Anion radical of nitrobenzene

If the styryl substituent retained its donor nature in the anion-radical state, an increase, not a decrease in the value of the nitrogen HFC constant (a(N)) would have been observed. Experiments show that fl(N) values for anion-radicals of nitrostilbenes decrease (not increase) in comparison with the fl(N) value for the anion-radical of nitrobenzene (Todres 1992). Both naked anion-radicals and anion-radicals involved in forming complexes with the potassinm cations obey such regularity. In the cases of potassinm complexes with THF as a solvent, a(N) = 0.980 mT for PhNOj anion-radical and fl(N) = 0.890 mT for PhCH=CHCgH4N02-4 anion-radical. In the presence of 18-crown-6-ether... 

An attempt to detain an unpaired electron was made by means of the second nitro group in the anion radical of nitrobenzene (Todres, Hovsepyan et al. 1988). The potassium salt of the anion radical of ort/zo-dinitrobenzene did yield an azo-coupled product according to Scheme 1-5 (the nitrogen oxide evolved was detected). 

As can be seen from Table 4-2, the relative rates of chlorine substitution in nitrochlorobenzenes under the action of different nucleophilic reagents are in agreement with af of the anion radicals. The constants af and af of the 4-chloronitrobenzene anion radical are close to the af and af constants of the nitrobenzene anion radical. The pair of anion radicals of 2-chloronitrobenzene and nitrobenzene show the same agreement between af and af. In the anion radical of nitrobenzene, af is larger than af. The substitution of ethoxyl for chlorine in 4-chloronitrobenzene proceeds much more easily and requires a lower activation energy than the same substitution in 2-chloronitrobenzene. The spin density in position 4 of the anion radical of 1,3-dinitrobenzene is greater than that in position 2 (af > af). Therefore, l,3-dinitro-4-chlorobenzene is more active in nucleophilic substitution than l,3-dinitro-2-chlorobenzene. 

The nitrogen splitting for 6 in acetonitrile may be compared with those for the nitro groups in the anion-radicals of nitrobenzene and the three nitropyridines in the same solvent (Part I Section III,A,l,b). The relative orders of magnitude are 2-nitrofuran > nitrobenzene > 3-nitropyridine >... 

A profound rebuilding of a molecule, which is associated with bond cleavage, cannot take place at a negative or zero activation energy. Thus, disintegration of the anion-radicals of 4-iodo (or 4-bromo) nitrobenzene in AN or DMF proceeds with positive activation energy of 70-85 kJ mol" (Parker 1981). The disintegration follows the equation ... 

The anion-radicals from aromatic nitro compounds preserve the second-order axis of symmetry. The analysis of superfine structure of the ESR spectrum of the nitrobenzene anion-radical reveals equivalency of the ortho and meta protons (Ludwig et al. 1964, Levy and Myers 1965). With the anion-radical of nitrosobenzene, the situation is quite different. This was evidenced from the ESR data (Levy and Myers 1965, Geels et al. 1965). Following electron transfer, the bent nitroso group fixes in the plane of the benzene ring to a certain extent. This produces five different types of protons, since both meta and ortho protons become nonequivalent. The nonequivalence of the ortho and meta protons has also been established for the anion-radicals of acetophenone (Dehl and Fraenkel 1963) and 5-methylthiobenzoate (Debacher et al. 1982 Scheme 6.17). 

Give the number of lines in the ESR spectrum of the anion radical of each of the following molecules. (Assume all lines are resolved.) (a) Naphthalene (b) anthracene (c) pentacene (d) azulene (e) o-xylene (f) w-xylene (g) p-xylene (h) nitrobenzene (i) />-fluoronitrobenzene. 

Table 4-3 presents the relative rate constants of chlorine-to-methoxy substitution in the nitrobenzene series (Epiotis 1973). The methoxide ion replaces chlorine in 4-chloro-3-methylnitrobenzene more rapidly than in 6-chloro-3-methylnitrobenzene (Table 4-3, nos. 1 and 2). This agrees with the fact that the anion radical of 3-methylnitrobenzene has a greater spin density in position 4 than in position 6. The aforementioned HFC constants of the 3-nitrochlorobenzene anion radical and the relative rate constants (kre ) for the substitution of methoxyl for chlorine at the / th carbons (Table 4-3, nos. 3 and 4) correlate in the same way. 

Perhaps the most striking example of such behavior is the reduction of halonitrobenzenes and haloben-zonitriles [i]. Even when the reduction of haloben-zenes occurs at more negative potentials than that of nitrobenzenes or benzonitriles, in halonitrobenzenes and halobenzonitriles the acceptance of the first electron yields a radical anion, sufficiently stable to eliminate the halide ion. The resulting radical of nitrobenzene or ben-zonitrile accepts the second electron and a proton. This is followed by the usual reduction of the NO2 or CN group. Thus formally, the C-X bond is reduced before the NO2 or CN group. 

Secondly, the rates and modes of reaction of the intermediates are dependent on their detailed structure. For example, the stability of the cation radical formed by the oxidation of tertiary aromatic amines is markedly dependent on the type and degree of substitution in the p-position (Adams, 1969b Nelson and Adams, 1968 Seo et al., 1966), and the rate of loss of halogen from the anion radical formed during the reduction of haloalkyl-nitrobenzenes is dependent on the size and position of alkyl substituent and the increase in the rate of this reaction may be correlated with the degree to which the nitro group is twisted out of the plane of the benzene ring (Danen et al., 1969). 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

MgO, respectively. Photolysis by an incandescent lamp is necessary to produce the radical on MgO, and the light significantly alters, both in amplitude and shape, the spectrum of the species on silica-alumina. Both spectra are interpreted in terms of very anisotropic hyperfine interactions. This is consistent with work on radical anions such as nitrobenzene on MgO (90) however, it is a bit surprising in light of the motional averaging found for most cations on silica-alumina. 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

With these radicals, spontaneous C(6)-0 heterolysis is slow ( < 10 s" ). However, if the electron density of the system is increased by OH -induced de-protonation of N(l)-H, 02 elimination is observed [23, 24, 25]. With the peroxyl radical from 5,6-dihydrouracil-6-yl, the heterolysis rate constant is 8.3 X 10 s the reaction leading to the isopyrimidine derivative shown [37]. The reaction is perfectly analogous to the eliminations of the radical anions of nitrobenzenes (Eq. 15) or anthraquinone-2,6-disulfonate (Eq. 18). 

Of course, it is the entire molecule that receives an electron on reduction. However, the nitro group is the part where the excess electrons spend the majority of their time. Consideration of quantum-chemical features of the nitrobenzene anion-radical is of particular interest. The model for the calculation includes a combination of fragment orbitals for Ph and NO2, and the results are represented in Scheme 1.1. The left part of the scheme refers to the neutral PhN02 and the right part refers to the anion-radical, PhN02 (Todres 1981). 

This means that 4-nitrostilbene is a more effective electron acceptor than nitrobenzene. This theoretical conclusion is verified by experiments. The charge-transfer complexes formed by nitrobenzene or 4-nitrostilbene with Af,Af-dimethylaniline have stability constants of 0.085 L mol or 0.296 L mol respectively. Moreover, the formation of the charge-transfer complex between cis-4-nitrostilbene and A/,Af-dimethylaniline indeed results in cis-to-trans conversion (Dyusengaliev et al. 1995). This conversion proceeds slowly in the charge-transfer complex, but runs rapidly after one-electron transfer leading to the nitrostilbene anion-radical (Todres 1992). The cis trans conversion of ion-radicals will be considered in detail later, (see sections 3.2.5.1, 6.4, and 8.2.1). 

Introduction of nitrobenzene sulfenates into the same mixture of trichlorosilane and tributylamine results in the evolution of hydrogen. As proven by Todres and Avagyan (1978), trichlorosilane with tributylamine yields the trichlorosilyl anion and tributylammonium cation. This stage starts the process involving one-electron transfer from the anion to a nitrobenzene sulfenate. At that time, nitrobenzene sulfenate produces the stable anion-radical with the tributylammonium counterion. The anion-radical gives off an unpaired electron to the proton from the counterion (see Scheme 1.14). 

Vacuum UV irradiation of aqueous solutions containing formate is one of the methods to generate CO2 . Under such conditions, the carbon dioxide anion-radical is formed in the excited state. The excited anion-radical transfers an unpaired electron to nitrobenzene, benzoic acid, or benzal-dehyde (Rosso et al. 2000). 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

An interesting but still unexplained case refers to nitrobenzene. The reversible electron exchange between nitrobenzeneand sodium salt of the nitrobenzene- N anion-radical is characterized by the usual constant of 0.40. Stevenson et al. (1987b) used NH3(liq) as a solvent for these measurements at -75°C. Under the same conditions, they obtained the equilibrium constant of 2.1( ) for the electron exchange between nitrobenzene- N and the potassium salt of nitrobenzene- " N anion-radical. Perhaps, the difference between ion radii of sodium and potassium cations is crucial for the stability of the corresponding ion pair with nitrobenzene anion-radical. Such diversity can be pivotal when the electron prefers the heavy or light nitrobenzene. 

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