A number of transient free-radical spectra produced during the pulse radiolysis of benzoquinone, and hydroquinone, have been observed and characterized. The semiquinone radical and radicalanion are produced directly by reactions of e,; and H atoms with benzoquinone and also by the unimolecular elimination of water from trihydroxycyclohexadienyl radicals. The latter species arise directly by OH addition to hydroquinone. The extinction coefficients of the various species and the reaction rate constants for their formation and decay have been measured. The equilibrium constant for the benzsemiquinone radical and radical-anion equilibrium is 10-4 M.Benzoquinone (Q) is reduced and hydroquinone (QH2) is oxidized by ionizing radiation,l-4 both processes occurring via the formation of the radical intermediate benzsemiquinone (QH) :In the pH range 11.5-13.5, durosemiquinone is present in equilibrium with both duroquinone and durohydroquinone 5 ; the durosemiquinone is in the ionized form (Q-) and the equilibrium constant for the reaction is 1028.6 Baxendale and Hardy measured the absorption spectrum of the durosemiquinone anion, and reported two maxima at 4150A.6 Flash photolysis has been used to measure the absorption spectra of the neutral and ionized forms of duroand other related semiquinones,79 * and the equilibrium constant for the durosemiquinone protonation reaction was 1.2 x 10-6 M-1 in ethanol+ water mixtures.Diebler et aZ.99 10 using a combination of flow and temperature jump techniques, measured the absorption spectrum of the radical and anion of benzoquinone and reported a value of 7 x 107 M-1 sec-1 at 11°C for the alkaline disproportionation reaction rate constant k2 (reverse). Pulse radiolysis studies have shown that the benzsemiquinone anion is produced in a unimolecular water elimination reaction 1171 Q + Q2-+2Q-QT + H++QH.
Pulse radiolysis has been used to measure directly the absolute rates of oxidation by ferricyanide ion of various radicals produced by OH attack on organic solutes. These include mono, di-and polyhydroxylic compounds, hydroxy acids, polyethylene oxides of molecular weight 200, 6000 and 20 OOO and the amino acid serine. Radicals produced by hydrogen abstraction from tc carbon atoms in alcohols are oxidized at, or near, diffusion-controlled rates, whereas the reactions are much slower for radicals formed by OH-attack elsewhere. The technique has been used to measure the percentage OH-attack at the a position for a series of straight and branched-chain alcohols.Oxygen competes with ferricyanide for radical oxidation. The data for oxygen-containing solutions fit a simple radical-competition scheme which has been used to measure rates of peroxy-radical formation. These approach diffusion-controlled limits.
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