Chlorine radicals radical scavengers

Fig. 24. Reductive dehalogenation of a chlorinated hydrocarbon in the presence of a metal forming an alkyl radical, showing (Pathway (I)) the alkyl radical scavenging a hydrogen atom, and (Pathway (II)) the alkyl radical losing a second halogen to form an alkene...

A number of sulfur-centered radical scavengers have been employed for Meerwein type carbothiolation reactions [109, 110]. The most prominent of those are certainly xanthates [111-113] and thiocyanates, among which the latter have received special attention recently. As shown in Scheme 21, thiocyanates are well-suited for the functionalization of activated and non-activated alkenes [114, 115]. Remarkably, the reaction of 56 with 2-methallyl chloride to give 57 is not significantly impeded by the possible (3-fragmentation of a chlorine radical, which would lead to allylation products [116]. With an activated and a non-activated alkene present in a substrate... 

It has been assumed that the reactions of chlorine and hypochlorite with carbohydrates proceed mainly by the radical mechanism because they are retarded in the presence of radical scavengers, such as chlorine dioxide ... 

Commercially available 85% MCPBA is generally employed in chlorinated hydrocarbon solvents at room temperature. Reaction times are typically a few hours to several days. Buffers utilized include disodium hydrogen phosphate, sodium acetate and sodium bicarbonate, the catalytic effect of which has been occasionally noted. Acid catalysis with sulfuric acid or Nafion-H are alternatives. Oxidations have been performed at elevated temperature with the aid of radical scavengers. "... 

Chloride ions are used in the Fricke dosimeter as hydroxyl radical scavengers (2). Since adding chloride ions does not decrease the yield of ferric ion (except in the presence of organic impurities), it can be argued that chlorine atoms and hence Cl2" radical ions react to oxidize ferrous ion. Using the present technique we have measured the effect of ferrous ions on the rate of decay of the Cl2" transient. Ferrous ions increased this rate, and a rate constant for Reaction 3 was determined h = 3.8 0.3 X 107M 1 sec."1 at pH 2.1... 

Scheme 8. In fact, when placed in the reaction medium in the dark, (23) gives a product distribution similar to that observed in the photochemical reaction. In contrast with reduction in alcohols, the quantum yield in aqueous HC1 drops rapidly with decreasing HC1 concentration ( = 0.11 in 12moll-1 HC1, but = 0.012 in 6 mol l-1 HC1 48 whereas in 50% aqueous propan-2-ol, is constant at >0.1 mol l-1 HQ 47). Both this dependence of quantum yield on acidity and some radical scavenging observed when phenol or anisole (chlorine atom traps) are present are in accord with the mechanism outlined in Scheme 8. Cu and Testa49 have found that the 313 nm irradiation of protonated 5-nitroquinoline...

Like heat and light stabilizers, flame retardants containing bromine or chlorine act as free-radical scavengers. They interrupt combustion essentially by replacing free hydrogen and hydroxyl radicals with halide (bromide or chloride) radicals that prevent the combustion reaction from continuing, according to a cyclic process (simplified here) ... 

The stabilization of products of decomposition of primary ozonide, and as a consequence secondary reactions with ozone and olefines existence. One can underline that corpuscular chlorine plays role in such reactions with olefines [21]. This fact has been taken into account in Ref [21] by inducing of radical scavengers. Experimental rate constant is should be considered as an upper edge of rate constant of elementary act, which is an object of present work. 

SCHEME 14.1 Scavenging of chlorine radicals by PMMA to minimize dehydrochlorination... 

The substitution of the hydrogen in the aliphatic side-chain by chlorine occurs in as a radical chain mechanism. In industrial chlorinations the formation of chlorine radicals is achieved either by irradiation (ultraviolet light, beta-radiation), or by the use of elevated temperature (100 - 200 °C). The reactants must be free of dissolved iron salts (build-up of Friedel-Crafts-catalysts), oxygen (radical scavenger), and water (build-up of hydrochloric acid). 

The procedure can be extended to achieve selective a-bromination and iodination of carboxylic acids and the general mechanism of the a-halogenation is outlined in Chapter 5, p 170. The autocatalytic effects in the selective a-chlorination of propionic acid to the 2-chloro and 2,2-dichloro acids have been studied in a semibatch reaction at 90-130 °C. The reaction was performed in the presence of chlorosulfonic acid and dichloroacetic acid as catalysts and oxygen as the radical scavenger. Kinetic experiments indicated autocatalytic formation of both products and that the selectivity was independent of the chlorine concentration in the liquid phase. The results confirmed the validity of the proposed reaction scheme which involved formation of the reaction intermediate, propanoyl chloride from propionic acid and chlorosulfonic acid, the acid-catalysed enolization of the acid, and a hydroxyl-chlorine exchange reaction. The acid-catalysed enolization was the rate-determining step in the reaction sequence. ... 

The selective a-chlorination of propanoic acid to 2-chloro- and 2,2-dichloropro-panoic acid at 80-130 °C in the presence of chlorosulfonic and dichloroethanoic acid as catalysts and oxygen as the radical scavenger was examined. The results confirmed previous studies (Chapter 5, Section 5.6), indicating that the intermediate was propanoyl chloride formed from propanoic acid and chlorosulfonic acid by acid-catalysed enolization and substitution of the hydroxyl group by chlorine. 

Assume that a scavenger is present that could remove a chlorine free radical.] [Tribology) Why you change your motor oil One of the major reasons for engine oil degradation is the oxidation of the motor oil. To retard the degradation process, most oils contain an antioxidant see Ind. Eng. C/i n. 26, 902 (1987)]. Without an inhibitor to oxidation present, the suggested mechanism at low temperatures is... 

What, specifically, are the first-formed X intermediates Their identity is not entirely clear. Nevertheless, some reasonable possibilities are polyenyl cation monoradicals, polyenyl cation diradicals, and chlorine atoms formed by the 3-scission of one or both of those species [12], In any event, it now appears that the prevention of autoacceleration by the ester thiols is likely to result, at least in part, from free-radical scavenging. Thiols, in general, are well-known to react very readily with a large variety of free radicals via equation 5, and the resultant... 

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