Photosensitised oxidation of amines: mechanism of oxidation of triethylamine

Abstract

The rates of photo-oxygenation of triethylamine sensitised by Rose Bengal have been measured for a variety of amine concentrations utilising methanol, methanol–water, and pyridine as solvents. In order to explain the results a kinetic scheme is used in which account is taken of chemical reaction occurring via singlet oxygen and interaction of the excited triplet state of the dye and the amine. The excited singlet state of Rose Bengal is quenched by the amine. That the oxidation of the amine can occur by two concurrent mechanisms is supported by a study of solvent isotope effects. Oxygenation reactions using tris-(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate as sensitiser are less complicated since only one excited state of the sensitiser is involved in the reactions. The effect of change in oxygen concentration upon the rate of oxygenation reactions involving both singlet oxygen and radical pathway is discussed. Calculations show that whether the change causes retardation or acceleration is dependent upon the magnitude of the various rate constants and the efficiency with which the singlet oxygen and radical pathways lead to products.