Partial rate factors for hydrogen exchange in some substituted aromatic compounds

Data on hydrogen exchange have been of particular interest for comparison with theoretical predictions of aromatic reactivity. Because the electrophile is well defined and small, calculation of the stability of various competing intermediates by 

MO methods is feasible. We will return to the comparison of exchange data with theoretical predictions in Section 9.5. 

The most general method for direct introduction of alkyl groups on an aromatic ring system is the Friedel-Crafts reaction. It involves generation of a carbonium ion intermediate or a related electrophilic carbon species. The most general method for generating these electrophiles involves reaction between an alkyl halide and a Lewis acid. The most common Lewis acid for preparative work is aluminum chloride. Alternative routes to the alkylating species include protonation (followed by dehydration) of alcohols and protonation of alkenes. 

There are relatively few kinetic data on the Friedel-Crafts reaction. Alkylation of benzene or toluene with methyl bromide or ethyl bromide with gallium bromide as catalyst is first-order in each reactant and in catalyst. With aluminum bromide as catalyst, the rate of reaction changes with time, apparently because of heterogeneity. The initial rate data fit the following kinetic expression  

Good kinetic results, in general, have been difficult to come by in the Friedel-Crafts reaction. With the common catalysts, the reactions are very fast and often complicated by other problems, including heterogeneity. For this reason, most studies of structure-reactivity trends have been done using competitive reactivity data, rather than direct rate measurements. 

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Table 10.8. Partial Rate Factors for Hydrogen Exchange in Some Substituted Aromatic Compounds...

Partial rate factors for hydrogen exchange in some substituted aromatic compounds... 

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