The kinetics of nitration

Although the nitronium ion cannot be detected by physical methods in these media, kinetic studies using these solutions have provided compelling evidence for the formation and effectiveness of this species in nitration. 

Much of the early work was inconclusive confusion sprang from the production by the reaction of water, which generally reduced the rate, and in some cases by production of nitrous acid which led to autocatalysis in the reactions of activated compounds. The most extensive kinetic studies have used nitromethane,acetic acid, sulpholan,i and carbon tetrachloride as solvents. 

It was from studies of nitration with solutions of nitric acid in nitromethane, and later in acetic acid, that Ingold and his co-workers first established the fundamental features of these reactions, and also correctly interpreted them. The use in these experiments of a large excess of nitric acid removed the problem caused by the formation of water. 

The rates of nitration of benzene, toluene, and ethylbenzene in solutions of nitric acid c. 3-7 mol 1 ) in nitromethane were independent 

Under conditions in which benzene and its homologues were nitrated at the zeroth-order rate, the reactions of the halogenobenzenes ([aromatic] = c. o-1 mol 1 ) obeyed no simple kinetic law. The reactions of fluorobenzene and iodobenzene initially followed the same rates as that of benzene but, as the concentration of the aromatic was depleted by the progress of the reaction, the rate deviated to a dependence on the first power of the concentration of aromatic. The same situation was observed with chloro- andjbromo-benzene, but these compounds could not maintain a zeroth-order dependence as easily as the other halogenobenzenes, and the first-order character of the reaction was more marked. 

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The kinetics of nitration of anisole in solutions of nitric acid in acetic acid were complicated, for both autocatalysis and autoretardation could be observed under suitable conditions. However, it was concluded from these results that two mechanisms of nitration were operating, namely the general mechanism involving the nitronium ion and the reaction catalysed by nitrous acid. It was not possible to isolate these mechanisms completely, although by varying the conditions either could be made dominant. 

TABLE 4.4 The kinetics of nitration of nitrobenzene in various medial... 

The kinetics of nitration in acetic anhydride are complicated. In addition to the initial reaction between nitric acid and the solvent, subsequent reactions occur which lead ultimately to the formation of tetranitromethane furthermore, the observation that acetoxylation accompanies the nitration of the homologues of benzene adds to this complexity. 

The kinetics of nitration of benzene in solutions at c. 20 °C in carbon tetrachloride have been investigated. In the presence of an excess of benzene (c. 2-4 mol 1 ) the rate was kinetically of the first order in the concentration of benzoyl nitrate. The rate of reaction was depressed by the addition of benzoic anhydride, provided that some benzoic acid was present. This result suggested that benzoyl nitrate itself was not responsible for the nitration, but generated dinitrogen pentoxide... 

First-order nitrations. The kinetics of nitrations in solutions of acetyl nitrate in acetic anhydride were first investigated by Wibaut. He obtained evidence for a second-order rate law, but this was subsequently disproved. A more detailed study was made using benzene, toluene, chloro- and bromo-benzene. The rate of nitration of benzene was found to be of the first order in the concentration of aromatic and third order in the concentration of acetyl nitrate the latter conclusion disagrees with later work (see below). Nitration in solutions containing similar concentrations of acetyl nitrate in acetic acid was too slow to measure, but was accelerated slightly by the addition of more acetic anhydride. Similar solutions in carbon tetrachloride nitrated benzene too quickly, and the concentration of acetyl nitrate had to be reduced from 0-7 to o-i mol 1 to permit the observation of a rate similar to that which the more concentrated solution yields in acetic anhydride. 

The kinetics of nitration have been studied fairly intensively over a number of years and show that the reaction can be represented by the general steps... 

Extension of this work to the kinetics of nitration of N-methyl- and N, N-dimethylaniline48 gave the rate coefficients given in Table 7 together with those... 

The kinetics of nitration by nitric acid in nitromethane and acetic acid have been studied in detail40, and interpretation of the data here is perhaps made a little more difficult by the fact that spectroscopic and conductimetric analyses give no evidence for the existence of the nitronium ion in these media57. 

Similar conclusions have been reached by Champetier and Foex [70] and by Trombe, Foex and Champetier [71]. Wilson and Miles also studied the kinetics of nitration in nitric acid vapour. On the basis of these investigations they concluded that two molecules of nitric acid take part in the reaction with one OH group ... 

Data have been presented on the kinetics of nitration of acetanilide in mixtures of nitric and sulfuric acids.29 A review discusses the several mechanisms operative in the nitration of phenol including /> / -sclective nitrosation-oxidation and mechanisms involving phenoxy radical-nitrogen dioxide reaction yielding a 55 45 ortho para nitration ratio.30 The kinetics of mononitration of biphenyl-2-carboxylic acid have been investigated in several solvents. The maximum ortho para product ratio of 8.4 is observed in tetrachloromethane.31 Nitration products were not formed in the presence of dioxane.31,32 Quantum-chemical AMI calculations were performed and the predominant formation of the ortho-nitro product is accounted for by stabilization of the cr-complex by the carboxyl group.33... 

The kinetics of nitration in sulphuric acid of both pyrazole and imidazole have been studied.87 Data have already been quoted (tables 8.1, 8.3) to support the view that the nitration of both of these compounds at Q4)... 

Ingold, Hughes and Reed [39] studied the kinetics of nitration of aromatic compounds with nitric acid only and found that it was a first order reaction. Its rate, VN/ may be expressed by an approximate equation ... 

The kinetics of nitration of aromatic hydrocarbons with nitric acid in the presence of acetic anhydride was first studied by Tronov, Kamay and Kovalenko [110], Their studies will be discussed later. 

Ingold, Hughes and Reed [39] have studied the kinetics of nitration with nitric acid in the presence of nitromethane and found the reaction to be of zero order in the case of readily nitrated aromatic compound, and of first order with compounds... 

The kinetics of nitration of imidazole have been studied in sulfuric acid medium. The yields of 4-nitroimidazoIe are dependent on the acidity, e.g. %H2S04, yield (%) 83.7, 46 89.6, 26 93.8, 19 98.8, 19 1% SO3, 90. Ring opening accompanies the process giving rise to ammonia and, presumably, oxidation products. The kinetic results which could be separated indicate that the species being nitrated is the imidazole cation, but side reactions complicate matters. The suggested mechanism is outlined in Scheme 30 (B-76MI40701). 

Macisaac, J., and Dugdale, R. C. (1969). The kinetics of nitrate and ammonia uptake by natural populations of marine phytoplankton. Deep Sea Res. 116, 45—57. 

Electrons are transferred by the respiratory electron chain to a cytochrome b which then donates electrons to the Mo-protein (nitrate reductase). By using reduced viologens it should be possible to determine the kinetics of nitrate reduction by the nitrate reductase however, no information on this aspect is currently available. The sensitivity of nitrate reduction to inhibition by cyanide is considered to be due to association of the inhibitor with molybdenum residues in the nitrate reductase (Enoch and Lester, 1975). Cytochrome b involved in nitrate reduction does not bind cyanide. 

The rate constant (/ i) for the formation of the nitronium ion from nitric acid in aqueous sulfuric, perchloric, and methanesulfonic acid [equation (14)] has been obtained by studying the kinetics of nitration of the reactive amine... 

In a study of the kinetics of nitration of benzene, toluene, m-xylene, mesity-lene, and anisole by nitric acid in AC2O, the rate of nitration and the order with respect to acid are both sensitive to prior solvent treatment. Conductivity measurements show that the effect is due to some trace impurities in the solvent which on protonation give rise to NOs" ions. Rates of nitration in purified solvent are sensitive to added NOs ions and water but almost unchanged by added AcOH. It has been deduced from arguments based on the true rate constant for electrophilic attack that reaction occurs through the N02 ion. ... 

Properties of Sulphonic Acids.—In the course of a study of the kinetics of nitration of arenesulphonic acids in H2SO4 it was shown that the sulphonate anion is a deactivating substituent even though it carries a negative charge. 

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