Mechanistic Principles of Electrophilic Aromatic Substitution

Recall the general mechanism for electrophilic addition to alkenes  

The first step is rate-determining. In it a carbocation forms when the pair of tt electrons of the alfcene is used to form a bond with the electrophile. The carbocation then undergoes rapid capture by some Lewis base present in the medium. 

The first step in the reaction of electrophilic reagents with benzene is similar. An electrophile accepts an electron pair from the -ir system of benzene to form a carbocation. 

This carbocation, called a cyclohexadienyl cation, arenium ion, or a-complex, then undergoes deprotonation to restore the aromaticity of the ring. If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon, the product would have been a nonaromatic cyclohexadiene derivative. Substitution occurs preferentially because a substantial driving force is present that favors rearomatization. 

Eigure 12.1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution. Eor electrophilic aromatic substitution reactions to 

The carbocation formed in this step is an arenium ion or cyclohexadienyl cation, also known as a a-complex. It is an allylic carbocation and is stabilized by the electron delocalization represented by resonance among the contributing structures  

Most of the resonance stabilization of benzene is lost when it is converted to the cyclohexadienyl cation intermediate. In spite of being allylic, a cyclohexadienyl cation is not aromatic and possesses only a fraction of the resonance stabilization of benzene. 

This particular carbocation is a resonance-stabilized one of the allylic type. It is a cyclo-hexadienyl cation (often referred to as an arenium ion). 

Representative Electrophilic Aromatic Substitution Reactions of Benzene 

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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... 

In many ways, the principles of substitution, elimination, and addition converge in aromatic systems in what is genetically called aromatic substitution.256 Addition to electrophilic centers, substitution of carbocations, nucleophilic displacement, and elimination of leaving groups are all mechanistic features of various aromatic substitution reactions. 

In their study of aromatic substitution reactions, Olah and his coworkers have based much of their mechanistic analysis on the reactivity-selectivity principle. It was observed that electrophilic substitution reactions conducted with powerful nitrating agents... 

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