Inverse temperature-reactivity relationship

A modification of the Hammett approach, suggested by Brown, called the selectivity relationship is based on the principle that reactivity of a species varies inversely with selectivity. Table 11.3 shows how electrophiles can be arranged in order of selectivity as measured by two indexes (i) their selectivity in attacking toluene rather than benzene, and (2) their selectivity between the meta and para positions in toluene. As the table shows, an electrophile more selective in one respect is also more selective in the other. In many cases, electrophiles known to be more stable (hence less reactive) than others show a higher selectivity, as would be expected. For example, the tert-butyl cation is more stable and more selective than the isopropyl (p. 236), and Br2 is more selective than Br+. However, deviations from the relationship are known. Selectivity depends not only on the nature of the electrophile but also on the temperature. As expected, it normally decreases with increasing temperature. 

Inverse gas chromatography parameters can also be applied in the field of cafaly-sis. In fhis way, as example, parent NaX and CaA zeolites, as well as transition metal (Co +, Mn +, Fe " )-exchanged zeolites, were evaluated for the catalytic oxidation of n-hexane. It was observed [51, 52], that although there was linear correlation between the acidity and the adsorption enthalpy of the n-hexane, there was no relationship between the acidity and the activity for n-hexane oxidation. However, if a reactivity parameter (such as Tso, temperature at which 50 % of conversion is attained) is plotted versus the adsorption heat, a so-called Volcano plot is obtained (Fig. 16.12), an optimum value of (-AH ) being observed, higher and lower values yielding to worst catalytic performance. 

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