Arenium ions thermodynamic control

The orientation and reactivity effects of each group are explained on the basis of resonance and field effects on the stability of the intermediate arenium ion. To understand why we can use this approach, it is necessary to know that in these reactions the product is usually kinetically and not thermodynamically controlled (see p. 214). Some of the reactions are irreversible and the others are usually stopped well before equilibrium is reached. Therefore, which of the three possible intermediates is formed is dependent not on the thermodynamic stability of the products but on the activation energy necessary to form each of the three... 

On the whole the effect of substituents on the relative stability of isomeric arenium ions (for details see Sect. IV, 1) is described in the same terms as those used to explain the influence of substituents on the orientation and relative rates of electrophilic aromatic substitution. However, the isomeric composition of electrophilic substitution products is often controlled by kinetic factors while the equilibrium composition of isomeric arenium ions formed in aromatic compound protonation is determined by thermodynamic equilibrium. Therefore, no quantitative agreement may be observed between the relative hydrogen substitution rates at different positions of this compound and the ratio of equilibrium concentrations of the respective arenium ions formed in protonating the same compound even under identical conditions (cf. Sect. IV, 7). 

In recent years, however, large evidence indicates that the orientation effects in the reactions of aromatic compounds with electrophiles are not only determined by the thermodynamic characteristics of c-complexes. This is evidenced, in particular, by kinetic control in the protonation of aromatic compounds, i.e. cases of the primary formation of less stable arenium ions which are then rearranged into thermodynamically more favourable ions (sec the preceding sec-... 

Our ultimate goal, the control of the reactivity of silylium ions is very ambitious, and we are still far away from a chemistry of cationic silicon in solution, in particular from a controlled chemistry. The benefits of our model system 1, however, are already obvious. The two silicon substituents confer to the cyclic cations 1 a high thermodynamic stability. An example is shown for the arenium ions 9 and the vinyl cations 11 and 12 and can be easily extended to every substituent X acting as a Tt-donor, such as alkenes, nitriles, allenes or azides. In addition the very weakly nucleophilic conditions applied in our reactions allow the generation of only weakly stabilized cations with very uncommon structural features, as for example the hydrogen bridged ion 16. 

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