MECHANISM OF CATALYTIC ASCORBIC-ACID OXIDATION SYSTEM CU2+ ASCORBIC ACID O2

Analysis is made of reported results on the kinetics and mechanism of ascorbic acid oxidation with oxygen in the presence of cupric ions. The diversities due to methodological reasons are cleared up. A kinetic study of the mechanism of Cu2+ anaerobic reaction with ascorbic acid (DH2) is carried out. The true kinetic regularities of catalytic ascorbic acid oxidation with oxygen are established at 2.7 ≤ pH < 4, 5 × 10−4 ≤ [DH2] ≤ 10−2M, 10−4 ≤ [Cu2+] ≤ 10−3M, and 10−4 ≤ [O2] ≤ 10−3M: (Formula Presented.) where𝔨1 (25°C) = 0.13 ± 0.01 M−0.5˙sec−1. The activation energy for this reaction is E1 = 22 ± 1 kcal/mol. It is found by means of adding Cu+ acceptors (acetonitrile and allyl alcohol) that the catalytic process is of a chain nature. The Cu+ ion generation at the interaction of the Cu2+ ion with ascorbic acid is the initiation step. The rate of the chain initiation at [Cu2+] ± 10−4M, [DH2] ± 10−2M, 2.5 < pH < 4, is (Formula Presented.) where𝔨i,1 (25°C) = (1.8 ± 0.3)M−1˙sec−1, Ei,1 = 31 ± 2 kcal/mol. The reaction of the Cu+ ion with O2 is involved in a chain propagation, so that the rate of catalytic ascorbic acid oxidation for the system Cu2+DH2O2 is (Formula Presented.) where𝔨1 (25°C) = (5 ± 0.5) × 104 M−1˙sec−1. The Cu+ ion and a species interacting with ascorbate are involved to quadratic chain termination. By means of photochemical and flow electron spin resonance methods we obtained data characteristic of the reactivities of ascorbic acid radicals and ruled out their importance for the catalytic chain process. A new type of chain mechanism of catalytic ascorbic acid oxidation with oxygen is proposed: (Formula Presented.) . Copyright © 1979 John Wiley & Sons, Inc.