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
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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