Of nitronium ion

The nitronium ions produced in this way tend to repress the selfdehydration of the nitric acid and therefore the net concentration of nitronium ions is not proportional to the concentration of the catalyst. When sufficient sulphuric acid has been added to make the self-ioniza-tion of nitric acid relatively unimportant, the nitronium ions will be produced predominantly from the above ionization, and the acceleration will follow a linear law. 

In the process of O-exchange the nitronium ion mechanism requires that the rate of nitronium ion formation be the rate at which the label... 

Raman spectroscopy provides the easiest way of estimating the concentration of nitronium ions in different media ( 2.4.1). The concentration, determined by infra-red spectroscopy, of nitronium ions in nitric acid was increased markedly by the addition of sulphuric acid. ... 

Raman spectroscopy I c.i.so showed that the addition of up to io% of water does not affect the concentration of nitronium ions further dilution reduces the concentration of this species, which is not detectable in solutions containing < 85 % sulphuric acid. The introduction of... 

Addition of water to solutions of nitric acid in 90% sulphuric acid reduces rates of nitration. Between 90% and 85% sulphuric acid the decrease in rate parallels the accompanying fall in the concentration of nitronium ions. This is good evidence for the operation of the nitronium ion as the nitrating agent, both in solutions more acidic than 90% and in weakly diluted solutions in which nitronium ion is still spectroscopically detectable. 

That the rate profiles are close to parallel shows that the variations in rates reflect the changing concentration of nitronium ions, rather than idiosyncrasies in the behaviour of the activity coefficients of the aromatic compounds. The acidity-dependences of the activity coefficients of / -nitrotoluene, o- and -chloronitrobenzene (fig. 2.2, 2.3.2), are fairly shallow in concentrations up to about 75 %, and seem to be parallel. In more concentrated solutions the coefficients change more rapidly and it... 

The relative abilities of nitromethane, sulpholan, and acetic acid to support the ionisation of nitric acid to nitronium ions are closely similar to their efficiencies as solvents in nitration. Raman spectroscopy showed that for a given concentration of mixed acid (i i nitric and sulphuric acids) the concentration of nitronium ions in these three solvents varied in the order nitromethane > sulpholan > acetic acid. The concentration of mixed acid needed to permit the spectroscopic detection of nitronimn ions was 25 %, 50 % and 60 % in the three solvents, respectively (see 4.4.3). 

NO3-] oc [N20J and so [NOai oc Now nitrate ions reduce the rate of formation of nitronium ion by de-protonating nitric acidium ions, and this effect must also depend upon [HN02]"toich> as was observed. 

In first-order nitration the anticatalysis is of the same form because the deprotonation of nitric acidium ion diminishes the stationary concentration of nitronium ion and therefore diminishes the rate of nitration. 

The use of i i mixed aeid in sulpholan and in aeetie acid was examined (table 4.1, eolumns (/)-(i))- The variation of the eoncentration of nitronium ions with the eoneentration of mixed aeids ([H2SO4] [HNO3], 1 1), in sulpholan (a), aeetie aeid (6), and nitromethane (c) are illustrated in fig. 4.1. The results for aeetie aeid and sulpholan were determined by Raman speetroseopy, and those for nitromethane from the infra-red speetra. 

Using sulpholan and acetic acid as solvents competitive nitrations were performed with solutions containing 75% and 30% of mixed acid (table 4.1, columns h, i and /, g, respectively). In the former the concentration of nitronium ions was substantial [c. 5-7 % by weight), whereas in the latter the concentration was below the level of spectroscopic detection. 

An investigation of the infra-red spectra of mixtures of nitric acid and acetic anhydride supports these conclusions. The concentration of nitronium ions, measured by the absorption band at 2380 cm, was... 

Ridd - has reinterpreted the results concerning the anticatalysis of the first-order nitration of nitrobenzene in pure and in partly aqueous nitric acid brought about by the addition of dinitrogen tetroxide. In these media this solute is almost fully ionised to nitrosonium ion and nitrate ion. The latter is responsible for the anticatalysis, because it reduces the concentration of nitronium ion formed in the following equilibrium ... 

Considering first pure nitric acid as the solvent, if the concentrations of nitronium ion in the absence and presence of a stoichiometric concentration x of dinitrogen tetroxide are yo and y respectively, these will also represent the concentrations of water in the two solutions, and the concentrations of nitrate ion will be y and x- y respectively. The equilibrium law, assuming that the variation of activity coefficients is negligible, then requires that ... 

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. 

The role of nitronium ion in the nitration of benzene was demonstrated by Sir Christo pher Ingold—the same per son who suggested the SnI and Sn2 mechanisms of nu cleophilic substitution and who collaborated with Cahn and Prelog on the R and S notational system... 

Acid mixtures containing nitric acid and a strong acid, eg, sulfuric acid, perchloric acid, selenic acid, hydrofluoric acid, boron trifluoride, or an ion-exchange resin containing sulfonic acid groups, can be used as the nitrating feedstock for ionic nitrations. These strong acids are catalysts that result in the formation of nitronium ions, NO" 2- Sulfuric acid is almost always used industrially since it is both effective and relatively inexpensive. 

The existence of the nitronium ion in sulfuric-nitric acid mixtures was demonstrated both by cryoscopic measurements and by spectroscopy. An increase in the strong acid concentration increases the rate of reaction by shifting the equilibrium of step 1 to the right. Addition of a nitrate salt has the opposite effect by suppressing the preequilibrium dissociation of nitric acid. It is possible to prepare crystalline salts of nitronium ions, such as nitronium tetrafluoroborate. Solutions of these salts in organic solvents rapidly nitrate aromatic compounds. ... 

Like N2O4 (p. 457) it dissociates ionically in strong anhydrous acids such as HNO3, H3PO4, H2SO4, HSO3F and HCIO4, and this affords a convenient source of nitronium ions and hence a route to nitronium salts, e.g. ... 

Steps (1) and (2) are fast while the production of nitronium ion in step (3) is slow . A great mass of evidence exists in support of the formation of N02+ in mixed acids of low water content, but there is some controversy about the presence of H2N03+ in MA. Hantzsch s cryo-metric studies (Refs 9 18) were interpreted to be in support, of the existence of the nitra-cidium ion (fijNOj ). These views were later supported by the studies of Hammet coworkers (Refs 20 24). Furthermore, Hantzsch isolated crystalline nitracidium perchlorate (H2N03+)(C104-)(Refs 14a 14b)... 

Euler was the first to suggest the presence of nitronium ions in mixed acids (Ref 12), This view was later supported by the conductometric studies of Walden (Ref 14). Titov (Ref 28) reinterpreted Hantzsch s data and suggested that the simplest way to explain them was ... 

The rates increase up to a maximum at about 90 wt. % sulphuric acid (this point varies slightly according to the aromatic reactivity) and the increase with increasing acid concentration is consistent with the increase in the concentration of nitronium ions. The occurrence of a maximum indicates an opposing factor and is thought42 to be partly due to protonation of the aromatic (most of the measured compounds contain the group >X=0) but since it also occurs for PhNMe3, medium effects must be involved, i.e. the activities of the species present varies, whilst the concentrations remain the same. The kinetic equation for reaction of nitronium ion with an aromatic is... 

By contrast, the rate-acidity profiles for nitration of 3-nitro-4-pyridone, 3-and 5-methyl-2-pyridone and l,5-dimethyl-2-pyridone resemble each other and differ from the above-indicated reaction upon the free base, and correction of the observed rates to allow for the concentration of free base actually present gave rate-acidity profiles of the expected form the corrected entropies of activation then turned out to be positive. Furthermore, if the logarithms of the corrected rate coefficients obtained in media of low acidity were plotted against +log aHlQ, then slopes of near unity were obtained (see above, p. 18), but not otherwise. A similar result was obtained from the nitration data for 4-pyridone in media of low acidity suggesting that here it reacts as the free base. A further test which was applied was to calculate the concentration of nitronium ions in the various media and to correct the observed rate coefficients for this the logarithms of these coeffi-... 

Scheme 9.9 Lanthanide catalysed generation of nitronium ions...

Scheme 3.1 Formation of nitronium ion and its reaction with primary and secondary amines.

When considering aromatic nitration, it seems reasonable to examine other nitrating agents (besides the nitrating mixture leading to the formation of nitronium ion) and outline the scope of ion-radical mechanism in each case. 

The reaction (NO + N02 NO+ + NO2 ) is recommended by Ridd (1998) to put in parentheses to imply that it illustrates the stoichiometry of the process, not the mechanism. The mechanism is more complex because the rate of these nitrous acid-catalyzed nitrations greatly exceed the rate of formation of nitronium ions in the solution. 

Further work by Baum and co-workers showed that the nitration of l,l-diamino-2,2-dinitroethylenes with trifluoroacetic anhydride and nitric acid in methylene chloride yields 1,1,1-trinitromethyl derivatives via addition of nitronium ion to the double bond of the enamine such treatment also resulting in the A-nitration of the products. In this way, trini-tromethyl derivatives like (185) and (188) are obtained. Further treatment of these trinitromethyl derivatives with aqueous potassium iodide results in reductive denitration and the formation... 

The rate of a nitration is fastest at the start of a reaction when a large excess of nitric acid is present. As the reaction progresses the water formed during nitration dilutes the mixed acid and slows the rate of reaction, and as such, it is common towards the end of a nitration, when most of the substrate has reacted, to heat the reaction to completion. This dilution of the acid with water is an important point and the amount of sulfuric acid used should be enough to take up all the water formed during the reaction otherwise, nitration may be incomplete and result in an unfavourable mixture of product and starting material. Increased amounts of water in mixed acid rapidly reduce the concentration of nitronium ions. When concentrated nitric acid is used for the nitration of some of the more reactive substrates a large excess of sulfuric is often used to compensate for the water present. 

Strong Brpnsted acids promote the formation of nitronium ions when mixed with nitric acid. Perchloric, hydrofluoric, phosphoric, polyphosphoric, trifluoroacetic, " ... 

Numerous Lewis acids promote the formation of nitronium ions when in the presence of nitric acid. Nitric acid-boron trifluoride, and the nitric acid-hydrogen fluoride-boron trifluoride reagents described by Olah are practical nitrating agents the latter provides a convenient preparation of nitronium tetrafluoroborate. Olah reports that nitric acid-magic acid (FSOsH-SbFs) is extremely effective for the polynitration of aromatic substrates. 

Unlike the direct nitration of amines under acidic conditions, nucleophilic nitration is an excellent route to both primary and secondary nitramines. In these reactions the amine or the conjugate base of the amine is used to attack a source of NO2. This source may be a nitrogen oxide, nitronium salt, cyanohydrin nitrate, alkyl nitrate ester or any other similar source of nitronium ion. 

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