CHAPTER TWELVE Reactions of Arenes: Electrophilic Aromatic Substitution Complexation of bromine with iron(in) bromide makes bromine more elec- trophilic, and it attacks benzene to give a cyclohexadienyl intermediate as shown in step I of the mechanism depicted in Figure 12. 4. In step 2, as in nitration and sulfonation, loss of a proton from the cyclohexadienyl cation is rapid and gives the product of elec- trophilic aromatic substitution Only small quantities of iron(I bromide are required. It is a catalyst for the bromination and, as Figure 12. 4 indicates, is regenerated in the course of the reaction We'll see later in this chapter that some aromatic substrates are much more reactive than benzene and react rapidly with bromine even in the absence of a catalyst. Chlorination is carried out in a manner similar to bromination and provides a read route to chlorobenzene and related aryl chlorides. Fluorination and iodination of benzene and other arenes are rarely performed. Fluorine is so reactive that its reaction with benzene is difficult to control. lodination is very slow and has an unfavorable equilibrium constant. Syntheses of aryl fluorides and aryl iodides are normally carried out by way of functional group transformations of arylamines; these reactions will be described in Chapter 22 12.6 FRIEDEL-CRAFTS ALKYLATION OF BENZENE Alkyl halides react with benzene in the presence of aluminum chloride to yield alkyl benzenes H C(CH3)3 +(Ch3)cCL Benzene tert-Butyl chloride tert-Butylbenzene Hydrogen Step 1: The bromine-iron(i) bromide complex is the active electrophile that attacks benzene. Two of the T electrons of benzene are used to form a bond to bromine and give a cyclohexadienyl cation intermediate B fe br 3 H Br-FeBr3 Benzene and bromine-iron(li) cation intermediate Step 2: Loss of a proton from the cyclohexadienyl cation yields bromobenzene Br-FeBr3 H—Br Tetrabromoferrate Bromobenzer Hydrogen cation intermediate FIGURE 12. 4 The mechanism of bromination of benzene Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteComplexation of bromine with iron(III) bromide makes bromine more elec￾trophilic, and it attacks benzene to give a cyclohexadienyl intermediate as shown in step 1 of the mechanism depicted in Figure 12.4. In step 2, as in nitration and sulfonation, loss of a proton from the cyclohexadienyl cation is rapid and gives the product of elec￾trophilic aromatic substitution. Only small quantities of iron(III) bromide are required. It is a catalyst for the bromination and, as Figure 12.4 indicates, is regenerated in the course of the reaction. We’ll see later in this chapter that some aromatic substrates are much more reactive than benzene and react rapidly with bromine even in the absence of a catalyst. Chlorination is carried out in a manner similar to bromination and provides a ready route to chlorobenzene and related aryl chlorides. Fluorination and iodination of benzene and other arenes are rarely performed. Fluorine is so reactive that its reaction with benzene is difficult to control. Iodination is very slow and has an unfavorable equilibrium constant. Syntheses of aryl fluorides and aryl iodides are normally carried out by way of functional group transformations of arylamines; these reactions will be described in Chapter 22. 12.6 FRIEDEL–CRAFTS ALKYLATION OF BENZENE Alkyl halides react with benzene in the presence of aluminum chloride to yield alkyl￾benzenes. H Benzene (CH3)3CCl tert-Butyl chloride C(CH3)3 tert-Butylbenzene (60%) HCl Hydrogen chloride AlCl3 0°C 450 CHAPTER TWELVE Reactions of Arenes: Electrophilic Aromatic Substitution H H Benzene and bromine–iron(III) bromide complex slow Br±Br±FeBr3 Cyclohexadienyl cation intermediate Step 2: Loss of a proton from the cyclohexadienyl cation yields bromobenzene. Step 1: The bromine–iron(III) bromide complex is the active electrophile that attacks benzene. Two of the π electrons of benzene are used to form a bond to bromine and give a cyclohexadienyl cation intermediate. Br Tetrabromoferrate ion H Cyclohexadienyl cation intermediate Tetrabromoferrate ion fast Bromobenzene Hydrogen bromide Iron(III) bromide   Br±FeBr3 Br H±Br FeBr3 Br Br±FeBr3  FIGURE 12.4 The mechanism of bromination of benzene. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website