| Results obtained on the alkaline autoxidation of 4-ethylguaiacol suggest strongly that unstable p- and o-hydroperoxides are intermediate products of the reaction. At pH-levels 10 or below, the hydroperoxide intermediates decompose to phenoxy radicals which undergo coupling. At higher pH-values, two competing reactions appear and become dominant above pH 13: conversion to dioxetane intermediates via hydroperoxide anions and formation of quinols by displacement of the hydroperoxy group. Dioxetanes can further rearrange to form epoxy quinols or ring opening products. Quinols have been shown to rearrange forming quinones with a subsequent alkyl migration. The structures of fourteen identified reaction products and the kinetics of the autoxidation reaction are in full accordance with the proposed mechanism.;Lignin oxidized under conditions similar to 4-ethylguaiacol suggested that similar reaction mechanisms are in effect. Low pH (0.1 N NaOH) resulted in ring coupling (44%) and ring opening (56%). Raising the pH (2.0 N NaOH) resulted in a 98% conversion of lignin, as measured by disappearance of free phenolic hydroxyl, by ring opening. Carbonyl conjugated phenols increased with increasing pH resulting in stabilization of the ring and a consequent retardation of the ring opening reaction. |
