Roger C. Sealy

Electron spin resonance studies. Part XLVI. Oxidation of thiols and disulphides in aqueous solution: formation of RS˙, RSO˙, RSO2˙, RSSR–, and carbon radicals

Radicals which mediate in the oxidation of thiols with the TiIII–H2O2 couple and CeIV and of disulphides with the former reagent, in aqueous solution at room temperature, have been studied by e.s.r. spectroscopy. The e.s.r. evidence establishes that signals with g ca. 2·01 and 2·005 are those of sulphinyl (RSO˙) and sulphonyl (RSO2˙) radicals, respectively. Radicals of both these types are detected in almost all instances; thiyl radicals (RS˙) cannot be detected directly but their involvement is demonstrated by spin-trapping methods; and two dithiols yield spectra with g ca. 2·013 which are attributed to cyclic disulphide radical-anions. Reaction mechanisms consistent with the e.s.r. data and earlier product studies are suggested.

Electron spin resonance studies. Part XXXIX. A kinetic investigation of the role of radical reduction in metal ion–hydrogen peroxide systems

Reduction of carbonyl-conjugated radicals by TiIII and FeII in aqueous solution has been investigated by e.s.r. spectroscopy and pulse radiolysis. Kinetic data have enabled the importance of such reactions in metal ion–hydrogen peroxide systems to be assessed. For example, reduction by TiIII at pH 1 is negligible under typical flow system conditions whereas at pH 7 reduction by TiIII–EDTA can provide an important pathway for radical termination. The dependence of reduction rates on radical structure has been investigated.

Electron spin resonance studies. Part XLIII. Reaction of dimethyl sulphoxide with the hydroxyl radical

A kinetic analysis is given of the reaction between the hydroxyl radical (generated from the TiIII–H2O2 couple in a continuous flow system) and dimethyl sulphoxide in aqueous acid at ambient temperature; steady-state concentrations of both Me˙ and MeSO2˙ have been monitored. Evidence is presented that the former radical arises from addition of ˙OH to the sulphoxide followed by rapid decomposition of the intermediate species Me2SO2˙. There was no evidence for the formation of ˙CH2S(O)Me. Rate constants for the reaction between hydroxyl and Me2SO, for that between Me˙ and MeSO2H, and for the dimerisation of methyl are estimated as 5 × 109, ca. 106, and 5·6 × 109 l mol–1 s–1, respectively.

Electron spin resonance studies. Part XLIV. The formation of alkylsulphonyl radicals by the oxidation of aliphatic sulphoxides with the hydroxyl radical and by the reaction of alkyl radicals with sulphur dioxide

E.s.r. spectra are reported for a range of alkylsulphonyl and carbon-centred radicals detected during the oxidation of sulphoxides with the TiIII–H2O2 couple in aqueous solution. Alkylsulphonyl radicals are also formed in some cases by reaction of substituted alkyl radicals with SO2 in aqueous solution; however, hydroxy-conjugated radicals under these conditions behave as one-electron reducing agents and yield SO2– and carbonyl- and carboxy-conjugated radicals do not appear to react. Evidence is adduced for the ready desulphonylation of HO2C·CH2·SO2˙.

Electron spin resonance studies. Part XL. A kinetic investigation of the oxidation of oxygen-substituted carbon radicals by hydrogen peroxide

E.s.r. spectroscopy has been employed to investigate the reaction of the hydroxyl radical with some alcohols and ethers, and the oxidation of hydroxy- and alkoxy-conjugated radicals by hydrogen peroxide. The TiIII–H2O2 couple has been used to generate radicals, and application of a competitive kinetic method involving the steady-state approximation leads to the estimation of rate constants for some of these reactions. For example, reaction of diethyl ether with ·OH leads to the radicals ·CHMeOEt and ·CH2CH2OEt; the observed steady-state concentrations at low [H2O2] are in the ratio ca. 13 : 1. At higher concentrations of hydrogen peroxide, this ratio is reduced and we estimate that the rate constant for oxidation of ·CHMeOEt by H2O2 is 5·5 × 104 l mol–1 s–1. The influence of structural features on the oxidation of related radicals has been investigated. Evidence is also presented for oxidation and reduction processes involving radicals generated during the oxidation of 2-methoxyethanol in aqueous solution.

Electron spin resonance studies. Part XLVII. Sulphinyl- and sulphonyl-substituted aliphatic radicals

E.s.r. spectroscopy has been employed to characterise sulphinyl-[·CR1R2S (O) R3] and sulphonyl-conjugated [·CR1R2S(O2)R3] carbon radicals. The former group are generated during the reaction between 1,3-bis-sulphoxides and the hydroxyl radical and from sulphoxides with the phenyl radical; the latter are formed from sulphones with either phenyl or hydroxyl (the reactivities and selectivities of which are compared). There is evidence that both groups of radicals are coplanar at the tervalent carbon atom, and that the sulphinyl group withdraws ca. 6% of the spin from that carbon atom, and that the sulphonyl group has no capacity for spin-delocalisation. Evidence is also presneted that acyclic radicals which posses a β-sulphinyl or β-sulphonyl substituent readily fragment to the corresponding alkene and a radical RSO· or RSO2·.

Electron spin resonance studies. Part XLVIII. Radicals derived from organic sulphites and related compounds

The reactions of organic sulphites and some related compounds with the hydroxyl radical have been investigated by e.s.r. spectroscopy in conjunction with a flow-system technique. There is evidence for a rapid reaction with sulphites, probably at sulphur, which yields an alkoxyl radical; this can be trapped by the nitromethane aci-anion, but otherwise is rapidly transformed into the isomeric 1-hydroxyalkyl radical.

Kinetic electron spin resonance spectroscopy. Part II. Termination rates for ketyl and semiquinone radicals in alcoholic solvents

An intermittent illumination method using e.s.r. detection has been employed to measure rate constants for the termination reactions of free radicals generated during the photoreduction of pyruvic acid, 2-oxobutyric acid, biacetyl, 1,4-benzoquinone, and 1,4-naphthoquinone in organic solvents. For these ketyl and semiquinone radicals, second-order termination rate constants are in the range 108–109 l mol–1 s–1 and show a dependence on solvent viscosity in accord with theories of diffusion-controlled reactions which include rotational diffusion within the solvent cage as a rate-influencing factor. Activation energies were measured for the combination of MeĊ(OH)CO2H and MeĊ(OH)COMe radicals in alcoholic solvents. Observed values of 10–20 kJ mol–1 are in general less than the temperature coefficients for viscous flow in these solvents, again in agreement with this model.

Electron spin resonance studies. Part XLV. Reactions of the methyl radical with some aliphatic compounds in aqueous solution

Results are reported of an e.s.r. investigation of the reactions of the methyl radical, generated from dimethyl sulphoxide with the TiIII–H2O2 couple, with a variety of aliphatic compounds in aqueous solution. For example, with carboxylic acids and nitriles, the spectra of radicals formed by hydrogen-atom abstraction are detected: it is shown that the methyl radical is more selective than hydroxy, having a relatively greater propensity for abstracting hydrogen from the carbon atom adjacent to the functional group, and rate constants for these reactions, which are in the range ca. 102–104 l mol–1 s–1, have been determined. Reaction with nitromethane is more complex. The spectra observed are those of the nitroethane radical-anion or its conjugate acid, depending on the pH, and it is concluded that abstraction by the methyl radical to give ·CH2NO2 is followed by rapid one-electron reduction by TiIII to give the aci-compound to which methyl then adds. Nitroethane behaves analogously, and both EtNO2H and PriNO2H radicals are estimated to have pKa 4·4.

Kinetic electron spin resonance spectroscopy. Part III. Proton exchange rates for ketyl radicals in organic solvents

Measurements of linewidths and hyperfine splittings in the e.s.r. spectra of some photochemically generated ketyl radicals have been used to determine the rates of acid-catalysed carboxy-proton exchange reactions in these species. For CH3Ċ(OH)CO2H radicals in chloroform, ethanol, and propan-2-ol the second-order rate constants k are 1·7, 2·0, and 2·8 × 107 l mol–1s–1 respectively at 298 K and for HO2CCH2Ċ(OH)CO2H radicals in ethanol k= 2·8 × 107 l mol–1s–1. Measured activation energies are ca. 30 kJ mol–1.