Hydrogen reaction 4- hydrated electron

Anbar, M. and Neta, P. (1967). A compilation of specific biomolecular rate constants for the reaction of hydrated electrons, hydrogen atoms and hydroxyl radicals with inorganic and organic compounds in aqueous solutions. Int. J. Appl. Radiat. Isot. 18, 493-497. 

Buxton, G. V., Greenstock, G. L., Helman, W. P., Ross, A. B. (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solutions. J. Phys. Chem. Ref. Data 17, 513-886. 

Ionizing radiations (a, ft and y) react unselectively with all molecules and hence in the case of solutions they react mainly with the solvent. The changes induced in the solute due to radiolysis are consequences of the reactions of the solute with the intermediates formed by the radiolysis of the solvent. Radiolysis of water leads to formation of stable molecules H2 and H2O2, which mostly do not take part in further reactions, and to very reactive radicals the hydrated electron eaq, hydrogen atom H" and the hydroxyl radical OH" (equation 2). The first two radicals are reductants while the third one is an oxidant. However there are some reactions in which H atom reacts similarly to OH radical rather than to eaq, as e.g. abstraction of an hydrogen atom from alcohols, addition to a benzene ring or to an olefinic double bond, etc. 

The second-order rate constant for the reaction of a hydrogen atom with a hydroxide ion to give an electron and water (hydrated electron) is 2.0 x 10 M s . The rate constant for the decay of a hydrated electron to give a hydrogen atom and hydroxide ion is 16M s. Both rate constants can be determined by pulse radiolytic methods. Estimate, using these values, the pA of the hydrogen atom. Assume the concentration of water is 55.5M and that the ionization constant of water is 10 M. 

The yields of these so-called primary species, present at the time when radical combination in, and diffusive escape from, the spurs is complete, were obtained by adding solutes to the water to capture the radicals and by measuring the stable identifying products. It was from a number of these studies that it became clear that the reducing radical must exist in two forms, which turned out to be the hydrogen atom and the hydrated electron (e q). For example, Hayon and Weiss [6] found that the yields of H2 and Cl produced by irradiating solutions of chloroacetic acid varied with pH in a manner that was consistent with the following reactions ... 

Reaction of the hydrated electron via Reaction 2 or 3 and of the hydrogen atom via Reaction 5 or 6 will ultimately yield an H02 radical. From known values of the rate constants (13)—viz., k2 = k3 = 2 X 1010 liters mole"1 sec."1—it can be calculated that under the experimental conditions of Table I, virtually all hydrated electrons react via Reaction 2. 

Nucleophilic addition to C=0 (contd.) ammonia derivs., 219 base catalysis, 204, 207, 212, 216, 226 benzoin condensation, 231 bisulphite anion, 207, 213 Cannizzaro reaction, 216 carbanions, 221-234 Claisen ester condensation, 229 Claisen-Schmidt reaction, 226 conjugate, 200, 213 cyanide ion, 212 Dieckmann reaction, 230 electronic effects in, 205, 208, 226 electrons, 217 Grignard reagents, 221, 235 halide ion, 214 hydration, 207 hydride ion, 214 hydrogen bonding in, 204, 209 in carboxylic derivs., 236-244 intermediates in, 50, 219 intramolecular, 217, 232 irreversible, 215, 222 Knoevenagel reaction, 228 Lewis acids in, 204, 222 Meerwein-Ponndorf reaction, 215 MejSiCN, 213 nitroalkanes, 226 Perkin reaction, 227 pH and, 204, 208, 219 protection, 211... 

Hydrated electrons are also formed as a product of the interaction of hydroxide ions with hydrogen atoms. This reaction was first established kinetically (4, 6, 81, 82, 99) and then corroborated spectro-photometrically using flash radiolysis (95, 96). It should be noted that the rate of the H + OH- - e aq reaction is only 1.8 X 107 M l sec.-1 (66) thus, this step may become rate determining in many reactions with reactive substrates. 

Hydrated electrons react with many Bronsted acids. This reaction is not a proton transfer process but an incorporation of the electron into the acid to form an AH - ion radical, which may subsequently undergo decomposition. This decomposition may occasionally yield a hydrogen atom, but in many cases other pathways of dissociation have been observed. 

A very fast second-order disappearance of hydrated electrons with the production of molecular hydrogen has been observed in water using pulse radiolysis (9). In our system this reaction would be expected to cause large deviations from first-order behavior at high water concentrations. The absence of such deviation shows that this reaction depends strongly upon the solvent structure and not merely upon the concentration of water molecules. 

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