Aromaticity. Reactions of Benzene Chapter 8

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Aromaticity.Reactions of Benzene Chapter 8

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Contents of Chapter 15

• Aromaticity
• Heterocyclic Compounds
• Chemical Consequences of Aromaticity
• Nomenclature
• Reactivity Considerations
• Mechanism for Electrophilic Substitution
• Halogenation/Nitration/Sulfonation of Benzene
• FriedelCrafts Reactions
• Substituent Effects
• Retrosynthetic Analysis

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Aromaticity

• Benzene is a cyclic compound which has a planar
  structure with a delocalized cloud of p electrons
  above and below the plane of the ring

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Criteria for Aromaticity

• There must be an uninterrupted ring of p
  orbital-bearing atoms leading to a delocalized ?
  cloud
• For the p cloud to be cyclic, the molecule must
  be cyclic
• For the p cloud to be uninterrupted, every ring
  atom must have a p orbital
• For the p cloud to form, each p orbital must be
  able to overlap the p orbital on either side

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Criteria for Aromaticity

• The ? cloud must have an odd number of pairs of ?
  electrons, or (2n1)2 4n2 ? electrons
• Hückels rule

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Aromaticity

• cyclooctatetraene is nonaromatic
• It is not planar

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Aromaticity
resonance broken 2 ? electrons 4 ? electrons
nonaromatic aromatic antiaromatic
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Aromaticity
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Aromaticity

• The criteria for aromaticity also can be applied
  to polycyclic hydrocarbons
• Naphthalene (5 pairs of p electrons),
  phenanthrene (7 pairs of p electrons), and
  chrysene (9 pairs of p electrons) all are aromatic

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Heterocyclic Compounds

• Lone pair cant be in p orbital because p orbital
  used to build ? bond with adjacent carbon(s)
• The lone pair on pyridines nitrogen is in an sp2
  hybrid, not part of the 3-pair aromatic ? system

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Heterocyclic Compounds

• In pyrrole the lone pair could be put into either
  an sp3 hybrid or a p orbital with bonds in sp2
  hybrid
• Pyrrole puts the lone pair in a p orbital, making
  3 pairs of ? electrons (aromatic is more stable)

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Heterocyclic Compounds

• In above structures the N lone pairs could be put
  into either sp3 hybrids or p orbitals with bonds
  to N in sp2 hybrids
• Because the lone pairs give these rings an EVEN
  number of pi electrons these rings are not
  aromatic
• In these cases the lone pairs are put into sp3
  hybrid orbitals because nature doesnt like even
  numbers of pairs of pi electrons in cyclic pi
  systems

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Heterocyclic Compounds

• In furan and thiophene there are 2 pairs of
  unshared electrons - one is an sp2 hybrid orbital
  and one pair is in a p orbital, like pyrrole (3
  pairs of ? electrons, aromatic)

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Heterocyclic Compounds
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Heterocyclic Compounds

• Quinoline, indole, imidazole, purine, and
  pyrimidine also are aromatic heterocyclic
  compounds

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Chemical Consequences of Aromaticity
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Chemical Consequences of Aromaticity

• Cyclopentadiene has such a low pKa because of the
  stability of the anion formed when the hydrogen
  ionizes - the anion is aromatic

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Chemical Consequences of Aromaticity

• Cycloheptatrienyl bromide is ionic because of the
  stability of the aromatic cycloheptatrienyl cation

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Naming Monosubstituted Benzenes
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Naming Common Monosubstituted Benzenes
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Reactivity Considerations

• The benzene ring consists of a ring with p
  electrons above and below
• Electrophiles are attracted to a benzene ring and
  form a nonaromatic carbocation intermediate (a
  cyclohexadienyl cation)

carbocation intermediate
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Electrophilic Substitution

• Electrophilic addition doesnt occur (would
  destroy aromaticity)

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Reactivity Considerations
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Mechanism for Electrophilic Substitution Reactions
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Halogenation of Benzene
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Nitration of Benzene
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Anilines From Nitrobenzenes

• Anilines (aminobenzenes) are always made from
  nitrobenzenes.
• Anilines decompose and make black tar when
  exposed to electrophilic aromatic substitution
  (EAS) reaction conditions
• For this reason nitrobenzenes are converted to
  anilines in the very LAST step AFTER all other
  groups are added by EAS reactions
• There are several ways to convert nitrobenzenes
  to anilines but this course teaches H2 and Pd/C

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Sulfonation of Benzene
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FriedelCrafts Acylation
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FriedelCrafts Alkylation
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Electron-donating Substituents
Resonance contributors increase electron density
in ortho and para positions. Overall electron
density is bigger.
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Electron-donating Substituents

• A pi system can be considered to be polarized in
  an alternating fashion by substituents for
  product analysis purposes.
• Electrophiles ( groups) add to positions
• Alkyl groups and atoms with lone pairs polarize
  the ring carbon thhey are attached to

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Electron-donating Substituents
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Electron-withdrawing Substituents
Atoms with charge polarize the attached ring
carbon negative ()
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Electron-withdrawing Substituents
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Naming Disubstituted Benzenes
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Naming Disubstituted Benzenes
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Retrosynthetic Analysis

• Work backwards from an aromatic compound to
  figure out how to make it
• First if amino group (NH2) is present work it
  backwards to nitro (NO2)
• Next remove one substituent and polarize the ring
  according to positions of other substituents
• If the substituent you removed came from a
  carbon remove another substituent, polarize the
  ring again, and repeat
• If remaining substituents dont agree on ring
  polarization or latest removed substituent
  doesnt remove from a carbon replace removed
  substituent and try to remove another one.
• Continue until all substituents have been removed
• Reverse the retrosynthetic analysis to figure out
  what reagents to add to benzene in what order

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Retrosynthetic Analysis
Work out synthesis of 4-bromo-3-chloroaniline
using retrosynthetic analysis