Substrates not involved in prototropic equilibria

The clearest evidence for microscopic diffusion control in nitration comes from the kinetic studies of Coombes et al. (1968), with low concentrations of nitric acid in 68.3% sulphuric acid as solvent. In this medium, the concentration of nitronium ions is proportional to the concentration of molecular nitric acid as required by (24) and, since the concentration of nitronium ions is very small, the concentration of molecular nitric acid is effectively equal to the stoicheiometric concentration of nitric acid. At a given acidity, the reactions have the kinetic form (25). Nitric acid is written out in full in this equation to show that the rate coefficient is calculated with reference to the stoicheiometric concentration of the acid. This convention assists the comparison of reaction rates over a wide range of acidity. 

Relative rate coefficients from (25) are listed in Table 6, taking that for benzene as unity. The most interesting feature of these results is the apparent limiting reaction rate at about forty times the reactivity of benzene. This limit is quite inconsistent with the predictions of the additivity principle. Thus, from the additivity principle and the partial rate factors for the nitration of toluene, the reactivities of m-xylene and mesitylene would be expected to exceed that of benzene by factors of 400 and 16 000 respectively (Coombes et al., 1968). 

One test of this approach is to calculate the rate coefficient under the limiting conditions in terms of the concentrations of the aromatic substrate and the nitronium ion as in (26). The concentration of nitronium ions in the reaction medium cannot be measured directly, but an indirect estimation is possible from the fact that nitric acid is entirely converted to nitronium bisulphate in 90% sulphuric acid and the assumption that the change in reaction rate with the concentration of sulphuric acid comes essentially from the change in the position of the nitric acid-nitronium ion equilibrium. Then, from the rate coefficient for the nitration of mesitylene in 68.3% sulphuric acid (2.1 mol-1 s 1 dm3) (Coombes et al., 1968), the rate coefficient for the nitration of the phenyltrimethylammonium ion in 90.1% sulphuric acid (3.5 x 10 2 mol-1 s dm3) (Gillespie and Norton, 1953), and the relative reactivity of mesitylene and the phenyltrimethylammonium ion (a factor of 1.0 x 109) (Table 6 see also Gastaminza et al., 1969) it is possible to calculate that the concentration of nitronium ions in 68.3% sulphuric acid is less than the stoichiometric concentration of nitric acid by a factor of ca. 6 x 10 8. The value of the rate coefficient for mesitylene in (26) then becomes 3.5 X 107 mol-1 s dm3. This is 

Relative rates of nitration at 25 ° by nitric acid in various media 

Substrate Sulphuric acid Perchloric acid 61.05% Sulpholan 7.5% aq. Nitromethane 15% aq/ Acetic acid 8-19% aq. Trifluoroacetic acid I mol dmH,0 

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