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Oxidation of caffeic acid (3,4-dihydroxycinnamic acid) 1H₃ has been studied by electrochemical methods and by pulse radiolysis in aqueous and organic solvents. The results have been compared with the behaviour of 4-coumaric acid 2H₂ and ferulic acid 3H₂. The first oxidative intermediates have been characterised by their UV spectra and oxidation potentials. In the case of 2H₂ and 3H₂, the initial radicals decay by a second order process indicating a radical-radical coupling mechanism. On the contrary, for caffeic acid the oxidation leads to the formation of the corresponding o-quinone through disproportionation of the initial semiquinone radical.