Rate of nitration

Sulphuric acid catalysed nitration in concentrated nitric acid, but the effect was much weaker than that observed in nitration in organic solvents ( 3.2.3). The concentration of sulphuric acid required to double the rate of nitration of i-nitroanthraquinone was about 0-23 mol 1, whereas typically, a concentration of io mol 1 will effect the same change in nitration in mixtures of nitric acid and organic solvents. The acceleration in the rate was not linear in the concentration of catalyst, for the sensitivity to catalysis was small with low concentrations of sulphuric acid, but increased with the progressive addition of more catalyst and eventually approached a linear acceleration. 

Nitration in aqueous solutions of nitric acid Added water retards nitration in concentrated nitric acid without disturbing the kinetic order of the reaction. The rate of nitration of nitrobenzene was depressed sixfold by the addition of 5 % of water, (c. 3 2 mol 1 ), but because of the complexity of the equilibria involving water, which exist in these media, no simple relationship could be found between the concentration of water and its effect on the rate. 

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 ... 

Nitration at a rate independent of the concentration of the compound being nitrated had previously been observed in reactions in organic solvents ( 3.2.1). Such kinetics would be observed if the bulk reactivity of the aromatic towards the nitrating species exceeded that of water, and the measured rate would then be the rate of production of the nitrating species. The identification of the slow reaction with the formation of the nitronium ion followed from the fact that the initial rate under zeroth-order conditions was the same, to within experimental error, as the rate of 0-exchange in a similar solution. It was inferred that the exchange of oxygen occurred via heterolysis to the nitronium ion, and that it was the rate of this heterolysis which limited the rates of nitration of reactive aromatic compounds. 

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

The rates of nitration of mesitylene-a-sulphonate anion (iii) and iso-durene-a -sulphonate anion (iv) in mixtures of aqueous nitric and perchloric acid followed a zeroth-order rate law. Although the rate of exchange of oxygen could not be measured because of the presence of perchloric acid, these results again show that, under conditions most amenable to its existence and involvement, the nitric acidium ion is ineffective in nitration. 

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. 

The activity coefficients in sulphuric acid of a series of aromatic compounds have been determined. The values for three nitro-com-pounds are given in fig. 2.2. The nitration of these three compounds over a wide range of acidity was also studied, and it was shown that if the rates of nitration were corrected for the decrease of the activity coefficients, the corrected rate constant, varied only slightly... 

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. 

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. ... 

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. 

Rates of nitration in perchloric acid of mesitylene, luphthalene and phenol (57 I-6i-i %), and benzene (57 i-64 4%) have been deter-mined. The activated compounds are considered below ( 2.5). A plot of the logarithms of the second-order rate coefficients for the nitration of benzene against — ( f + log over the range of acidity... 

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... 

In both media a limit was reached beyond which the introduction of further activating substituents did not increase the rate of nitration this limit was identified as the rate of encounter of the nitronium ions and the aromatic molecules. 

The rates of nitration, under a variety of conditions (56-80% sulphuric acid, 57-62% perchloric acid), of mesitylene and benzene... 

The rates of nitration of benzene, toluene, and ethylbenzene in solutions of nitric acid c. 3-7 mol 1 ) in nitromethane were independent... 

In acetic acid the rates of nitration of chlorobenzene and bromo-benzene were fairly close to being first order in the concentration of aromatic, and nitration fully according to a first-order law was observed with O, m-, and/i-dichlorobenzene, ethyl benzoate and 1,2,4-trichloro-benzene. 

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 ... 

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... 

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 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. 

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). 

First-order nitrations. The kinetics of nitrations in solutions of acetyl nitrate in acetic anhydride were first investigated by Wibaut. He obtained evidence for a second-order rate law, but this was subsequently disproved. A more detailed study was made using benzene, toluene, chloro- and bromo-benzene. The rate of nitration of benzene was found to be of the first order in the concentration of aromatic and third order in the concentration of acetyl nitrate the latter conclusion disagrees with later work (see below). Nitration in solutions containing similar concentrations of acetyl nitrate in acetic acid was too slow to measure, but was accelerated slightly by the addition of more acetic anhydride. Similar solutions in carbon tetrachloride nitrated benzene too quickly, and the concentration of acetyl nitrate had to be reduced from 0-7 to o-i mol 1 to permit the observation of a rate similar to that which the more concentrated solution yields in acetic anhydride. 

The rates of nitration of benzene in solutions at 25 °C containing 0-4-2-0 mol 1 of acetyl nitrate in acetic anhydride have been deter-mined.2 The rates accord with the following kinetic law ... 

Under these first-order conditions the rates of nitration of a number of compounds with acetyl nitrate in acetic anhydride have been determined. The data show that the rates of nitration of compounds bearing activating substituents reach a limit by analogy with the similar phenomenon shown in nitration in aqueous sulphuric and perchloric acids ( 2.5) and in solutions of nitric acid in sulpholan and nitro-methane ( 3.3), this limit has been taken to be the rate of encounter of the nitrating entity with the aromatic molecule. 

Zeroth-order nitrations. The rates of nitration at 25 °C in solutions of acetyl nitrate (6xio —0-22 mol 1 ) in acetic anhydride of 0- and jw-xylene, and anisole and mesitylene were independent of the concentration and nature of the aromatic compound provided that... 

Fig. 5.1. Zeroth-order rates of nitration with acetyl nitrate compared with those for other systems, (a) HNOa/ sulpholan, (i) HNO3/CCI4, (c) AcONOj/AcjO/O % AcOH, (d) AcONOj/AcaO/O % AcOH/[mesitylene] = o-8 mol l-i,n (e) AcONOj/AcaO/ [AcOH] = 2-2moll-i,ii >(/)AcONOa/AcaO/[AcOH] = 1-96 mol l-i and (g) AcONOa/ AcaO/[AcOH] 3 9i mol l-i/[mesitylene] = o-8 mol 1-i H.

The results in fig. 5.1 show that zeroth-order rates of nitration in solutions of acetyl nitrate in acetic anhydride are much greater than the corresponding rates in solutions in inert organic solvents of nitric acid of the same stoichiometric concentration as that of acetyl nitrate. Thus, for corresponding concentrations of nitric acid and acetyl nitrate, nitration in acetic anhydride is e. 5 x 10 and 10 times faster than nitration in sulpholan and nitromethane respectively. This fact, and the fact that the fraction of free nitric acid in solutions of acetyl nitrate in acetic... 

Nitrations of the zeroth order are maintained with much greater difficulty in solutions of acetyl nitrate in acetic anhydride than in solutions of nitric acid in inert organic solvents, as has already been mentioned. Thus, in the former solutions, the rates of nitration of mesi-tylene deviated towards a dependence on the first power of its concentration when this was < c. o-05-o-i mol 1 , whereas in nitration with nitric acid in sulpholan, zeroth-order kinetics could be observed in solutions containing as little as 10 mol 1 of mesitylene ( 3.2.1). 

The effects of added species. The rate of nitration of benzene, according to a rate law kinetically of the first order in the concentration of aromatic, was reduced by sodium nitrate, a concentration of io mol 1 of the latter retarding nitration by a factor of about Lithium nitrate... 

The addition of sulphuric acid increased the rate of nitration of benzene, and under the influence of this additive the rate became proportional to the first powers of the concentrations of aromatic, acetyl nitrate and sulphuric acid. Sulphuric acid markedly catalysed the zeroth-order nitration and acetoxylation of o-xylene without affecting the kinetic form of the reaction. ... 

Despite the fact that solutions of acetyl nitrate prepared from purified nitric acid contained no detectable nitrous acid, the sensitivity of the rates of nitration of very reactive compounds to nitrous acid demonstrated in this work is so great that concentrations of nitrous acid below the detectable level could produce considerable catalytic effects. However, because the concentration of nitrous acid in these solutions is unknown the possibility cannot absolutely be excluded that the special mechanism is nitration by a relatively unreactive electrophile. Whatever the nature of the supervenient reaction, it is clear that there is at least a dichotomy in the mechanism of nitration for very reactive compounds, and that, unless the contributions of the separate mechanisms can be distinguished, quantitative comparisons of reactivity are meaningless. 

Fig. 5.2. Initial rates of nitration of mesitylene and phenol under zeroth-order conditions. Temperature 25 °C. [AcONOo] = r. 7 x io mol 1 . [HNO ] < 10" mol 1. ...

As regards sulphuric acid, there is here again an increase in polarity and an increase in rates of nitration when comparison is made with other solvents in the series. This gross fact would be difficult to reconcile with any mechanism, such as the one, which contains an essential forward step which would be retarded by increased polarity of the solvent. 

Generally the determination of the reactivity of a particular compound depends upon comparison of its rate of nitration with that of benzene at the same acidity and temperature. Because of the spread of rates this may not be practically possible and, in any case, is usually not necessary because of the parallelism existing among rate profiles (fig. 2.4). Reactivities in aqueous sulphuric acid are, in fact, very nearly independent of acidity, and stepwise comparison of data for a compound with those of benzene determined at different acidities is possible. 

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