Nitrate rate constants

In determining the values of Ka use is made of the pronounced shift of the UV-vis absorption spectrum of 2.4 upon coordination to the catalytically active ions as is illustrated in Figure 2.4 ". The occurrence of an isosbestic point can be regarded as an indication that there are only two species in solution that contribute to the absorption spectrum free and coordinated dienophile. The exact method of determination of the equilibrium constants is described extensively in reference 75 and is summarised in the experimental section. Since equilibrium constants and rate constants depend on the ionic strength, from this point onward, all measurements have been performed at constant ionic strength of 2.00 M usir potassium nitrate as background electrolyte . 

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. 

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

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... 

The value of the second-order rate constant for nitration of benzene-sulphonic acid in anhydrous sulphuric acid varies with the concentration of the aromatic substrate and with that of additives such as nitromethane and sulphuryl chloride. The effect seems to depend on the total concentration of non-electrolyte, moderate values of which (up to about 0-5 mol 1 ) depress the rate constant. More substantial concentrations of non-electrolytes can cause marked rate enhancements in this medium. Added hydrogen sulphate salts or bases such as pyridine... 

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. 

Like added nitrate, sulphuric acid is not involved in the condition which determines the order of the reaction, and therefore its only effect will be to increase the observed rate constants. The rate under zeroth-order conditions is given by the first of the two expressions below,... 

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. ... 

Relative rate meaning, here, simply the ratio of nitro-alkylbenzene to nitrobenzene, multiplied by the initial ratio of alkylbenzene to benzene. This is not precisely the same as the ratio of rate constants for nitration. ... 

The equilibrium constant K, the rate constants and and the dependences of all these quantities on temperature were determined. In the absence of added acetic acid, the conversion of nitric acid into acetyl nitrate is almost quantitative. Therefore, to obtain at equilibrium a concentration of free nitric acid sufficiently high for accurate analysis, media were studied which contained appreciable concentrations (c. 4 mol 1 ) of acetic acid. 

The dependence of the zeroth-order rate constants on the concentration of acetyl nitrate is shown in fig. 5.1 in the absence of added acetic acid the rate increases according to the third power of the concentration of acetyl nitrate, but when acetic acid is added the dependence becomes... 

Nitration may or may not involve the predominant form of the substrate. In the latter case, if the predominant form is the conjugate acid, the observed second-order rate constant can be corrected to give one (/iafb.) appropriate to the reacting free base. With a reaction of the form... 

For a base the stoichiometric second-order rate constant which should be observed, imder conditions where ionisation to the nitronium ion is virtually complete, namely > 90 % H2SO4, if nitration were limited to the free base and occurred at every encounter with a nitronium ion, would be ... 

If the observed second-order rate constant is greater than calc., reaction via the free base is precluded. If /taobs. is less than k czlc., reaction via the conjugate acid or the free base is possible. The first compound reported to be nitrated via its conjugate acid, and yet to have 2 calc. > obs. at the acidities concerned, was pyrazoleother examples are mentioned later ( 9.3 10.4.2). 

At lower acidities the observed rate constant for nitration of p-nitroaniline becomes greater than that for nitration of anilinium. ... 

The similarity of their rate profiles, and the similarity of their rate constants for nitration at a particular temperature and acidity show that 4-pyridone, i-methyl-4-pyridone, and 4-methoxypyridine are all nitrated as their cations down to about 85 % sulphuric acid. The same is true of 2-methoxy-3-methylpyridine. In contrast, 3- and 5-methyl-2-pyridone, i,5-dimethyl-2-pyridone and 3-nitro-4-pyridone all react... 

It should be noted that reported kinetic data on the nitration of biphenyl are hmited to one rate constant for reaction in 68-3 % sulphuric acid at 25 °C k = 0-92 1 mol s i relative rate, 15-8). Until they have been extended the above discussion must be regarded as provisional. 

Recently kinetic data have become available for the nitration in sulphuric acid of some of these hydroxy compounds (table 10.3). For 4-hydroxyquinoline and 4-methoxyquinoline the results verify the early conclusions regarding the nature of the substrate being nitrated in sulphuric acid. Plots of log Q against — (Lf + logioflHao) fo " these compounds and for i-methyl-4-quinolone have slopes of i-o, i-o and 0-97 at 25 C respectively, in accord with nitration via the majority species ( 8.2) which is in each case the corresponding cation of the type (iv). At a given acidity the similarity of the observed second-order rate constants for the nitrations of the quinolones and 4-methoxy-quinoline at 25 °C supports the view that similarly constructed cations are involved. Application of the encounter criterion eliminates the possibilities of a... 

Since the first-order rate constant for nitration is proportional to y, the equilibrium concentration of nitronium ion, the above equations show the way in which the rate constant will vary with x, the stoichiometric concentration of dinitrogen tetroxide, in the two media. An adequate fit between theory and experiment was thus obtained. A significant feature of this analysis is that the weak anticatalysis in pure nitric acid, and the substantially stronger anticatalysis in partly aqueous nitric acid, do not require separate interpretations, as have been given for the similar observations concerning nitration in organic solvents. 

The kinetics of the nitration of benzene, toluene and mesitylene in mixtures prepared from nitric acid and acetic anhydride have been studied by Hartshorn and Thompson. Under zeroth order conditions, the dependence of the rate of nitration of mesitylene on the stoichiometric concentrations of nitric acid, acetic acid and lithium nitrate were found to be as described in section 5.3.5. When the conditions were such that the rate depended upon the first power of the concentration of the aromatic substrate, the first order rate constant was found to vary with the stoichiometric concentration of nitric acid as shown on the graph below. An approximately third order dependence on this quantity was found with mesitylene and toluene, but with benzene, increasing the stoichiometric concentration of nitric acid caused a change to an approximately second order dependence. Relative reactivities, however, were found to be insensitive... 

Rates of nitration determined over a range of temperatures in two-phase dispersions have been used to calculate energies of activation from 59—75 kj/mol (14—18 kcal/mol). Such energies of activation must be considered as only apparent, since the tme kinetic rate constants, NO2 concentrations, and interfacial area all change as temperature is increased. 

Note that all the zero-order rate constants are essentially equivalent except those for the poly-hydric alcohols which are exactly half the value of the others. Ingold et al (Ref 49a) interpret this to mean that the rate of attack of nitronium is the same for both OH groups of the glycol molecule. Since there are two such groups the overall rate constant k0 is Vi that for monohydric alcohols. The explanation for the observed k0 for glycerol is more complex. In essence it consists of postulating that the two outside OH s are readily nitrated, ie, the 1-OH is nitrated at the same rate as the 3-OH, but the middle OH is nitrated much more slowly... 

As implied in the preceding discussion, deliberate introduction of water into the nitration medium should make step (8) competitive with step (16) even for a reactive XH, and the overall nitration rate changes from zero-order to first order. These phenomena have been observed (Refs 49 49a). Under nearly identical conditions (2.5M HN03, 1.09M H20, ca 0.02M XH in NM soln at 25°), the measured first-order rate constants are 2.5xl0 4, 2.0xl0 4, 1.2xlQ s sec 1 respectively for MeOH, toluene, and n-methyl-2,4,6-triiutroaniiine. Combined with some earlier data, the reactivities"... 

Table I. Trace gas rate constants and lifetimes for reaction with ozone, hydroxyl radical, and nitrate radical. Lifetimes are based upon [O3]=40ppb [HO ]=1.0x10 molecules cm (daytime) [NO3 ]=10ppt (nighttime).

Figure 13. Specific 2-CP (open symbols) and 2,4-DCP (solid symbols) hydrodechlorination rate constant K) as a function of the average Ni particle diameter ( nO for reaction over Ni catalysts prepared via impregnation with nitrate (0,0), deposition-precipitation (A,A) and impregnation with nickel ethanediamine ( , ) r= 423K reaction data refer to aqueous solutions. (Reprinted from Reference [147], 2003, with permission from Royal Society of Chemistry).

When the kinetic parameters are obtained by eliminating C, as has been done in the previous cases, k°2 is always higher than k , in accordance with earlier observations.55 It is interesting to note that the rate constant in a potassium nitrate medium is very fast (100 times higher) as compared to that in other media. The k°2 values are, in general, of the order of 10 2cm/s except in potassium nitrate, for which the rate constant is 1.37 cm/s. In general, k is also of the order of 10-2 cm/s but lesser in magnitude than k°. 

It is interesting to observe that in nitrate media both rate constants are 100 times higher than in the other two media. The high values of rate constants in NO3 and SO4- media are due to their preferential adsorption at the electrode/solution interface.7,75 77... 

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