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1、443 CHAPTER 12 REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION I n the preceding chapter the special stability of benzene was described, along with reac- tions in which an aromatic ring was present as a substituent. In the present chapter we move from considering the aromatic ring as a subs
2、tituent to studying it as a functional group. What kind of reactions are available to benzene and its derivatives? What sort of reagents react with arenes, and what products are formed in those reactions? Characteristically, the reagents that react with the aromatic ring of benzene and its derivativ
3、es are electrophiles. We already have some experience with electrophilic reagents, particularly with respect to how they react with alkenes. Electrophilic reagents add to alkenes. A different reaction takes place when electrophiles react with arenes. Substitution is observed instead of addition. If
4、we represent an arene by the general formula ArH, where Ar stands for an aryl group, the electrophilic portion of the reagent replaces one of the hydrogens on the ring: ArH Arene ?EY ? Electrophilic reagent ?ArEHY Product of electrophilic aromatic substitution CC Alkene ?EY ? Electrophilic reagent E
5、CCY Product of electrophilic addition BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website We call this reaction electrophilic aromatic substitution; it is one of the fundamental processes of organic chemistry. 12.1REPRESENTATIVE ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS OF BENZENE The
6、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 subst
7、ituted 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 a
8、t the general mecha- nism of electrophilic aromatic substitution. 12.2MECHANISTIC PRINCIPLES OF ELECTROPHILIC AROMATIC SUBSTITUTION Recall from Chapter 6 the general mechanism for electrophilic addition to alkenes: The fi rst step is rate-determining. It is the sharing of the pair of ? electrons of
9、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 fi rst step in the reaction of electrophilic reagents with benzene is similar. An electrophile accepts an electron pair from the ? sy
10、stem 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 EY ? Benzene and electrophile H ? E
11、Carbocation slow Y ? ECC Alkene and electrophile ECC? Carbocation ?Y? ? fast ECCY Product of electrophilic addition ECC? Carbocation Y? Nucleophile 444CHAPTER TWELVEReactions of Arenes: Electrophilic Aromatic Substitution BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website PROBLEM 12.1In t
12、he simplest molecular orbital treatment of conjugated sys- tems, it is assumed that the ? system does not interact with the framework of ? bonds. When this MO method was used to calculate the charge distribution in cyclohexadienyl cation, it gave the results indicated. How does the charge at each ca
13、rbon compare with that deduced by examining the most stable resonance struc- tures for cyclohexadienyl cation? 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 H HH H H H 0 ?0.3
14、3 0 H 0 ?0.33 ?0.33 12.2Mechanistic Principles of Electrophilic Aromatic Substitution445 TABLE 12.1Representative Electrophilic Aromatic Substitution Reactions of Benzene Reaction and comments 1. Nitration Warming benzene with a mix- ture of nitric acid and sulfuric acid gives nitrobenzene. A nitro
15、group (NO2) replaces one of the ring hydrogens. 3. Halogenation Bromine reacts with ben- zene in the presence of iron(III) bromide as a catalyst to give bromobenzene. Chlorine reacts similarly in the presence of iron(III) chloride to give chlorobenzene. 4. Friedel-Crafts alkylation Alkyl halides rea
16、ct with benzene in the presence of alu- minum chloride to yield alkylbenzenes. 5. Friedel-Crafts acylation An analogous reaction occurs when acyl halides react with benzene in the presence of alumi- num chloride. The products are acylben- zenes. 2. Sulfonation Treatment of benzene with hot concentrated sulfuric acid gives ben- zenesulfonic acid. A sulfonic acid group (SO2OH) replaces one of the ring hydro- gens. Equation H Benzene ? Sulfuric acid HOSO2OH Benzenesulfonic acid (100%
