The Nitrating Species

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 

A more recently recognized kinetic complication is that, for all aromatics more reactive than toluene, Eq. (3.3) is encounter limited, so that all these substrates react at the same rate [68JCS(B)800] in 68.3 wt% H2S04 this is —40 times that of benzene (67CC352). A problem here is that although there is no intermolecular selectivity under these conditions, the 

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Nitration at a rate independent of the concentration of the compound being nitrated had previously been observed in reactions in organic solvents ( 3.2.1). Such kinetics would be observed if the bulk reactivity of the aromatic towards the nitrating species exceeded that of water, and the measured rate would then be the rate of production of the nitrating species. The identification of the slow reaction with the formation of the nitronium ion followed from the fact that the initial rate under zeroth-order conditions was the same, to within experimental error, as the rate of 0-exchange in a similar solution. It was inferred that the exchange of oxygen occurred via heterolysis to the nitronium ion, and that it was the rate of this heterolysis which limited the rates of nitration of reactive aromatic compounds. 

If acetoxylation were a conventional electrophilic substitution it is hard to understand why it is not more generally observed in nitration in acetic anhydride. The acetoxylating species is supposed to be very much more selective than the nitrating species, and therefore compared with the situation in (say) toluene in which the ratio of acetoxylation to nitration is small, the introduction of activating substituents into the aromatic nucleus should lead to an increase in the importance of acetoxylation relative to nitration. This is, in fact, observed in the limited range of the alkylbenzenes, although the apparently severe steric requirement of the acetoxylation species is a complicating feature. The failure to observe acetoxylation in the reactions of compounds more reactive than 2-xylene has been attributed to the incursion of another mechan-104... 

The nitration can be considered a typical esterification equiUbrium. The removal of water with sulfuric acid forces the reaction to completion. It is beheved the nitrating species is N02, the nitronium ion (49,50). 

Possible nitrating entities in these media are the nitronium ion (the concentration of which can be measured spectroscopically), molecular nitric acid or the nitricacidium ion. Nitric acid is not the nitrating species because dramatic changes in the reaction rate are observed in the nitration of 2-phenylethylsulphonic acid in aqueous nitric acid on addition of perchloric acid (Table 1). Also, when nitric... 

Excitation spectra have been of considerable use recently in studying both hydration numbers (by lifetime measurements) and inner-sphere complexation by anions (by observing appearance of the characteristic frequencies for e.g. the Eu3+ 5D0-+ 7F0 transition for the different possible species). Thus using a pulsed dye laser source, it was possible to demonstrate the occurrence of inner sphere complexes of Eu3+ with SCN, CI or NO3 in aqueous solution, the K values being 5.96 2, 0.13 0.01 and 1.41 0.2 respectively. The CIO4 ion did not coordinate. Excited state lifetimes suggest the nitrate species is [Eu(N03)(HzO)6,s o.4]2+ the technique here is to compare the lifetimes of the HzO and the corresponding D20 species, where the vibrational deactivation pathway is virtually inoperative.219 The reduction in lifetime is proportional to the number of water molecules complexed.217 218... 

The process for the production of nitrobenzene from benzene involves the use of mixed acid (Fig. 1), but there are other useful nitrating agents, e.g., inorganic nitrates, oxides of nitrogen, nitric acid plus acetic anhydride, and nitric acid plus phosphoric acid. In fact, the presence of sulfuric acid in quantity is vital to the success of the nitration because it increases the solubility of the hydrocarbon in the reaction mix, thus speeding up the reaction, and promotes the ionization of the nitric acid to give the nitronium ion (N02+), which is the nitrating species. Absorption of water by sulfuric acid favors the nitration reaction and shifts the reaction equilibrium to the product. 

The nitrate ion activities in the aqueous phase were measured with a nitrate ion selective electrode taking into account the presence of high hydrogen ion concentration by calibration of the nitrate electrode with nitric acid. The nitrate ion concentration in the organic phase owing to the extraction of neodymium complexes by HDEHP was determined by back-extraction of the organic phase with 3M sulfuric acid, dilution, and analysis with a nitrate ion electrode calibrated for different nitrate and sulfate concentrations. The amount of the nitrate species extracted into the organic phase increases as the initial neodymium nitrate concentration increases. 

It is possible to nitrate highly activated aromatic compounds such as phenols using dilute nitric acid. The nitrating species is considered to be... 

A number of manganese(I) derivatives are known, e.g., Mn(CO)3-LX whereX = NO3- (2), NCS (735), and 01 , Br , or I" (377). The nitrate species undergoes substitution reactions with one or two moles of tri-phenylphosphine with the loss of one carbon monoxide ligand it may be oxidized to a pair of isomeric manganese(II) complexes [Mn(bipy)2(N03)2]- These have been designated cis and trans isomers on... 

For the next experiment a 1 m solution of 4-(phenyl)morpholin-3-one in acetic acid was treated with neat fuming nitric acid. Then the nitrating species was used in roughly 24-fold molar excess. Now (Table 15.3) the reaction proceeds much faster. 

No further work was done with N(V) nitration however, on the basis of these observations, we assert that nitric acid itself does not nitrate pyrene in methylene chloride. This finding can be contrasted with the results of Grosjean et al. (3), who concluded in atmospheric pollutant studies that the opposite view is correct. Specifically, their results from vapor-phase experiments suggested that N02 was not active in nitration of filter-deposited PAHs, whereas nitric acid was necessary for nitration. The differences in conditions must be considered when comparing these data with those from this work. Nonetheless, it is difficult to refute the absence of nitration activity by N(V) under our conditions, and the important questions concerning the nitrating species in the environmental work remain. 

The nitration data of this investigation were first used to discriminate between the kinetic models developed for the slow, fast, and instantaneous reaction systems (or regimes) and second to pick the best kinetic model. In order to do this, values had to be estimated for the concentration of the nitrating species (or entity) in the acid phase, the interfacial concentration of benzene, the kinetic rate constants for acid-phase nitrations, interfacial area (between two liquid phases), and diffusivity values or mass transfer coefficients for benzene. In analyzing the data, the concentration of the nitrating species was assumed to be that of the nitric acid in the acid phase it is appreciated that this assumption is open to question as will be discussed later but based on available information is probably the best assumption possible. The interfacial concentration of benzene (hC., .) was estimated based in part on the results obtained in making distribution measurements h is the distribution coefficient for benzene and C. - is the benzene concentration in the hydrocarbon phase. KinetTc rate constants (for the second-order nitration reactions occurring in the acid phase) were estimated based on the results of Deno and Stein (10),... 

The second broad grouping of papers (Chapters 11—16) considers both the chemistry and physical transfer steps between phases which often occur during nitration. In aromatic nitrations using mixed acids, for example, the presence of two immiscible liquid phases complicates the nitration reaction. Agitation to emulsify the two phases is necessary to obtain adequate contact between the hydrocarbon and the nitrating species. Transfer of reactants and products, heat transfer, nature of emulsion, etc. are key factors. 

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