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Reac%ons)of)Benzene)and) Subs%tuted)Benzenes) 19...¢  THE BIRCH REDUCTION: Aromatic rings...

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  • Reac%ons  of  Benzene  and   Subs%tuted  Benzenes  

  • This  Chapter  Begins  the  Discussion  of   the  Families  of  Compounds  in  Group  IV  

  • Many  Subs%tuted  Benzenes     are  Found  in  Nature  

  • The  Nomenclature  of  Subs%tuted   Benzenes  

    some  monosubs%tuted  benzenes  have  names     that  incorporate  the  subs%tuent  

  • •  Benzene  is  aroma%c:  a  cyclic  conjugated  compound  with  6  π   electrons  

    •  Reac%ons  of  benzene  lead  to  the  reten%on  of  the  aroma%c   core  

    Subs%tu%on  Reac%ons  of  Benzene     and  Its  Deriva%ves  

  • The  Way  Benzene  Reacts  

    Aroma%c  compounds  such  as  benzene  undergo   electrophilic  aroma%c  subs%tu%on  reac%ons.    

    The  π  electrons  above  and  below  the  ring  make  benzene  a   nucleophile.  

  • Benzene  Undergoes  Subs%tu%on,  Not   Addi%on  

    Aroma%city  is  restored  in  the  product  from  electrophilic   subs%tu%on  

  • Benzene  Undergoes  Subs%tu%on,  Not   Addi%on  

    The  reac%on  of  benzene  with  an  electrophile  forms  the     aroma%c  subs%tu%on  product,  not  the  nonaroma%c  addi%on  product.    

  • The  Mechanism  for   Electrophilic  Aroma%c  Subs%tu%on  

  • •  Bromina%on  or  chlorina%on  of  benzene  requires  a  Lewis  acid   catalyst  because  benzene’s  aroma%city  causes  it  to  be  less  reac%ve   than  an  alkene.  

    •  Ferric  bromide  (FeBr3)  or  ferric  chloride  (FeCl3)  is  usually  used.  

    •  FeBr3  is  added  as  a  catalyst  to  polarize  the  bromine  reagent  

    •  Benzene’s  π  electrons  par%cipate  as  a  Lewis  base  in  reac%ons  with   Lewis  acids  

    •  The  product  is  formed  by  loss  of  a  proton,  which  is  replaced  by   bromine  

    Electrophilic  Aroma%c     Subs%tu%on  Reac%ons:  Bromina%on  or  Chlorina%on  

  • Addi%on  Intermediate  in  Bromina%on  

    The  intermediate  is  not  aroma%c  and  therefore   high  in  energy  

  • •  The  ca%onic  addi%on   intermediate  transfers  a   proton  to  FeBr4-­‐  (from  Br-­‐   and  FeBr3)  

    •  This  restores  aroma%city  (in   contrast  with  addi%on  in   alkenes)  

    Forma%on  of  Product  from  Intermediate  

  • •  Chlorine  and  iodine  (but  not  fluorine,  which  is  too  reac%ve)   can  produce  aroma%c  subs%tu%on  with  the  addi%on  of  other   reagents  to  promote  the  reac%on  

    •  Chlorina%on  requires  FeCl3     •  Iodine  must  be  oxidized  to  form  a  more  powerful  I+  species  

    (with  Cu+  or  peroxide)  

    Other  Aroma%c  Subs%tu%ons    

  • •  The  combina%on  of  nitric  acid  and  sulfuric  acid  produces  NO2+   (nitronium  ion)  

    •  The  reac%on  with  benzene  produces  nitrobenzene  

    Aroma%c  Nitra%on  

  • •  Subs%tu%on  of  H  by  SO3H  (sulfona%on)   •  Reac%on  with  sulfuric  acid  and  heat,  or  a  mixture  of  sulfuric  

    acid  and  SO3   •  Reac%ve  species  is  sulfur  trioxide  or  its  conjugate  acid  

    Aroma%c  Sulfona%on  

  • Alkali  Fusion  of  Aroma%c  Sulfonates  

          Benzensulfonic  Acid                              Phenol  

    SO3H

    1) NaOH

    2) H3O+

    OH

  • Sulfona%on  of  Benzene  is  Reversible  

    The  Mechanism  for  Desulfona%on:  

    If  benzenesulfonic  acid  is  heated  in  dilute  acid,   an  H+  adds  to  the  ring  and  the  sulfonic  acid  group  comes  off  the  ring.  

  • Aroma%c  Hydroxyla%on  

    •  Direct  hydroxyla%on  of  an  aroma%c  ring   difficult  in  the  laboratory  

    •  Usually  occurs  via  an  enzyme  in  biological   pathways  

  • Friedel–Craas  Subs%tu%ons  

    • Two  electrophilic  subs%tu%ons  are  named  for   the  chemists  Charles  Friedel  and  James  Craas  

    •  Friedel–Craas  acyla%on  places  an  acyl  group   on  a  benzene  ring  

    •  Friedel–Craas  alkyla%on  places  an  alkyl  group   on  a  benzene  ring.  

  • •  Alkyla%on  among   most  useful   electrophilic   aroma%c   subsitu%on   reac%ons  

    •  Aroma%c   subs%tu%on  of  R+   for  H+  

    •  Aluminum  chloride   promotes  the   forma%on  of  the   carboca%on  

     Alkyla%on  of  Aroma%c  Rings:     The  Friedel–Craas  Reac%on    

  • •  Only  alkyl  halides  can  be  used  (F,  Cl,  I,  Br)   •  Aryl  halides  and  vinylic  halides  do  not  react  (their  carboca%ons  

    are  too  hard  to  form)   •  Will  not  work  with  rings  containing  an  amino  group  subs%tuent  

    or  a  strongly  electron-­‐withdrawing  group  

    Limita%ons  of  the  Friedel-­‐Craas  Alkyla%on  

  • •  Mul%ple  alkyla%ons  can  occur  because  the  first  alkyla%on  is   ac%va%ng  

    Control  Problems  

  • •  Similar  to  those  that  occur  during  electrophilic  addi%ons  to   alkenes  

    •  Can  involve  H  or  alkyl  shias  

    Carboca%on  Rearrangements  During  Alkyla%on  

  • •  Reac%on  of  an  acid  chloride  (RCOCl)  and  an  aroma%c  ring  in  the   presence  of  AlCl3  introduces  acyl  group,  ⎯COR     – Benzene  with  acetyl  chloride  yields    acetophenone    

    Acyla%on  of  Aroma%c  Rings    

  • Friedel–Craas  Acyla%on  

    Mechanism:  

  • •  Similar  to  alkyla%on   •  Reac%ve  electrophile:  resonance-­‐stabilized  acyl  ca%on     •  An  acyl  ca%on  does  not  rearrange  

    Mechanism  of  Friedel-­‐Craas  Acyla%on  

  • The  Gaderman–Koch  Reac%on  

    •  Benzaldehyde  cannot  be  made  by  a  Friedel–Craas   acyla%on  because  the  needed  acyl  chloride   (formyl  chloride)  is  unstable  

    •  Formyl  chloride  is  generated  in  the  reac%on   mixture  

  • Electrophilic  Aroma%c     Subs%tu%on  Reac%on  

    Pufng  a  Straight  Chain  Alkyl  Group  on  a  Ring  

  • Other  Ways  to  Convert  a  Carbonyl   Group  to  a  Methylene  Group  

    Mechanism  for  the  Wolff–Kishner  Reduc%on  

  • Coupling  Reac%ons  Can  Be  Used  to  Put     a  Straight  Chain  Alkyl  Group  on  a  

    Benzene  Ring  

  • Why  it  is  Important  to  Have  More  Than   One  Way  to  Carry  Out  a  Reac%on  

    •  Cataly%c  hydrogena%on  reduces     aroma%c  nitro  groups  and  carbonyl  groups.  

    •  Wolff–Kishner  reduc%on  reduces     only  the  carbonyl  group.  

  • Reduc%on  of  Benzene  

    O H

    H

    H

    THE BIRCH REDUCTION: Aromatic rings are inert to catalyzed hydrogenation except under industrially extreme conditions. A useful alternative to hydrogenation is the BIRCH REDUCTION. The resulting diene can then be readily hydrogenated to the corresponding alkene or alkane using H2/Pd.

    H

    HLi(0) NH3, EtOH

    Li(0)

    O H

    Li(0)

  • •  Aroma%c  rings  are  inert  to

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