Nitrosation, nitration through

Moodie, Schofield and their co-workers have extended their studies to the other media listed in Table 6, but the evidence for a limiting reaction rate is then less clear cut. This is, however, to be expected since the other media are less viscous than 68% sulphuric acid and so the influence of diffusion control in reducing the apparent reactivity of the more reactive compounds should be less marked. It is possible also that nitration through nitrosation is significant with some of the most reactive compounds, although Moodie, Schofield and their co-workers took care to minimize the interference by this reaction. 

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 kinetic study of nitration by nitric acid in carbon tetrachloride has been briefly reported and is of interest because of the third-order dependence of rate upon nitric acid concentration, for nitration of N-methyl-N-nitrosoaniline. This is believed to arise from equilibria (28) and (29) below, which give rise to a nitrosating species and nitration is achieved through subsequent oxidation of the nitrosated aromatic69. 

Aromatic A A -dialkylamines react rapidly with HN02 and undergo ring nitration and nitrosative dealkylation both reactions are linked through the formation of a nitrosammo-nium ion R1 RiV—N=O (R1 = Ar, R2 = Aik). This nitorosoammonium ion then undergoes reversible homolysis to NO and a cation radical (Loeppky et al. 1998). 

The introduction of the 5-nitro group can easily be explained by nitrosation of 8-hydroxyquinoline in position 5 and subsequent oxidation of the nitroso compound. No similar explanation can be given with regard to the mechanism of the introduction of the second nitro group, in position 7, as no nitrosation of nitro-phenols is known and the formation of 8-hydroxy-5-nitro-7-nitrosoquinoline , does not seem to be possible. The formation of 5,7-dimtroso-8-hydroxyquinoline as an intermediate is also improbable because no instance of the introduction of two nitroso groups into a monophenol is known. Thus nitration most likely precedes through the formation of the 5-nitroso derivative only. 

Alternatively, the reaction may proceed through nitrosation by the nitrosonium ion, NO", and subsequent oxidation of the nitroso compound by nitric acid (Scheme 7.3). There is only a small concentration of NO" in dilute nitric acid and so catalytic amounts of sodium nitrite are sometimes added to increase the quantity. This technique is a useful means of effecting smooth, low-temperature nitrations. 

Also Ross and co-workers (87] pointed out that the accepted scheme of nitration of phenol in 56.2% sulphuric acid through nitrosation prior to nitration, is inconsistent with the results and expressed the view that another route should exist. 

The problem of nitration with aqueous nitric acid was reviewed by Hanson and associates [8]. They confirmed the idea of nitracidium ion being a nitrating agent and pointed out that attention should be paid to the presence of nitrous acid in the system, as nitration with dilate nitric acid can proceed through the nitrosation by nitrous acid formed as the result of oxidation-reduction. T. Urbahski and Kutkiewicz [9] (Vol. l,p. 85) found that 8-hydroxyquinoIine can be nitrated by boiling with 0.5% nitric acid to yield 5,7-dinitro-8-hydroxyquino-line. It was also found that 8-hydroxy-5-nitroquinoIine yielded the same dinitro derivative. 

In the absence of sulfuric acid, the nitration of certain reactive benzene derivatives appears to proceed through a preliminary nitrosation step rather than the sequence outlined above. Thus phenol,40 anisole,41 and... 

It was deduced that the exchange of oxygen occurred through heterolysis to produce the nitronium ion (reaction 2) which will have an equal rate to that of zeroth-order nitration. A possible point of criticism is that it is not absolutely clear whether the acid solutions used were positively shown to be free from nitrous acid. If any nitrous acid is present in nitrating acid, it can lead to nitration by means of nitrosation involving the NO ion (16)... 

It was stated by T. Urbanski and Kutkiewicz [9] that 8-hydroxyquinoline could be nitrated by boUing with a dilute (ca. 0.3%) solution of NaNOj in 10% hydrochloric acid to yield 5,7-dinitro-8-hydroxyquinoline. If the reaction passes through the nitrosation it cannot give a dinitro product, as the nitrosation can introduce only one nitroso group. Still more convincing is the fact the 8-hy-droxy-5-nitroquinoline when heated to ca. 90 C with ca. 0.4% solution of sodium nitrite in 10% hydrochloric acid produced 5,7-dinitro-8-hydroxyquinoline with 60% yield. It is known that a mononitrophenol (and thus the 8-hy-droxy-5-nitroquinoline) cannot be nitrosated. Subsequently the formation of the dinitro compound is the result of nitric acid formed from NO2 and yielding NO according to the scheme rationalized already — reactions (5a) and (5b) (p. 24). 

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