An experimental and mechanistic investigation of the complexities arising during inhibition of the Briggs-Rauscher reaction by antioxidants

A method to determine the relative antioxidant capacity of radical scavengers based on the inhibition of the oscillations of the Briggs-Rauscher (BR) oscillating reaction was previously reported. A semiquantitative mechanistic interpretation of the inhibitory effects required two steps to obtain simulated inhibition times in very good agreement with the experimental ones. The first step is inhibitory, involving H-atom transfer from antioxidant to the HOO. radical; the second step is a first-order degradation of the antioxidant to unspecified products. Since the degradation may be due to oxidation and/or iodination of the antioxidant, we studied the kinetics of the subsystems IO3- (H+) + antioxidant and I-2(H+) + antioxidant. We used 2,5- and 2,6-dihydroxybenzoic acids, caffeic acid (= 3-(3,4-dihydroxyphenyl)prop-2-enoic acid), ferulic acid (= 3-(4-hydroxy-3-methoxyphenyl)prop-2-enoic acid), pyrocatechol (= benzene-1,2-diol), and hydroquinone (= benzene-1,4-diol) as antioxidants. Spectra in the wavelength range 500-250 nm were repeated at given time intervals to follow the peaks of the iodine and oxidation products, which were mainly quinones. For the iodination of the above diphenols (= benzenediol derivatives) the substitution and/or addition reactions with I-2 or HOI were found to be relatively slow compared to oxidation by IO3-. Approximate rate constants for oxidation were obtained on the basis of a reasonable kinetic model by using a suitable numerical integration program. Although these complexities can arise also in the completely inhibited BR oscillator, we believe that the inhibitory effects are due to the HOO. scavenging action by diphenols or by quinones since HOO. radicals are also potential reducing agents. We propose two steps that could maintain a small reservoir of diphenol, while both quinone and diphenol deplete HOO. radicals. In short, the complexities do not affect the method for monitoring the relative activity of antioxidants based on the BR oscillating reaction. The effects of temperature on the inverse of the oscillatory time in the BR-uninhibited system, on the inverse of inhibition times, and on the time length of the resumed oscillations for four antioxidants were also investigated. Apparent average activation energies were obtained.