Nitrobenzene anion radical

For those systems where Ri = R2 = H or Ri = H, R2 = CH3, i.e. where the number of alkyl groups at C, is <1, and R3 = H to NO3, the alkoxynitroxyl radicals formed according to Eq. 7 under steady-state-ESR or pulse radiolysis conditions do not give rise to nitrobenzene radical anions. This means that the rate constants for heterolysis of the nitroxyls are < 10 s . This is not only true in weakly acidic (pH 4) or neutral but also in strongly alkaline solution (pH 13-14). The latter observation means that the nitroxyls are not susceptible to base catalyzed heterolysis. From this the rate constant for OH catalyzed decomposition can be estimated to be < 10 M s [19]. This low number for... 

Nitrobenzene radical-anion is more stable in aprotic solvents than its aliphatic counterparts. Nitrobenzene shows two one-electron polarographic waves in acetonitrile with By, -1,15 and -1.93 V vj. see, Tire first wave is due to tlie formation of the radical-anion and this species has been characterised by esr spectroscopy [6]. Nitrobenzene radical-anion can also be generated in steady-state concentration by electrochemical reduction in aqueous solutions at pH 13 [7] and in dimethyl-formamide [8]. It is yellow-brown in. solution with A., ax 435 nm. Protonation initiates a series of reactions in which niti osobenzene is formed as an intermediate and... 

On the other hand, it has also been suggested that the reaction of nitrobenzene radical anion or dianion with r-butyl iodide to give p-( -butyl)nitrobenzene (18%) occurs by an S l reaction140. 

A one-radical system can sometimes be achieved via intermediate radicals. For example, the nitrobenzene radical anion can be produced by reduction with eaq [reaction (28)], while H and OH can be... 

Therefore the electron substituent effects in radical dianions of 2-substituted 5(6)-nitrobenzimidazoles are transmitted with approximately equal contributions of inductive and resonance components. A similar picture is observed for para-substituted nitrobenzene radical anions also [689, 883], Actually, the aN(N02) correlation of 2-substituted 5(6)-nitrobenzimidazoles RDA (RDA BI) with the aN(N02) of para-substituted nitrobenzene RA (RA Bz) indicates the same mechanism of substituent effects transmission but with different intensity, as shown in equation (3.4) ... 

The nitrobenzene radical anion (17,18) then abstracts either a proton (Scheme 2) or a hydrogen atom (Scheme 1) to form a benzylic alkyloxy radical. 

On continuous cycling of the potential through the two reduction processes, the magnitude of the current associated with the first reduction process gradually drops whereas that due to the second process increases. This behaviour is expected for an ECE process in which the p-iodonitrobenzene is reduced irreversibly to the nitrobenzene radical anion. Further, the addition of iodide anions to the solution decreases the rate of formation of nitrobenzene as evidenced by noting that the rate at which the peak current associated with the p-iodonitrobenzene reduction decreases less rapidly on successive scans through both reduction processes. The dependence on the iodide concentration in bulk solution suggests that the C step of the ECE mechanism actually consisted of processes (51) and (52),... 

A comparable study was carried out on the cleavage reactions of p-bromo-, m-iodo- and p-iodo-nitrobenzene radical anions in DMF and CH3CN. The data reproduced in Table 22 show that the relationship between activation parameters with bromo- and iodo-substitution does not exhibit the anomaly observed with the substituted anthracene radical anions. The value of... 

As is the case with NMR, the ESR determinations are often carried out in the limit of slow exchange. The fast-exchange limit is difficult to reach experimentally because it requires inordinately high concentration of reagents (> 1 M), which exceeds the solubility limit of many compounds. Nonetheless, some electron-exchange constants, such as that between nitrobenzene radical anion and nitrobenzene have been measured in the limit of fast exchange [41], In this example, the diamagnetic species, nitrobenzene, was used as solvent. 

Certain aromatic nitro compounds which contain electronegative substituents undergo elimination when reduced. Fujinaga et al. (1964) found that the electrochemical reduction of several halogenonitro-benzenes gives initially the corresponding radical-anion whose e.s.r. spectrum is then replaced by that of the nitrobenzene radical-anion, e.g. 

The ease of loss of the substituent decreases in the order, o-, m-, p-1 > o-Br >p-Br >o-Cl>m-Br >m-, p-Cl, indicating that steric effects are important. The authors of this Review have observed that the reduction of ethyl o-nitrophenylacetate by electron transfer in a basic solution of dimethyl sulphoxide gives rise to the nitrobenzene radical-anion it seems probable that in this case the enolate anion, CHg.COaEt, is the leaving group. In a related reaction, Rieger et al. (1963) observed that p-cyanofluorobenzene, when reduced electrochemically, gives the radical-anion of 4,4 -dicyanobiphenyl, probably as follows ... 

For a-substitution, we can compare, for instance, a for the p-fluoro-nitrobenzene radical-anion with a. for the nitrobenzene radical-anion in aqueous solution the ratio is 7-95 3-40, i.e. 2-3 1 (Kolker and Waters, 1964) for the o-fluoro and p-fluoro-substituted phenoxy radicals the corresponding ratios are 2-6 (Stone and Waters, 1964a). There are probably two contributions to the observed fluorine-splitting (Carrington et al., 1965) a spin-polarization term, as for C— H, giving negative spin density at the fluorine nucleus, and a spin-polarization of the fluorine Is and 2s electrons which arises from spin density in a 2p orbital on fluorine (C—— F) and results in a positive spin density at the... 

III, and IV are expected to be less stable than the addncts I and V. Besides in the rednction of (of C - O bond heterolysis, which is measured in terms of the build up kinetics of the nitrobenzene radical anion), this leads to an increased tendency to deprotonate at an early stage of the transition state, i.e. deprotonation tends to precede the heterolysis of C - O bond. This is symbolized as shown below ... 

We have shown that the reaction of benzamide, and excess nitrobenzene in the presence of base under anaerobic conditions generated the tetramethylammonium salt of N-(4-nitrophenyl)- benzamide, in 98% yield under mild conditions (7). The only other observable product in this reaction was azoxybenzene, 13 which was formed in 33% yield based on benzamide charged (Figure 8). That azoxybenzene is observed as a by-product of this reaction under anaerobic conditions indicates that nitrobenzene is functioning as the primary oxidant in this system. This was confirmed by the analysis of the reaction mixture by EPR spectroscopy which revealed that nitrobenzene radical anion was also formed during the course of the reaction as evident by its distinct 54 line pattern (8). 

A mechanism which explains the simultaneous formation of 12 and 13 is shown in Figure 9. Intermolecular oxidation of a-complex 14 by nitrobenzene generates 12 and initially nitrobenzene radical anion, 15, which can decompose by a variety of pathways including disproportionation to give nitrosobenzene (9). The... 

In contrast to the case where aniline is used as the nucleophile, the benzamide reaction can be improved by utilizing dioxygen in the reaction mixture since 11 is resistent to autoxidation. Under aerobic conditions the nitrobenzene radical anion is readily trapped by O2 generating superoxide and nitrobenzene (Figure 10) (11). This reaction pathway inhibits the formation of azoxybenzene by diverting the electron transfer cascade and ultimately utilizing dioxygen as the terminal oxidant. Thus, under aerobic reaction conditions 12 is the only observed reaction product. 

Fig.3. Low field part of the ESR-spectrum of nitrobenzene radical anion. The center of the spectrum is indicated by an arrow, the line used for time dependent measurements by a cross.

It is important to mention that even when the one-electron mechanism to form the stable radical anion (Eq. 15.54) is also observed in a range of organic solvents (e.g., acetonitrile, dimethyl sulfoxide, and Af,Af-dimethylformamide), the debromination step only occurs in organic solvents after the absorption of light by the l-bromo-4-nitrobenzene radical anion (photochemical reaction) [50,52-54]. 

Ernst S, Ward KR, Norman SE, Hardacre C, Compton RG (2013) Changed reactivity of the l-bromo-4-nitrobenzene radical anion in a room temperature ionic liquid. Phys Chem Chem Phys 15(17) 6382-6389. doi 10.1039/c3cp51004b... 

The stabilization of the radical cation in SDS micelles is consistent with the formation of the surface complex already mentioned in Sect. 2.1.1. There are a number of examples of the stabilization of radical ions in micelles of the opposite sign. The electrochemical reduction of nitrobenzene in homogeneous solutions is two-electronic, but in micellar CTAB solutions it is resolved into two separate reactions [135]. So the presence of cationic micelles inhibits the disproportionation of nitrobenzene radical anions. It may be caused by the reduced mobility of the radical ions in the surface layer of the micelles due to complex formation with the end groups of the micelles. It can account for the inhibition of the disproportionation because the latter needs a coUision of two radical ions. The salt formation of the radical cation of N-methylphenothiazine with the end groups of SDS micelles was proposed in the study of electrochemical oxidation of N-methylphenothiazine in micellar solution [136]. Tetracyanoquinodi-methane is solubilized in dodecylpyridinium-inverted micelles in the form of a radical anion with the oxidation of the iodide ion [137]. 

Assuming that the reduction is a one-electron process forming the nitrobenzene radical anion, calculate the diffusion coefficient D of nitrobenzene and obtain an accurate value for the concentration studied. 

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