Studies on the formation of complex oxidation and condensation products of phenols. Part II.—Coupling of simple phenols and quinones to diphenyl derivatives

Abstract

General .—The oxidative coupling of phenols with formation of diphenyl derivatives has long been known, but the mechanism of the reaction has not been discussed. On the basis of the Englar-Bach peroxide theory it seems possible to assume the intermediate formation of a peroxide, which would yield the diphenyl derivative by means of a rearrangement of the hydrazobenzene-benzidine type. There are, however, many objections to such a view. For example, it is characteristic of the benzidine migration that it takes place in the presence of acids. The oxidative coupling phenols occurs readily in alkaline solution also. The peroxides are known to dissociate into radicals containing monovalent oxygen, and these radicals are very reactive and easily undergo spontaneous decomposition. On the other hand, the dissociation is reversible, and it seems to be more justifiable to regard the peroxide formation as an attempt by the free radicals to acquire stability, and to assume that the free radicals as such are responsible for the coupling of the phenols. Evidence in favour of this view has been provided by Pummerer and his co-workers who have studied in particular the dehydrogenation of naphtols. That the "oxidation" of phenols is in reality a dehydrogenation has been conclusively demonstrated by Wieland, who discovered that phenols can easily be dehydrogenated to quinones dipheny derivatives by the action of platinum or palladium in absence of free oxygen.