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Stone and Waters. 213 35. Aryloxy-radicals. Part I . Electron Spin Resonance Spectm.

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Fieser's critical oxidation potentials can be used to predict the relative ease of 1 -electron abstraction from phenols, but the equation for Edt is pH dependent. For further oxidation to cations (ArO)+, is not pH dependent and so there should be an equipotential pH below which ArO-is metastable with respect to (ArO)+ and (Ar0)-. Oxidative C-0-C coupling and aryloxy-polymer formation is ascribed to reactions of ArO* radicals and many C-C coupling reactions to electrophilic attack on phenol molecules by (ArO)+ cations.It is suggested that in plants those'biochemical oxidations of phenols which lead to Aryl-O-Aryl structures probably involve the one-electron oxidation of aryloxy-anions to ArO* radicals which then dimerise, but that those oxidations which lead to aryl-aryl coupled products involve the electrophilic attack on phenol molecules by (ArO)+ cations. The diversity of the reaction paths which have now been established for biological oxidations of phenols in plants may be due to the involvement of two factors, (a) the oxidation potential of the reagent or active enzyme and ( 6 ) the pH of the reacting mixture, in the selection of the preferred oxidation route. (a)

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The normal or " triplet" ground state (%g-) of molecular oxygen is that of a biradical, -0-0.. but " singlet " oxygen (lAg) O=O, which is structurally similar to ethylene, is the initial product of heterolytic decompositions of hydrogen peroxide or of per-acids. " Singlet " oxygen reverts to normal " triplet " oxygen by a bimolecular reaction that produces chemiluminescence but is sufficiently long-lived to be able to react as a dienophil; it combines with anthracenoid hydrocarbons at their meso positions, or adds to suitable 1.3-dienes.Singlet oxygen has been produced (i) by the reaction of alkaline hydrogen peroxide with either sodium hypochlorite or bromine and (ii) by decompositions of alkaline solutions of per-acids, either alone or with added hydrogen peroxide. By these methods the endo-peroxides (the " photoperoxides ") of a number of 9.1 O-disubstituted anthracenes have been prepared in the dark, whilst 9-substituted anthracenes have given typical hydrolysis products of the primary endo-peroxides. The most effective route uses the H202-Br2 reaction, but gives side-products owing to concurrent bromine addition unless a two-phase reaction mixture is used. With the H202-NaOCI system the dienophilic addition of oxygen to lumisteryl acetate and to 2.4-cholestadiene has also been established.Similar evidence is given to show that the chemiluminescent Trautz reaction also yields singlet oxygen.SPECTROSCOPISTS have shown that molecular oxygen ated,l contains no unpaired electron. These allotropes can exist for finite periods of time in a number of different of oxygen differ chemically as well as physically. Thus electronic states. Of these the ground state (3Xg-) is ground-state, or "triplet," oxygen can rationally be paramagnetic, having two unpaired electrons with written as *O-Oa because it is a stable biradical; its parallel spins in degenerate V , orbitals, whilst the more chemical reactions are of free-radical type since they energy-rich lag state, with which the visible lumin-normally proceed by chain mechanisms whilst again escence of excited oxygen molecules is probably associ-the vast majority of the reactions of molecular oxygen September 1965.

The initial stage of oxidation of phenols by alkaline ferricyanide appears to be a reversible reaction between the phenol anion and the ferricyanide ion, giving ferrocyanide and a mesomeric aryloxy-radical. Irreversible reactions then follow, giving mixtures of dimeric and polymeric products. Though the rate of oxidation is dependent upon the alkalinity of the solution and on the [Ferricyanide] /[Ferrocyanide] ratio, Fieser's theory 1 concerning a " critical oxidation potential " for attack on a phenol gives far too simple an interpretation of the reactions involved.New dimeric products, both of ketonic and of diphenolic type, have been obtained from a number of mono-and di-substituted phenols, and a-(4-hydroxy-3 : 5dimethylphenyl) benzyl alcohol has been obtained from 4-benzyl-2 : 6-dimethylphenol.

The free hydroxyl radical, produced by the action of ferrous salts on aqueous hydrogen peroxide, has been found to oxidise organic compounds of many types. A kinetic examination has shown that these oxidations fall into well-defined groups as ( a ) chain reactions or ( b ) nonchain reactions, and this grouping can be related to the structure of the free organic radical which is formed during the oxidation process. Certain of these organic free radicals are capable of reducing mercuric salts, or iodine.Several substances, e.g., acetic acid, acetone, chlorides, and bromides, which are not themselves oxidised under these conditions, have been found to retard the oxidations of other oxidisable organic substrates. This retarding action can be ascribed to a radical transfer which replaces the active hydroxyl radical by a much less active radical derived from the retarding agent. A preliminary kinetic study of this action indicates that characteristic " retardation coefficients " can probably be assigned to these radical-transfer agents.