Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution

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					            Chapter 12
       Reactions of Arenes:
Electrophilic Aromatic Substitution


  H                            E
      d+   d–
      +E   Y                       +H   Y
                12.1
Representative Electrophilic Aromatic
 Substitution Reactions of Benzene

 H                                E
      d+   d–
     +E    Y                          +H   Y
       H                                 E
            d+   d–
           +E    Y                           +H   Y


  Electrophilic aromatic substitutions include:

Nitration
  Sulfonation
       Halogenation
            Friedel-Crafts Alkylation
                  Friedel-Crafts Acylation
Table 12.1: Nitration of Benzene




H           H2SO4              NO2
 + HONO2
                                   + H2O


                    Nitrobenzene
                        (95%)
Table 12.1: Sulfonation of Benzene




H          heat                 SO2OH
 + HOSO2OH
                                     + H2O


                Benzenesulfonic acid
                      (100%)
Table 12.1: Halogenation of Benzene




 H           FeBr3              Br2
  + Br2
                                      + HBr


                     Bromobenzene
                        (65-75%)
Table 12.1: Friedel-Crafts Alkylation of Benzene




       H              AlCl3            C(CH3)3
        + (CH3)3CCl
                                           + HCl


                          tert-Butylbenzene
                                (60%)
Table 12.1: Friedel-Crafts Acylation of Benzene


                                      O
                 O
     H            AlCl3                CCH2CH3
      + CH3CH2CCl
                                          + HCl


                       1-Phenyl-1-propanone
                              (88%)
                12.2
      Mechanistic Principles
                 of
Electrophilic Aromatic Substitution
    Step 1: attack of electrophile
on p-electron system of aromatic ring



                 E+
    H       H                       H          H E
H                H              H        +
                                                H
    H        H                      H          H


    highly endothermic
    carbocation is allylic, but not aromatic
    Step 2: loss of a proton from the carbocation
                     intermediate




     H       H                       H        H E
H                 E              H        +
                                               H
    H         H       H+             H        H


     highly exothermic
     this step restores aromaticity of ring
                H        H E
            H        +
                          H
                H        H




    H   H                          H   H
H           H + E+             H           E + H+
    H   H                          H   H
     Based on this general mechanism:



what remains is to identify the electrophile in
nitration, sulfonation, halogenation, Friedel-
Crafts alkylation, and Friedel-Crafts acylation
to establish the mechanism of specific
electrophilic aromatic substitutions
         12.3
Nitration of Benzene
Nitration of Benzene




     H                H2SO4               NO2
      + HONO2
                                            + H2O



                                 +
    Electrophile is       •           •
                          •O••
                                 N   O•
                                     ••
    nitronium ion
Step 1: attack of nitronium cation
on p-electron system of aromatic ring



               NO2+
    H     H                      H          H NO2
H              H             H          +
                                             H
    H      H                     H          H
Step 2: loss of a proton from the carbocation
intermediate




    H     H                      H        H NO2
H              NO2           H        +
                                           H
    H      H    H+               H        H
      Where does nitronium ion come from?


••          •• –                         ••                 •• –
•              •                     •                      O• •
•O    +     O•                       •O           +         ••
            ••      H2SO4                         N
      N

     •
                                              +O
     •O                                           ••
       ••   H                        H                      H



                          +                            ••
                   •             •   +                 O
                   •O••
                          N   O•
                              ••              H        ••       H
          12.4
Sulfonation of Benzene
   Sulfonation of Benzene




         H          heat                     SO2OH
          + HOSO2OH
                                               + H2O


                                 ••          •• –
                                 •           O• •
                                 •O    +     ••
Several electrophiles present:         S
a major one is sulfur trioxide
                                      •
                                      •O
                                        ••
Step 1: attack of sulfur trioxide
on p-electron system of aromatic ring



               SO3
    H     H                      H          H SO3–
H              H             H          +
                                             H
    H      H                     H          H
Step 2: loss of a proton from the carbocation
intermediate




    H     H                      H        H SO3–
H              SO3–          H        +
                                           H
    H      H    H+               H        H
Step 3: protonation of benzenesulfonate ion




    H     H           H2SO4       H    H
H              SO3–           H             SO3H
    H      H                      H     H
         12.5
Halogenation of Benzene
   Halogenation of Benzene




         H            FeBr3               Br2
          + Br2
                                                + HBr




Electrophile is a Lewis acid-Lewis base
complex between FeBr3 and Br2.
                 The Br2-FeBr3 Complex



                                            +
 ••      ••                         ••      ••   –
• Br
•        Br •    + FeBr3           • Br
                                   •       Br    FeBr3
            •
  ••     ••                          ••     ••

 Lewis base        Lewis acid             Complex


       The Br2-FeBr3 complex is more electrophilic
       than Br2 alone.
    Step 1: attack of Br2-FeBr3 complex
    on p-electron system of aromatic ring


                      +     –
                Br    Br    FeBr3
    H       H                            H       H Br
H               H                    H       +
                                                  H
    H       H                            H       H


                                         + FeBr4–
Step 2: loss of a proton from the carbocation
intermediate




    H     H                      H        H Br
H              Br            H        +
                                           H
    H      H    H+               H        H
                12.6
Friedel-Crafts Alkylation of Benzene
Friedel-Crafts Alkylation of Benzene




     H              AlCl3              C(CH3)3
      + (CH3)3CCl
                                          + HCl


                        H3C
Electrophile is               +
                              C    CH3
tert-butyl cation
                        H3C
                     Role of AlCl3

     acts as a Lewis acid to promote ionization
     of the alkyl halide

                                               +
          ••                                   ••   –
(CH3)3C   Cl •
             •   +   AlCl3           (CH3)3C   Cl   AlCl3
          ••                                   ••
                      Role of AlCl3

     acts as a Lewis acid to promote ionization
     of the alkyl halide

                                                 +
          ••                                     ••   –
(CH3)3C   Cl •
             •   +    AlCl3           (CH3)3C    Cl   AlCl3
          ••                                     ••




                           +     ••      –
                     (CH3)3C + • Cl
                               •         AlCl3
                                  ••
Step 1: attack of tert-butyl cation
on p-electron system of aromatic ring


               +
                C(CH3)3
    H     H                      H          H C(CH3)3
H              H             H          +
                                             H
    H      H                     H          H
Step 2: loss of a proton from the carbocation
intermediate




    H     H                      H        H C(CH3)3
H              C(CH3)3       H        +
                                           H
    H      H    H+               H        H
Rearrangements in Friedel-Crafts Alkylation


    Carbocations are intermediates.
    Therefore, rearrangements can occur


   H                                      C(CH3)3
                           AlCl3
    + (CH3)2CHCH2Cl

       Isobutyl chloride      tert-Butylbenzene
                                    (66%)
Rearrangements in Friedel-Crafts Alkylation


       Isobutyl chloride is the alkyl halide.
       But tert-butyl cation is the
       electrophile.
   H                                            C(CH3)3
                           AlCl3
    + (CH3)2CHCH2Cl

       Isobutyl chloride       tert-Butylbenzene
                                     (66%)
Rearrangements in Friedel-Crafts Alkylation

           H
                        +    –
                        ••
    H3C    C      CH2   Cl   AlCl3
                        ••
           CH3


                  H
           +                  ••     –
     H3C   C      CH2   +    • Cl
                             •       AlCl3
                               ••
            CH3
Reactions Related to Friedel-Crafts Alkylation


      H
                      H2SO4
          +



                              Cyclohexylbenzene
                                   (65-68%)
Cyclohexene is protonated by sulfuric acid,
giving cyclohexyl cation which attacks the
benzene ring
                12.7
Friedel-Crafts Acylation of Benzene
       Friedel-Crafts Acylation of Benzene


                                           O
                   O
       H            AlCl3                  CCH2CH3
        + CH3CH2CCl
                                                + HCl


Electrophile is an acyl cation

          +       ••                           +
    CH3CH2C       O•
                   •             CH3CH2C       O••
Step 1: attack of the acyl cation
on p-electron system of aromatic ring

               O
                                         O
            CCH2CH3
    H     H +                  H        H CCH2CH3
H           H              H        +
                                         H
    H      H                   H        H
    Step 2: loss of a proton from the carbocation
    intermediate


                                               O
                O
    H       H                        H        H CCH2CH3
H               CCH2CH3          H        +
                                               H
    H       H       H+               H        H
             Acid Anhydrides

can be used instead of acyl chlorides
                                         O
              O O
     H                    AlCl3          CCH3
         + CH3COCCH3


                                   Acetophenone
                                     (76-83%)

                                        O

                                   + CH3COH
        12.8
Acylation-Reduction
                    Acylation-Reduction

      permits primary alkyl groups to be attached
                  to an aromatic ring

                O                    O
           H                         CR
               RCCl
                AlCl3                     Zn(Hg), HCl

Reduction of aldehyde and ketone                 CH2R
carbonyl groups using Zn(Hg) and HCl is
called the Clemmensen reduction.
                    Acylation-Reduction

      permits primary alkyl groups to be attached
                  to an aromatic ring

                O                  O
          H                         CR
              RCCl
                                          H2NNH2, KOH,
               AlCl3                      triethylene glycol,
                                          heat
Reduction of aldehyde and ketone
carbonyl groups by heating with H2NNH2
                                                 CH2R
and KOH is called the
Wolff-Kishner reduction.
    Example: Prepare isobutylbenzene



      (CH3)2CHCH2Cl               CH2CH(CH3)3

           AlCl3


No! Friedel-Crafts alkylation of benzene
using isobutyl chloride fails because of
rearrangement.
Recall




                                       C(CH3)3
                        AlCl3
 + (CH3)2CHCH2Cl

    Isobutyl chloride      tert-Butylbenzene
                                 (66%)
  Use Acylation-Reduction Instead

             O

+ (CH3)2CHCCl

AlCl3        Zn(Hg)           CH2CH(CH3)3
              HCl
        O

        CCH(CH3)2

				
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posted:11/23/2011
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