Nitrous ion

For a sensitive colour reaction in the presence of nitrous ions, see Falciola, Gazzetta, 1922, 52, i., 179. 

The reaction then takes place and its progress is checked until the absence of nitrous ions. After 40 minutes, the presence or absence of nitrous ions is checked at regular intervals in the reaction medium. Starch-iodine paper, for example, is used, checking every 5 minutes. After about 60 minutes of reaction, the nitrous acid had been entirely consumed and no more N02" ions remained in the reaction medium. The pH was then adjusted to 7 with pure caustic soda, and the products of the reaction were recovered by the addition of 31 liters of pure ethanol (2 volumes). The precipitate formed was collected by centrifugation, washed with ethanol and dried at 60°C under vacuum. 

The nitrous ion N02" attracts a proton, thus facilitating the nitrosating action of the nitrosonium ion NO+. 

Because of the activity of sulfions and polysuMde ions, O-N bonds would be broken to form alkoxy and thionitrate ions or polythionitrate ions. The effect of sulfides on the reduction of oxygen atoms should be considered in other same type reactions. It would form nitrous ions, which cannot be produced from nitrate reduction. 

In the products of the reaction between alkyl nitrate and hydrazine, the compounds, including nitric ions, nitrous ions, alcohols, alkyl hydrazine, nitrogen oxide, ammonium, and trace aldehyde, can be detected. If the reaction runs in solvent-free or excess hydrazine solvent, a reduction reaction would be happened [38]. 

These all contain the ion NO2. They are much more stable than nitrous acid, and those of the alkali metals can be fused without... 

Dissolve a few drops of nitromethane in 10% sodium hydroxide solution. Add a few crystals of sodium nitrite and shake. Now add dilute sulphuric acid drop by drop. A brownish-red coloration develops, but fades again when an excess of acid is added. The sulphuric acid has thus liberated nitrous acid, which has in turn reacted with the nitromethane to give a nitrolic acid, the sodium salt of which is CH3NO2 + ONOH = CH(N02) N0H + HgO reddish-brown in colour, probably owing to mesomeric ions of the type ... 

Primary aromatic amines differ from primary aliphatic amines in their reaction with nitrous acid. Whereas the latter yield the corresponding alcohols (RNHj — ROH) without formation of intermediate products see Section 111,123, test (i), primary aromatic amines 3neld diazonium salts. Thus aniline gives phcnyldiazonium chloride (sometimes termed benzene-diazonium chloride) CjHbNj- +C1 the exact mode of formation is not known, but a possible route is through the phenjdnitrosoammonium ion tlius ... 

The influence of added species upon the rates and kinetic forms of nitration in organic solvents were of the greatest importance in elucidating details of the processes involved, particularly of the steps leading to the nitronium ion. These influences will first be described, and then in the following section explained. The species to be considered are sulphuric acid, nitrate ions, urea and water. The effect of nitrous acid is considered later ( 4.3). 

In experiments on the nitration of benzene in acetic acid, to which urea was added to remove nitrous acid (which anticatalyses nitration 4.3.1), the rate was found to be further depressed. The effect was ascribed to nitrate ions arising from the formation of urea nitrate. In the same way, urea depressed the rate of the zeroth-order nitration of mesitylene in sulpholan. ... 

Solutions of dinitrogen tetroxide (the mixed anhydride of nitric and nitrous acids) in sulphuric acid are nitrating agents ( 4.3.2), and there is no doubt that the effective reagent is the nitronium ion. Its formation has been demonstrated by Raman spectroscopy and by cryoscopy ... 

In aqueous solutions of sulphuric (< 50%) and perchloric acid (< 45 %) nitrous acid is present predominantly in the molecular form, although some dehydration to dinitrogen trioxide does occur.In solutions contairdng more than 60 % and 65 % of perchloric and sulphuric acid respectively, the stoichiometric concentration of nitrous acid is present entirely as the nitrosonium ion (see the discussion of dinitrogen trioxide 4.1). Evidence for the formation of this ion comes from the occurrence of an absorption band in the Raman spectrum almost identical with the relevant absorption observed in crystalline nitrosonium perchlorate. Under conditions in which molecular nitrous... 

If we consider the effect of nitrous acid upon zeroth-order nitration in organic solvents we must bear in mind that in these circumstances dinitrogen tetroxide is not much ionised, so the measured concentration of nitrous acid gives to a close approximation the concentration of dinitrogen tetroxide. Further, the negligible self-ionisation of nitric acid ensures that the total concentration of nitrate ions is effectively that formed from dinitrogen tetroxide. Consequently as we can see from the equation for the ionisation of dinitrogen tetroxide ( 4.3.1),... 

The weak effect of nitrous acid upon nitration in nitric acid is a consequence of the already considerable concentration of nitrate ions supplied in this case by the medium. 

The more powerful anticatalysis of nitration which is found with high concentrations of nitrous acid, and with all concentrations when water is present, is attributed to the formation of dinitrogen trioxide. Heterolysis of dinitrogen trioxide could give nitrosonium and nitrite ions 2N2O4 + HjO N2O3 + 2HNO3. 

In contrast to its effect upon the general mechanism of nitration by the nitronium ion, nitrous acid catalyses the nitration of phenol, aniline, and related compounds. Some of these compounds are oxidised under the conditions of reaction and the consequent formation of more nitrous acids leads to autocatalysis. 

The kinetics of nitration of anisole in solutions of nitric acid in acetic acid were complicated, for both autocatalysis and autoretardation could be observed under suitable conditions. However, it was concluded from these results that two mechanisms of nitration were operating, namely the general mechanism involving the nitronium ion and the reaction catalysed by nitrous acid. It was not possible to isolate these mechanisms completely, although by varying the conditions either could be made dominant. 

The catalysis was very strong, for in the absence of nitrous acid nitration was very slow. The rate of the catalysed reaction increased steeply with the concentration of nitric acid, but not as steeply as the zeroth-order rate of nitration, for at high acidities the general nitronium ion mechanism of nitration intervened. 

The effect of nitrous acid on the nitration of mesitylene in acetic acid was also investigated. In solutions containing 5-7 mol 1 of nitric acid and < c. 0-014 mol of nitrous acid, the rate was independent of the concentration of the aromatic. As the concentration of nitrous acid was increased, the catalysed reaction intervened, and superimposed a first-order reaction on the zeroth-order one. The catalysed reaction could not be made sufficiently dominant to impose a truly first-order rate. Because the kinetic order was intermediate the importance of the catalysed reaction was gauged by following initial rates, and it was shown that in a solution containing 5-7 mol 1 of nitric acid and 0-5 mol 1 of nitrous acid, the catalysed reaction was initially twice as important as the general nitronium ion mechanism. 

Nitration at the encounter rate and nitrosation As has been seen ( 3.3), the rate of nitration by solutions of nitric acid in nitromethane or sulpholan reaches a limit for activated compounds which is about 300 times the rate for benzene imder the same conditions. Under the conditions of first-order nitration (7-5 % aqueous sulpholan) mesitylene reacts at this limiting rate, and its nitration is not subject to catalysis by nitrous acid thus, mesitylene is nitrated by nitronium ions at the encounter rate, and under these conditions is not subject to nitration via nitrosation. The significance of nitration at the encounter rate for mechanistic studies has been discussed ( 2.5). 

Under the same conditions the even more reactive compounds 1,6-dimethylnaphthalene, phenol, and wt-cresol were nitrated very rapidly by an autocatalytic process [nitrous acid being generated in the way already discussed ( 4.3.3)]. However, by adding urea to the solutions the autocatalytic reaction could be suppressed, and 1,6-dimethyl-naphthalene and phenol were found to be nitrated about 700 times faster than benzene. Again, the barrier of the encounter rate of reaction with nitronium ions was broken, and the occurrence of nitration by the special mechanism, via nitrosation, demonstrated. 

Figure 22 5 shows what happens when a typical primary alkylamine reacts with nitrous acid Because nitrogen free products result from the formation and decomposition of diazonium ions these reactions are often referred to as deamination reactions Alkyl... 

FIGURE 22 5 The diazo mum ion generated by treatment of a primary al kylamine with nitrous acid loses nitrogen to give a car bocation The isolated prod ucts are derived from the carbocation and include in this example alkenes (by loss of a proton) and an al cohol (nucleophilic capture by water)... 

Aryl diazonium ions prepared by nitrous acid diazotization of primary arylamines are substantially more stable than alkyl diazonium ions and are of enormous synthetic value Their use m the synthesis of substituted aromatic compounds is described m the following two sections... 

Nitrous acid or nitrite salts may be used to catalyze the nitration of easily nitratable aromatic hydrocarbons, eg, phenol or phenoHc ethers. It has been suggested that a nitrosonium ion (NO + ) attacks the aromatic, resulting initially in the formation of a nitro so aromatic compound (13). Oxidation of the nitro so aromatic then occurs ... 

The nitrosonium ion is produced from nitrous acid and nitric acid. 

Exceptions are salts of oxidizing anions, which decompose with oxidation of the ammonium ion to nitrous oxide [10024-97-2], N2O, or nitrogen, N2. 

When 6/3-aminopenicillanic acid (6-APA) is diazotized in the presence of chloride ion, the principal product obtained is 6a-chloropenicillanic acid (38) (62JOC2668), presumably by way of the diazo intermediate (39 Scheme 29) (72JCS(P1)895). If the diazotization is carried out in the presence of excess bromide instead of chloride, significant amounts of the 6,6-dibromo derivative are obtained, and in the case of excess iodide the 6,6-diiodopenicillanic acid becomes the predominant product (69JCS(C)2123). The 6,6-dihalo products presumably arise from nitrous acid oxidation of halide to halogen, which then reacts with (39). 

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