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Aromatic Substitution . Chapter 22. 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. Problems. In text: 4 - 6 8 - 13 20 - 29 End of Chapter: 1 - 5 9 - 13 . Sect. 22.1 Nomenclature.

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aromatic substitution

Aromatic Substitution

Chapter 22

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sections to skip
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
Problems
  • In text: 4 - 6 8 - 13 20 - 29
  • End of Chapter: 1 - 5 9 - 13

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catechols
Catechols

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chapter 22 aromatic substitutions
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 5 nitration
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 nitration step 1 where does the electrophile come from
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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sect 22 6 halogenation
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
Sect. 22.7: Friedel-Crafts Alkylation
  • alkyl halide + AlCl3 -->carbocation + AlCl3X-
  • watch out for carbocation rearrangements!
  • more than one alkylation can occur --> mixtures!

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friedel crafts acylation
Friedel-Crafts Acylation
  • acid chloride + AlCl3 --> 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
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
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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slide20

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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now let s look at a meta directing group
Now let’s 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
Why does the nitration reaction take place preferentially at the meta position?

Let’s ask a “what if” question.

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slide25

ortho

BAD!

meta

para

BAD!

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activation during substitution
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
Deactivation during substitution
  • All meta directors arestrongly deactivating relative to benzene.

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slide30

ortho/para directors can work together with meta directors. They reinforce each other.

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slide33

Some groups can be modified to change their directing effects.

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an explosive
An explosive!

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some miscellaneous examples
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 huckel s rule
Sect. 22.16 Aromaticity and Huckel’s Rule

Aromatic compounds 4n + 2 pi electrons

n = 1 6 pi electrons systems

:

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other n 1 aromatics
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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some antiaromatic compounds
Some Antiaromatic compounds

:

Not aromatic!

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diazonium ions azo dyes and the sandmeyer reaction from chapter 23 not covered 06
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

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