The electron spin resonance studies of some nitrogen free radicals

A survey of the literature showed that nitrobenzene and related radical-anions could be obtained from a variety of electron transfer reactions. The possibility of using hydroxylarnine and N-substituted hydroxylamines as electron donors to aromatic mono nitro compounds, in oxygen free, aqueous, alkaline ethanol, was investigated. Five of the seven hydroxylamine compounds investigated, hydroxylarnine, N-methylhydroxylamine, N-phenylhydroxylamine, N-benzylhydroxylamine and N-benzhydrylhydroxylamine, were effective in transferring an electron to a wide range of nitro compounds, the radical-anions produced being detected by electron spin resonance spectroscopy.
The spectra of these radicals were recorded, and hyperfine splitting constants measured and compared with values in the literature. g-Values of the radicals were also measured and it was found for the nitro radical-anions containing halogen substituents that this increased - Br > Cl > F > H.
The limit of the reducing power of the hydroxylamine compounds was reached with para-nitroanisole, E1/4 = 0.957v, para-nitrophenol not being reduced by this method.
An investigation into the mechanism of this reaction and its products then followed.
The build-up and decay of the nitro radical-anions were followed using stopped flow techniques. The general pattern which emerged was that of a very rapid build-up of the radical, reaching the maximum intensity within a minute of mixing the reactants, followed by a very slow decay. This is characteristic of consecutive reactions in which a stationary state intermediate is being destroyed much more rapidly than it is being produced. It was found that:
Initial rate ∝ [OH-] [ArNO2] [RNHOH]
and that
[Radical]max ∝ [OH-]½ [ArNO2]½ [RNHOH]½
and the following scheme was proposed:
RNHOH + OH- ⇌k RNHO- + H2O
RHHO- + ArNO2 →k1 RNHO• + ArNO•̅2
2ArNO•̅2 →k2 products.
2RNHO• →fast products.
This was verified by the evaluation of k2 as a rate constant for the bimolecular disappearance of the nitro radical-anion, in a series of experiments with para-nitrobenzoic acid and a range of hydroxylamine compounds.
k2 = 6.5 × 102 l.mol.-1 sec.-1
This value for k2 is higher than literature values of rate constants for the disproportionation of nitro radical-anions. This may be due to traces of oxygen, present in the current work, or to the fact that k2 may not be the rate constant for a direct disproportionation reaction.
The relative rates at which the various hydroxylamines will transfer an electron was found to be:
MeNHOH > PhCH2NHOH > Ph2CHNHOH > NH2OH
the sequence that would be expected from the electron releasing powers of the substituents on the nitrogen atom of the hydroxylamine group.
Using a single hydroxylamine in reaction with a range of nitro compounds an approximate value of p = + 3.3 was obtained for the electron transfer reaction. This indicated that a low electron density at the site of reaction in the nitro compound would be favourable i.e. the rates with the following substituents would vary:
p -CN > p -COO- > p -OCH3
With hydroxylamine itself, kinetic measurements were much less accurate, owing to a pseudo unimolecular gas evolution which appeared to come from the reactions:
2NH2O• → (H2NO)2 → N2 + 2H2O
From the mechanism that has been proposed for the general reaction, it would be expected that nitroso compounds would be among the products. However these compounds were not isolated.
With hydroxylamine, itself, the reaction proceeded further via a diazotate and a diazonium salt to yield an azide.
From N-alkylhydroxylamines, R2CH.NHOH, the intermediate aliphatic nitroso compounds in part tautomerized to the oximes, R2C=NOH, which have been isolated.
The subsequent reactions of the aromatic nitroso compounds fall into two categories.
In the reactions involving N-benzhydryl and N-phenylhydroxyl-amines, the intermediate nitroso compounds derived from the original nitro compound underwent a further one electron reduction, followed by dimerisation to give an azoxy compound. In the case of N-phenyl-hydroxylamine this was also the fate of the PhNO•̅ radicals, derived from the hydroxylamine.
In the reactions involving N-alkylhydroxylamines, R-CH2-NHOH, deep red products were isolated and, with difficulty, purified. These were ultimately identified as formazans (I), by comparison of a range of their physical properties with those of synthesized materials.
[For the diagram omitted here, please consult the PDF.]
The mechanism proposed to account for their formation from the intermediate nitroso compounds had to satisfy the following:
In the reaction, variations in the nitro compound alters the Ar-group in I,
In the reaction, variation in the hydroxylamine compound alters the R- group in I,
The lack of formation of formazans with other hydroxylamines indicates that an active methylene group adjacent to the nitrogen atom in the hydroxylamine is essential.
The mechanism proposed depended upon the cross combination of nitroso radical-anions derived from both reactants to give an unsymmetrical azoxy compound, R.CH2N=N(O)Ar. Attack by base at the active methylene group resulted in the formation of formazan (I). Evidence to support this mechanism has been gained by synthesizing the intermediate unsymmetrical azoxy compound and allowing it to react with base, with the result that traces of the predicted formazan were detected.