Chapter 9 Aromatic compounds ?????

1
Chapter 9 Aromatic compounds?????

• Text 1 chapter 16, 17
• Text 2 ???

2
Contents

• 1. The structure of benzene
• 2. Aromaticity
• 3. Nomenclature of benzene derivatives
• 4. Physical properties
• 5. Reactions of benzene and its derivatives
• Electrophilic aromatic substitution
  ----Effects of substituents Reactivity and
  orientation
• Nucleophilic aromatic substitution
• Addition reactions of benzene derivatives
• Side-chain reactions of benzene
  derivatives
• Reactions of phenols

3
Discovery of Benzene

• Isolated in 1825 by Michael Faraday who
  determined CH ratio to be 11.
• Synthesized in 1834 by Eilhard Mitscherlich who
  determined molecular formula to be C6H6.
• Other related compounds with low CH ratios had a
  pleasant smell, so they were classified as
  aromatic(????).

4
The Kekule Structure for benzene ???????
1. The Structure of benzene (16-2, 3)

• Proposed in 1866 by Friedrich Kekulé, shortly
  after multiple bonds were suggested.
• Failed to explain existence of only one isomer
  of 1,2-dichlorobenzene.

Same compund
5
Modern theories of the structure of benzene
RT
VB
MO
6
The unusual stability of benzene
Hydrogenation of just one double bond in benzene
is endothermic(??)!
7
Differences in Reactions between alkene and
benzene
8
Aromatic Requirements
2. Aromaticity???

• Structure must be cyclic with conjugated pi
  bonds.
• Each atom in the ring must have an unhybridized p
  orbital.
• The p orbitals must overlap continuously around
  the ring. (Usually planar structure)
• Compound is more stable than its open-chain
  counterpart

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Anti- and Nonaromatic?????????

• Antiaromatic(?????) compounds are cyclic,
  conjugated, with overlapping p orbitals around
  the ring, but the energy of the compound is
  greater than its open-chain counterpart.
• Nonaromatic(?????) compounds do not have a
  continuous ring of overlapping p orbitals and may
  be nonplanar.

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Hückels Rule (?????)
The (4n2) pi-Electron Rule

• If the compound has a continuous ring of
  overlapping p orbitals and has 4N 2 pi
  electrons, it is aromatic.
• If the compound has a continuous ring of
  overlapping p orbitals and has 4N electrons, it
  is antiaromatic.

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Other aromatic compounds
(1) polynuclear aromatic hydrocarbons
(PAH, ????,16-10)
Fused aromatic compounds????
12
As the number of aromatic rings increases, the
resonance energy per ring decreases, so larger
PAHs will add Br2.
(mixture of cis and trans isomers)
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(2) Nannulenes (??)

• 4Annulene is antiaromatic (4N e-s)
• 8Annulene would be antiaromatic, but its not
  planar, so its nonaromatic.
• 10Annulene is aromatic except for the isomers
  that are not planar.
• Larger 4N annulenes are not antiaromatic because
  they are flexible enough to become nonplanar.
  

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Nonaromatic ????
8annulene
10annulene
nonaromatic
14annulene
nonaromatic
16annulene
Antiaromatic if planar
18annulene
Aromatic if planar
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Azulene ?
The dipole moment is 1.0D
( aromatic ?????)
(aromatic)
Answer (A)
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MO Derivation of Hückels Rule

• Lowest energy MO has 2 electrons.
• Each filled shell has 4 electrons.

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The Polygon Rule?????
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Cyclopentadienyl Ions ??????
Aromatic ions
4 electrons, antiaromatic.
6 electrons, aromatic.
Acidity
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Tropylium Ion ????
Aromatic ions
Cycloheptatrienyl cation ???????

• 6 p electrons and an empty p orbital.
• Aromatic more stable than open chain ion

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Dianion of 8Annulene 8???????
Aromatic ions
Cyclooctatetraenyl dianion ?????????

• 10 p electrons, aromatic.

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(3) Heterocyclic aromatic compounds (16-9)
???????(????)
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Pyridine ??

• Nonbonding pair of electrons in sp2 orbital, so
  weak base, pKb 8.8.

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Pyrrole ??

• Also aromatic, but lone pair of electrons is
  delocalized, so much weaker base.

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Basic or Nonbasic?
Pyrimidine(??) has two basic nitrogens.
Imidazole(??) has one basic nitrogen and one
nonbasic.
Purine(??) ?
25
gt
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3. Nomenclature of benzene derivatives

• Monosubstituted benzenes
• XXbenzene , XX?

Fluorobenzene (??)
Chlorobenzene (??)
Bromobenzene (??)
Nitrobenzene (???)
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Methylbenzene Toluene (??)
Ethylbenzene (??)
Hydroxybenzene Phenol (??)
Aminobenzene Aniline (???, ??)
Benzenesulfonic acid ???
Benzoic acid ???
Acetophenone ???
Anisole ???
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(2) disubstituted benzenes 1,2-
1,3- 1,4- o- (?), m- (?), p- (?)
1,3-Dibromobenzene m-Dibromobenzene ????
1,2-Dibromobenzene o-Dibromobenzene ????
1,4-Dibromobenzene p-Dibromobenzene ????
29
1,2-Dimethylbenzene 1,3-Dimethylbenzne
1,4-Dimethylbenzene o-xylene
m-xylene p-xylene
2-Nitrobenzoic acid 3-Nitrobenzic acid
4-Nitrobenzic acid
o-Nitrobenzoic acid m-Nitrobenzoic acid
p-Nitrobenzoic acid
30
(3) benzene as substituent phenyl
group (??), Ph-
Butylbenzene
2-Phenyl-2-butene
2-Phenylheptane
Benzyl group (??),Bn-
Aryl group (??),Ar-
Arene Aromatic hydrocarbon
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4. Physical properties and spectroscopy of
aromatic compounds

• Melting points More symmetrical than
  corresponding alkane, pack better into crystals,
  so higher melting points.
• Boiling points Dependent on dipole moment, so
  ortho gt meta gt para, for disubstituted benzenes.
• Density More dense than nonaromatics, less dense
  than water.
• Solubility Generally insoluble in water.

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spectroscopy of aromatic compounds
UV 254 nm (B ?)
IR 3030 (C-H), 1600 (skeleton), 1500 cm-1
1H NMR 79ppm
MS
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(No Transcript)
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1-phenylpropan-2-one
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Chapter 9 Aromatic compounds

• Assignments
• T-1 16-27, 28, 30, 33, 36

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Chapter 9 Aromatic compounds
5. Reactions of benzene and its derivatives
(Chapter 17)
37
Many benzene and naphthalene(?) derivatives are
useful drugs
38

• 1) Electrophilic aromatic substitution reactions

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Electrophilic addition-elimination
mechanism ????-????
Step 1 addition
Arenium ion (?????) intermediate s- complex
(s-???)
Step 2 elimination
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E
TS 1
rate-limiting step
TS 2
Eact2
Eact1
s-complex
reactants
product
reaction coordination
The reaction-energy diagram (?????)
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Halogenation of benzene X2, FeX3
Reactivity F2 gt Cl2 gt Br2 gt I2
42
Nitration of benzene HNO3 H2SO4
Nitronium ion srong electrophile
43
Sulfonation of benzene fuming sulfuric acid
(????) 7
SO3 in H2SO4
44
The Friedel-Crafts alkylation of benzene
RX, AlCl3( or ether Lewis acid)
E
CC, H
Other carbocation source
C-C-OH, acid
Limitations

• R can rearrangement
• Polyalkylations often occur.

45
Problems Predict products for the reactions
of benzene with the following reagents.
46
The Friedel-Crafts Acylation of benzene
RCOCl, AlCl3 (Lewis acid)
E
Acylium ion ?????
47
The Gatterman-Koch formylation synthesis of
genzaldehyde
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The Clemmensen reduction synthesis of
alkylbenzenes.
The Clemmensen reduction
How to shnthesize n-propylbenzene from benzene?
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The effects of substituents on the electrophilic
aromatic substitution Reactivity and Orientation

• Reactivity
• Electron donating group (???) Activiting
  (??)
• Electron withdrawing (???) deactivating
  (??)
• (2) Orientation o-, m-, p-, which is major?

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Nitration of toluene
Yield 60 4 36

• Toluene is about 25 times faster than benzene.

Why?
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A) Activating groups ortho-para
directors??,???????
17-6
-O-, -NR2, HO-, RO-, CH3CONH-, R-
Alkyl groups, substituents with nonbonding
electrons (O, N) can activate the aromatic ring.
(???????????????, ??????, ?)???????.)
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B) Deactivating groups Meta-Directors??,?????
17-7
-NR3, -NO2, -CF3, -CCl3, -CN, -SO3H, -CHO, RCO-,
-COOH, -COOR, CH3CO-,
In general, deactivating substituents are groups
with a positive charge (or a partial positive
charge) on the atom bonded to the aromatic ring.

(???????????????, ????????.)
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C) Halo substituents Deactivating but
ortho-para directors (????, ????????)
17-8
Halo substitution electron withdrawing groups,
????
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Inductive effect explanation ??????
Explanation of the orientaion
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Resonance effect explanation??????
m- director
o-, p- director
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d
o-
charges adjacent, unstable
d
p-
Conclusion -CF3 is a m-director?
m-
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o-
charges adjacent, unstable
p-
Conclusion -NO2 is a m-director?
m-
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o-
3favorable
Conclusion -CH3 is a o- and p-director.
p-
3favorable
m-
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o-
Especially stable
p-
Conclusion -OH is a o- and p-director.
m-
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o-
Especially stable
p-
Conclusion -Cl is a o- and p-director.
m-
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Notice Friedel-Crafts reactions fail with
strongly deactivated systems.
Aniline and phenol (especialy aniline )does not
undergo a Friedel-Crafts reaction
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Orientation in disubstituted benzenes

• ?????????????????

• ??????????????????????? ??????????????,?????????
  ?

biphenyl ??
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2) Nucleophilic aromatic substitution
Substrate aryl halide (???)
????????????
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Addition-elimination mechanism
Elimination-addition mechanism benzyne ????
base NaNH2 or high pressure
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3) Addition reactions of benzene derivatives
(1) Chlorination
Free radical mechanism
(2) Addition of hydrogen (reduction)
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The Birch reduction ????
Developed by the Australian chemist A.J. Birch
Mechanism similar as trans- reduction of alkyne
with Na/NH3(liq.) (p-756)
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electron donating ????,???2,5-?
electron withdrawing ????, ???1,4- ?
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4) Reactions of the side chain of alkylbenzenes
(1) Halogenation of the side chain benzylic
radicals
useful in synthesis
Notice ??????,???????ß??????
69
(2) Oxidation of the side chain
The alkyl side chain with a - H on a
benzene ring is oxidized to benzoic acid by
chromic acid(??) or KMnO4.
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5) phenols (17-15)
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Synthesis of phenol
in laboratory
industrial synthesis
72
Reactions of phenols
A) Acidity
B) Acylation of hydroxyl group
Aspirin ????
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C) Electrophilic aromatic substitution
White precipitate.
74
The o- and p- nitrophenol can be separated by
steam distillation(?????).
rate control
equilibrium control
75
D) Oxidation of phenols
p-Benzoquinone (?-??)
Notice ????????????,?????
E) Reaction with FeCl3
phenol , ???,???????????????????
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6) Naphthalene ?
1-nitronaphthalene a-nitronaphthalene
5-methyl-2-naphthalenesulfonic acid 5-methyl- ß
-naphthalenesulfonic acid
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Reactions of naphthalene
Reactivity naphthalene gt benzene
a- gtß-
A) Substitution
rate control
equilibrium control
78
Notice ??????????????????????
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B) Oxidation and reduction
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1,4-dihydronaphthalene 1,4-???
decalin???
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Summary

1. ????????,???RT,VB,MO?????????????
2. ??Hückel rule ????????????????
3. ????????,????????????
4. ?????????
5. ???????????????????????????,???????????Fridel-Craf
   ts ?????????,????????????
6. ??????????????????????????????
7. ??????????????????????
8. ????????,????????Berch reduction?????????
9. ????????????????????

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Problems
How to synthesis the following compounds?
???,????
???-??-??-??
83
?????????????????????????
?????? ???-??-??
???-??-????????
84
Propose mechanism and predict the structure of
the product.
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Assignments

• T-1 17-44, 46, 47, 49, 50, 51,
• 57,59, 61, 65
• T-2 (selected) 5, 6, 7, 11, 12, 18, 19