Proton-driven disproportionation

Such a proton-driven disproportionation process means that 02 can deprotonate acids much weaker than water (< pATa = 23).13... 

Such a proton-driven disproportionation process means that 02 can deprotonate acids much weaker than water (up to pK 23). " The data of Tables 1 and 2 indicate that 02 - is a moderate one-electron redncing agent. Thus, cytochrome c (Fe ) is reduced in H20 and iron(lll) porphyrins in DMF (equation 28). 

Complications which militate against the observance of pure [Mn(H20)6]3+ in aqueous solution are its oxidizing properties, the usual electroneutrality-driven hydrolysis (or, more exactly, proton transfer to solvent) and the disproportionation reactions (14) and (15). The instability of die cation is shown in the behaviour of the alums MMn(S04)2 12H20 (M = K, Rb, Cs, NH4) all four were isolated by Christensen in 1901, as red crystalline compounds, but only the Cs compound is stable at normal room temperatures and even this one begins to blacken and decompose at 33°C. No other solids containing the cation appear to have been described. 

The pKa values for such radicals were determined from changes in absorption spectra and found to be in the mildly alkaline range. On the other hand, metalloporphyrin radical anions cannot protonate on the nitrogen unless this leads to demetallation (see below). They may protonate on a carbon atom, but such species have not been shown to exist in the microsecond time scale under ambient conditions. Therefore, the above disproportionation mechanism is likely to be driven by protonation of the dianion rather than protonation of the radical anion. 

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