Chlorine abstraction

The most common way to generate sulfonyl radicals for spectroscopic studies has been the photolysis of solutions containing di-t-butyl peroxide, triethylsilane and the corresponding sulfonyl chloride in a variety of solvents (equations 4-6). The slowest step in this sequence is the reaction between t-butoxyl radicals and triethylsilane (ks = 5.3 x 106m 1s-1)26 since that for chlorine abstraction (equation 6) is extremely efficient (cf. Table 4). 

Many of the quantitative rate data on radical reactions which are to be found in the scientific literature have been obtained by comparison of reaction rates rather than by direct measurement of absolute rate constants (Ingold, 1973). For example, it is a straightforward matter to compare the rate of chlorine abstraction from CC14 by phenyl radicals with the rate of hydrogen abstraction from cyclohexane by the same species, simply by comparing the PhCl/PhH product ratio from a suitable competition experiment (Bridger and Russell, 1963). In contrast, direct measurements of the absolute rates of these reactions have yet to be carried out (although indirect estimates are available). 

The use of benzene as a solvent eliminates the chlorinated organic products in Table II. This point and the products in the table are consistent with radical abstraction (hydrogen atom transfer) from the substrate, cyclohexene, by the high valent (and likely oxometal) intermediate form of the TMSP complexes, followed by chlorine abstraction from the solvent by intermediate organic radicals. Separation and analysis of the two phases after the reaction reveals that the polyoxometalate is intact. 

The results demonstrate that the rate of the chlorine abstraction reaction (Eq. (47)), which is usually the rate determining step in this process, is affected by (a) the redox potential of the Cun/IL couple (solvents such as acetonitrile, that form complexes with Cu1 and anodically shift its redox potential, decrease the reactivity),... 

Such a process would have an intensity exponent of unity as observed. A further fact in support of the disproportionation mechanism is that the yield of CF2CI2 is largely independent of the ketone pressure at room temperature an abstraction mechanism would require a first-order dependence upon ketone pressure. While the dimer of CF2, tetrafluoro-ethylene, has never been observed in the reaction mixture, a white solid collected in the cell which was probably a polymer of CF2. While the experimental conditions are not strictly comparable, it is significant that the absorption spectrum of CF2 has been observed in the flash photolysis of 1,3-dichlorotetrafluoroacetone.39 When the temperature is raised, however, the yield of CF2C12 in normal photolysis, increases rapidly suggesting an energy of activation and this process can only be chlorine abstraction. The rate function ... 

Photolysis of aryl or pyridyl oxime esters in pyridine provides a-phenylpyridines as the major products together with bipyridyls (84TL3887). Rate constants for the addition of phenyl radical to protonated and non-protonated 4-substituted pyridines have been determined by studing the competition between phenyl radical addition and chlorine abstraction from carbon. The 4-arylpyridines were the major products, and no 3-substituted pyridines were observed. Among the solvents studied (MeCN, DMF, DMSO, and HMPA), MeCN gave the highest yields and selectivity (910PP438). 

A recent study showed that 152 behaves mechanistically different from other catalysts in addition reactions of more activated halides 140, such as trichloroacetate to styrene [222]. After initial reduction to Ru(II), chlorine abstraction from substrates 140 is in contrast to all other ruthenium complexes not the rate limiting step (cf. Fig. 36). ESR spectroscopic investigations support this fact. The subsequent addition to styrene becomes rate limiting, while the final ligand transfer step is fast and concentration-independent. For less activated substrates 140, however, chlorine abstraction becomes rate-determining again. Moreover, the Ru(III) complex itself can enter an, albeit considerably slower Ru(III)-Ru(IV) Kharasch addition cycle, when the reaction was performed in the absence of magnesium. This cycle operates, however, for only the most easily reducible halides, such as trichloroacetate. 

The methyl radical reacts either by addition to or by chlorine abstraction from the substrate as represented in steps 8 and 9. The free-radical process may not be a surprise, since the reaction was carried out in a silica vessel and in the absence of a chain suppressor. 

Bromo- and iodo-aliphatic compounds react quite rapidly k 108 and 109 M 1 s-1 respectively, and the reaction is predominantly halogen abstraction. Chlorine abstraction is a slower process and usually takes place concurrently with H-abstraction, k 106-107 M-1 s 1. Fluoro-compounds undergo only H-abstraction at... 

Bottoni A. Theoretical study of the hydrogen and chlorine abstraction from chloromethanes by silyl and trichlorosilyl radicals a comparison between the Hartree-Fock method, perturbation theory, and density functional theory. J Phys Chem A 1998 102(49) 10142-10150. 

Potassium aquo ferricyanide, K2Fe(CN)5.H20, is obtained by the prolonged action of chlorine upon potassium ferrocyanide solution. At first potassium ferricyanide is formed, which undergoes further decomposition, the chlorine abstracting one (CN) group, water taking its place. Thus —1... 

Table III classifies the bridging ligands by their bite distances and correlates them with the known oxidation states of the diplatinum complexes. When ligands with bite distances longer than ca. 2.7 A react with Pt(II) salts, Pt(II) complexes are obtained. On the contrary, in those cases with a shorter bite distance than 2.7 A, Pt(III) instead of Pt(II) dimers are obtained. Chlorine abstraction of [Pt2"(pyt)4] from chloroform to produce [Pt2 (pyt)4Cl2] is taken as an intermediate phenomenon, and the bite distance of ca. 2.7 A is considered a limit for...

The same dehydrohalogenation reactions discussed in Section 5.2.2.1.1.1. have been used to prepare a number of alkylidenecyclopropanes in which the side chain containing the exo double bond is larger than methylene. In most of these procedures bromocyclopropanes have served as the starting materials, and only a few examples of chlorine abstraction from the ring position are known. 

However, with the 3, 5) -cyclocholestan-6-yl radical 13, formed by chlorine abstraction from 6)ff-chloro-3 8,5-cyclo-5 -cholestane (3fi,5fS-10) by tin-centred radicals, the SOMO overlaps best with the 4,5-bond, and it rearranged highly selectively to give the norsteroid 14 with a five-membered A-ring. ... 

Chlorine abstraction from (R-RuCl)2(p-Cl)2 (R = C6H6, p-C6H4Pr ) by AgBp4, followed by complexation with Ph2P(E)N(H)P(E)Ph2 (E = S, Se) led to the formation of ruthenium complexes (53). Chlorine abstraction from (CeHe-... 

Copper(I) complexes catalyse a variety of organic reactions which are of synthetic and industrial importance.305 In such processes that involve halide abstraction from aryl or alkyl halides, the abstraction step by a Cu(I) catalyst is believed to be the rate-determining step. In order to circumvent the property of facile disproportionation of Cu, various methods of stabilising Cu(I) and influencing reaction rates were considered.306 A kinetics study of ligand (L) effects on the reactivity of Cu(I)L complexes towards C13CC02 was undertaken. The results indicated that the rate of the chlorine abstraction reaction was affected by several factors. These were the redox potential of the Cu(II/I)L couple, the hybridisation on Cu(I) in the Cu(I)L complex, steric hindrance, and electron density on the central Cu(I) cation at the binding site of the chlorine atom to be abstracted. The volume of activation,... 

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