Nitration by nitronium ions

Nitration at the encounter rate and nitrosation As has been seen ( 3.3), the rate of nitration by solutions of nitric acid in nitromethane or sulpholan reaches a limit for activated compounds which is about 300 times the rate for benzene imder the same conditions. Under the conditions of first-order nitration (7-5 % aqueous sulpholan) mesitylene reacts at this limiting rate, and its nitration is not subject to catalysis by nitrous acid thus, mesitylene is nitrated by nitronium ions at the encounter rate, and under these conditions is not subject to nitration via nitrosation. The significance of nitration at the encounter rate for mechanistic studies has been discussed ( 2.5). 

The nature of the electrophile in this nitrating mixture is still not wholly agreed upon whereas kinetic evidence can be interpreted as consistent with nitration by nitronium ion, the fact that substituents with lone pairs of electrons or it-electrons give markedly different ortho para ratios from other nitrating mixtures is usually conceded to be consistent with the electrophile being something other than the nitronium ion. The balance of evidence at present is in favour of pro-tonated acetyl nitrate being the electrophile. 

Kinetic studies with benzene in acetic anhydride containing 0.4-2 M nitric acid at 25 °C show the reaction to be first-order in benzene and approximately second-order in nitric acid this falls to first-order in nitric acid on addition of sulphuric acid, which also increases the first-order rate coefficient (first-order in benzene) from 4.5 x 10-4 to 6.1 x 10 4. By contrast the addition of as little as 0.001 M sodium nitrate reduced the rate to 0.9 x 10-4 without affecting the kinetic order70. These results were, therefore, interpreted as nitration by nitronium ion via equilibria (21a) and (22). 

Dinitrogeri pentoxide is the anhydride of nitric acid and is prepared by removing water from pure nitric acid by means of phosphorus (V) oxide. It is a crystalline solid having the ionic structure of (N02) (N03) , nitronium nitrate (the nitronium ion is mentioned later). It decomposes above 273 K, thus ... 

The Raman spectrum of nitric acid shows two weak bands at 1050 and 1400 cm. By comparison with the spectra of isolated nitronium salts ( 2.3.1), these bonds were attributed to the nitrate and nitronium ion respectively. Solutions of dinitrogen pentoxide in nitric acid show these bands , but not those characteristic of the covalent anhydride , indicating that the self-dehydration of nitric acid does not lead to molecular dinitrogen pentoxide. Later work on the Raman spectrum indicates that at —15 °C the concentrations of nitrate and nitronium ion are 0-37 mol 1 and 0 34 mol 1 , respectively. The infra-red spectrum of nitric acid shows absorption bands characteristic of the nitronium ion. The equivalence of the concentrations of nitronium and nitrate ions argues against the importance of the following equilibrium ... 

Because of the chemical similarity between benzoyl nitrate and the acetyl nitrate which is formed in solutions of nitric acid in acetic anhydride, it is tempting to draw analogies between the mechanisms of nitration in such solutions and in solutions of benzoyl nitrate in carbon tetrachloride. Similarities do exist, such as the production by these reagents of higher proportions of o-substituted products from some substrates than are produced by nitronium ions, as already mentioned and further discussed below. Further, in solutions in carbon tetrachloride of acetyl nitrate or benzoyl nitrate, the addition of acetic anhydride and benzoic anhydride respectively reduces the rate of reaction, implying that dinitrogen pentoxide may also be involved in nitration in acetic anhydride. However, for solutions in which acetic anhydride is also the solvent, the analogy should be drawn with caution, for in many ways the conditions are not comparable. Thus, carbon tetrachloride is a non-polar solvent, in which, as has been shown above,... 

The form of potential energy curve deduced by Olah from kinetic evidence on the nitration of benzene, and some alkyl- and halo-benzenes, by nitronium ions derived from NOJ BIV is shown in Fig. 18. In this diagram, position D is associated with a localized structure analogous to that of Fig. 16 and 19b. 

Interaction within encounter pairs. The above arguments have assumed that there is no interaction between the components in the encounter pairs A. B and B.X (Scheme 3) but this is probably unrealistic. Stabilization of the encounter pair A. B by charge-transfer interaction should favour the pre-association path by reducing the value of k 3 and possibly also increasing that of k4 [cf. equation (46)].22 Thus, in the nitration of neutral amine molecules (X) by nitronium ions (B) in concentrated sulphuric acid, interaction between the components in the encounter pair ArNHJ.NOJ should increase the acidity of the N—H hydrogens and facilitate the formation of the free amine. In the bromination of aromatic compounds (B) by HOBr(A), interaction between the components should increase the concentration of the encounter pair ArH. HOBr and facilitate the protonation of the hypobromous acid. 

HMordenite, HFaujasite-780, HFaujasite 720 and Na-Faujasite zeolites. Among the different catalysts, HFaujasite-720 was the most active and selective catalyst towards 2,4-dinitrotoluene, achieving a yield of dinitrotoluenes of 92 % with a ratio of 2,4- to 2,6- isomers of 4.3 1 in 3 min reaction time. Using this zeolite, l-chloro-2-nitrobenzene and pyrazole were also nitrated regioselectively to obtain l-chloro-2,4-dinitrobenzene in a l-chloro-2,4-dinitro l-chloro-2,6-dinitro ratio of 30 1, and 1,4-dinitropyrazole in 80% yield, respectively. The authors proposed a nitration mechanism in which the protons in the zeolite are replaced by nitronium ions derived from N2Os in a fast pre-equilibrium process. This produces active sites for transfer of nitronium ion from faujasite to aromatic in the rate-controlling step. 

Virtually all nitration reactions involve electrophillic attack by nitronium ions, NO2. Consequently, reactions can be regulated by controlling the concentration of nitronium ions in solution. The conditions required for the nitration reaction vary greatly with the reactivity of the aromatic substrate. The nitration mixture required for introduction of the second nitro group into benzene to prepare dinitrobenzene and concentrated nitric and sulfuric acids at 95°C is unsuitable for dinitration of alkylated phenol because it provides the conditions for an uncontrollable exothermic reaction. The DNBP process described above was based on controlling the nitration conditions and has the advantage of having very few side reactions because it is a two-phase system with nitration reactivity based on mass transfer between phases. 

The sequence of steps for nitration and sulfonation of benzene is similar to that for chlorination and bromination. For nitration, the nitronium ion, N02, a very strong electrophile, is generated by the reaction between nitric acid and sulfuric acid. 

The long-held formulation of electrophiHc nitration of benzene and other arenes (ArH) by nitronium ions can be described in terms of the successive formation of a 7r-complex from the reactants and its direct transformation into a covalently bound tr-complex, followed by proton transfer to... 

Parker and Bethell expressed doubt concerning the conclusions that the Wheland intermediates are not involved in aromatic brominations which differ principally from, say nitration with nitronium ion, by the fact that a strong nucleophile (bromide ion) is present under the former conditions and not the latter. It seemed reasonable that computations would not reveal the presence of the positive sigma complex since its formation might be expected to be very slow compared to the very rapid subsequent reactions. It is commonly observed that it is difficult to observe an intermediate of the latter nature by computation if the transition state for its formation is not found. Several transition states were found in the previous study but all of them were depicted as complex processes (see Table 2 in Ref 157) involving the simultaneous making and breaking of more than one bond. 

A theoretical study of nitration of amines MeaNL (L = H, CHaPh, CHO, COR, X, SO2R, SiMcs) by nitronium ion has used the AMI method to calculate... 

Nitration can be effected under a wide variety of conditions, as already indicated. The characteristics and kinetics exhibited by the reactions depend on the reagents used, but, as the mechanisms have been elucidated, the surprising fact has emerged that the nitronium ion is preeminently effective as the electrophilic species. The evidence for the operation of other electrophiles will be discussed, but it can be said that the supremacy of one electrophile is uncharacteristic of electrophilic substitutions, and bestows on nitration great utility as a model reaction. 

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

The two absorption bands, at 1050 and 1400 cm , which appear in the Raman spectra of solutions of nitric acid in concentrated sulphuric acid are not attributable to either of the acid molecules. In oleum the lower band appears at 1075-1095 cm. That these bands seemed to correspond to those in the spectra of anhydrous nitric acid and solid dinitrogen pentoxide caused some confusion in the assignment of the spectrum. The situation was resolved by examining the Raman spectra of solutions of nitric acid in perchloric or selenic acids , in which the strong absorption at 1400 cm is not accompanied by absorption at about 1050 cm . Thus, the band at 1400 cm arises from the nitronium ion, and the band at about 1050 cm can be attributed in the cases of nitric acid and solid dinitrogen pentoxide to the nitrate ion formed according to the following schemes ... 

Concentrated solutions are here considered to be those containing > c. 89 % by weight of sulphuric acid. In these solutions nitric acid is completely ionised to the nitronium ion. This fact, and the notion that the nitronium ion is the most powerful electrophilic nitrating species, makes operation of this species in these media seem probable. Evidence on this point comes from the effect on the rate of added water ( 2.4.2)... 

Second-order rate coefficients for nitration in sulphuric acid at 25 °C fall by a factor of about 10 for every 10 % decrease in the concentration of the sulphuric acid ( 2.4.2). Since in sulphuric acid of about 90% concentration nitric acid is completely ionised to nitronium ions, in 68 % sulphuric acid [NO2+] io [HNO3]. The rate equation can be written in two ways, as follows ... 

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. 

The zeroth-order rates of nitration depend on a process, the heterolysis of nitric acid, which, whatever its details, must generate ions from neutral molecules. Such a process will be accelerated by an increase in the polarity of the medium such as would be produced by an increase in the concentration of nitric acid. In the case of nitration in carbon tetrachloride, where the concentration of nitric acid used was very much smaller than in the other solvents (table 3.1), the zeroth-order rate of nitration depended on the concentrationof nitric acid approximately to the fifth power. It is argued therefore that five molecules of nitric acid are associated with a pre-equilibrium step or are present in the transition state. Since nitric acid is evidently not much associated in carbon tetrachloride a scheme for nitronium ion formation might be as follows ... 

The most crucial observation concerning the effects of added species is that nitrate ion anticatalyses nitration without changing the kinetic form of the reaction. This shows that nitrate does not exert its effect by consuming a proportion of the nitronium ion, for, as outlined above, this would tend to bring about a kinetically first-order reaction. Nitrate ions must be affecting the concentration of a precursor of the nitronium... 

Solutions of dinitrogen tetroxide (the mixed anhydride of nitric and nitrous acids) in sulphuric acid are nitrating agents ( 4.3.2), and there is no doubt that the effective reagent is the nitronium ion. Its formation has been demonstrated by Raman spectroscopy and by cryoscopy ... 

This reaction showed certain characteristics which distinguish it from nitrations in solutions of nitric acid in organic solvents. Thus, in changing the solvent from carbon tetrachloride to nitromethane, the rate increased by a factor of only 6, whereas nitration involving the nitronium ion was accelerated by a factor of about 30 when the solvent was changed from acetic acid to nitromethane. It was held that the... 

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