Acid dependence nitrate

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

If it be assumed that the ionising characteristics of nitric acid are similar to those of the organic indicators used to define the scales of acidity, then a correspondence between the acidity-dependence of nitration and would suggest the involvement of the nitronium ion, whereas a correspondence with Hq would support the h)rpothesis that the nitric acidium ion were active. The analogies with and Hg are expressed in the first and last pairs of the followii equations respectively. The symbol AQ represents anthraquinone, the indicator originally used in this way for comparison with the acidity dependence of the rate of nitration of nitrobenzene ... 

There is increasing evidence that the ionisation of the organic indicators of the same type, and previously thought to behave similarly, depends to some degree on their specific structures, thereby diminishing the generality of the derived scales of acidity. In the present case, the assumption that nitric acid behaves like organic indicators must be open to doubt. However, the and /fp scales are so different, and the correspondence of the acidity-dependence of nitration with so much better than with Hg, that the effectiveness of the nitronium ion is firmly established. The relationship between rates of nitration and was subsequently shown to hold up to about 82 % sulphuric acid for nitrobenzene, />-chloronitrobenzene, phenyltrimethylammonium ion, and p-tolyltrimethylammonium ion, and for various other compounds. ... 

Although the difference between the Hg and scales is sufficient to permit such a gross mechanistic distinction to be made, the acidity-dependence of nitration deviates from a close correspondence with the... 

The correlations of rates with acidity functions provide a convenient means of treatii results, and their uses will frequently be illustrated. However, their status is empirical, for whilst the acidity dependence of nitration becomes less steep with increasing temperature, the slope of... 

Another reason for treating with caution the results for benzene in solutions more acidic than 68% is discussed below ( 2.5). The acidity-dependences of rates of nitration at 25 °C have been established for the compounds listed in table 2.5. 

If the concentration of effective aromatic species does vary with acidity, as sometimes happens if the compound is substantially proto-nated, then the acidity-dependence of the rate will be less steep than usual, because the concentration of the active free base diminishes significantly with increasing acidity. This situation has been observed in certain cases ( 8.2). The fall in the concentration of the active species can be allowed for from a knowledge of its pK and the acidity function which, for the particular compound, gives the best measure of the acidity of the medium. Then the corrected acidity-dependence of the rate resembles that observed with compounds the concentration of which does not change significantly with acidity. The nitration of minor species is discussed later ( 8.2). 

Unlike the effect of sulphuric acid upon nitration in nitric acid ( 2.2.3 where zeroth-order reactions are unknown), the form of the catalysis of zeroth-order nitration in nitromethane by added sulphuric acid does not deviate from a first-order dependence with low concentrations of catalyst. ... 

The anticatalytic effect of nitrous acid in nitration The effect of nitrous acid was first observed for zeroth-order nitrations in nitromethane ( 3.2). The effect was a true negative catalysis the kinetic order was not affected, and nitrous acid was neither consumed nor produced by the nitration. The same was true for nitration in acetic acid. In the zeroth-order nitrations the rate depended on the reciprocal of the square root of the concentration of nitrous acid =... 

TABLE 8.1 The acidity dependence and Arrhenius parameters for the nitration of some cations in... 

TABLE 8.2 The acidity dependence of rates of nitration of some free bases in sulphuric acid... 

The case of i-methyl-4-quinolone is puzzling. The large proportion of the 3-nitro isomer formed in the nitration (table 10.3 cf. 4-hydroxyquinoline) might be a result of nitration via the free base but this is not substantiated by the acidity dependence of the rate of nitration or by the Arrhenius parameters. From r-methyl-4-quinolone the total yield of nitro-compounds was not high (table ro.3). 

Among the W-oxides of this series of compounds isoquinoline 2-oxide shows the simplest behaviour on nitration. The acidity dependence of the rate of nitration (table 8.1), and comparison with the 2-methoxyiso-quinolinium cation (v, R = Me) (table 10.3) show the oxide to be nitrated as its conjugate acid (v, i = H) in 76-83 % sulphuric acid. The... 

The same orientation is found in the nitration of 2-methylbenzimi-dazole, whilst 5-nitro- and 2-methyl-5-nitrobenzimidazole are further nitrated at C(g). The acidity dependence of the rate of nitration of... 

The kinetics are the same in sulphuric and phosphoric acid media as in perchloric acid. The main kinetic features are reproduced in a nitric acid medium although a term, k[Ag(II)]V[Ag(I)], is also involved. Both terms include a strong inverse acidity dependence however, the role of the nitrate ions obscures this. 

Recently, Gunther et al. [41] proposed that nitric oxide may directly react with enzymes without intermediate formation of peroxynitrite. It is known that the oxidation of arachido-nic acid by prostaglandin H oxidase is mediated by the formation of enzyme tyrosyl radical (see Chapter 26). Correspondingly, it has been suggested that NO is able to react with this radical to form the tyrosine iminoxyl radical and then nitrotyrosine. Therefore, the NO-dependent nitration of protein tyrosine residue may occur without the formation of peroxynitrite or other nitrogen oxides. 

The nitration of phenols can result in anomalous and large differences in product isomer ratios, showing a high dependence on both nitrating agent and reaction medium. Here the situation is complicated by the intervention of an alternative nitration mechanism - that of nitrous acid catalyzed nitration, which proceeds via in situ nitrosation-oxidation (see Section 4.4). 

In other compounds, reaction can occur in both rings. Under such circumstances the orientation can depend on the conditions frequently reaction in the benzene ring involves the cationic species, whereas that in the pyridine ring involves the free base. Thus, the temperature dependent nitration of quinoline 1-oxide (578) reflects the decrease in intrinsic acidity as the temperature rises, which in turn increases the available amount of free base species. 

When a competition with potential nitration of a phenyl ring exists, as in compound 39, the site of nitration is dependent upon the reaction conditions.25 In the presence of sulfuric acid, initial nitration takes place on the... 

According to R. Ihle, the formation of ammonia at the cathode in the electrolysis of nitric acid depends on the current density and the cone, of the acid. Thus, for acids of 14-67, 28-73, 43-34, and 85-37 per cent. HN03, current densities of 0-00159, 0-01122, and 0-0564, and 8-6 amps, per sq. cm. were respectively required before any trace of ammonia was obtained. With increased current, the quantity of ammonia formed was also increased. J. F. Daniell noted the formation of ammonia at the cathode during the electrolysis of an aq. soln. of potassium nitrate. G. E. Cassel obtained ammonia by the electrolysis of soln. of nitrates. 

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