Nitration theories

Gariy nitration theories considered nitration as an addition reaction in which the initial step was the direct addition of the nitrating agent to the molecule undergoing nitration, eg ... 

This scheme, however, has the disadvantage of ignoring the influence of the nitronium ion (NO ) on the reaction, whereas, as expounded in the chapter on nitration theories (Vol. I), the nitronium ion is of enormous importance for such a reaction. In this connection Lamberton [43] suggests alternative schemes which appear more probable. Scheme (8) leads to the formation of a nitramine and scheme (9) to a salt of nitric acid ... 

The nitration theory, which assumes that a free radical takes part, seems to be more probable than the addition theory and is in better agreement with the experimental data. An interesting piece of evidence for the participation of free radicals in the reaction was given by McCleary and Degering [36], who obtained a mixture of nitroethane and ethyl nitrate when nitrating tetraethyl lead with nitric acid in an atmosphere of C02, at 150°C. It is probable that the reaction proceeds as follows ... 

The theory that the catalysed nitration proceeds through nitrosation was supported by the isolation of some />-nitrosophenol from the interrupted nitration of phenol, and from the observation that the ortho.-para ratio (9 91) of strongly catalysed nitration under aqueous conditions was very similar to the corresponding ratio of formation of nitrosophenols in the absence of nitric acid. ... 

A familiar feature of the electronic theory is the classification of substituents, in terms of the inductive and conjugative or resonance effects, which it provides. Examples from substituents discussed in this book are given in table 7.2. The effects upon orientation and reactivity indicated are only the dominant ones, and one of our tasks is to examine in closer detail how descriptions of substituent effects of this kind meet the facts of nitration. In general, such descriptions find wide acceptance, the more so since they are now known to correspond to parallel descriptions in terms of molecular orbital theory ( 7.2.2, 7.2.3). Only in respect of the interpretation to be placed upon the inductive effect is there still serious disagreement. It will be seen that recent results of nitration studies have produced evidence on this point ( 9.1.1). 

Most correlations of rates with localisation energies have used values for the latter derived from the Hiickel approximation. More advanced methods of m.o. theory can, of course, be used, and fig. 7.1 illustrates plots correlating data for the nitration of polynuclear hydrocarbons in acetic anhydride -" with localisation energies derived from self-... 

M.o. theory has had limited success in dealing with electrophilic substitution in the azoles. The performances of 7r-electron densities as indices of reactivity depends very markedly on the assumptions made in calculating them. - Localisation energies have been calculated for pyrazole and pyrazolium, and also an attempt has been made to take into account the electrostatic energy involved in bringing the electrophile up to the point of attack the model predicts correctly the orientation of nitration in pyrazolium. ... 

As with cases mentioned earlier, Hiickel m.o. theory performs satisfactorily in predicting the orientation of nitration in these oxides, but again fails to reproduce their strong deactivation. ... 

Since the first-order rate constant for nitration is proportional to y, the equilibrium concentration of nitronium ion, the above equations show the way in which the rate constant will vary with x, the stoichiometric concentration of dinitrogen tetroxide, in the two media. An adequate fit between theory and experiment was thus obtained. A significant feature of this analysis is that the weak anticatalysis in pure nitric acid, and the substantially stronger anticatalysis in partly aqueous nitric acid, do not require separate interpretations, as have been given for the similar observations concerning nitration in organic solvents. 

Numerous theories exist as to how the Chilean deposits formed and survived. It has been postulated that the unique nitrate-rich caUche deposits of northern Chile owe their existence to an environment favorable to accumulation and preservation of the deposits, rather than to any unusual source of the saline materials (2). The essential conditions are an extremely arid climate similar to that of the Atacama desert in the 1990s, slow accumulation during the late Tertiary and Quaternary periods, and a paucity of nitrate-utilizing plants and soil microorganisms. 

When activating substituents are present in the benzenoid ring, substitution usually becomes more facile and occurs in accordance with predictions based on simple valence bond theory. When activating substituents are present in the heterocyclic ring the situation varies depending upon reaction conditions thus, nitration of 2(177)-quinoxalinone in acetic acid yields 7-nitro-2(177)-quinoxalinone (21) whereas nitration with mixed acid yields the 6-nitro derivative (22). The difference in products probably reflects a difference in the species being nitrated neutral 2(177)-quinoxalinone in acetic acid and the diprotonated species (23) in mixed acids. 

Frontier orbital theory predicts that electrophilic substitution of pyrroles with soft electrophiles will be frontier controlled and occur at the 2-position, whereas electrophilic substitution with hard electrophiles will be charge controlled and occur at the 3-position. These predictions may be illustrated by the substitution behaviour of 1-benzenesulfonylpyr-role. Nitration and Friedel-Crafts acylation of this substrate occurs at the 3-position, whereas the softer electrophiles generated in the Mannich reaction (R2N=CH2), in formylation under Vilsmeier conditions (R2N=CHC1) or in formylation with dichloromethyl methyl ether and aluminum chloride (MeO=CHCl) effect substitution mainly in the 2-position (81TL4899, 81TL4901). Formylation of 2-methoxycarbonyl-l-methylpyrrole with... 

Aceording to Frontier Molecular Orbital (FMO) theory, thiophene s most reactive site can be identified by examining the shape of its highest-occupied molecular orbital (HOMO). Which atoms contribute to thiophene s HOMO Which atom(s) contributes the most Which nitration product should form preferentially ... 

Valence bond theory (Chapter 7) explains the fact that the three N—O bonds are identical by invoking the idea of resonance, with three contributing structures. MO theory, on the other hand, considers that the skeleton of the nitrate ion is established by the three sigma bonds while the electron pair in the pi orbital is delocalized, shared by all of the atoms in the molecule. According to MO theory, a similar interpretation applies with all of the resonance hybrids described in Chapter 7, including SO S03, and C032-. 

Theory. The anion exchange resin, originally in the chloride form, is converted into the nitrate form by washing with sodium nitrate solution. A concentrated solution of the chloride and bromide mixture is introduced at the top of the column. The halide ions exchange rapidly with the nitrate ions in the resin, forming a band at the top of the column. Chloride ion is more rapidly eluted from this band than bromide ion by sodium nitrate solution, so that a separation is possible. The progress of elution of the halides is followed by titrating fractions of the effluents with standard silver nitrate solution. 

The most important precipitation processes in titrimetric analysis utilise silver nitrate as the reagent (argentimetric processes). Discussion of the theory will, therefore, be confined to argentimetric processes the same principles can, of... 

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