Rate and Regioselectivity in Electrophilic Aromatic Substitution

So far we ve been concerned only with electrophilic substitution of benzene. Two important questions arise when we turn to substitution on rings that already bear at least one substituent  

What is the effect of a substituent on the rate of electrophilic aromatic substitution  

To illustrate substituent effects on rate, consider the nitration of benzene, toluene, and (trifluoromethyl)benzene. 

Just as there is a marked difference in how methyl and trifluoromethyl substituents affect the rate of electrophilic aromatic substitution, so too is there a marked difference in how they affect its regioselectivity. 

Toluene undergoes nitration some 20-25 times faster than benzene. Because toluene is more reactive than benzene, we say that a methyl group activates the ring toward electrophilic aromatic substitution. (Trifluoromethyl)benzene, on the other hand, undergoes nitration about 40,000 times more slowly than benzene. We say that a triflu-oromethyl group deactivates the ring toward electrophilic aromatic substitution. 

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Would you expect the substituent —N(CH3)3 to more closel resemble —NfCHj), or —NO, in its effect on rate and regioselectivity in electrophilic aromatic substitution Why ... 

How substituents control rate and regioselectivity in electrophilic aromatic substitution results from their effect on carbocation stability. An electron-releasing substituent stabilizes the cyclohexadienyl cation intermediates corresponding to ortho and para attack more than meta. 

Representative Electrophilic Aromatic Substitution Reactions of Benzene 457 Mechanistic Principles of Electrophilic Aromatic Substitution 458 Nitration of Benzene 459 Sulfonation of Benzene 461 Halogenation of Benzene 462 Biosynthetic Halogenation 464 Friedel-Crafts Alkylation of Benzene Friedel-Crafts Acylation of Benzene Synthesis of Alkylbenzenes by Acylation-Reduction 469 Rate and Regioselectivity in Electrophilic Aromatic Substitution 470 Rate and Regioselectivity in the Nitration ofToluene 472... 

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