Thursday, February 21, 2008

very important note on benzene

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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