General nitrating agents

Replacement of hydrogen by the nitro group is one of the most important reactions in the whole of organic chemistry, particularly in the aromatic series. It is effected not merely by nitric acid itself in various concentrations or in mixtures or solvents, but also by a number of other reagents which, however, are less important than nitric acid the commonest reagent is the mixture of nitric and sulfuric acid known as nitrating acid. 

Urbanski, Chemie und Technologie der Explosivstoffe, VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1961, Vol. 1 G. A. Olah and St. J. Kuhn in G. A. Olah, Friedel-Crafts and Related Reactions, Interscience Publ., New York-London-Sydney, 1964, Vol. 3, Part 2, p. 1393. 

If mercury salts are added, the oxidizing action of dilute nitric acid can be elegantly utilized for simultaneous introduction of hydroxyl and several nitro groups. This oxynitration , discovered by Wolffenstein and Boters,103 permits the one-step conversion of, e.g., benzene into picric acid in about 50% yield104 and of benzoic acid into 3-hydroxy-2,4,6-trinitrobenzoic acid, although in less good yield in the latter case105 (for the mechanism see West-heimer et a/.106). 

The preparation of 2,4-dinitrophenol107 serves as example of the procedure Mercuric oxide (60 g) is dissolved in 70% nitric acid (541 ml) contained in a 1.5-1 flask fitted with a stirrer, reflux condenser, and dropping funnel. The solution is diluted wih water to 750 ml, then sodium nitrite (0.1 g) is added. Benzene (50 g) is dropped in at 50° during 200 min and the mixture is stirred for a further 160 min at 50° and then allowed to cool. 12 Hours later the product is filtered off and washed with dilute nitric acid and then with cold water it has m.p. 112-113°, the yield being 67 g. If the filtrate is worked up, a total of 68% of dinitro-phenol is obtained. 

Concentrated nitric acid allows nitration to be carried out at lower temperatures and tends to cause less of the undesired oxidation however, it is being constantly diluted by the water formed, so that attainment of satisfactory yields requires extremely large excesses of the nitrating agent, and this severely limits its utility. 

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The general mechanism of nitrating alcohols to form nitrate esters is described under Nitration in this Vol. Several specific remarks about PETN are contained in that article. The industrial nitration of PE differs from most nitrate esters in that it employs coned nitric acid rather than mixed acid. Nevertheless nitration via the nitronium ion mechanism, which is the preferred mechanism in mixed acid nitrations, is also feasible in coned nitric acid. However, Eremenko and co-workers claim that the nitrating agent in PE nitrations, in mixed acid, is unionized nitric acid (Refs 39 76). The present writer does not find Eremenko s arguments to be very convincing. In any case, commercial production of PETN employs nitric acid and not the mixed acids of Eremenko s studies... 

The outstanding problem is to decide how much, if any, association exists between N02 and X" in the generally rate-determining step of the reaction. Kinetic studies tend to indicate the presence of different electrophiles under different conditions whereas the derived partial rate factors are closely similar and therefore indicate one electrophile common to most, if not all, nitrating agents. The more electron-attracting is X , the more easily is N02 displaced from it and hence a reactivity sequence should be... 

Most nitrations are highly exothermic and hence release a lot of reaction heat for most experimental protocols [37, 94]. This high exofhermidty may even lead to explosions [37, 38]. Nitration agents frequently display acid corrosion [37]. For these reasons, nitrations generally are regarded as being hazardous [37, 38]. 

The nitration of active methylene compounds generally proceeds via the reaction of carbanionic intermediates with an electrophilic nitrating agent such as alkyl nitrate (alkyl nitrate nitration). Details of this process are well documented in the reviews.38 The alkyl nitrate nitration method has been used extensively for the preparation of arylnitromethanes. The toluene derivatives, which have electron-withdrawing groups are nitrated with alkyl nitrates in the presence of KNH2 in liquid ammonia (Eqs. 2.19 and 2.20).39... 

As useful as tetranitromethane is as a charge-transfer nitrating agent, it is generally too unreactive to effect the comparable electrophilic nitration of most aromatic donors, except the most electron-rich ones. Thus in order to make the direct comparison between the photochemical and thermal nitration of the same ArH, we now turn to the A-nitropyridinium acceptor (PyNOj) as the alternative nitrating agent (Olah et al., 1965, 1980). For example, PyNO can be readily prepared as a colourless crystalline salt, free of any adventitious nitrosonium impurity, and used under essentially neutral conditions. Most importantly, the electrophilic reactivity of this... 

It is generally accepted that nitric acid-acetic anhydride mixtures contain acetyl nitrate (Equation 4.4). Acetyl nitrate is a weak nitrating agent but in the presence of a strong acid, like nitric acid, it can ionize to nitronium and acetate ions. Protonated acetyl nitrate could also be an active nitrating agent under these conditions. 

Cross-linked polystyrene can be directly nitrated with fuming nitric acid at low temperatures (-25 °C to 0°C) [189,203,374] polymers with up to one nitro group per arene result [203]. Partial nitration can be achieved with milder nitrating agents, such as acetyl nitrate [203]. Because direct nitrations are not compatible with most linkers (which are often acid- or oxidant-labile), nitro compounds are generally not prepared on supports but in solution, and are then linked to the support. 

Abstract Synthesis methods of various C- and /V-nitroderivativcs of five-membered azoles - pyrazoles, imidazoles, 1,2,3-triazoles, 1,2,4-triazoles, oxazoles, oxadiazoles, isoxazoles, thiazoles, thiadiazoles, isothiazoles, selenazoles and tetrazoles - are summarized and critically discussed. The special attention focuses on the nitration reaction of azoles with nitric acid or sulfuric-nitric acid mixture, one of the main synthetic routes to nitroazoles. The nitration reactions with such nitrating agents as acetylnitrate, nitric acid/trifluoroacetic anhydride, nitrogen dioxide, nitrogen tetrox-ide, nitronium tetrafluoroborate, V-nitropicolinium tetrafluoroborate are reported. General information on the theory of electrophilic nitration of aromatic compounds is included in the chapter covering synthetic methods. The kinetics and mechanisms of nitration of five-membered azoles are considered. The nitroazole preparation from different cyclic systems or from aminoazoles or based on heterocyclization is the subject of wide speculation. The particular section is devoted to the chemistry of extraordinary class of nitroazoles - polynitroazoles. Vicarious nucleophilic substitution (VNS) reaction in nitroazoles is reviewed in detail. 

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