Hydroquinone, viii

A linear hydrogen-bonded structure VIII has also been proposed, but accounts less satisfactorily for the color. The surprising thing about these complexes is the lack of any measurable exchange of the hydroxyl hydrogen atoms from the hydroquinone moiety to the quinone moiety... 

It should be noted that one of these diols, the hydroquinone, did not provide any oligomer in the first step. This was due to the formation of the quinone structure which made it impossible to use hydroquinone directly in the substitution reaction. An alternate method was used to overcome this problem which involved the use of 4-methoxyphenol to obtain the sulfone product, followed by cleavage of the methyl ether to the diol (VIII) with boron tribromide. This set of reactions is outlined in Figure 5. 

Fig. la and b. IR-spectra of epoxy-amine polymers and model compounds at 298 K 36 a. 1 linear polymer VIII (see Table 3), 2-4 stoichiometric networks based on diglycidyl ether of resorcinol and m-phenylenediamine (2) and 4,4 -diaminodiphenylsulfone (3) and diglycidyl ether of hydroquinone and m-phenylenediamine (4). b. Model compounds II, III, VI (Table 3), (a) crystal (b) melt (c) glass... 

Oxidation of Tetramethylethylene. Tetramethylethylene, TME, was an excellent model olefin since it was rapidly and selectively oxidized in the presence of many transition metal complexes (12). Oxidation of TME in the presence of the group VIII metal complexes [MCI(CO)-(Ph3P)2] (M = Rh, Ir) at 50°C gave two major products 2,3-dimethyl-2,3-epoxybutane, I, and 2,3-dimethyl-3-hydroxy-l-butene, II (Reaction 5). Reaction mixtures were homogeneous with no observable deposits of insoluble materials. Little oxidation occurred under these conditions in the absence of the metal complexes, but low yields of I and II were obtained in the presence of a radical initiator (Table I). Reactions were severely inhibited by hydroquinone. The ruthenium (II) complex, [RuCl2(Ph3P)3]2, also promoted efficient oxidation of TME yielding I... 

Alternatively, one may polarize the platinum disk to the quinone-hydroquinone equilibrium potential with the help of a current supplied by an auxiliary circuit. Then one may determine the required current 7, and the rate of consumption of quinone or the rate of formation of hydroquinone. At the quinone-hydroquinone equilibrium potential the electrochemical reduction of hydroquinone vanishes. Consequently, a finite rate of the formation of hydroquinone at the quinone-hydroquinone equilibrium potential equals the partial rate due to the nonelectrochemical mechanism according to Eqs. (VIII.lla)-(VIII.11c). 

Phenolase Complex. Phenolase (= tyrosinase, or phenol oxidase) converts tyrosine to dopa (= dihydroxyphenylalanine) and oxidizes the dihydroxy derivative further to the quinone stage. Through a series of subsequent reactions, some of which occur spontaneously and wathout enzymic catalysis, the black or brownish black melanin is finally formed (for a schematic representation of the reactions see Chapt. VIII-11). The phenolase is an oxidase with mixed functions, where the product of oxygenation, the hydroquinone derivative, simultaneously acts as... 

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