CHAPTER TWELVE Reactions of Arenes: Electrophilic Aromatic Substitution call this reaction electrophilic aromatic substitution; it is one of the fundamental cesses of organic chemistry 12.1 REPRESENTATIVE ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS OF BENZENE The scope of electrophilic aromatic substitution is quite large; both the arene and the lectrophilic reagent are capable of wide variation. Indeed, it is this breadth of scope that makes electrophilic aromatic substitution so important. Electrophilic aromatic substitu tion is the method by which substituted derivatives of benzene are prepared. We can gain a feeling for these reactions by examining a few typical examples in which benzene is the substrate. These examples are listed in Table 12.1, and each will be discussed in more detail in Sections 12.3 through 12.7. First, however, let us look at the general mecha- nism of electrophilic aromatic substitution 12.2 MECHANISTIC PRINCIPLES OF ELECTROPHILIC AROMATIC SUBSTITUTION Recall from Chapter 6 the general mechanism for electrophilic addition to alkenes +:Y Alkene and electrophile Carbocation E-C-C++:Y →E-C-C-Y Carbocation Nucleophile Product of electrophilic first step is rate-determining. It is the sharing of the pair of T electrons of the alkene with the electrophile to form a carbocation. Following its formation, the carbocation 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 T system of benzene to form a carbocation: H +: Y Benzene and electrophile Carbocation 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). H Resonance forms of a cyclohexadienyl cation Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteWe call this reaction electrophilic aromatic substitution; it is one of the fundamental processes of organic chemistry. 12.1 REPRESENTATIVE ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS OF BENZENE The scope of electrophilic aromatic substitution is quite large; both the arene and the electrophilic reagent are capable of wide variation. Indeed, it is this breadth of scope that makes electrophilic aromatic substitution so important. Electrophilic aromatic substitu￾tion is the method by which substituted derivatives of benzene are prepared. We can gain a feeling for these reactions by examining a few typical examples in which benzene is the substrate. These examples are listed in Table 12.1, and each will be discussed in more detail in Sections 12.3 through 12.7. First, however, let us look at the general mecha￾nism of electrophilic aromatic substitution. 12.2 MECHANISTIC PRINCIPLES OF ELECTROPHILIC AROMATIC SUBSTITUTION Recall from Chapter 6 the general mechanism for electrophilic addition to alkenes: The first step is rate-determining. It is the sharing of the pair of electrons of the alkene with the electrophile to form a carbocation. Following its formation, the carbocation 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 system of benzene to form a carbocation: 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). H E H E H E Resonance forms of a cyclohexadienyl cation slow Y H E Y  Benzene and electrophile H E Carbocation slow Y  E C C Alkene and electrophile E C C Carbocation Y fast E C C Y Product of electrophilic addition E C C Carbocation Y Nucleophile 444 CHAPTER TWELVE Reactions of Arenes: Electrophilic Aromatic Substitution Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website