Radical halogens

Halogen radicals account for about one-third of photochemical ozone loss observed in the spring in the lower stratosphere (below 21 km) at 15—60°N latitude (76). The following three cycles (4—6) are the most important. Rate constant data are given in Reference 11. 

The active chain carriers are replaced with the much less active halogen radical slowing the rate of energy production and helping flame extinguishment. 

Triaryl phosphate esters are thermally stable, high-boiling (>350°C) materials. They can volatilize without significant decomposition into the flame zone, where they decompose. Flame inhibition reactions, similar to the halogen radical trap theory, have been proposed (6) ... 

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

UV radiation. Thus, halogen molecules are easily photolysed to generate halogen radicals. 

As discussed in Chapters 7, 8, and 9, there are a number of free radical species whose reactions in the aqueous phase drive the chemistry of clouds and fogs. These include OH, HOz, NO-, halogen radicals such as Cl2, sulfur oxide radicals, and R02. Generation of these radicals in the liquid phase for use in kinetic... 

Tuckermann, M., R. Ackermann, C. Golz, H. Lorenzen-Schmidt, T. Senne, J. Stutz, B. Trost, W. Unold, and U. Platt, DOAS-Ob-servation of Halogen Radical-Catalysed Arctic Boundary Layer Ozone Destruction during the ARCTOC-Campaigns 1995 and 1996 in Ny-Alesund, Spitsbergen, Tellus, 49B, 533-555 (1997). 

The loss of 03 at latitudes where CIO has a high concentration is consistent with the known chemistry of catalytic destruction of 03 by halogen radicals. [Prom J. G. Anderson, W. H. Prune, and M. H. Proffitt, J. Geophys. Res. 1989, 94D, 11465.]... 

The ClO-BrO instrument was designed to test the theory that chlorine and bromine were the cause of the rapid loss of ozone over Antarctica. Its pedigree includes several versions of balloon-borne halogen radical measuring... 

Potential Role of Halogen Radicals in Ozone Destruction... 

The corrinoid-mediated reduction of polyhaloethenes has been the subject of a recent study, which reports reaction via homolytic C-halogen bond fission. The elimination of a further halogen radical affords haloalkynes, which lead to acetylene itself.56 The electron transfer-induced reductive cleavage of alkyl phenyl ethers with lithium naphthalenide has been re-examined in a study which showed that it is possible to reverse regioselectivity of the cleavage (i.e. ArOR to ArH or ArOH) by introduction of a positive charge adjacent to the alkyl ether bond.57 A radical intermediate has been detected by ESR spectroscopy in the reduction of imines to amines with formic acid58 which infers reacts takes place via Lukasiewicz s mechanism.59... 

Behnke W, Elend M, Kruger U, Zetzsch C (1999) The Influence of NaBr/NaCl Ratio on the BU-Catalysed Production of Halogenated Radicals. J Atmos Chem 34 87... 

Another concern in halogenation is multi-halogena ted products. Increased concentration of the halogen will result in the di-, tri-, and tetra-halogenatcd products, while dilute solutions will yield only monohalogenated products. This is because in a dilute solution the halogen radical is more likely to collide with an alkane than an alkyl halide. 

In contrast to these concepts above obtained from numerous ab-initio molecular orbital calculations (usually from small radicals, e. g. H, F, H3C or HOCH2 to fluoroalkenes), some semi-empirical calculation on more longer halogenated radicals were performed by Rozhkov et al. [333, 334] and Xu et al. [335]. The former team determined equilibrium geometries and electronic properties of perfluoroalkyl halogenides and showed that fluorine atoms in vinyl position strongly stabilise all the sigma molecular orbital. 

The intense absorption by halogen radical anions in the near UV (for Br2 , Xmax = 360 nm, e = 9900 M 1 cm-1) provides a convenient means to monitor these reactions. 

The cathodic removal of one halogen radical from perhalogenated quinolines and isoquinolines 450) occurs with high selectivity ... 

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