COMPARATIVE KINETICS OF THE INDUCED RADICAL AUTOCONDENSATION OF POLYFLAVONOID TANNINS .2. FLAVONOID UNITS EFFECTS

摘要

Comparative kinetics of the radical autocondensation induced by SiO2 on a series of polyflavonoid tannins, namely, pine, pecan, mimosa, quebracho, gambler, sumach, and on the catechin monomer as a model compound were carried out by electron spin resonance. The induced radical autocondensation appeared to be independently catalyzed by the known base mechanism, as well as SiO2 and Lewis acid attack directly at the heterocycle oxygen. The reaction occurs in two definite steps: the first, the radical-anion formation, the second, the condensation proper with other flavonoid units of the reactive sites formed. The rate determining step depends on both the main flavonoid unit structure of each tannin and particularly on the level of colloidal state of the tannin solution and the number-average degree of polymerization (DPn), with the latter two parameters being the main determining ones for the second reaction step and the first two for the first reaction step. It is, however, the combination of the three parameters that determines the total observable effect for each of the flavonoid tannins. The SiO2 attack. at the heterocycle ether oxygen is of such an intensity that the A-rings' phenoxide radicals, which drive the reaction, surge very rapidly to such a higher proportion than the B-rings phenoxide radicals that the B-rings also start to surge later by shifting to the left of the B-. reversible arrow A(.) equilibrium. There are also indications that ionic mechanisms might be more important for the second step of the reaction. Different radical-anion species and the relative movements of the relevant equilibria involved can be clearly identified from the spectra peaks. The initial, maximum intensity of the peaks has been shown to be the parameter defining the first step of the reaction, while the radical decay rate has been shown to refer to the second step of the reaction. Hydrolyzable tannins have been shown not to undergo neither any silica-induced radical surge nor autocondensation as predictable from their structures. (C) 1997 John Wiley & Sons, Inc. [References: 33]