Chemistry 331 - Fall 1996
Elements of Organic Chemistry I

Professor Carl C. Wamser

Chapter 5 - 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