Reactivity effects nitration

Concentrated nitric acid can effect nitration but it is not as reactive as a mixture of nitric acid with sulfuric acid. The active nitrating species in both media is the nitronium ion, NOz+, which is formed by protonation and dissociation of nitric acid. The concentration of NOz+ is higher in the more strongly acidic sulfuric acid than in nitric acid. 

The function of the sulphuric acid is to convert the nitric add into the highly reactive, electrophilic, nitronium ion, N02, which is the effective nitrating agent. 

Since the early days of organic chemistry, nitration has been considered to be an important reaction and has been widely used. As early as 1825 Faraday discovered benzene and recorded its reaction with nitric acid. Shortly after, the use of nitric acid sulfuric acid mixtures to effect nitration was reported and was soon quoted in a patent. Nitration figured prominently in the development of ideas of theoretical organic chemistry in the early part of the twentieth century and, as the most widely applicable and most widely used example of electrophilic substitution, it played an important role in the consideration of aromatic stability and reactivity. In 1910 the first report of orientation and deactivation in aromatic electrophilic substitution was published (10MI1). 

Since the classic papers by Ingold and his co-workers,110, 111 nitration has for a long time been considered as the standard electrophilic substitution. Many orientation and relative rate data on the nitration of both carbocyclic and heterocyclic substrates have been accumulated and the results have been generalized as valid for all electrophilic substitutions. As a matter of fact, this popularity is partially undeserved nitration is a complicated reaction, which can occur by a multiplicity of parallel mechanisms.112 In particular, in the case of the very reactive substrates that five-membered heterocycles are, two complications may make meaningless both kinetic measurements and competitive experiments.113 (i) Due to the great reactivity of both partners the encounter limiting rate may be achieved in this case, of course, all the substrates react at the same rate and the effect of structure on the reactivity cannot be studied. (ii) Nitrous acid, always present in traces, may exert an anticatalytic effect in some cases and a markedly catalytic effect in others with a very reactive substrate, nitration may proceed essentially via nitrosa-tion, followed by oxidation. For these reasons, the nitration data must be handled with much caution. 

The formation of the nitronium ion is critical to the success of nitration. The purpose of the sulfuric acid is to generate the nitronium ion other acids such as perchloric acid, hydrogen fluoride and boron trifluoride are also effective. Nitric acid alone contains only relatively small concentrations of nitronium ion and hence is not an effective nitrating agent, except with very reactive substrates. 

An amino group may be introduced into an aromatic compound by nitration, followed by reduction25. Most aromatic compounds, whether of high or low reactivity, undergo nitration with one of a wide variety of nitrating agents26-28. The reduction of the nitro compounds to amines can also be effected by a variety of methods (equation 3). This... 

There are several reaction systems that are capable of effecting nitration. A major factor in the choice of reagent is the reactivity of the aromatic ring to be nitrated. Concentrated nitric acid can effect nitration, but it is not nearly so reactive as mixtures of nitric acid with sulfuric acid. In both media, the active nitrating species is the nitronium ion. A variety of physical measurements provide evidence for the existence of this species and permit estimation of its concentration under some conditions. In concentrated sulfuric acid, the dissolution of nitric acid results in the formation of nearly 4 ions per nitric acid molecule, as determined by freezing-point depression ... 

Bicycloheptane and Bicyclo-octane Derivatives.—Relative positional reactivities in nitration of benzonorbornene, benzonorbornadiene, (727), and the endo-isomer of (727) have been determined for the ipso-, a-, and /S-positions. The lower a-reactivity of (727) as compared to its endo-isomer is explained by the buttressed fused ortho effect , arising in (727) because steric hindrance between bridge and cyclopropane methylene hydrogens is sufficient to cause bridge bending, thus moving the sy -H... 

Hence, in the presence of NO2 not only the activity of SCR catalysts, but also their selectivity appears to be governed by the reactivity of nitrates. When NO is available in addition to NH3, and the temperature exceeds a threshold related to the dissociation of ammonium nitrate, the desired reduction of the surface nitrates by NO can proceed effectively, resulting in the most efficient selective DeNO pathway, i.e., the Fast SCR reaction. On the other hand, if an excess of NO2 prevails in comparison to NO, the unselective pathways (9.10) and (9.11) will prevail at lower and higher temperatures, respectively. 

For nitrations in sulphuric and perchloric acids an increase in the reactivity of the aromatic compound being nitrated beyond the level of about 38 times the reactivity of benzene cannot be detected. At this level, and with compounds which might be expected to surpass it, a roughly constant value of the second-order rate constant is found (table 2.6) because aromatic molecules and nitronium ions are reacting upon encounter. The encounter rate is measurable, and recognisable, because the concentration of the effective electrophile is so small. 

We are not concerned here with the mechanism of nitrosation, but with the anticatalytic effect of nitrous acid upon nitration, and with the way in which this is superseded with very reactive compounds by an indirect mechanism for nitration. The term nitrous acid indicates all the species in a solution which, after dilution with water, can be estimated as nitrous acid. 

Despite the fact that solutions of acetyl nitrate prepared from purified nitric acid contained no detectable nitrous acid, the sensitivity of the rates of nitration of very reactive compounds to nitrous acid demonstrated in this work is so great that concentrations of nitrous acid below the detectable level could produce considerable catalytic effects. However, because the concentration of nitrous acid in these solutions is unknown the possibility cannot absolutely be excluded that the special mechanism is nitration by a relatively unreactive electrophile. Whatever the nature of the supervenient reaction, it is clear that there is at least a dichotomy in the mechanism of nitration for very reactive compounds, and that, unless the contributions of the separate mechanisms can be distinguished, quantitative comparisons of reactivity are meaningless. 

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). 

If this electrostatic treatment of the substituent effect of poles is sound, the effect of a pole upon the Gibbs function of activation at a particular position should be inversely proportional to the effective dielectric constant, and the longer the methylene chain the more closely should the effective dielectric constant approach the dielectric constant of the medium. Surprisingly, competitive nitrations of phenpropyl trimethyl ammonium perchlorate and benzene in acetic anhydride and tri-fluoroacetic acid showed the relative rate not to decrease markedly with the dielectric constant of the solvent. It was suggested that the expected decrease in reactivity of the cation was obscured by the faster nitration of ion pairs. 

Table 9.7 contains recent data on the nitration of polychlorobenzenes in sulphuric acid. The data continue the development seen with the diehlorobenzenes. The introduetion of more substituents into these deactivated systems has a smaller effect than predicted. Whereas the -position in ehlorobenzene is four times less reactive than a position in benzene, the remaining position in pentachlorobenzene is about four times more reactive than a position in 1,3,4,5-tetraehlorobenzene. The chloro substituent thus activates nitration, a circumstance recalling the faet that o-chloronitrobenzene is more reactive than nitrobenzene. As can be seen from table 9.7, the additivity prineiple does not work very well with these compounds, underestimating the rate of reaction of pentachlorobenzene by a factor of nearly 250, though the failure is not so marked in the other cases, especially viewed in the circumstance of the wide range of reactivities covered. 

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