One-electron reduced organic nitro and nitroso compounds

1 One-electron reduced organic nitro and nitroso compounds 

The question whether or not radical ions are formed upon irradiation of liquids and stabilized enough for detection or engagement in bimolecular chemical reactions has moved radiation chemists ever since the early days of this research field. This was particularly exciting with respect to low polarity solvents, but even for aqueous solutions conclusions had to rely mainly on indirect evidence. A real breakthrough came with the experimental discovery of the hydrated electron and other powerful one-electron reductants (e.g., a-hydroxyalkane radicals such as (CH3)2C OH). Applying these new tools a large number of organic radical anions were detected and characterized with respect to their optical and chemical properties, particularly by pulse radiolysis. 

Among the early examples were the reduction of organic nitro and nitroso compounds which yielded relatively long-lived nitrogen-centered radical anions. One of the questions raised in those days, especially by electrochemists, was whether the reduced nitro moiety existed exclusively in an anionic form or could establish an acid/base equilibrium as formulated in eq. 1 for the nitrobenzene case. 

It was radiation chemistry which provided an unambiguous answer and established the pK = 3.2 based on the different optical absorptions of the two conjugates.9 

Substitution of the aromatic phenyl by the aliphatic methyl, incidentally, does not cause too much of a change with respect to the pK. It shifted by just about one order of magnitude to pK = 4.4 for equilibrium 2. 0 

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