Toluene-benzene reactivity ratios in Friedel-Crafts alkylation reactions

Steric effects also play a major role in determining the o p ratio in Friedel-Crafts alkylations. The amount of or /io-substitution of toluene decreases as the size of the entering alkyl group increases along the series methyl, ethyl, 2-propyl.No ar /ia-substitution product is found when the entering group is -butyl.  

A good deal of experimental care is often required to ensure that the product mixture at the end of a Friedel-Crafts reaction represents the composition governed by kinetic control The strong Lewis acid catalysts, in particular, can catalyze the isomerization of alkylbenzenes, and if isomerism takes place, the product composition is not informative about position selectivity. Isomerization of the reaction product usually favors formation of the me a-isomer in the case of dialkylbenzenes, since this isomer is thermodynamically the most stable. 

Alcohols and alkenes can also serve as sources of electrophiles in Friedel-Crafts reactions in the presence of strong acids  

The generation of carbonium ions from these sources is well documented, and the ions can be observed by NMR when they are generated under appropriate conditions. There has, however, been little detailed mechanistic study of reactions involving alkylation of aromatics using alcohols and alkenes as the carbonium ion sources. 

Friedel-Crafts acylation usually involves the reaction of an acyl halide, a Lewis acid catalyst, and the aromatic substrate. Two possible electrophiles can be envisaged. A discrete positively charged acylium (or oxocarbonium) ion can be formed and act as the electrophile, or the active electrophile could be a complex formed by the acid halide and Lewis acid catalyst. 

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Table 9.10. Toluene-Benzene Reactivity Ratios in Friedel-Crafts Alkylation Reactions... 

Toluene-benzene reactivity ratios in Friedel-Crafts alkylation reactions... 

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