Substrates involved in prototropic equilibria

The comparison of the observed values of k and EA with those calculated from (32) and (33) therefore provides evidence on whether or not the reaction occurs on encounter. At lower acidities, allowance has to be made for the extent of conversion of nitric acid to nitronium salts. 

Several pieces of evidence can be used to determine whether the nitration of a given substrate occurs through the conjugate acid (SH+) or through the small amount of neutral substrate in equilibrium with it these approaches include the determination of the rate profile and comparisons with the reactivities of model compounds (Ridd, 1971b). From such evidence, it appears that a number of nitration reactions involve the small amount of neutral substrate in equilibrium with the conjugate acid the substrates include 2,6-dichloropyridine, pyridine-1-oxide, l-methylpyrazole-2-oxide, 3-methyl-2-pyridone, acetophenone and p-nitroaniline. These reactions have been reviewed recently (Hoggett et al., 1971 see especially Chap. 8). However, for the majority of these substrates, the 

This conclusion is reached also by considering directly the dependence of the rate of proton transfer on the acidity function for the protonation of the amine. The unit slope indicates that these reactions occur through the stages shown in (34),15 where S.H30+ and SH+.H20 represent encounter pairs in the correct 

13 The nitration of l,4,5-trimethylimidazole-3-oxide and l-methylpyrazole-2-oxide may also occur at about the encounter rate (Ferguson et al., 1977). 

14 In the original paper (Hartshorn and Ridd, 1968), the calculated activation energy was given as ca. 54 kJ mol-1 but this was based on the earlier work on the temperature dependence of Ht (Gel bshtein et al., 1956). The revised values for this temperature dependence (Johnson et al., 1969) give the value in the text. 

---