Search

E.s.r. spectra are reported for the radicals obtained by the oxidation of a variety of organic compounds by the titanium(III)–persulphate redox system. Evidence is obtained that the sulphate radical anion is formed throughout the pH range employed (1–10·5) and that it can react with organic compounds in at least three ways: by abstracting a hydrogen atom from saturated carbon, by adding to unsaturated or aromatic carbon, and by removing one electron from carboxylate anions and from certain neutral molecules. It displays a marked degree of selectivity in its addition reactions and it behaves differently from the hydroxyl radical in several respects.

The reactions of some enols and enol ethers with the hydroxyl and the amino-radical have been investigated by e.s.r. spectroscopy. Adducts from enols with the hydroxyl radical, but not apparently the amino-radical, undergo acid-catalysed elimination of water. Evidence is adduced that the two adducts from an enol ether with the hydroxyl radical, but again not the amino-radical, undergo acid-catalysed interconversion, presumably by way of an oxonium ion; results from the oxidation of 2-methoxyethanol and 1,2-dimethoxyethane with the hydroxyl radical are helpful in elucidating the significance of this rearrangement. A further inference from the results is that the radical MeO·CH(OH)·CH2· is thermodynamically more stable than its alkoxy-conjugated isomer ·CH(OMe)·CH2OH, at least in aqueous acid, and the basis for this is discussed.

Earlier studies by e.s.r. spectroscopy of the radicals formed by the interaction of titanium(III) ions and hydrogen peroxide, in the presence or absence of organic compounds, have been extended to the examination of the reactions of the initially formed radicals. Particular use has been made of flow cells which allow one or more reactants to be added shortly after the others have been mixed and before the solution reaches the spectrometer cavity. The results are consistent with the view that the free hydroxyl radical is the primary oxidising agent but that the one-electron oxidation of the resulting organic radicals by both metal ions and hydrogen peroxide is important in determining the relative concentrations of the various radicals which are observed; in this respect, hydrogen peroxide is of special interest because its reduction by organic radicals leads to the formation of further hydroxyl radicals and thence the perpetuation of a chain reaction. This observation has led to the rationalisation both of the dependence of the observed concentrations of readily oxidised radicals on the concentrations of the redox reactants, and of the increase in concentration of the relatively stable radicals derived from titanium(IV)–peroxide complexes which is effected by the addition of small amounts of certain organic compounds. Evidence is also adduced that the markedly different relative concentrations of radicals which are observed when iron(II) ion is used in place of titanium(III) ion are not the result of there being different types of oxidising radicals in the two systems but reflect the fact that iron(III) ion is both a much stronger oxidising agent than titanium(IV) ion for organic radicals and shows a significant degree of selectivity in this capacity between tervalent carbon atoms in different environments.