Radical ions competitive addition

The competition between the radical and cationic pathway in the Kolbe reaction is strongly influenced by the presence of foreign anions, e.g. CIO4 and F", as has been known for a long time (for reviews, see Eberson, 1968, 1973). Exactly how strong this influence is was recently shown for the oxidation of phenylacetate ion the addition of only one hundredth molar proportion of perchlorate relative to phenylacetate ion completely suppressed formation of the... 

The photochemical Diels-Alder reactions of anthracene with fumarodinitrile and 1,4-benzoquinone have been studied in chloroform solution. Not surprisingly, the addition occurs in competition with dimerization of the arene and proceeds by way of electron transfer from anthracene to the dienophiles. The radical ion pair has been detected by transient absorption spectroscopy, and the resulting diradical precursor of adduct formation from the quinone was observed by ESR at 77 K. 2,7-Dibromotropone is reported to undergo (871+471) photoaddition to 9,10-dicyanoanthracene in benzene-methanol (9 1), giving (25) as the primary adduct which is then proposed to react with methanol and water (solvent contaminant) to yield the final product (26). In contrast, 2-bromotropone and the anthracene in CH2CI2 solution afford the substitution products (27) (62%) and (28) (25%). 

The experiments with 2-(3-butenyloxy)benzenediazonium ions (10.55, Z = 0, n = 2, R=H) and benzenethiolate showed a significant shift of the product ratio in favor of the uncyclized product 10.57. They also indicated that the covalent adduct Ar — N2 — SC6H5 is formed as an intermediate, which then undergoes homolytic dissociation to produce the aryl radical (Scheme 10-83). Following the bimolecular addition of the aryl radical to a thiolate ion (Scheme 10-84), the chain propagation reaction (Scheme 10-85) yielding the arylphenylsulfide is in competition with an alternative route leading to the uncyclized product 10.57. 

Both CO and C02 are reduced by eh. The immediate product of the first reaction is CO-, which reacts with water, giving OH and the formyl radical the latter has been identified by pulse radiolysis. The product of carbon dioxide reduction, C02-, is stable in the condensed phase with an absorption at 260 nm. It reacts with various organic radicals in addition reactions, giving carboxylates with rates that are competitive with ion-ion or radical-radical combination rates. 

The competition between ion-pair and radical-pair collapse can be predictably modulated by the polarity of the solvent and by the addition of inert salt. 

Such competition between ion-pair collapse of MT+, C(N02)f and the radical-pair collapse of MT+, NO is also readily modulated by the addition of inert salt.14 The description of the solvent and salt effects in equations (82) and (83) is further confirmed by direct kinetics analysis of the decay of the cation radical MT+ on the nanosecond/microsecond timescale. 

Workentin et al. (1994) described another interesting solvent effect on the competition between electron transfer and the addition reaction between organic cation-radicals and azides. TEE and AN were compared as solvents. In TEE, the cation-radicals of 4-methoxystyrene (R =R =H), P-methyl-4-methoxystyrene (R =Me, R =H), or p,p-dimethyl-4-methoxystyrene (R =R =Me) react with the azide ion according to the following equation ... 

Substituted benzaldehydes have proved useful as acid-base indicators for reactions involving the addition of hydroxide ions n strongly alkaline aqueous media (19). It seemed logical to extend their use to solutions of sodium hydroxide in water-ethanol and water-DMSO mixtures. In ethanol-water, it was of interest whether the competition between addition of hydroxide and ethoxide ions will be reflected in the dependence of the J- function on ethanol concentration. In water-DMSO mixtures, it was important to investigate whether the radical change at higher DMSO concentrations, observed for H values and attributed to changes in solvation of the hydroxide ion, will be observed for the addition reaction as well. 

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