Regioselective oxidation of aliphatic acids by complexed hydroxyl radicals
Hewgill, F.R.; Proudfoot, G.M.
Australian Journal of Chemistry 29(3): 637-647
ISSN/ISBN: 0004-9425 DOI: 10.1071/ch9760637
The oxidation of fatty acids is of both metabolic and industrial importance. Radicals produced by the TiCl3/H2O2 oxidation of various saturated and unsaturated aliphatic acids were identified by ESR, and their relative concentrations were determined by computer simulation of the spectra. In acid solution saturated acids are oxidized principally at the .beta. and .omega.-1 (penultimate) positions and in basic solution at the .alpha. and .omega.-1 positions. A mechanism involving intermolecular oxidation of the terminal methylene group of 1 acid by .OH complexed through Ti to the carboxyl group of another acid in a head to tail oriented dimer is proposed to explain the prevalence of .omega.-1 oxidation. Explanations for other selective oxidations can be made in terms of inductive and mesomeric effects. The lower unsaturated acids react by addition of .OH to the double bond. Failure to detect radicals other than from .alpha.-H abstraction in the larger acids is attributed to micelle formation. In some cases assignments were confirmed by the oxidation of related hydroxy acids and by reduction of halogenated acids.