Organic radical ions bimolecular reactions

The question whether or not radical ions are formed upon irradiation of liquids and stabilized enough for detection or engagement in bimolecular chemical reactions has moved radiation chemists ever since the early days of this research field. This was particularly exciting with respect to low polarity solvents, but even for aqueous solutions conclusions had to rely mainly on indirect evidence. A real breakthrough came with the experimental discovery of the hydrated electron and other powerful one-electron reductants (e.g., a-hydroxyalkane radicals such as (CH3)2C OH). Applying these new tools a large number of organic radical anions were detected and characterized with respect to their optical and chemical properties, particularly by pulse radiolysis. [Pg.343]

Since the sensitivity towards water in many organic reactions lies in the order carbanion > carbonium ion > free radical, it appears likely that as water is progressively removed from a-methylstyrene—and, perhaps, other vinyl monomers—the free radical propagation is augmented or supplanted by a carbonium ion mechanism, which, in turn, is further enhanced at low water content, by a carbanion mechanism. Under the latter conditions, one would expect a termination mechanism which is bimolecular with regard to the total concentration of propagating species and hence a square-root dependence of the polymerization rate on the dose rate. This is the order dependence observed in a-methylstyrene at the highest polymerization rates and lowest water content. [Pg.191]

Net reaction between hydrophilic oxidants and reductants separated by bilayer membranes has been convincingly demonstrated in numerous asymmetrically organized vesicle and planar bilayer systems [2a-c]. Particularly compelling examples are those in which electrogenic electron transport is accompanied by net translocation of lipophilic cations or anions, as is required to maintain electroneutrality. For example, MV + occluded within the inner aqueous phase of DHP vesicles can be completely reduced to the MV+ radical cation by 8204 ion in the bulk phase, but only if an equal or greater amount of viologen is initially present in the external environment [40]. Neither MV-+ nor 8204 is membrane permeable. The reaction is biphasic, with rapid bimolecular reduction of external being followed by... [Pg.2982]

In this Scheme the ligand is often bipyridyl if the metal is Cul and, in early ATRP, ruthenium compounds were used. The nature of the ligand controls the effectiveness of the catalyst. The system is living since the concentration of the transition metal in the higher oxidation state is sufficient to ensure that deactivation to the dormant state is much faster than any other polymer radical termination steps, including bimolecular recombination. Because of the absence of a Trommsdorff effect the reaction may be carried out in the bulk (Matyjaszewski, 1999). The detailed mechanism is complex since the reaction rate depends on the metal, the organic halide, the ligand and the counter-ion (Fischer, 2001). [Pg.83]

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