Free-radical reaction

About: Free-radical reaction is a research topic. Over the lifetime, 1919 publications have been published within this topic receiving 56787 citations.

Radical generation by the interaction of transition metals with common oxidants.

Abstract: Nine transition metals were tested for the activation of three oxidants and the generation of inorganic radical species such as sulfate, peroxymonosulfate, and hydroxyl radicals. From the 27 combinations, 14 M/Ox couples demonstrated significant reactivity toward transforming a model organic substrate such as 2,4-dichlorophenol and are further discussed here. It was found that Co(II) and Ru(III) are the best metal catalysts for the activation of peroxymonosulfate. As expected on the basis of the Fenton reagent, Fe(III) and Fe(II) were the most efficient transition metals for the activation of hydrogen peroxide. Finally, Ag(I) showed the best results toward activating persulfate. Quenching studies with specific alcohols (tert-butyl alcohol and ethanol) were also performed to identify the primary radical species formed from the reactive M/Ox interactions. The determination of these transient species allowed us to postulate the rate-determining step of the redox reactions taking place when a metal is coupled with an oxidant in aqueous solution. It was found that when Co(II), Ru(III), and Fe(II) interact with peroxymonosulfate, freely diffusible sulfate radicals are the primary species formed. The same was proven for the interaction of Ag(I) with persulfate, but in this case caged or bound to the metal sulfate radicals might be formed as well. The conjunction of Ce(III), Mn(II), and Ni(II) with peroxymonosulfate showed also to generate caged or bound to the metal sulfate radicals. A combination of sulfate and hydroxyl radicals was formed from the conjunction of V(III) with peroxymonosulfate and from Fe(II) with persulfate. Finally, the conjunction of Fe(III), Fe(II), and Ru(III) with hydrogen peroxide led primarily to the generation of hydroxyl radicals. It is also suggested here that the redox behavior of a particular metal in solution cannot be predicted based exclusively on its size and charge. Additional phenomena such as metal hydrolysis as well as complexation with other counterions present in solution might affect the thermodynamics of the overall process and are further discussed here.

Free radical mechanisms in tissue injury

Abstract: Free radicals are molecules or molecular fragments containing a single unpaired electron. In general, free radicals are reactive chemically, some (e.g. HO•) being extremely reactive. However, certain types of free radical, such as nitroxyl-radicals and free radicals stabilized by steric or derealization features, are much less reactive and a few (e.g. diphenyl picryl hydrazyl) are stable enough to be crystallised and stored at temperatures above 0°. Table 1 gives the general structures of free radicals that will be discussed in this short review.

Ultrafast synthesis of ultrahigh molar mass polymers by metal-catalyzed living radical polymerization of acrylates, methacrylates, and vinyl chloride mediated by SET at 25 degrees C.

Abstract: Conventional metal-catalyzed organic radical reactions and living radical polymerizations (LRP) performed in nonpolar solvents, including atom-transfer radical polymerization (ATRP), proceed by an inner-sphere electron-transfer mechanism. One catalytic system frequently used in these polymerizations is based on Cu(I)X species and N-containing ligands. Here, it is reported that polar solvents such as H(2)O, alcohols, dipolar aprotic solvents, ethylene and propylene carbonate, and ionic liquids instantaneously disproportionate Cu(I)X into Cu(0) and Cu(II)X(2) species in the presence of a diversity of N-containing ligands. This disproportionation facilitates an ultrafast LRP in which the free radicals are generated by the nascent and extremely reactive Cu(0) atomic species, while their deactivation is mediated by the nascent Cu(II)X(2) species. Both steps proceed by a low activation energy outer-sphere single-electron-transfer (SET) mechanism. The resulting SET-LRP process is activated by a catalytic amount of the electron-donor Cu(0), Cu(2)Se, Cu(2)Te, Cu(2)S, or Cu(2)O species, not by Cu(I)X. This process provides, at room temperature and below, an ultrafast synthesis of ultrahigh molecular weight polymers from functional monomers containing electron-withdrawing groups such as acrylates, methacrylates, and vinyl chloride, initiated with alkyl halides, sulfonyl halides, and N-halides.

Cobalt-mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds. implications of chloride ions.

Abstract: The sulfate radical pathway of the room-temperature degradation of two phenolic compounds in water is reported in this study. The sulfate radicals were produced by the cobalt-mediated decomposition of peroxymonosulfate (Oxone) in an aqueous homogeneous system. The major intermediates formed from the transformation of 2,4-dichlorophenol were 2,4,6-trichlorophenol, 2,3,5,6-tetrachloro-1,4-benzenediol, 1,1,3,3-tetrachloroacetone, pentachloroacetone, and carbon tetrachloride. Those resulting from the transformation of phenol in the presence of chloride ion were 2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol, 1,1,3,3-tetrachloroacetone, and pentachloroacetone. In the absence of chloride ion, phenol transformed into 2,5-cyclohexadiene-1,4-dione (quinone), 1,2-benzenediol (catechol), and 1,4-benzenediol (hydroquinone). Several parameters were varied, and their impact on the transformation of the organic compounds is also discussed. The parameters varied were the initial concentration of the organic substrate, the dose of Oxone used, the cobalt counteranion, and in particular the impact of chloride ions and the quenching agent utilized for terminating the reaction. This is one of the very few studies dealing with intermediates formed via sulfate radical attack on phenolic compounds. It is also the first studythat explores the sulfate radical mechanism of oxidation, when sulfate radicals are generated via the Co/Oxone reagent. Furthermore, it provides strong evidence on the interaction of chloride ions with sulfate radicals leading to halogenation of organics in water.

Free radical initiators as source of water- or lipid-soluble peroxyl radicals.

Abstract: Publisher Summary This chapter discusses the role of free radical initiators as source of water- or lipid-soluble peroxyl radicals. Free radicals can be generated in either an aqueous or the lipid phase as required by using water-soluble 2,2′-Azo-bis(2-amidinopropane) dihydrochloride (AAPH) or lipid-soluble 2,2′-azobis(2,4-dimethylvaleronitrile) (AMVN). Admittedly, such azo compounds are not present in biological systems, but they are useful tools for studying quantitatively (1) the damage induced by free radicals on biological and related molecules and membranes and (2) the inhibition in model systems. The advantages are that the radicals can be generated at a constant rate at a specific site and that the rate of radical flux can be measured and controlled. Obviously, the most important characteristic of the free radical reaction is that it proceeds by a chain mechanism—that is, the rate of the overall reaction or the extent of damage can be quite significant even if the rate of initial radical formation or the amount of attacking radical is very small. It is, therefore, quite important to know how long the kinetic chain lasts. The chain length can never be known without knowing the rate of chain initiation or the radical flux. In fact, in the in vitro experiment, the kinetic chain length is as long as 100 in the oxidation of erythrocyte membranes induced by AAPH. Another advantage in using azo compounds is that, unlike peroxides, they are not explosive and can be handled easily and safely.