Nitration polar mechanism

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 structure of the ferroelectric, tri-glycine fluoberyllate [8, 9], has been determined, and participation of H-bonds in the polarization mechanism at the Curie point is described. Glycine silver nitrate is also ferroelectric [10], and a polarization mechanism, also involving H-bonds, is suggested. It is evident, from dielectric and structural... 

The catalysed reaction was considered to arise from the heterolysis of dinitrogen pentoxide induced by aggregates of molecules of nitric acid, to yield nitronium ions and nitrate ions. The reaction is autocatalytic because water produced in the nitration reacts with the pentoxide to form nitric acid. This explanation of the mechanism is supported by the fact that carbon tetrachloride is not a polar solvent, and in it molecules of nitric acid may form clusters rather than be solvated by the solvent ( 2.2). The observation that increasing the temperature, which will tend to break up the clusters, diminishes the importance of the catalysed reaction relative to that of the uncatalysed one is also consistent with this explanation. The effect of temperature is reminiscent of the corresponding effect on nitration in solutions of nitric acid in carbon tetrachloride ( 3.2) in which, for the same reason, an increase in the temperature decreases the rate. 

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,... 

As regards sulphuric acid, there is here again an increase in polarity and an increase in rates of nitration when comparison is made with other solvents in the series. This gross fact would be difficult to reconcile with any mechanism, such as the one, which contains an essential forward step which would be retarded by increased polarity of the solvent. 

A slight but systematic decrease in the wave number of the complexes bond vibrations, observed when moving from sodium to cesium, corresponds to the increase in the covalency of the inner-sphere bonds. Taking into account that the ionic radii of rubidium and cesium are greater than that of fluorine, it can be assumed that the covalent bond share results not only from the polarization of the complex ion but from that of the outer-sphere cation as well. This mechanism could explain the main differences between fluoride ions and oxides. For instance, melts of alkali metal nitrates display a similar influence of the alkali metal on the vibration frequency, but covalent interactions are affected mostly by the polarization of nitrate ions in the field of the outer-sphere alkali metal cations [359]. 

Analogies for such a mechanism in diazotization are found in the nitrous acid-catalyzed nitration of A,A-dimethylaniline, mesitylene, 4-nitrophenol, and some related compounds, which were investigated by 15N NMR spectroscopy in Ridd s group (Ridd and Sandall, 1981 Ridd et al., 1992 Clemens et al., 1984a, 1984b, 1985 Johnston et al., 1991 review Ridd, 1991). Ridd and coworkers were able to demonstrate clearly that not only the nitration proper, but also the preceding C-nitrosation, is accompanied by a marked 15N nuclear polarization. This was at-... 

They recorded such a polarization curve for zinc, for copper in the presence of gelatin and for silver in nitrate solution. Under this mechanism, a negative fluctuation in concentration drives the current density up, resulting in further reduction in interfacial concentration. For this instability to be expressed, the surface concentration must be free to respond to variations in current. As a result, the instability is seen only far from the limiting current, where the interfacial concentration is pinned at zero. At high Peclet numbers, the concentration disturbance is propagated downstream by convection, and the striations follow the streamlines. 

While at Leeds from 1924 to 1930, Ingold s laboratory focused on three main topics of research (1) the nature and mechanism of orienting effects of groups in aromatic substitution (mainly nitration) (2) the study of prototropic rearrangements (shifts of H+) and aniontropic rearrangements (shifts of anions) as the ionic mechanisms of tautomerism and (3) the effect of polar substituents on the velocity and orientation of addition reactions to unsaturated systems. One of Ingold s students at Leeds, John William Baker, wrote a widely read book on tautomerism. 16... 

Several reactions of imines of synthetic utility are reported. Nitric oxide reacts with A-benzylidene-4-methoxyaniline (18) in ether to give 4-methoxybenzenediazonium nitrate (19) and benzaldehyde. Two mechanisms are proposed, both involving nitrosodiazene (20), and the preferred route is suggested to involve direct electrophilic reaction of NO to the imine double bond, favoured by the polarity of the latter. 

In their thoughtful 1983 review, Nielsen and coworkers noted that particles of diesel soot or wood smoke can absorb significant amounts of water. Thus, they suggested that the most plausible mechanism(s) for nitration (and possibly other electrophilic reactions) of particle-associated PAHs in ambient air may involve reactions both in a liquid film and on solid surfaces and that fundamental laboratory studies of the rates, products, and mechanisms of PAHs in polar solvents would be atmospherically relevant for reactions in the liquid films. Based on this, they proposed a classification scheme for the reactivities of key PAHs in electrophilic reactions, which was subsequently described in detail (Nielsen, 1984). 

The accepted reaction mechanism for the electrophilic aromatic nitration was postulated by Ingold in 1969[3] and involves several steps (Scheme 5.1). Firstly, the nitric acid is protonated by a stronger acid (sulfuric). The protonated nitric acid gives water and the nitronium ion (N02+) which is the electrophilic active species for nitration of aromatics. Nitric acid heterolysis is considered to be accelerated by the polarity of the solvent, and solvation of nitronium ion in different media affects its reactivity and the selectivity of the reaction. Combination of nitronium ion and an aromatic molecule form an intermediate named the Wheland complex or er-complex. The loss of a proton from the er-complex gives the aromatic nitrocompound (Scheme 5.1). 

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