Nitrate, 221 table

What happens if solid lead (II) nitrate, Pb(N03)2, is placed in water According to the solubility table, nitrate compounds are soluble with no exceptions, so lead (II) nitrate would separate into its ions Pb and NOs. ... 

Nitro derivatives. No general experimental details for the preparation of nitro derivatives can be given, as the ease of nitration and the product formed frequently depend upon the exact experimental conditions. Moreover, some organic compounds react violently so that nitrations should always be conducted on a small scale. The derivatives already described are usually more satisfactory for this reason the nitro derivatives have been omitted from Table IV,9. 

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. 

TABLE 2.1 Relative rates of nitration in aqueous nitric acid ... 

For the last two compounds, first-order rates were observed towards the end of the reactions, enabling the reactivities of these compounds relative to that of water to be estimated (table 2.1). The nitration of 2-mesitylethanesulphonate anion was independent of the concentration of the aromatic over 80 % of its course, and because the final part of the reaction did not obey a truly first-order law its reactivity could not be estimated. 

TABLE 2.2 Zeroth-order rates of nitration and of O-exchange in aqueous nitric acid at o°C... 

A simple kinetic order for the nitration of aromatic compounds was first established by Martinsen for nitration in sulphuric acid (Martin-sen also first observed the occurrence of a maximum in the rate of nitration, occurrii for nitration in sulphuric acid of 89-90 % concentration). The rate of nitration of nitrobenzene was found to obey a second-order rate law, first order in the concentration of the aromatic and of nitric acid. The same law certainly holds (and in many cases was explicitly demonstrated) for the compounds listed in table 2.3. 

Although the proportion of nitric acid present as nitronium ions does not change between 90% and 100% sulphuric acid, the rate constants for nitration of most compounds decrease over this rai e. Fig. 2.1 illustrates the variation with acidity of the second-order rate constants of the nitration of a series of compounds of widely differing reactivities. Table 2.4 lists the results for nitration in 95% and 100% acid of a selection of less completely investigated compounds. 

TABLE 2.3 Nitration in concentrated sulphuric acid compounds which have been studied kinetically... 

Many aromatic compounds are sufficiently basic to be appreciably protonated in concentrated sulphuric acid. If nitration occurs substantially through the free base, then the reactivity of the conjugate acid will be negligible. Therefore, increasii the acidity of the medium will, by depleting the concentration of the free base, reduce the rateof reaction. This probably accounts for the particularly marked fall in rate which occurs in the nitration of anthraquinone, benzoic acid, benzenesulphonic acid, and some nitroanilines (see table 2.4). 

Fig. 2.1. Rate profiles for nitration in 80-100% sulphuric acid. For references see table 3.3.

TABLE 2.4 Second-order rate constants for nitration at 25 °C in 95 and 100 % sulphuric acid... 

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. 

TABLE 2.5 Some compounds for which the dependences of rate of nitration upon acidity, in aqueous sulphuric acid at 25 °C, have been determined... 

The phenomenon was established firmly by determining the rates of reaction in 68-3 % sulphuric acid and 61-05 % perchloric acid of a series of compounds which, from their behaviour in other reactions, and from predictions made using the additivity principle ( 9.2), might be expected to be very reactive in nitration. The second-order rate coefficients for nitration of these compounds, their rates relative to that of benzene and, where possible, an estimate of their expected relative rates are listed in table 2.6. 

TABLE 2.6 Second-order rate coefficients and relative rates for nitration at 2 -0 °C in 68- % sulphuric acid and 6i-o % perchloric acid ° ... 

TABLE 3.1 Zeroth-order rate constants for nitrations in organic solvents... 

Data for zeroth-order nitration in these various solvents are given in table 3.1. Fig. 3.1 shows how zeroth-order rate constants depend on the concentration of nitric acid, and table 3.2 shows how the kinetic forms of nitration in organic solvents depend on the reactivities of the compounds being nitrated. 

TABLE 3.2 Comparison of organic solvents as media for nitration... 

The data for nitration in carbon tetrachloride were obtained with much lower concentrations of nitric acid than those tabulated (see table 3.1). 

Accepting, for the moment without further evidence, that the nitro-nium ion formed by heterolysis of nitric acid is the active reagent in the solutions imder discussion, it remains to consider briefly why nitration in such solutions depends on the concentrations of nitric acid to such high powers (fig. 3.1), and why different solvents behave so differently (table 3.2). 

The zeroth-order rates of nitration depend on a process, the heterolysis of nitric acid, which, whatever its details, must generate ions from neutral molecules. Such a process will be accelerated by an increase in the polarity of the medium such as would be produced by an increase in the concentration of nitric acid. In the case of nitration in carbon tetrachloride, where the concentration of nitric acid used was very much smaller than in the other solvents (table 3.1), the zeroth-order rate of nitration depended on the concentrationof nitric acid approximately to the fifth power. It is argued therefore that five molecules of nitric acid are associated with a pre-equilibrium step or are present in the transition state. Since nitric acid is evidently not much associated in carbon tetrachloride a scheme for nitronium ion formation might be as follows ... 

Nitration in organic solvents is strongly catalysed by small concentrations of strong acids typically a concentration of io mol 1 of sulphuric acid doubles the rate of reaction. Reaction under zeroth-order conditions is accelerated without disturbing the kinetic form, even under the influence of very strong catalysis. The effect of sulphuric acid on the nitration of benzene in nitromethane is tabulated in table 3.3. The catalysis is linear in the concentration of sulphuric acid. 

TABLE 3.4 The effects of potassium nitrate on rates of nitration in nitromethane... 

The addition of water depresses zeroth-order rates of nitration, although the effect is very weak compared with that of nitrate ions concentrations of 6x io mol 1 of water, and 4X io mol 1 of potassium nitrate halve the rates of reaction under similar conditions. In moderate concentrations water anticatalyses nitration under zeroth-order conditions without changing the kinetic form. This effect is shown below (table 3.5) for the nitration of toluene in nitromethane. More strikingly, the addition of larger proportions of water modifies the kinetic... 

TABLE 3.5. The ejfect of added water on the zeroth-order rates of nitration of toluene (faromatic = o-og mol l ) in a solution at —10 °C of nitric acid - yo mol l ) in nitromethane... 

For nitrations in sulphuric and perchloric acids an increase in the reactivity of the aromatic compound being nitrated beyond the level of about 38 times the reactivity of benzene cannot be detected. At this level, and with compounds which might be expected to surpass it, a roughly constant value of the second-order rate constant is found (table 2.6) because aromatic molecules and nitronium ions are reacting upon encounter. The encounter rate is measurable, and recognisable, because the concentration of the effective electrophile is so small. 

A similar circumstance is detectable for nitrations in organic solvents, and has been established for sulpholan, nitromethane, 7-5 % aqueous sulpholan, and 15 % aqueous nitromethane. Nitrations in the two organic solvents are, in some instances, zeroth order in the concentration of the aromatic compound (table 3.2). In these circumstances comparisons with benzene can only be made by the competitive method. In the aqueous organic solvents the reactions are first order in the concentration of the aromatic ( 3.2.3) and comparisons could be made either competitively or by directly measuring the second-order rate constants. Data are given in table 3.6, and compared there with data for nitration in perchloric and sulphuric acids (see table 2.6). Nitration at the encounter rate has been demonstrated in carbon tetrachloride, but less fully explored. ... 

TABLE 4.1 Nitration of aromatic compounds relative rates at 25 °C... 

TABLE 4.2 Nitration of aromatic compounds isomer proportions and partial rate factors ... 

TABLE 4.3 Product ratios arising from nitration under conditions where mixing is slow... 

The data of table 4.1, column [a), do not suggest that nitration of the alkylbenzenes with nitronium tetrafluoroborate in sulpholan occurs upon encoimter mesitylene might be so reacting ( 3.3) but comparison with a more inherently reactive substance would be necessary before this possibility could be considered. 

It has already been noted that, as well as alkylbenzenes, a wide range of other aromatic compounds has been nitrated with nitronium salts. In particular the case of nitrobenzene has been examined kinetically. Results are collected in table 4.4. The reaction was kinetically of the first order in the concentration of the aromatic and of the nitronium salt. There is agreement between the results for those cases in which the solvent induces the ionization of nitric acid to nitronium ion, and the corresponding results for solutions of preformed nitronium salts in the same solvent. 

Olah and his co-workers compared the behaviour of nitronium salts in competitive nitrations with the behaviour of other nitrating systems. The results are given in table 4.1, columns (a)-(j), and also in table 4.2. The results obtained from competitive nitrations using solutions of nitric acid in organic solvents (table 4.1, columns (6)-(e)) are in line with those obtained by earlier workers. The evidence that in nitromethane,... 

TABLE 4.4 The kinetics of nitration of nitrobenzene in various medial... 

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. 

TABLE 4.5 Competitive nitrations of toluene and benzene with 1 -nitropyridinium tetrafluoroborates in acetonitrile at 25 °... 

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