12.2 Mechanistic Principles of Electrophilic Aromatic Substitution TABLE 12.1 Representative Electrophilic Aromatic Substitution Reactions of Benzene Reaction and comment Equation 1. Nitration Warming benzene with a mix ture of nitric acid and nitrobenzene. a nitro H2o places one of the ring hydrogens Benzene Nitrobenzene(95%)Water 2. Sulfonation Treatment of benzene with H hot concentrated sulfuric acid gives ben- zenesulfonic acid. A sulfonic acid group HOSO2OH H2o (-SO2OH)replaces one of the ring hydro- gens. Sulfuric acid Benzenesulfonic acid Water (100%) 3. Halogenation Bromine reacts w zene in the presence of iron(Ill)bi FeBr a catalyst to give bromobenzene. chlorine HBr reacts similarly in the presence chloride to give chlorobenzene Benzene Bromine Bromobenzene Hydrogen bromide 4. Friedel-Crafts alkylation Alkyl halides H C(CH3)3 react with benzene in the presence of alu- minum chloride to yield alkylbenzenes CH3)aCCI HCl Benzene tert-Butyl chloride 5. Friedel-Crafts acylation An analogous reaction occurs when acyl halides react with benzene in the presence of alumi- H CCH2 CH3 num chloride. The products are acylben CH3 CH2 CCI + HCl Benzene Propanoyl 1-Phenyl-1 Hydroger anone chloride PROBLEM 12.1 In the simplest molecular orbital treatment of conjugated sys- tems, it is assumed that the system does not interact with the framework of g bonds. When this Mo method was used to calculate the charge distribution in A model showing the electrostatic potential of this yclohexadienyl cation, it gave the results indicated How does the charge at each irbocation can be viewed or carbon compare with that deduced by examining the most stable resonance struc-Learning ures for cyclohexadienyl cation +033 0.33 H Most of the resonance stabilization of benzene is lost when it is converted to the yclohexadienyl cation intermediate. In spite of being allylic, a cyclohexadienyl cation Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsitePROBLEM 12.1 In the 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 carbon 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 H H H H H 0 0.33 0 H 0 0.33 0.33 12.2 Mechanistic Principles of Electrophilic Aromatic Substitution 445 TABLE 12.1 Representative 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 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 react 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%) SO2OH Water H2O heat H Benzene Bromine Br2 Bromobenzene (65–75%) Br Hydrogen bromide HBr FeBr3 H Benzene tert-Butyl chloride (CH3)3CCl tert-Butylbenzene (60%) C(CH3)3 Hydrogen chloride HCl AlCl3 0°C H Benzene Hydrogen chloride HCl Propanoyl chloride CH3CH2CCl O 1-Phenyl-1- propanone (88%) CCH2CH3 O AlCl3 40°C Nitric acid HNO3 Nitrobenzene (95%) NO2 Water H2O H2SO4 30–40°C Benzene H A model showing the electrostatic potential of this carbocation can be viewed on Learning By Modeling. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website