Cl radicals

Initiator (Section 5.3) A substance with an easily broken bond that is used to initiate a radical chain reaction. For example, radical chlorination of alkanes is initiated when light energy breaks the weak Cl-Cl bond to form Cl-radicals. 

The mechanism is usually electrophilic (see p. 972), but when free-radical initiators (or UV light) are present, addition can occur by a free-radical mechanism. Once Br-or Cl- radicals are formed, however, substitution may compete (14-1 and 14-2). This is espiecially important when the alkene has allylic hydrogens. Under free-radical conditions (UV light) bromine or chlorine adds to the benzene ring to give, respectively, hexabromo- and hexachlorocyclohexane. These are mixtures of stereoisomers (see p. 161). ... 

Chain reactions ivere discovered around 1913, ivhen Bodenstein and Dux found that the reaction betv een H2 and CI2 could be initiated by irradiating the reaction mixture ivith photons. They ivere surprised to find that the number of HCl molecules per absorbed photon, called the quantum yield, is around 10 Nernst explained this phenomenon in 1918 the photon facilitated the dissociation of CI2 into Cl radicals (the initiation step), which then started the following chain process ... 

For n-alkanes, n-alcohols, 1-chloroalkanes, n-ethers, and chloroethenes, the carbon chain length influences the reactivity, and the clear linear correlations indicate that the attack mechanism of these pollutants by OH or Cl radicals occurs via the same pathway. However, such correlations do not hold true for aromatics, ketones, and aldehydes, for reasons discussed in our previous paper [3]. We also estimated missing values of kci by analogy for ethylbenzene, we take kci = 1.5e-10 cm molecule S, greater than that for m-xylene, but smaller than the 2.0e-10 cm molecule- s-i value for very reactive compoxmds. Also we estimate a similar value for butyraldehyde kci = le-10 cm molecule- s-, only 10% larger than kci of acetaldehyde to remain consistent with the equivalent koH value. 

We can correlate our experimental conversions and rates with the extent of (dark) contaminant adsorption (O), and the literature homogeneous second order rate constants for OH and Cl radicals, and the product of the rate constant times coverage. 

Once UV photons have been absorbed by the polymer, excited states are formed they disappear by various routes, one of them leading to the formation of free radicals by cleavage of the C-Cl bonds. The very reactive Cl radicals evolved are most likely to abstract an hydrogen atom from the surrounding CHC1 sites to generate a-B,B ... 

The initiation is the same as for 5.5(b). In the propagation part, Sn- abstracts T from C5. The C5 radical then adds to C7 of CO to make a new C7 radical. The C7 radical adds to C2 to make a Cl radical, which adds to Cl of a second equivalent of CO to make a Cl radical. Cl then abstracts H from B SnH to give the product and regenerate B Sn-. 

In both products, the C1-C2 bond has cleaved. Cleavage of this bond can occur by fragmentation of the Cl radical to give the C2 radical and CO. The Cl radical is generated by abstraction of H-. 

The C1-C2 bond is quite weak. Homolysis of this bond gives a 1,3-diradical at Cl and C2. The Cl radical is allylically delocalized onto C4, also. Combination of the C2 radical with with the C4 radical gives the product. 

Evidence for the generation of these reactive intermediates was obtained from a study of the effect of added cydohexene on the sonication of chloroform. The presence of free radicals in the system was confirmed by the appearance of chlorocydohexane as a product and by the increased rate of decomposition of CHCI3 in the presence of cyclohexene. The increased decomposition rate is a consequence of the presence, in the cavitation bubble, of the alkene which mops up the Cl radical as it is formed and prevents the regeneration of chloroform - i. e. the kinetic steps in Scheme 3.4 are driven from left to right. Carbene intermediates are implicated by the formation of tricyclic compounds such as (1) via dichlorocarbene addition to cydohexene. 

Once these form, highly exothermic channels involving H, OH, and the relatively abundant Cl radicals become available. For example, some pathways leading to SnCl2(OH)2 (the hydroxide with the highest concentration at equilibrium) are ... 

The absorptions at both 500 nm and 320 nm follow first order kinetics with a lifetime of 420 ns. This absorption species is neither the excimer of polystyrene nor free cationic species of polystyrene. Although the excimer of polystyrene has an absorption band around 500 nm, the lifetime is only 20 ns. Further the free cationic species of polystyrene should live for a longer time in this solution, and the absorption band should exist in a longer wavelength region (6). These considerations of lifetime and absorption spectrum lead us to conclude that the absorption spectrum shown in Figure 12 is due to the charge transfer-radical complex between polystyrene and Cl radical (2,4,17). A very similar... 

The formed PCI4 and Cl radicals subsequently can initiate free-radical chlorination reactions. In contrast, a predominant heterolytic cleavage and consequently ionic chlorination takes place in polar solvents. 

The mechanism that accounts for the oleofinic oxidation by hydroxyl radicals is the hydrogen abstraction. Moreover Cl radicals may also be an important mechanism for chlorinated organics. When the C-Cl bond is broken by photolysis, a Cl radical is released and can initiate additional oxidation reactions through a chain mechanism as follows ... 

Chatgilialoglu C, Ferreri C, Bazzanini R, Guerra M, Choi S-Y, Emanuel CJ, Horner JH, Newcomb M (2000) Model of DNA Cl radical. Structural, spectral and chemical properties of the thyminyl-methyl radical and the2 -deoxyuridin-1 -yl radical. J Am Chem Soc 122 9525-9533... 

The free radicals generated in these reactions then react with chlorine to form either 1-chloro-propane or 2-chloropropane and regenerate a Cl- radical. 

Fourier Transform IR Studies of the Reactions of Dimethyl Sulfoxide with OH, N03, and Cl Radicals... 

Dimethyl sulfoxide (DMSO) has recently been detected in marine air masses. To date nothing is known about the atmospheric fate of DMSO in the gas phase. Reported here are product and kinetic studies on the reactions of OH, NO3 and Cl radicals with DMSO. The investigations were performed in a 420 1 reaction chamber at atmospheric pressure using long path in situ Fourier transform (FTIR) absorption spectroscopy for detection of reactants and products. 

Cu11 complexes undergo photoreduction to the Cu1 species and may be the photocatalyst in photo-oxidation cycles of organic environmental matter, quite similar to the Fem species [20, 81] (see Figure 9.11). Dissolved copper compounds are important to transformation reactions, because they react with hydroperoxyl (H02) and superoxide (02 ) radicals much faster than other species present in the solution. Oxidation of Cu1 and Fe11 by H202 is a source of the OH radicals in oceans comparable with nitrite photolysis, whereas photochemistry of Cu11 chlorocom-plexes provides Cl radicals [81] ... 

Nearly always the phenyl groups contain attached NO., and Cl radicals, and often acid, ester or CF. ... 

Reaction 1 is the initiation step, and Cl, C2C13, and COC1 are the chain carriers. When methane is present, it intercepts and removes the Cl radical by... 

BDD anodes without impurities are not electrocatalytically active because water electrolysis is characterised by the formation of OH radicals (Marselli et al. 2003), ozone (Cho et al. 2005) and hydrogen peroxide (Drogui et al. 2001). One can conclude from radical chemistry that other radicals have to be expected in the anodic reaction layer and, maybe, in the bulk. Foerster and co-workers compared active chlorine formation on Pt and BDD anodes (Foerster et al. 2002). Formation of active chlorine was explained by a mechanism involving the formation of Cl radicals (Ferro et al. 2000) ... 

An intermediate of the pH-dependent Cl radical formation is ClOH- (Klaening and Wolff 1985). 

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