Mechanism aromatic nitration

The mechanism of aromatic aulphonation may be similar to that previously described for nitration and halogenation, involving attack of the electrophilic... 

Certain features of the addition of acetyl nitrate to olefins in acetic anhydride may be relevant to the mechanism of aromatic nitration by this reagent. The rapid reaction results in predominantly cw-addition to yield a mixture of the y -nitro-acetate and y5-nitro-nitrate. The reaction was facilitated by the addition of sulphuric acid, in which case the 3rield of / -nitro-nitrate was reduced, whereas the addition of sodium nitrate favoured the formation of this compound over that of the acetate. As already mentioned ( 5.3. i), a solution of nitric acid (c. i 6 mol 1 ) in acetic anhydride prepared at — 10 °C would yield 95-97 % of the nitric acid by precipitation with urea, whereas from a similar solution prepared at 20-25 °C and cooled rapidly to —10 °C only 30% of the acid could be recovered. The difference between these values was attributed to the formation of acetyl nitrate. A solution prepared at room... 

The authors of this work were concerned chiefly with additions to alkenes, and evidence about the mechanism of aromatic nitration arises by analogy. Certain aspects of their work have been repeated to investigate whether the nitration of aromatic compounds shows the same phenomena ( 5-3-6). It was shown that solutions of acetyl nitrate in acetic anhydride were more powerful nitrating media for anisole and biphenyl than the corresponding solutions of nitric acid in which acetyl nitrate had not been formed furthermore, it appeared that the formation of acetyl nitrate was faster when 95-98% nitric acid was used than when 70 % nitric acid was used. 

Feng et al. (1986) performed quantum-chemical calculations of aromatic nitration. The resnlts they obtained were in good accordance with the IPs of N02 and benzene and its derivatives. The radical-pair recombination mechanism is favored for nitration whenever the IP of an aromatic molecule is much less than that of N02. According to calculations, nitration of toluene and xylene with N02 most probably proceeds according to ion-radical mechanism. Nitration of nitrobenzene and benzene derivatives with electron-acceptor substituents can proceed through the classical polar mechanism only. As for benzene, both mechanisms (ion-radical and polar) are possible. Substituents that raise the IP of an aromatic molecule to a value higher than that of N02 prevent the formation of this radical pair (one-electron transfer appears to be forbidden). This forces the classical mechanism to take place. It shonld be nnderlined that a solvent plays the decisive role in nitration. 

The title system in AN forms a homogeneous solution. The generation of NO cation takes place. As known, NO is a remarkable, diverse reagent not only for nitrosation and nitration but also for oxidation. Kochi et al. (1973) christened a new general mechanism oxidative aromatic substitution to describe aromatic snbstitntion reactions (Kochi 1990, Bosch and Kochi 1994). This mechanism incorporates ground-state electron transfer before the substitution step (see also Skokov and Wheeler 1999). 

J. H. Reid, "Mechanism of Aromatic Nitration, Accounts of Chemical Research 4, 248 (1971). 

The mechanism of aromatic nitration, which is illustrated below in the case of benzene, is a two-step process involving electrophilic attack of the nitronium ion on the benzene molecule to form the intermediate mesomeric ion (1), followed by removal of a proton by the hydrogen sulphate ion, which is the most basic species in the reaction mixture. 

The mechanism of aromatic sulphonation is broadly analogous to that previously described for aromatic nitration and halogenation and may be represented in the following way, the neutral sulphur trioxide molecule functioning as the electrophilic species. Sulphonation differs from nitration and halogenation, however, in that the overall reaction is reversible. 

The mechanism of aromatic nitration is shown in Figure 1. It may be seen that the sulfuric acid serves as the source of hydrogen ion (proton) which protonates nitric acid to form nitronium ion and water. The NO, ion is the active electrophile that causes the nitration reactions. 

According to Michael [1], on nitrating aromatic hydrocarbons an intermediate addition product is formed, which has one hydrocarbon hydrogen atom attached to one oxygen atom of nitric acid, and a carbon atom of the aromatic ring directly attached to the nitrogen atom of the acid. The aldol formed gives off water in the presence of an excess of nitric or sulphuric acid. The mechanism was depicted by the author as follows ... 

Until the 1960s, the nitration of aromatic compounds by solutions of nitric acid in sulfuric acid or other mineral acids, as well as in organic solvents, was confidently discussed in terms of the attack by the nitro-nium ion, N02+. Support for this hypothesis came from spectroscopy, cryoscopic measurements, and the comparison between the rate of nitration and the rate of lsO exchange between nitric acid and the media. Accordingly the generally accepted mechanism of aromatic nitration involved the steps outlined in Equations (3.1)—(3.4). Depending on the conditions and the aromatic substrate, either of... 

The trapping group is the amide and it has trapped a cation formed by addition of NO to the aromatic ring. We are faced with the problem of drawing a mechanism for the formation of this remarkable compound and, when we discover that a necessary intermediate is also an intermediate in our preferred mechanism for aromatic nitration, we feel more confident about that mechanism. 

Stok LM (1976) A classic mechanism for aromatic nitration. In Taft RW (ed) Progress in Physical Organic Chemistry, vol 12, pp 21—47... 

CA 34,7285(1940) (Nitration of toluene in the presence of acetic acid and nitrobenzene) b)J.Chedin 8t S.Fene ant, MSCE 32,92-100(1945XNolecular composition of HNO,—AcOH mixtures studies by Raman spectroscopy) c)J.Ch6din et al, MSCE 34, 289-90(1948XMixtures of HNO AcOH, H,0 and metallic nitrates) d)M.Kirsch C.A. Vinkler, CanJRes 28B, 715 19(1950XNitrolysis of hexamine in acetic acid) e)E.D.Hughes et al, jeS 1950,2406-09(Mechanism of aromatic nitrations in the presence of AcOH)... 

Benzoyl and acetyl nitrates direct the nitro group to the ortho position of substituted benzenes however, detailed procedures are lacking. Acetyl nitrate is presumed to be present in solutions of nitric acid in acetic anhydride. Another reagent is the combination of nitrogen tetroxide and sulfuric acid. A review of these and other nitration processes to 1950 has been made in addition, the general mechanisms of aromatic nitrations have been extensively studied. ... 

L. Albright C. Hanson, Loss Prevention 3, 26 (1969) CA 72, 99685 (1970) (Kinetics and mechanism of aromatic nitrations are reviewed in terms of safety considerations)... 

Several papers appeared after the pioneer work of Mine and co-workers (Vol. I, p. 126). Papers by Chernova and co-workers [S3] and Sugimoto and co-workers [54] have shown that even dilute nitric acid (0.1-1.7 N) can nitrate aromatic compounds when subjected to gamma radiation from Co °. Benzene yielded nitrobenzene and p-nitrophenol as the result of free radical reactions. Nitric acid is decomposed by radical mechanism according to equation [55] ... 

Important for industry is the reaction of the substitution of a nilro group by a sulphonic group as was described in Vol. I (pp. 201, 207). The mechanism of the reaction is now given in the chapter dedicated to the manufacture of TNT and other higher nitrated aromatics. 

The decrease of the catalytic activity in the order W03/Zr02 > W03/Ti02 > HZSM-5 corresponds to decrease of their acid strength. This agrees with the well-known mechanism of aromatic compounds nitration via formation of intermediate nitronium cation N02". ... 

Melander43 has carried out a detailed and thorough investigation of the effect of tritium substitution at the aromatic nucleus on the mechanism of aromatic nitration. He has studied the... 

In the Gas Phase. Schmitt et al. (32, 33) found that ArH + and N02 couple in the gas phase to give the a complex (ArHN02) +. The ArH + was produced by ionization of ArH with photons emitted from pulse-radiation excited argon atoms. Reactions were carried out in a flow system within the source of a quadrupole mass spectrometer. In contrast, reactions of the corresponding ArH with N02+ were found not to go directly to (ArHN02) + but to lead either to ArH + and N02 by fast electron transfer or to (ArHO) + and NO by oxygen-atom transfer. From these results, Schmitt et al. (32, 33) concluded that aromatic radical cations are a plausible intermediate in the mechanism for aromatic nitration. ... 

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