CHAPTER TWELVE Reactions of Arenes: Electrophilic Aromatic Substitution The aluminum chloride complexes of methyl and ethyl halides contain highly polarized carbon-halogen bonds, and these complexes are the electrophilic species that react with benzene AIX CH3,-X-AIX3 Methyl halide-aluminum Other limitations to One drawback to Friedel-Crafts alkylation is that rearrangements can occur, esp Friedel-Crafts will cially when primary alkyl halides are used. For example, Friedel-C be encountered in this chap- benzene with isobutyl chloride(a primary alkyl halide) yields only tert-butylbenzene Table 12.4 C(CH3)3 t(CH3),CHCH, CI Hcl Isobutyl chloride Here, the electrophile is tert-butyl cation formed by a hydride migration that accompa nies ionization of the carbon-chlorine bond CH3C—CH2Cl-AlCl3 CH3C一CH2+ sobutyl chloride- terl-Butyl cation Tetrachloroaluminate aluminum chloride complex PROBLEM 12.4 In an attempt to prepare propylbenzene a chemist alkylated benzene with 1-chloropropane and aluminum chloride. However, two isomeric hydrocarbons were obtained in a ratio of 2: 1, the desired propylbenzene being the minor component. What do you think was the major product? How did it bocao since electrophilic attack on benzene is simply another reaction available to a car- bocation, other carbocation precursors can be used in place of alkyl halides. For exam- ple, alkenes, which are converted to carbocations by protonation, can be used to alky late benzene Cyclohexylbenzene(65-68%) PROBLEM 12.5 Write a reasonable mechanism for the formation of cyclohexyl benzene from the reaction of benzene, cyclohexene and sulfuric acid Alkenyl halides such as vinyl chloride(CH2-CHCI) do not form carbocations on treatment with aluminum chloride and so cannot be used in friedel-crafts reactions Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteThe aluminum chloride complexes of methyl and ethyl halides contain highly polarized carbon–halogen bonds, and these complexes are the electrophilic species that react with benzene. One drawback to Friedel–Crafts alkylation is that rearrangements can occur, espe￾cially when primary alkyl halides are used. For example, Friedel–Crafts alkylation of benzene with isobutyl chloride (a primary alkyl halide) yields only tert-butylbenzene. Here, the electrophile is tert-butyl cation formed by a hydride migration that accompa￾nies ionization of the carbon–chlorine bond. PROBLEM 12.4 In an attempt to prepare propylbenzene, a chemist alkylated benzene with 1-chloropropane and aluminum chloride. However, two isomeric hydrocarbons were obtained in a ratio of 2:1, the desired propylbenzene being the minor component. What do you think was the major product? How did it arise? Since electrophilic attack on benzene is simply another reaction available to a car￾bocation, other carbocation precursors can be used in place of alkyl halides. For exam￾ple, alkenes, which are converted to carbocations by protonation, can be used to alky￾late benzene. PROBLEM 12.5 Write a reasonable mechanism for the formation of cyclohexyl￾benzene from the reaction of benzene, cyclohexene, and sulfuric acid. Alkenyl halides such as vinyl chloride (CH2œCHCl) do not form carbocations on treatment with aluminum chloride and so cannot be used in Friedel–Crafts reactions. H2SO4 Benzene Cyclohexene Cyclohexylbenzene (65–68%) CH3 CH3 H C CH2 AlCl3  Cl Isobutyl chloride– aluminum chloride complex CH3 CH3 C H CH2 tert-Butyl cation  AlCl4 Tetrachloroaluminate ion H Benzene (CH3)2CHCH2Cl Isobutyl chloride C(CH3)3 tert-Butylbenzene (66%) HCl Hydrogen chloride AlCl3 0°C CH3 X AlX3  Methyl halide–aluminum halide complex CH3CH2 X AlX3  Ethyl halide–aluminum halide complex 452 CHAPTER TWELVE Reactions of Arenes: Electrophilic Aromatic Substitution Other limitations to Friedel–Crafts reactions will be encountered in this chap￾ter and are summarized in Table 12.4. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website