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Basic Organic ChemistryCourse code: CHEM 12162
(Pre-requisites : CHEM 11122)
Chapter – 08Chapter – 08
Aromatic Compounds
Dr. Dinesh R. Pandithavidana
Office: B1 222/3
Phone: (+94)777-745-720 (Mobile)
Email: [email protected]
Discovery of Benzene
• Isolated in 1825 by Michael Faraday who
determined C:H ratio to be 1:1.
• Synthesized in 1834 by Eilhard Mitscherlich
who determined molecular formula to be who determined molecular formula to be
C6H6.
• Other related compounds with low C:H ratios
had a pleasant smell, so they were classified
as aromatic.
Resonance Structure
C
C
CC
C
C
H
H
H
H
H
H Proposed in 1866 by Friedrich
Kekulé, shortly after multiple bonds
were suggested.
Each sp2 hybridized C in the ring has an unhybridized
p orbital perpendicular to the ring which overlaps
around the ring.
Unusual Reactions
• Alkene + KMnO4 → diol (addition)
Benzene + KMnO4 → no reaction.
• Alkene + Br2/CCl4 → dibromide (addition)
Benzene + Br /CCl → no reaction.Benzene + Br2/CCl4 → no reaction.
• With FeCl3 catalyst, Br2 reacts with benzene
to form bromobenzene + HBr (substitution!).
Double bonds remain.
Annulenes
• All cyclic conjugated hydrocarbons were proposed to be aromatic.
• However, cyclobutadiene is so reactive that it is so reactive that it dimerizes before it can be isolated.
• And cyclooctatetraene adds Br2 readily.
Hückel’s Rule
• If the compound has a continuous ring of
overlapping p orbitals and has 4N + 2 electrons, it
is aromatic (molecule should be planar).
• If the compound has a continuous ring of • If the compound has a continuous ring of
overlapping p orbitals and has 4N electrons, it is
antiaromatic.
• Nonaromatic compounds do not have a
continuous ring of overlapping p orbitals and may
be nonplanar
[N]Annulenes
• [4]Annulene is antiaromatic
• [6]Annulene is aromatic, it’s a
planar, molecule.
• [8]Annulene would be antiaromatic,
• [10]Annulene is aromatic except for
the isomers that are not planar.
Cyclopentadienyl Ions
• The cation has an empty p orbital, 4 electrons, so antiaromatic.
• The anion has a nonbonding pair of electrons in a p orbital, 6 electrons, aromatic.
Acidity of Cyclopentadiene
pKa of cyclopentadiene is 16, much more
acidic than other hydrocarbons.
pKa = 19pKa = 16
HOC(CH3)3+
H
OC(CH3)3_
+
H H
Tropylium Ion
• The cycloheptatrienyl cation has 6 p electrons
and an empty p orbital.
• Aromatic: more stable than open chain ion
H OH
H+ , H2O
H
+
Dianion of [8]Annulene
• Cyclooctatetraene easily forms a 2- ion.
• Ten electrons, continuous overlapping porbitals, so it is aromatic.
+ 2 K + 2 K+
Pyridine
• Heterocyclic aromatic compound.
• Nonbonding pair of electrons in sp2 orbital, so
weak base, pKb = 8.8.
Pyrrole
Also aromatic, but lone pair of electrons is
delocalized, so much weaker base.
Basic or Nonbasic-.???
NN
Pyrimidine has two basic
nitrogens.
N N HImidazole has one basic
nitrogen and one nonbasic.
N
N
N
N
H
PurineB?
Other Heterocyclic Compounds
Fused Ring Hydrocarbons
• Naphthalene
• Anthracene
• Phenanthrene
Common Names of Benzene
Derivatives
OH OCH3NH2CH3
phenol toluene aniline anisolephenol toluene aniline anisole
C
H
CH2 C
O
CH3
C
O
HC
O
OH
styrene acetophenone benzaldehyde benzoic acid
Disubstituted Benzenes
The prefixes ortho-, meta-, and para- are commonly
used for the 1,2-, 1,3-, and 1,4- positions,
respectively.
BrBr
Br
o-dibromobenzene or1,2-dibromobenzene
HO
NO2
p-nitrophenol or4-nitrophenol
3 or More Substituents
Use the smallest possible numbers, but the
carbon with a functional group is #1.
OH
NO2
NO2
O2N
1,3,5-trinitrobenzene
NO2
NO2
O2N
OH
2,4,6-trinitrophenol
Common Names for
Disubstituted Benzenes
CH3 CH3
CH
CO OH
OH
CH3 CH3H3C
CH3OH
H3Cm-xylene mesitylene o-toluic acid p-cresol
Phenyl and Benzyl
Phenyl indicates the benzene ring attachment.
The benzyl group has an additional carbon.
Br
phenyl bromide
CH2Br
benzyl bromide
=>
Physical Properties
• Melting points: More symmetrical than
corresponding alkane, pack better into
crystals, so higher melting points.
• Density: More dense than nonaromatics, less
dense than water.
• Solubility: Generally insoluble in water.
Electrophilic Aromatic Substitution
Electrophile substitutes for a hydrogen on
the benzene ring.
Mechanism
Bromination of Benzene
• Requires a stronger electrophile than Br2.
• Use a strong Lewis acid catalyst, FeBr3.
Br Br FeBr3 Br Br FeBr3
Br Br FeBr3
H
H
H
H
H
H
H
H
H
H
HH
Br+ + FeBr4
_
Br
HBr+
Chlorination and Iodination
• Chlorination is similar to bromination. Use
AlCl3 as the Lewis acid catalyst.
• Iodination requires an acidic oxidizing agent,
like nitric acid, which oxidizes the iodine to an like nitric acid, which oxidizes the iodine to an
iodonium ion.
H+
HNO3 I21/2 I+
NO2 H2O+ ++ +
Nitration of Benzene
Use sulfuric acid with nitric acid to form the
nitronium ion electrophile.
H O N
O
O
H O S O H
O
O
+ HSO4
_H O N
OH
O+
H O N
OH
O+
H2O + N
O
O
+
Sulfonation
Sulfur trioxide, SO3, in fuming sulfuric acid is
the electrophile.
S
O
S
O
S
O
S
O
+ + +
_
_ _
2 H2SO4 SO3 + H3O+ + HSO4
-
O OS
O OS
O OS
O O
+ +_ _
S
O
OO
H
S
O
O
OH
+
_
S
HOO
O
benzenesulfonic acid
Nitration of Toluene
• Toluene reacts 25 times faster than benzene. The
methyl group is an activator.
• The product mixture contains mostly ortho and para
substituted molecules.
Sigma Complex
Intermediate
is more
stable if
nitration nitration
occurs at the
ortho or para
position.
Energy Diagram
Activating, Ortho-, Para- Directing
Substituents
• Alkyl groups stabilize the sigma complex by
induction, donating electron density through the
sigma bond.
• Substituents with a lone pair of electrons stabilize
the sigma complex by resonance.
OCH3
H
NO2
+
OCH3
H
NO2
+
The Amino Group
Aniline reacts with bromine water (without a
catalyst) to yield the tribromide.
Sodium bicarbonate is added to neutralize the
HBr that’s also formed.
NH2
Br23
H2O, NaHCO3
NH2
Br
Br
Br
Summary of Activators
Deactivating Meta-Directing
Substituents
• Electrophilic substitution reactions for nitrobenzene are 100,000 times slower than for benzene.
• The product mix contains mostly the meta isomer, • The product mix contains mostly the meta isomer, only small amounts of the ortho and para isomers.
• Meta-directors deactivate all positions on the ring, but the meta position is less deactivated.
Ortho Substitution on Nitrobenzene
Para Substitution on Nitrobenzene
Meta Substitution on Nitrobenzene
Meta-Deactivators
The atom attached to the aromatic ring will have a partial
positive charge.
Electron density is withdrawn inductively along the sigma
bond, so the ring is less electron-rich than benzene.
More Deactivators
Halobenzenes
• Halogens are deactivating toward electrophilic
substitution, but are ortho, para-directing!
• Since halogens are very electronegative, they
withdraw electron density from the ring inductively
along the sigma bond.
• But halogens have lone pairs of electrons that can
stabilize the sigma complex by resonance.
Summary of Directing Effects
Multiple Substituents
The most strongly activating substituent will
determine the position of the next substitution.
May have mixtures.
OCH3
O2N
SO3
H2SO4
OCH3
O2N
SO3H
OCH3
O2N
SO3H
+
Friedel-Crafts Alkylation
• Synthesis of alkyl benzenes from alkyl halides and
a Lewis acid, usually AlCl3.
• Reactions of alkyl halide with Lewis acid produces
a carbocation which is the electrophile.a carbocation which is the electrophile.
• Other sources of carbocations:
alkenes + HF or alcohols + BF3.
Examples of Carbocation Formation
CH3 CH CH3
Cl
+ AlCl3
CH3
C
H3C H
Cl AlCl3+ _
F_
H2C CH CH3
HFH3C CH CH3
F+
H3C CH CH3
OHBF3
H3C CH CH3
OH BF3+
H3C CH CH3
++ HOBF3
_
Formation of Alkyl Benzene
C
CH3
CH3
H+
H
H
CH(CH3)2+
H
H
CH(CH3)2
B
F
F
F
OH
CH
CH3
CH3
+
HF
BF
OHF+
-
Limitations of Friedel-Crafts Reactions
• Reaction fails if benzene has a substituent that is more
deactivating than halogen.
• Carbocations rearrange. Reaction of benzene with n-
propyl chloride and AlCl3 produces isopropylbenzene.
• The alkylbenzene product is more reactive than benzene,
so polyalkylation occurs.
(CH3)2CHCl
AlCl3
CH(CH3)2 CH(CH3)2
CH(CH3)2
+
Friedel-Crafts Acylation
• Acyl chloride is used in place of alkyl
chloride.
• The acylium ion intermediate is resonance
stabilized and does not rearrange like a stabilized and does not rearrange like a
carbocation.
• The product is a phenyl ketone that is less
reactive than benzene.
Mechanism of Acylation
R C
O
Cl AlCl3 R C
O
AlCl3Cl+ _
R C
O
AlCl3Cl+ _
AlCl4 +
_ +R C O R C O
+
C
O
R
+
H
C
H
O
R
+
Cl AlCl3
_C
O
R +
HCl
AlCl3
Clemmensen Reduction
Acylbenzenes can be converted to
alkylbenzenes by treatment with aqueous HCl
and amalgamated zinc.
+ CH3CH2C
O
Cl1) AlCl3
2) H2O
C
O
CH2CH3Zn(Hg)
aq. HCl
CH2CH2CH3
Nucleophilic Aromatic
Substitution
• A nucleophile replaces a leaving group on
the aromatic ring.
• Electron-withdrawing substituents activate
the ring for nucleophilic substitution.
Examples of Nucleophilic Substitution
Addition-Elimination Mechanism
Benzyne Mechanism
• Reactant is halobenzene with no electron-withdrawing groups on the ring.
• Use a very strong base like NaNH2.
Benzyne Intermediate
CH3
HH
H NH2
CH3
HH
H NH2
_
or
CH3
HH
H
NH2
CH3
HH
H
NH2_
meta-toluidine para-toluidine
Catalytic Hydrogenation
• Elevated heat and pressure is required.
• Possible catalysts: Pt, Pd, Ni, Ru, Rh.
• Reduction cannot be stopped at an
intermediate stage.
CH3
CH3
Ru, 100°C
1000 psi3H2,
CH3
CH3
Birch Reduction
Mechanism of Birch Reduction
Mechanism of Birch Reduction
Side-Chain Oxidation
Alkylbenzenes are oxidized to benzoic acid by
hot KMnO4 or Na2Cr2O7/H2SO4.
CH(CH3)2
CH CH2
KMnO4, OH-
H2O, heat
COO
COO
_
_
Side-Chain Halogenation
• Benzylic position is the most reactive.
• Chlorination is not as selective as bromination,
results in mixtures.
• Br2 reacts only at the benzylic position.
CHCH2CH3
Br
hνBr2,
CH2CH2CH3
SN1 Reactions
• Benzylic carbocations are resonance-
stabilized, easily formed.
• Benzyl halides undergo SN1 reactions.
CH2BrCH3CH2OH, heat
CH2OCH2CH3
SN2 Reactions
• Benzylic halides are 100 times more
reactive than primary halides via SN2.
• Transition state is stabilized by ring.