12.6 Friedel-Crafts Alkylation of Benzene Alkylation of benzene with alkyl halides in the presence of aluminum chloride was dis- covered by Charles Friedel and James M. Crafts in 1877. Crafts, who later became pres- ident of the Massachusetts Institute of Technology, collaborated with Friedel at the Sor- bonne in Paris, and together they developed what we now call the Friedel-Crafts reaction into one of the most useful synthetic methods in organic chemistry Re Alkyl halides by themselves are insufficiently electrophilic to react with ben- zene. Aluminum chloride serves as a Lewis acid catalyst to enhance the elec trophilicity of the alkylating agent. With tertiary and secondary alkyl halides, the addi tion of aluminum chloride leads to the formation of carbocations, which then attack the aromatic ring. (CH3)3C--CI: AlCI3->(CH3)3C-CI-AICI tert-Butyl chloride Aluminum Lewis acid-Lewis base chloride complex (CH3)3CCI-AlCI3 (CH3)3C++ tert-Butyl chloride- tert-Butyl Tetrachloroaluminate Figure 12.5 illustrates attack on the benzene ring by tert-butyl cation( step 1) and subsequent formation of tert-butylbenzene by loss of a proton from the cyclohexadienyl cation intermediate(step 2) Secondary alkyl halides react by a similar mechanism involving attack on benzene by a secondary carbocation. Methyl and ethyl halides do not form carbocations when treated with aluminum chloride, but do alkylate benzene under Friedel-Crafts conditions Step 1: Once generated by the reation of tert-butyl chloride and aluminum chloride, tert-butyl cation attacks the Tr electrons of benzene and a carbon-carbon bond is formed CH CH C(CH3)3 Benzene and tert-butyl cation Step 2: Loss of a proton from the cyclohexadienyl cation intermediate yields tert-butylbenzene C(CH3)3 +:C!1-A1Cl3 t HCI AlCl3 Tetrachloroalumina Aluminum cation intermediate boride chloride FIGURE 12.5 The mechanism of Friedel-Crafts alkylation. An electrostatic potential map of tert-butyl cation can be viewed on Learning By Modeling Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteAlkylation of benzene with alkyl halides in the presence of aluminum chloride was dis￾covered by Charles Friedel and James M. Crafts in 1877. Crafts, who later became pres￾ident of the Massachusetts Institute of Technology, collaborated with Friedel at the Sor￾bonne in Paris, and together they developed what we now call the Friedel–Crafts reaction into one of the most useful synthetic methods in organic chemistry. Alkyl halides by themselves are insufficiently electrophilic to react with ben￾zene. Aluminum chloride serves as a Lewis acid catalyst to enhance the elec￾trophilicity of the alkylating agent. With tertiary and secondary alkyl halides, the addi￾tion of aluminum chloride leads to the formation of carbocations, which then attack the aromatic ring. Figure 12.5 illustrates attack on the benzene ring by tert-butyl cation (step 1) and subsequent formation of tert-butylbenzene by loss of a proton from the cyclohexadienyl cation intermediate (step 2). Secondary alkyl halides react by a similar mechanism involving attack on benzene by a secondary carbocation. Methyl and ethyl halides do not form carbocations when treated with aluminum chloride, but do alkylate benzene under Friedel–Crafts conditions. AlCl3  (CH Cl 3)3C tert-Butyl chloride– aluminum chloride complex tert-Butyl cation (CH3)3C AlCl4  Tetrachloroaluminate anion (CH3)3C Cl tert-Butyl chloride AlCl3 Aluminum chloride AlCl3  (CH Cl 3)3C Lewis acid-Lewis base complex 12.6 Friedel–Crafts Alkylation of Benzene 451 H Benzene and tert-butyl cation slow Step 1: Once generated by the reation of tert-butyl chloride and aluminum chloride, tert-butyl cation attacks the electrons of benzene, and a carbon-carbon bond is formed. Step 2: Loss of a proton from the cyclohexadienyl cation intermediate yields tert-butylbenzene. C H Cyclohexadienyl cation intermediate C(CH3)3 C(CH3)3 C(CH3)3 Cyclohexadienyl cation intermediate fast H Cl Tetrachloroaluminate ion tert-Butylbenzene  HCl Hydrogen chloride CH3 CH3 CH3 AlCl3 Aluminum chloride AlCl3 FIGURE 12.5 The mechanism of Friedel–Crafts alkylation. An electrostatic potential map of tert-butyl cation can be viewed on Learning By Modeling. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website