AROMATIC COMPOUNDS

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AROMATIC COMPOUNDS
by Parinya Theramongkol Department of
Chemistry Khon Kaen University
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ORGANIC COMPOUNDS
Aliphatic (fatty)
Aromatic (fragrant)
Open-chain cyclic compounds
Benzene others that resemble benzene in
chemical behavior
AROMATIC CHARACTER
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Aromatic character The HÜckel 4n2 rule
An aromatic compound must have a molecule that
contains cyclic clouds of delocalized p
electrons above and below the plane of the
molecule and the p clouds must contain a total
of ( 4n2 ) p electrons.
Benzene C6H6
Kekulé structures
Resonance hybrid
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Stability of the benzene ring resonance energy
Benzene contains 36 kcal less energy than
predicted !
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Resonance structure of benzene
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Resonance hybrid
Kekulé structures
Resonance structures structures that differ
only in the arrangement of electrons
Benzene is a resonance hybrid of I and II
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Orbital picture of benzene
Benzene is
a flat molecule
very symmetrical
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Nomenclature

1. Simply prefix the name of the substituent
   groupto the word -benzene

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2. Special names
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3. Disubstituted benzene
Tell group names relative positions
3.1 Same groups
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3.2 Different groups
3.2.1 neither is a special groupend the name
with -benzene
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3.2.2 a special group presentname as a
derivative of that special compound.
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4. Polysubstituted benzene
Use numbers to indicate their relative positions
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Polynuclear aromatic hydrocarbons
Aromatic rings share a pair of carbon atoms
fused - ring
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Reactivity benzene vs. alkene
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Reaction
Typical reactions of benzene Electrophilic
substitution
addition
substitution
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The first substitution
1. Bromination Lewis acid needed e.g., FeBr3 ,
AlBr3
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Benzenonium ion is stable via resonance

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2. Nitration
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3. Sulfonation
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4. Friedel-Crafts Alkylation (1877)
Charles Friedel 1832-1899
James Mason Crafts 1839-1917
Page out of Friedel's notebook
Alkylation of an aromatic ring with an alkyl
halide and a trace of AlCl3.
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But there are problems with this alkylation
method !
1. The electrophile may undergo rearrangement
because it is a carbocation.
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No rearrangement
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Rearrangement
1,2-H-shift
2o carbocation
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2. The alkyl substituent activates the ring so
that a second substitution may also occur.
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5. Friedel-Crafts Acylation
RCO- or ArCO- group is called an acyl group. This
is a method of choice to prepare an aryl ketone.
Linear alkyl benzene obtainedwithout
rearrangement !
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Mechanism of Friedel-Crafts Acylation
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The second substitution
The first substituent may affect the second
substitution in 2 ways.
1. Reactivity
the second substitution may be easier
Activating group
the second substitution may be harder
Deactivating group
2. Orientation
ortho
ortho , para - director
meta - director
meta
para
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Effect of the 1st substituent on the 2nd
substitution
o,p-Directors
m-Directors(all deactivating)
Increasingactivation
Increasingdeactivation
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Mechanism of the 2nd substitution with an
o,p-director

1. A ring is activated by increasing electron
   density.
2. Electron density can be increased by an electron
   donating group.
3. A group can donate electrons bya) inductive
   effect b) resonance effect

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No catalyst needed !
a benzenonium ion
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p-intermediate gets extra stabilization by
resonance effect
extra stabilization extra resonance form
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Alkyl group is electron donating by the inductive
effect
ortho
para
Extra stabilization next to e- donating group
meta
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Mechanism of the 2nd substitution with a
m-director
an e withdrawing group
A meta-director is
deactivates all positions
deactivates meta-positions less than the other
positions
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ortho
para
Destabilization next to ve center
meta
Favored no adjacent charges
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Summary of substituent effects
A substituent that is electron-donating activates
a benzene ring andis an o,p-director.
A halogen is an o,p-director (donating electrons
by resonance), butdeactivating by its electron
withdrawing inductive effect.
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A meta director deactivates all positions on the
ring by electronwithdrawal and deactivates the
o,p-positions especially by resonance-destabiliza
tion.
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Aromatic substitution as a synthetic tool
To prepare substituted benzene compounds the
order of substitution reactions is of importance.
Example to prepare o- and p-chloronitrobenzene
Nitration or chlorination in the first place ?
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How about m-chloronitrobenzene ?
Now try to prepare m-bromoaniline !!!!
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This one needs the conversion of one groupto
another ( reduction of nitro to amino).
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Notes about Friedel-Crafts alkylation and
acylation
Aniline does not undergo Friedel-Crafts reactions
because an aminogroup (a basic group) reacts
with Lewis acids !
Nitrobenzene does not undergo Friedel-Crafts
reactions because the ring is deactivated by the
strong e-withdrawing nitro group.
No aromatic substitution
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Alkylbenzenes The benzylic position
Benzylic position is a site of attack in many
reactions becausethe benzyl cation, a benzyl
radical, and benzyl carbanion are
allresonance-stabilized by the benzene ring !
Oxidation
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Free radical halogenation
Takes place on the alkyl sidechain preferentially
!
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Problems
1. ???????????????????? monobromination ?????
?????????????????????????????????????????????????
??????????????????????????????????????????a)
acetanilide(C6H5NHCOCH3) b) iodobenzenec)
sec-butylbenzene d) N-methylaniline(C6H5NHCH3)e)
ethyl benzoate(C6H5COOC2H5) f)
acetophenone(C6H5COCH3) 2. ??????????????????????
??????????????????????????????????????????????mono
nitration ??????????????a) o-nitrotoluene b)
m-dibromobenzenec) m-cresol(m-CH3C6H4OH) d)
o-cresole) terephthalic acid(p-C6H4(COOH)2 f)
anilinium hydrogen sulfate
(C6H5NH3HSO4-) 3. ???????????????????
???????nitration ????????????????? ?
???????????????????????????????????a) b)
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4. ???????????????????????????????????????????????
????????????? electrophilic substitution?????????
???????????????????????????????????????????
m-isomer ????????? ??????????????????????a)
C6H5N(CH3)3, C6H5CH2N(CH3)3, C6H5CH2 CH2
N(CH3)3,b) C6H5NO2 , C6H5 CH2 NO2 , C6H5 CH2
CH2 NO2 c) C6H5CH3 , C6H5CH2COOC2H5 ,
C6H5CH(COOC2H5)2 , C6H5C(COOC2H5)3
5. ??????????????????????????????????????????????
...a) ethylbenzene Cl2 b) ethylbenzene
Br2c) toluene 1-chloropropaned) toluene
propene
6. ?????????????????????????????????????????
aromatic a)
b)c) d)
cyclooctatetraenedianion