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Chain ion molecular mechanism of ascorbic acid oxidation with hydrogen peroxide. Cu 3+ ion as a chain carrier
Author(s) -
Skurlatov Yu.
Publication year - 1980
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550120506
Subject(s) - chemistry , ascorbic acid , catalysis , chain termination , rate equation , acetonitrile , hydrogen peroxide , ion , activation energy , reaction rate constant , chain propagation , reaction rate , hydrochloric acid , reaction mechanism , kinetics , photochemistry , inorganic chemistry , organic chemistry , physics , food science , radical polymerization , monomer , quantum mechanics , copolymer , polymer
Abstract The kinetics and mechanism of ascorbic acid (DH 2 ) oxidation have been studied under anaerobic conditions in the presence of Cu 2+ ions. At 10 −4 ≤ [Cu 2+ ] 0 < 10 −3 M , 10 −3 ≤ [DH 2 ] 0 < 10 −2 M , 10 −2 ≤ [H 2 O 2 ] ≤ 0.1 M , 3 ≤ pH < 4, the following expression for the initial rate of ascorbic acid oxidation was obtained:where χ 2 (25°C) = (6.5 ± 0.6) × 10 −3 sec −1 . The effective activation energy is E 2 = 25 ± 1 kcal/mol. The chain mechanism of the reaction was established by addition of Cu + acceptors (allyl alcohol and acetonitrile). The rate of the catalytic reaction is related to the rate of Cu + initiation in the Cu 2+ reaction with ascorbic acid by the expressionwhere C is a function of pH and of H 2 O 2 concentration. The rate equationwhere k 1 (25°C) = (5.3 ± 1) × 10 3 M −1 sec −1 is true for the steady‐state catalytic reaction. The Cu + ion and a species, which undergoes acid–base and unimolecular conversions at the chain propagation step, are involved in quadratic chain termination. Ethanol and terbutanol do not affect the rate of the chain reaction at concentrations up to ≈0.3 M . When the Cu 2+ –DH 2 –H 2 O 2 system is irradiated with UV light (λ = 313 nm), the rate of ascorbic acid oxidation increases by the value of the rate of the photochemical reaction in the absence of the catalyst. Hydroxyl radicals are not formed during the interaction of Cu + with H 2 O 2 , and the chain mechanism of catalytic oxidation of ascorbic acid is quantitatively described by the following scheme. Initiation:Propagation:Termination: