the role of a nucleophile and attacks a radical cation. 

For the lanthanide metals, Sm, Eu, and Yb, which have a readily accessible ( + 2) oxidation state, oxidative addition reactions of M(1I) complexes with halogens and organic halides are dominated by the atom transfer or free radical mechanism (cf. 5.S.2.9.1.) in which two metal ions are each oxidized to the -I- 3 state. Numerous examples illustrate the ability of cyclopentadienyl or indenyl Ln(II) complexes (Ln = Sm, Eu, Yb) to abstract halogen atoms from molecular halogens ", halogenated solvents sueh as CH2CI2 and and alkyl halides . An archetypieal example is ... 

To promote a polymerization, the newly formed carbon-halogen bond must be capable of being reactivated and the new radical must be able to add another alkene. This was accomplished for the radical polymerizations of St and methyl acrylate (MA), which were initiated by 1-phenylethyl bromide and catalyzed by a Cu(I)/2,2 -bipyridine (bpy) complex [42,79-81]. The process was called Atom Transfer Radical Polymerization (ATRP) to reflect its origins in ATRA. A successful ATRP relies on fast initiation, where all the initiator is consumed quickly, and fast deactivation of the active species by the higher oxidation state metal. The resulting polymers are well defined and have predictable molecular weights and low polydispersities. Other reports used different initiator or catalyst systems, but obtained similar results [43,82]. Numerous examples of using ATRP to prepare well-defined polymers can now be found [44-47,49]. Scheme 4 illustrates the concepts of ATRA and ATRP. To simplify schemes 3,4 and 5, termination was omitted. 

Detailed studies of the chlorine-atom-sensitized oxidation of chloromethane, dichloromethane, 1,1,2-trichloroethane, CH2CC12, and C2C14148 have been reported. Reports on the photolysis of Freons of interest to upper-atmosphere chemistry are discussed in the last section of this Report, and laser enhancement of some halogen-containing molecular reactions is discussed in Section 11. A paper concerned with the mechanism of photodissociation of alkyl and aryl halides was discussed earlier.38 The photochemical chlorination of 1,2-dichloroethane149 and fluorination of carbonyl fluoride,150 reactions of 2 radicals,151 and the photochemical decomposition of FaO at elevated temperatures152 have been reported. 

Free-radical reactions of tervalent phosphorus acids have been covered to some extent in Section II (addition of PX3 to alkenes, equation 15-17) and in Section IV (oxidations with molecular oxygen). Several other reactions occur via radicals, e.g. reactions with peroxides, certain disulphides and certain halogen compounds. However, these reactions are the subject of chapters by Bentrude and Danko wski in Vol. 1 of this series and will not be covered here. 

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