A study of radical addition to double and to triple bonds by means of electron spin resonance spectroscopy

The e.s.r. spectra of a variety of radical-adducts generated by the oxidation of a number of saturated organic compounds in a titanium(III) ion-hydrogen peroxide flow system and their subsequent addition to the unsaturated substrates maleic acid, benzene-1,3,5-tricarboxylic acid and acetylenedicarboxylic acid in both acidic and alkaline media have been studied. In the case of maleic acid, the radical-adduct exhibits long-range coupling constants leading to a positive identification of the species concerned. The configurations of these radicals were found to be partly pH dependent. Radical-adducts of benzene-1,3,5-tricarboxylic acid are positively identified by their long-range coupling constants. Hydroxyl radical addition provides two radicals which are highly specific to pH and one of which is seen in emission. Initial radical addition to acetylenedicarboxylic acid provides, in the first instance, a vinyl-type radical (adduct). However, this vinyl-type radical is believed to proceed to an allyl-type radical either by an intramolecular hydrogen shift, or addition to another substrate molecule followed by a hydrogen shift. Structures for these radicals and mechanisms for the transformations are suggested.