Methylbenzene relative basicity

An indication of the nature of the transition state in aromatic substitution is provided by the existence of some extrathermodynamic relationships among rate and acid-base equilibrium constants. Thus a simple linear relationship exists between the logarithms of the relative rates of halogenation of the methylbenzenes and the logarithms of the relative basicities of the hydrocarbons toward HF-BFS (or-complex equilibrium).288 270 A similar relationship with the basicities toward HC1 ( -complex equilibrium) is much less precise. The jr-complex is therefore a poorer model for the substitution transition state than is the [Pg.150]

Comparison of relative basicity with relative reactivity for the halogenation of some methylbenzenes (Brown and Brady, 1962)... 

The additivity rule governing the proton affinity of various multiply substituted aromatics may be useful in rationalizing many closely related properties of this family of compounds. For instance. Brown and Brady [47] determined the relative basicities of a number of aromatic compounds including methylbenzenes. Unfortunately, experimental PA values related to methylbenzenes are scarce. Hence, proton affinities obtained by the additivity rule may prove very useful here in predicting the missing experimental data... 

By describing the electronic effect of the methyl groups in benzenium ions as a combination of the inductive and the hyperconjugative effects the relative basicity of methylbenzenes is interpreted rather successfully by quantum chemical calculations... 

Nevertheless, Condon (1948) had already renewed interest in the possible application of a simple relationship for substitution reactions by a successful linear free-energy calculation of the rates of halogenation of the methylbenzenes. The parallelism in reactivity between the relative rates of halogenation and the measurements of the basicity of... 

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