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23056_Chapter 9- Aromatic Compounds

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					                 Chapter 9 Notes - Aromatic Compounds
Aromaticity

      pleasant odor
      unusually low reactivity
       - substitution, not addition
      unusually stable
      characteristic ring structurewith delocalized pi bonding

Benzene - stability

      C6H6 (1,3,5-cyclohexatriene ?)
      no typical C=C reactions
       unreactive with HX, X2, KMnO4
      reaction requires extreme conditions
      when reaction does occur, it is substitution not addition

Benzene - structure

      all C are sp2 (trigonal, 120° angles)
       ideal for a planar hexagon



      all C-C bonds are the same (139 pm)
      compare C-C (154 pm), C=C (134 pm)
      cyclic conjugated pi bonds are unusually stable (resonance)

Nomenclature of Aromatics

      monosubstituted benzenes:
       common names - see Table 5.1
      disubstituted benzenes:
       ortho (1,2-), meta (1,3-), para (1,4-)




p-nitrobenzoic acid / / / 2-chloro-6-ethylaniline

Nomenclature of Aromatics
      group names:
       phenyl C6H5
       benzyl C6H5CH2



       (E)-1-phenyl-1-butene

Electrophilic Aromatic Substitution

      benzene can be made to react with very strong electrophiles (E+)
      intermediate is a carbocation
       (like addition to one of the pi bonds)
      nucleophiles don't add to the cation
       (H+ leaves, regenerates benzene ring)
      reaction is substitution (E+ for H+)

Mechanism of Aromatic Substitution




Mechanism - why slower than alkenes ?

      Ea for electrophilic attack on benzene is greater than Ea for electrophilic attack on an
       alkene
      although the cation intermediate is delocalized and more stable than an alkyl cation,
       benzene is much more stable than an alkene

Mechanism - why substitution ?

      the substitution product regains the aromatic stability
      an addition product would be a conjugated diene, not as stable
Bromination of Benzene

      electrophile is Br+
      generated from Br2 + FeBr3




Chlorination of Benzene

      electrophile is Cl+
      generated from Cl2 + FeCl3




Nitration of Benzene

      electrophile is NO2+
      generated from H2SO4 + HNO3




Sulfonation of Benzene

      electrophile is HSO3+
      generated from H2SO4 + SO3




Friedel-Crafts Alkylation

      electrophile is an alkyl cation (R+)
      generated from RCl + AlCl3
Friedel-Crafts Acylation

      electrophile is an acyl cation (RCO+)
      generated from RCOCl + AlCl3




Substituent Effects

      substituents on the benzene ring can affect the reaction in two ways:
       reactivity - substituted benzene may react faster or slower than benzene itself reacts
       orientation - the new group may be oriented ortho, meta, or para with respect to the
       original substituent

Reactivity Effects

      activating - reaction is faster
       observed with electron-donating groups that make the ring more electron-rich
      deactivating - reaction is slower
       observed with electron-withdrawing groups that make the ring less electron-rich

Orientation Effects

      substituent already present on the benzene ring determines the location of the new group
      ortho,para-directors: electron-donating groups direct the new group mainly to ortho &
       para
      meta-directors: electron-withdrawing groups direct new group mainly meta

Ortho, Para Directors

      the best cation is formed when the electrophile adds either ortho or para
       (better than unsubstituted)




Meta Directors
      the best cation is formed when the electrophile adds meta
       (but this is worse than unsubstituted)




Classifying Substituents

      activating and o,p-directing:
       alkyl, aryl, O and N groups
      deactivating and m-directing:
       N+ groups, polar multiple bonds
      deactivating but o,p-directing:
       the halogens (F, Cl, Br, I)
       (electron-withdrawing atoms, but lone pairs can stabilize the cation when it is ortho or
       para)

Oxidation of Side Chains

      alkyl groups attached to aromatic rings are easily oxidized to carboxylic acids




Reduction of Aromatic Rings

      under extreme conditions, a benzene ring can be hydrogenated to a cyclohexane ring




Polycyclic Aromatics

      larger aromatic compounds can be made from fused benzene rings




naphthalene / / / anthracene
Heterocyclic Aromatics

       some aromatic rings have atoms other than carbon




pyridine / / / pyrrole / / / furan


Synthetic Strategy

       synthesis of complex compounds requires attention to the order in which groups are
        attached
       retrosynthetic analysis - think backwards one step at a time
        (What reaction could have made this target compound?)

Synthesis Example

       target compound: p-nitrobenzoic acid




Synthesis Example




Graphite

       extended sheets of benzene rings
        electrically conductive
        good lubricant
Fullerenes

      curved closed form of 60 carbon atoms in benzene rings with intervening 5-membered
       rings (soccer ball pattern)
      subject of the 1996 Nobel Prize in chemistry

				
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posted:3/1/2012
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