Nitrobenzene radical attack

Palmisano et al. [41] in a study on the selectivity of hydroxyl radical in the partial oxidation of different benzene derivatives have investigated how the substituent group affect the distribution of the hydroxylated compounds. The reported results show that the primary photocatalytic oxidation of compounds containing an electron donor group (phenol, phenylamine, etc.) leads to a selective substitution in ortho and para positions of aromatic molecules while in the presence of an electron-withdrawing group (nitrobenzene, benzoic acid, cyanobenzene, etc.) the attack of the OH radicals is nonselective, and a mixture of all the three possible isomers is obtained. 

Radical cations that are produced by electrochemical oxidation are not stable in solvents with appreciable base character. This results because such radicals are subject to attack by available nucleophiles, and solvents that contain donor electron pairs are good nucleophiles. Cation radicals are most stable in solvents that are good Lewis acids and show negligible basic properties. Some of the solvent systems that have been employed to stabilize electrochemically produced cation radicals include nitromethane and nitrobenzene,21 dichloro-methane,22 trifluoroacetic acid-dichloromethane (1 9),23 nitromethane-AlCl3,24 and AlCl3-NaCl (1 l).25 Organic chemists should be familiar with the stabilization of carbonium ions by superacid media.26 These media usually contain fluorosulfuric acid, or mixtures of fluorosulfuric acid with antimony pen-tachloride and sulfur dioxide, and are potent solvents for the production and stabilization of organic cations. 

With anisole, the SOMO/HOMO interaction (B) is strong, and with nitrobenzene the SOMO/LUMO interaction (A) is strong, but with benzene neither is stronger than the other. Product development control can also explain this, since the radicals produced by attack on nitrobenzene and anisole will be more stabilised than that produced by attack on benzene. However, this cannot be the explanation for another trend which can be seen in Table 7.1, namely that a p-nitrophenyl radical reacts faster with anisole and benzene than it does with nitrobenzene. This is readily explained if the SOMO of the p-nitrophenyl radical is lower in energy than that of the phenyl radical, making the SOMO/HOMO interactions (C and D) strong with the former pair. 

Hey and co-workers [119, 121, 124] gave a quantitative analysis of the rate of homolytic attack on nitrobenzene as compared with that on chlorobenzene. The phenylation of nitrobenzene gave proportions of ortho, meta and para products 58, 10 and 32% respectively, whereas the phenylation of chlorobenzene yielded 62, 24 and 14% respectively. However when the entering group is more electrophilic than benzene (e.g. o- and p-nitrophenyl radicals) the proportion of meta substituents increased. 

Hydroxyl radicals generated from ferrous sulfate and hydrogen peroxide at room temperature attack nitrobenzene to form small amounts of o, m-, and p-nitrophenol benzene yields small amounts of phenol and diphenyl. ... 

Let us consider nitrobenzene as an example. Table 8-1 gives the localisation energies, in units of J , for the three types of attack, nucleophilic, radical and electrophilic, in each of the ortho-, meta- and para-positions. 

The nitroxide 56 traps in its mesomeric form 56c radicals ROO . Quinone imine iV-oxide 62, is formed via alkylperoxycyclohexadieneimine intermediate. 62 is destroyed by the further attack of ROO . Benzoquinone (63) and nitroso- (64, n = l) and nitrobenzene (64, n=2) are formed in the ultimate phase of the lifetime of 62 [65]. An alternative pathway for oxidation of 56 with ROO in cumene autoxidation suggests formation of an olefin and hydroxylamine 66 as CB antioxidant species [66] ... 

Mechanistic assignment in terms of path a or b is not always straightforward. Recently, the proposal was advanced that also apparent two-electron processes, such as SN2 substitution reactions, proceed via single-electron transfer with synchronous coupling of the electron pair of the ensuing radical pair (7, 8). The opposite situation has also been encountered. Evidence was recently reported that the reaction of nitrobenzene with f-BuOK in THF, a system in which radicals are observed, is not a straightforward electron-transfer process but proceeds via initial nucleophilic attack (9). 

Evans et al. (10) have shown that OH radicals produced by the Fenton reagent can initiate the polymerization of vinyl compounds and that the OH radicals, which in the first instance attack the double bond of the monomer, are built into the polymer chain. Stein and Weiss (45) have used the hydroxylation of benzene and of other simple aromatic compounds (e.g., benzoic acid and nitrobenzene) to detect these radicals. In the action of OH radicals on benzene in aqueous systems the formation of phenol and of diphenyl indicated a free radical mechanism of the following type ... 

Fluorination at saturated carbon. Hesse et al. have observed that adamantanc in the presence of radical inhibitors (.e.g., nitrobenzene) reacts with CF3OF to form 1-fluoroadamantane (75% yield). The same reaction can be carried out with Fa itself. This reaction is characterized by almost exclusive attack at a tertiary position, pronounced tendency to monosubstitution, and a marked polar effect on the rate (adamantane substituted at the 1-position by NHCOCF3 is much less reactive than adamantane). Thus the reaction is considered to involve direct electrophilic fluorination. 

In general, radical cations are less stable than their anionic counterparts, but relatively stable cations can be obtained from reactants in which those positions which carry the highest charge density in the radical are blocked with respect to either proton loss or nucleophilic attack. For example the 9,10-diphenylanthracene and rubrene radical cations are sufficiently stable to give reversible cyclic voltammo-grams. Radical cations appear to be more stable in nitrobenzene than in acetonitrile solutions. Coating of the anode with insoluble insulating polymeric ffims is a common hazard in anodic oxidation systems but it can be alleviated by the use of scraped electrodes and periodic polarity-reversal techniques. 

Substitution reactions are characterized by the fact that a substrate reacts with a second molecule by incorporating the second molecule in its structure and hy releasing a part of the substrate. Substitution reactions can take place as electrophilic (see Section 2.2.5 for details), nucleophilic (Section 2.2.3), or radical suhstitu-tion reactions (Section 2.2.2) depending on the nature of the attacking reagent. Scheme 2.2.1 shows the electrophilic substitution of a hydrogen atom at henzene by the nitronium electrophile N02. This technically relevant reaction liberates a proton and forms nitrobenzene. It represents an important step in the synthesis of nitrobenzene, the key-intermediate for the production of aniline. 

In several cases the amine, RNH2, is also produced in significant quantities the source of the hydrogen atoms is apparently the solvent. The deoxygenation appears not to take place via free nitrene (R-N ) intermediates. Also, direct electron transfer to form a nitro radical anion (R-NO2 ) can be reasonably ruled out. It thus appears that the initial step of the reaction involves direct attack by a nitro oxygen atom at the uranium. That nitrobenzene reacts more rapidly than 4-nitrotoluene (a Hammett p value of —1.7 is measured) argues that the nitro compound is being reduced in the transition state. 

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