Reactions with solvated electrons

Schindewolf and WUnschel [112] have studied solvated electron reactions in liquid ammonia and water with several univalent anions and divalent cations. Ions such as NO3, N02, and BrO in water showed diffusion-limited behaviour and the ions Cd2+, Ni2+, Co2+, and Zn2+ in water displayed diffusion-limited behaviour or faster. Schindewolf and WUnschel considered that reactions of none of these ions were quite diffusion-limited in liquid ammonia. Applying the hydrodynamic correction suggests that the anionic reaction with solvated electrons may just be diffusion-limited, but the cations reaction with solvated electrons remains slower than diffusion-limited. 

Calcaterra, Closs and Miller observed rapid intramolecular ET in the radical anions 8-10 in which the chromophores are attached to a rigid steroid bridge (Figure 10) [33]. The edge-to-edge distance between the two chromophores in these molecules is about 11 A, and they are separated by nine C-C bonds. The radical anions of 7-10 in 2-methyltetrahydrofuran at room temperature were generated by reaction with solvated electrons formed by pulse radiolysis. These studies differ from those discussed above in that the ET processes in 7-10 are exergonic, rather... 

In addition to the chloroacetates, as shown by comparisons with the estimates in Table V, the half-lives for both lindane and mirex in the natural water samples would have been considerably longer than those observed, had reaction with solvated electrons in bulk solution been the dominant mechanism for photoreaction. The higher efficiency of these halocarbon reactions may be attributable to sorption of the chloroacetates on the NOM, which permits more facile electron capture. Other possible pathways for reactions of sorbed halocarbons include direct photoreduction by excited states of the NOM, which, like solvated electrons, also are quenched by oxygen. Alternatively, the enhancement may involve other direct electron-transfer mechanisms such as amine-halomethane reactions. These alternative possibilities are examined in the following section. 

Aquatic Sinks for Methylchloroform. Methylchloroform distributions in the troposphere have been used to estimate the concentrations of tropospheric hydroxyl radicals (65). The estimates assume that reaction with tropospheric OH radicals is the dominant sink. Aquatic sinks have been ignored. Wine and Chameides (66), however, presented model computations that indicated that hydrolysis and reaction with solvated electrons may be a significant sink for methylchloroform and other halocarbons in the open ocean. 

Note kh represents hydrolysis rate constants, kt denotes rate constants for reaction with solvated electrons, and kOH symbolizes rate constants for reaction with OH radicals. All values are given in reciprocal seconds. 

Early y-radiolysis experiments with p-bromophenol have shown that reaction with solvated electrons yields Br ions quantitatively and that the organic products include hydroxylated biphenyl and terphenyl". From pulse radiolysis experiments it was concluded that the hydroxyphenyl radical, formed by reaction of Caq with p-bromophenol, adds rapidly (k = 1 x 10 M s ) to the parent compound (equation 6). 

Dihydroxytoluenes, protonation of 309 Diiodotyrosine, reactions with solvated electrons 1099... 

It is interesting to compare direct cathodic reduction with reduction of the same substance using solvated electrons. Thus, phosphine cations, which are formed in hexamethylphosphotriamide solutions of iodine, tosyl and mesyl chlorides (see above), and also N-tosylcarbazol during direct cathodic reduction (whose potential is by 2.5 and 0.8 V, respectively, more positive than the generation potential of solvated electrons) yield, unlike the reaction with solvated electrons, unexcited particles chemiluminescence is absent in this case Thus, investigation of... 

Solvated electrons can be obtained in concentrations up to 10" moldm . Reactions with solvated electrons are very fast second-order reactions, with rate constants between 10 to 5x10 mor dm s", which means that their rates are close to the diffusion control. A reaction is diffusion-controlled when the reaction rate is dependent upon the rate at which reactants diffuse toward one another. A diffusion-controlled reaction must have a small activation energy, because if is high (EJRT 1), then the reaction rate is controlled by the number of molecules with energy higher than the activation energy, not by the diffusion rate. Reactions with high E are activation-controlled. 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

Steady photoemission currents can be realized when acceptors (scavengers) for the solvated electrons are present in the solution. A good scavenger should be nonelectroactive at the potenhal of interest, should react quickly with solvated electrons, and the reaction products should be either nonelectroactive or reducible. A reachon with acceptors implies that the current of reoxidation of the solvated electrons becomes lower, and thus a steady photoemission current appears. The acceptors most often used are nitrous oxide, N2O, and hydroxonium ions, HjO. In the former case, OH radical is produced in the scavenging process, which undergoes further reduction on the electrode, thus doubling the photocurrent ... 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

The review by Rice and Pilling [39] discussed some of the experimental studies of reactions of solvated electrons with scavengers and other more recent work has been included in Chap. 3, Sect. 2. For completeness, some of the more important studies are presented below. 

Effective encounter distances for reaction of solvated electrons with electron scavengers at room temperature compared with crystallographic encounter distances Unless otherwise noted, the solvent is water (containing an inert electrolyte in some cases). Corrections for ionic interactions according to eqn. (106) were applied and reaction rate coefficient were extrapolated to zero ionic strength (Chap. 3, Sect. 1.6 and 1.7). Many of these studies have been mentioned in Chap. 3, Sect. 2... 

Effective encounter distances for reactions of solvated electrons with electron scavengers at low temperatures, compared with crystallographic encounter distances, from a fit between experiment and eqn. (105)... 

Some reactions are difficult to classify an example is reduction by means of solvated electrons, a synthetically useful method. In such a reaction the electron is ejected from the electrode into the solvent, where it has a finite lifetime before it reacts with the substrate. Reductions with solvated electrons will not be discussed here. Another example, coupling reactions, will be discussed later. 

The number of solvents that have been used in SrnI reactions is somewhat limited in scope, but this causes no practical difficulties. Characteristics that are required of a solvent for use in SrnI reactions are that it should dissolve both the organic substrate and the ionic alkali metal salt (M+Nu ), not have hydrogen atoms that can be readily abstracted by aryl radicals (c/. equation 13), not have protons which can be ionized by the bases (e.g. Nth- or Bu O" ions), or the basic nucleophiles (Nu ) and radical ions (RX -or RNu- ) involved in the reaction, and not undergo electron transfer reactions with the various intermediates in the reaction. In addition to these characteristics, the solvent should not absorb significantly in the wavelength range normally used in photostimulated processes (300-400 nm), should not react with solvated electrons and/or alkali metals in reactions stimulated by these species, and should not undergo reduction at the potentials employed in electrochemically promoted reactions, but should be sufficiently polar to facilitate electron transfer processes. 

In the century since its discovery, much has been learned about the physical and chemical properties of the ammoniated electron and of solvated electrons in general. Although research on the structure of reaction products is well advanced, much of the work on chemical reactivity and kinetics is only qualitative in nature. Quite the opposite is true of research on the hydrated electron. Relatively little is known about the structure of products, but by utilizing the spectrum of the hydrated electron, the reaction rate constants of several hundred reactions are now known. This conference has been organized and arranged in order to combine the superior knowledge of the physical properties and chemical reactions of solvated electrons with the extensive research on chemical kinetics of the hydrated electron. 

---