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Studies on nitroxides (XII) : A kinetic ESR study on the oxidation of ascorbic acid by a nitroxide
Author(s) -
Liu YouCheng,
Wu LungMin,
Liu ZhongLi,
Han ZhengXu
Publication year - 1985
Publication title -
acta chimica sinica english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.28
H-Index - 41
eISSN - 1614-7065
pISSN - 0256-7660
DOI - 10.1002/cjoc.19850030409
Subject(s) - chemistry , disproportionation , ascorbic acid , redox , aqueous solution , nitroxide mediated radical polymerization , electron transfer , reaction rate constant , radical , activation energy , electron paramagnetic resonance , kinetic energy , photochemistry , radical ion , kinetics , ion , inorganic chemistry , catalysis , organic chemistry , radical polymerization , polymerization , nuclear magnetic resonance , polymer , physics , food science , quantum mechanics
A kinetic ESR study (flow‐stop method) was performed on the redox reaction between ascorbic acid(1)and 4‐hydroxy‐2,2,6,6‐tetramethylpiperidinyl‐1‐oxy radical (6) in oxygen‐free aqueous solution. Both 6 and the oxidation intermediate of 1 — ascorbate radical anion (3) were observed simultaneously in alkaline solution (pH 9‐11) and their decay rates monitored by ESR spectroscopy after stopping the flow. Based on the kinetic data a mechanism involving two step one‐electron transfer reactions and the disproportionation of 3 is proposed. The rate constants at 25°C are 1.95, 1.42 × 10 4 and 2.22 × 10 5 M −1 s −1 with activation energies 39.4, 32.5 and 24.7 kJ.mole −1 and activation entropies −29.9, −17.6 and −13.6 e.u. respectively. The results indicate that the formation of strictly oriented activation complexes may be involved during these reactions.