Aromatic Substitution

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Aromatic Substitution

• Chapter 22

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Sections to skip!!

• Skip sections 22.1 (most), 22.3, 22.4, and 2.13
  through 22.15
• Keep 22.16 (review from c. 351, for some of you)
• Keep 22.17

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Problems

• In text 4 - 6 8 - 13 20 - 29
• End of Chapter 1 - 5 9 - 13

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Sect. 22.1 Nomenclature
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Catechols
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Nomenclature-- examples
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Chapter 22 Aromatic Substitutions,

• Monosubstitution reactions on benzene
• Sect. 22.2 Electrophilic aromatic substitution
  mechanism
• Sect. 22.5 Nitration
• Sect. 22.6. Halogenation
• Sect. 22.7 Friedel-Crafts Reactions
• Sect. 22.8 Sulfonation Reactions (skip fall
  2006)

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Sect. 22.2 Electrophilic aromatic substitution.
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Sect. 22.5 Nitration

• conc. HNO3 and H2SO4 react to make electrophile,
  NO2
• nitro aromatics are important intermediates
• reduction of nitro groups give anilines

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Mechanism of Aromatic NitrationStep 1 Where
does the electrophile come from?
Nitronium ion (NO2) is the electrophile that
reacts with the benzene ring.
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Mechanism of Aromatic Nitration (Step 2)
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Mechanism of Aromatic Nitration (Step 3)
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Sect. 22.6 Halogenation

• active electrophile is a bromonium or chloronium
  ion
• need Lewis acid catalyst ( FeX3 ) to activate X2

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Sect. 22.7 Friedel-Crafts Alkylation

• alkyl halide AlCl3 --gtcarbocation AlCl3X-
• watch out for carbocation rearrangements!
• more than one alkylation can occur --gt mixtures!

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Friedel-Crafts Acylation

• acid chloride AlCl3 --gt acylium ion AlCl4-
• cation rearrangements are NOT observed!
• acylation will only occur ONCE...
• reaction VERY sensitive to substituents-- an acyl
  group prevents further reaction

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Aromatic substitution on Benzene

• Sect. 22.8 Sulfonation (skip, fall 06)
• Sect. 22.9 Summary

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Sect. 22.10 and 22.11 Directing effects

• methoxy group releases electrons by resonance
  effect ortho and para director
• nitro group withdraws electrons by inductive and
  resonance effect meta director

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These are ortho and para directors! All are
electron releasing!!
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All ortho/para directing groups have pairs of
electrons next to the benzene ring! The only
exception are alkyl groups. They are also
ortho/para directors.
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Why do ortho/para groups direct as they do?
Resonance!!
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These are meta directors! All are electron
withdrawing!!
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Now lets look at a meta directing group
This is an example of Electrophilic Aromatic
Substitution (EAS).
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Why does the nitration reaction take place
preferentially at the meta position?
Lets ask a what if question.
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ortho
BAD!
meta
para
BAD!
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meta substitution preferred because the charge
is never next to the CO2R group
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Activation during substitution

• All ortho and para directing groups are
  activating relative to benzene, except halogen
  substituents. Halogens are weakly deactivating
  but are still o, p- directors.

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Deactivation during substitution

• All meta directors are strongly deactivating
  relative to benzene.

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Sect. 22.12 and 22.17 Some synthetic examples
involving aromatic substitution
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• ortho/para directors can work together with meta
  directors. They reinforce each other.

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Strong o/ p directors win over weak o, p and
meta directors.
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Substitution RARELY occurs in-between two
substituents--too hindered!
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Some groups can be modified to change their
directing effects.
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Good stuff! Order of reaction is critical!
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Some more good stuff!
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An explosive!
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2,4,6-Trinitrotoluene TNT
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Some miscellaneous examples

• Nitration of 3-nitrobenzoic acid
• Acylation of 1,3-dimethylbenzene
• Acylation of 1,4-dimethylbenzene
• Make 2-methyl-1-phenylpropane

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Sect. 22.16 Aromaticity and Huckels Rule
Aromatic compounds 4n 2 pi electrons
n 1 6 pi electrons systems

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Other n 1 aromatics





6 electrons, one pair not involved!
6 electrons, one pair not involved!
6 electrons
All are aromatic!
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n 0 aromatic 2 pi electrons
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Some Antiaromatic compounds

Not aromatic!
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Diazonium ions, Azo Dyes and the Sandmeyer
Reaction- from Chapter 23 (not covered 06)
Sect 23.16 Diazonium ion formation Sect 23.17
Sandmeyer reaction Sect 23.19 Azo dyes