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WWU-Chemistry Aromatic Substitution Electrophilic.

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WWU-Chemistry WWU-Chemistry Aromatic Substitution Aromatic Substitution Electrophilic
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Page 1: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Aromatic Substitution Aromatic Substitution

Electrophilic

Page 2: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Sect. 22.1 NomenclatureSect. 22.1 Nomenclature

benzene

CH3

toluene

Cl

chlorobenzene

OH

phenol

NH2

O

OH

aniline benzoic acid

O

H

benzaldehyde

NO2

nitrobenzene

Page 3: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Nomenclature-- examples Nomenclature-- examples

O

OH

benzoic acid

O

H

Group ortho

meta or 3

para or 4

or 2

NO2

3-nitro (IUPAC)

m-nitrobenzoic acid (common)

Cl

4-chlorobenzaldehyde (IUPAC)

p-chlorobenzaldehyde (common)

Page 4: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

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)

Page 5: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Sect. 22.2 Electrophilic aromatic Sect. 22.2 Electrophilic aromatic substitution.substitution.

E+SLOW

E

HH

E

H

E

H

resonance stabilized cation

E

H

:B

"base"

re-aromatizeE

+ HB

delocalizedcation

Page 6: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Sect. 22.5: NitrationSect. 22.5: Nitration• conc. HNO3 and H2SO4 react to make electrophile, NO2

+

• nitro aromatics are important intermediates

• reduction of nitro groups give anilines

N OO

nitronium ion

NO2

H

HSO4

NO2

H2SO4

HNO3

H2SO4

NO2

NO2 reduce NH2

reducing agents: LiAlH4 or H2 / Pd(C) or Sn / HCl or Fe / HOAc

+ H2O

Page 7: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Mechanism of Aromatic NitrationMechanism of Aromatic NitrationStep 1: Where does the electrophile Step 1: Where does the electrophile

come from?come from?

HO N

O

O

+ H2SO4

OH H

+N

Nitronium ion

O O

O N

O

OH

H

O N

O

OH

H

HSO4

Nitronium ion (NO2+) is the electrophile that reacts

with the benzene ring.

Page 8: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Mechanism of Aromatic Mechanism of Aromatic Nitration (Step 2)Nitration (Step 2)

H

+ NO2

slow

H

NO2

H

NO2

H

NO2

a resonance-stabilized arenium ion

Page 9: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Mechanism of Aromatic Mechanism of Aromatic Nitration (Step 3)Nitration (Step 3)H

NO2 + HSO4

fast

NO2

+

this represents theresonance hybridof the arenium ion

H2SO4

H

NO2

O -S

O

O

OH

NO2

S

O

O

OHH-O

Page 10: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Sect. 22.6: HalogenationSect. 22.6: Halogenation• active electrophile is a bromonium or chloronium ion

• need Lewis acid catalyst ( FeX3 ) to activate X2

bomonium ion

Br

HBr

Br

HBr

Br2

FeBr3

Br

+ HBr

Br

Br2 + FeBr3

Cl2

FeCl3

Cl

+ HCl

(iodination requires special conditions)

mechanism:

Br + FeBr4

Page 11: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Sect. 22.7: Friedel-Crafts Sect. 22.7: Friedel-Crafts AlkylationAlkylation

• alkyl halide + AlCl3 -->carbocation + AlCl3X-

• watch out for carbocation rearrangements!

• more than one alkylation can occur --> mixtures!

+ Br + HBrAlCl3 (cat)

CH2Cl2

+ + HBrAlCl3 (cat)

CH2Cl2Br

Page 12: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Friedel-Crafts AcylationFriedel-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

+Cl

OAlCl3 (cat)

O

+ HCl

Cl

O

AlCl4C OAlCl3

O

an acylium ion

Page 13: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

HO H

O

Two otherresonancestructures

H+_

O

Page 14: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Aromatic substitution on Aromatic substitution on BenzeneBenzene

• Sect. 22.8: Sulfonation (skip, fall 06)

• Sect. 22.9: Summary

Page 15: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Sect. 22.10 and 22.11: Directing Sect. 22.10 and 22.11: Directing effectseffects

• methoxy group releases electrons by resonance effect: ortho and para director

• nitro group withdraws electrons by inductive and resonance effect: meta director

Page 16: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

These are These are ortho and paraortho and para directors! All are electron directors! All are electron

releasing!! releasing!!

R = H or alkyl

-O-R

O

-N-C-R

O

H

-O-C-R

-O-H -R -X-NR2

..

..

.. ..

..

..

..

..

..

..

..

Page 17: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

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.

Page 18: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Why do ortho/para groups direct as Why do ortho/para groups direct as they do? Resonance!!they do? Resonance!!

O CH3

H3C

O

ClAlCl3

H3C

O

AlCl4

_

O

CH3H

HO

CH3

OCH3

HO

CH3

OCH3

HO

CH3

OCH3

HO

CH3

OCH3

Extra resonancestructure from lonepair on oxygen!Ortho would worktoo.

Meta won't work!can't put + nextto O-CH3

Page 19: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

These are These are meta meta directors! All are directors! All are electron withdrawing!!electron withdrawing!!

R

O

H

O

O-R

O

NH2

O

N

CH3

CH3

CH3

C

F

FF

NO

OSO

O

OH

Notice! These groups have electronegative elements next to the benzene ring! There are NO non-bonded electrons next to the ring!

C N

Page 20: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Now let’s look at a meta directing Now let’s look at a meta directing groupgroup

CO

O

CH3

+ HNO3

H2SO4

15°C

CO

O

CH3

+ H2O

NO2H

This is an example of Electrophilic Aromatic Substitution (EAS).

Page 21: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Why does the nitration reaction take Why does the nitration reaction take place preferentially at the place preferentially at the metameta

position?position?

Let’s ask a “what if” question.

Page 22: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

ortho

meta

para

BAD!

BAD!

H

NO2

CO OCH3

H

NO2

CO OCH3

H

NO2

CO OCH3

CO OCH3

CO OCH3

CO OCH3

H

NO2

H

NO2

H

NO2

CO OCH3

CO OCH3

CO OCH3

H NO2 H NO2 H NO2

Page 23: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

meta substitution preferred because meta substitution preferred because the + charge is never next to the the + charge is never next to the

COCO22R groupR group

Page 24: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

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.G

H E

+Electron releasing groups (G) help stabilize the + in the ring introduced by the electrophile.This increases the rate of substitution!

Page 25: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Deactivation during substitutionDeactivation during substitution

• All meta directors are strongly deactivating relative to benzene.

G

H E

+Electron withdrawing groups (G) intensify the + in the ring introduced by the electrophile. This slows the substitution reaction!

Page 26: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Sect. 22.12 and 22.17: Some Sect. 22.12 and 22.17: Some synthetic examples involving synthetic examples involving

aromatic substitutionaromatic substitution

Page 27: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

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

O-CH3HNO3

O-CH3

NO2

HNO3

O-CH3

NO2

NO2

H2SO4H2SO4

ortho/ para directorortho/parameta

Page 28: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Strong o/ p directors win over weak o, p and meta directors.

OH

CH3

Br2

FeBr3

OH

CH3

Br

Page 29: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Substitution RARELY occurs in-between two substituents--too hindered!

CH3

CH3

H3C

O

Cl

AlCl3

CH3

CH3

O CH3

nothing here!

Page 30: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Some groups can be modified to change their directing effects.

CH3

ortho/para director

KMnO4

O OH

meta directorBr2

FeBr3

CH3

Br

CH3

Br

Br2

FeBr3

O OH

BrKMnO4 KMnO4

Br

O OH O OH

Br

3-bromobenzoic acid

2-bromobenzoic acid 4-bromobenzoic acid

Page 31: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Good stuff! Order of reaction is critical! Good stuff! Order of reaction is critical! CH3

KMnO4

O OH

CH3

NO2

CH3

NO2

HNO3

H2SO4

O OH

NO2

HNO3

H2SO4

O OHO OH

NO2

NO2

2-nitrobenzoic acid4-nitrobenzoic acid 3-nitrobenzoic acid

KMnO4KMnO4

Page 32: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

H3C

O

Cl

AlCl3

O CH3

Br2

FeBr3Br

O CH3

O CH3

Br

Br2

FeBr3

Br

H3CO

Cl

AlCl3

meta director

ortho/para director

Some more good stuff!Some more good stuff!

Page 33: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

An explosive!An explosive!

CH3

NO2

NO2

O2N

2,4,6-trinitrotoluene (TNT)

Page 34: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

CH3

HNO3

H2SO4

CH3

NO2

CH3

NO2

Easy!

HNO3

H2SO4

CH3

NO2

NO2

more difficultHeat it!

HNO3

H2SO4

Even more difficultso really heat it!

CH3

NO2

NO2O2N

2,4,6-Trinitrotoluene = TNT

Page 35: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Some miscellaneous examplesSome miscellaneous examples

• Nitration of 3-nitrobenzoic acid

• Acylation of 1,3-dimethylbenzene

• Acylation of 1,4-dimethylbenzene

• Make 2-methyl-1-phenylpropane

Page 36: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Sect. 22.16 Aromaticity and Sect. 22.16 Aromaticity and Huckel’s RuleHuckel’s Rule

Aromatic compounds 4n + 2 pi electronsn = 1 6 pi electrons systems

+ _

Cation Anion

:

Page 37: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

H

H

H

Empty p-orbital

6-pi electrons

This is a n = 1with 6 pi electrons

Aromatic!

Page 38: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Other n = 1 aromaticsOther n = 1 aromatics

N OS

H

: : : : :

6 electrons 6 electrons, one pairnot involved!

6 electrons, one pair not involved!

All are aromatic!

Page 39: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

O

Oxygen has two pairs, but only one pair is in thepi system (p-orbital).

rehydridize the oxygen atom from sp3 to sp2

This system is a n = 1 system, with 6 pi electrons

This compound is AROMATIC!

Page 40: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

n = 0 aromatic: 2 pi electronsn = 0 aromatic: 2 pi electrons

+

Page 41: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

HH

H

2 pi electrons

Aromatic!

H

H

H

H

Cyclopropene is not aromatic, you need p orbital at all positions!

Page 42: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Some Antiaromatic compoundsSome Antiaromatic compounds

4 electrons 8 electrons

_

4 electrons

:

Not aromatic!

Page 43: WWU-Chemistry Aromatic Substitution Electrophilic.

WWU-ChemistryWWU-Chemistry

Diazonium ions, Azo Dyes and Diazonium ions, Azo Dyes and the Sandmeyer Reactionthe Sandmeyer Reaction- from - from

Chapter 23 (not covered 06)Chapter 23 (not covered 06)

Sect 23.16: Diazonium ion formationSect 23.17: Sandmeyer reaction Sect 23.19: Azo dyes


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