Auto-Inhibitory Factors of p-Cresol Oxidative Coupling and Their Elimination

Abstract

Following the addition of p-cresol (1) to aqueous solution of ferric chloride (FeCl3), direct-liked oligomers of 1 were produced by oxidative coupling. HCl and FeCl2, were produced during the course of the reactions, which was shown to be auto-inhibitory, in that the synergistic effect of these compounds greatly inhibited oxidation. HCl appeared to prevent the coordination of 1 with oligomers to Fe3+. FeCl2 may possibly function as an competitive inhibitor by complexing with 1 and oligomers. These auto-inhibitory factors were tried to eliminate through the addition of ammonia, ethylenediamine, and pyridine. The mechanism for this elimination through ammonium salt formation is unclear though interactions with metal ions could be clearly explained based on the experimental results. Ammonia and ethylenediamine, hard bases according to the law of hard and soft ligands and metal ions, prevented oxidation. These compounds appeared strongly coordinate with Fe3+ (hard metal ion and oxidizing reagent) to inhibit the oxidation. Pyridine, an intermediate base, promoted the conversion of 1 to oligomers by forming a stable complex with Fe2+ (intermediate metal ion and the competitive inhibitor of the reaction).