Chapter 16 Aromatic Co mpounds Jo Blackburn Richland College, Dallas, TX Dallas County Community College District 2003, Prentice Hall Organic Chemistry , 5 th Edition L. G. Wade, Jr.
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Chapter 16 Aromatic Compounds
Jo BlackburnRichland College, Dallas, TX
Dallas County Community College District 2003, Prentice Hall
Organic Chemistry , 5th Edition
L. G. Wade, Jr.
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Chapter 16 2
Discovery of Benzene
• Isolated in 1825 by Michael Faraday
who determined C:H ratio to be 1:1.
• Synthesized in 1834 by EilhardMitscherlich who determined molecular
formula to be C6H6.
• Other related compounds with low C:Hratios had a pleasant smell, so they
were classified as aromatic.
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Chapter 16 3
Kekulé Structure
• Proposed in 1866 by Friedrich Kekulé, shortly
after multiple bonds were suggested.
• Failed to explain existence of only one isomerof 1,2-dichlorobenzene.
C
C
CC
C
C
H
H
H
H
H
H
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Chapter 16 4
Resonance Structure
Each sp2 hybridized C in the ring has anunhybridized p orbital perpendicular tothe ring which overlaps around the ring.
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Chapter 16 5
Unusual Reactions
• Alkene + KMnO4 diol (addition)
Benzene + KMnO4 no reaction.
• Alkene + Br 2/CCl4 dibromide (addition)Benzene + Br 2/CCl4 no reaction.
• With FeCl3 catalyst, Br 2 reacts with
benzene to form bromobenzene + HBr(substitution!). Double bonds remain.
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Chapter 16 6
Unusual StabilityHydrogenation of just one double
bond in benzene is endothermic!
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Chapter 16 7
Annulenes
• All cyclic conjugatedhydrocarbons wereproposed to be aromatic.
• However, cyclobutadiene
is so reactive that itdimerizes before it canbe isolated.
• And cyclooctatetraene
adds Br 2 readily.
• Look at MO’s to explainaromaticity. =>
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Chapter 16 8
MO Rules for Benzene
• Six overlapping p orbitals must form six
molecular orbitals.
• Three will be bonding, three antibonding.
• Lowest energy MO will have all bonding
interactions, no nodes.
• As energy of MO increases, the number of
nodes increases. =>
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Chapter 16 9
MO’s for Benzene
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Chapter 16 10
Energy Diagram for
Benzene• The six electrons fill three bonding pi orbitals.
• All bonding orbitals are filled (“closed shell”),
an extremely stable arrangement.
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Chapter 16 11
MO’s for Cyclobutadiene
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Chapter 16 12
Energy Diagram for
Cyclobutadiene
• Following Hund’s
rule, two electronsare in separateorbitals.
• This diradical would
be very reactive.
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Chapter 16 13
Polygon Rule
The energy diagram for an annulene has
the same shape as the cyclic compound
with one vertex at the bottom.
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Chapter 16 14
Aromatic Requirements
• Structure must be cyclic with conjugated
pi bonds.
• Each atom in the ring must have anunhybridized 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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Chapter 16 15
Anti- and Nonaromatic
• Antiaromatic compounds are cyclic,
conjugated, with overlapping p orbitals
around the ring, but the energy of thecompound is greater than its open-chain
counterpart.
• Nonaromatic compounds do not have acontinuous ring of overlapping p orbitals
and may be nonplanar. =>
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Chapter 16 16
Hückel’s Rule
• If the compound has a continuous ring
of overlapping p orbitals and has 4N + 2
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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Chapter 16 17
[N ]Annulenes
• [4]Annulene is antiaromatic (4N e-’s)
• [8]Annulene would be antiaromatic, but
it’s not planar, so it’s nonaromatic. • [10]Annulene is aromatic except for the
isomers that are not planar.
• Larger 4N annulenes are notantiaromatic because they are flexible
enough to become nonplanar. =>
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Chapter 16 18
MO Derivation of
Hückel’s Rule• Lowest energy MO has 2 electrons.
• Each filled shell has 4 electrons.
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Chapter 16 19
Cyclopentadienyl Ions
• The cation has an empty p orbital, 4 electrons,so antiaromatic.
• The anion has a nonbonding pair of electronsin a p orbital, 6 e-’s, aromatic.
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Chapter 16 20
Acidity of Cyclopentadiene
pK a of cyclopentadiene is 16, much more
acidic than other hydrocarbons.
=>
p K a = 19 p K a = 16
HOC(CH3)3+
H
OC(CH3)3
_
+
H H
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Chapter 16 21
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
+
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Chapter 16 22
Dianion of [8]Annulene
• Cyclooctatetraene easily forms a -2 ion.
• Ten electrons, continuous overlapping p
orbitals, so it is aromatic.
=>
+ 2 K + 2 K +
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Chapter 16 23
Pyridine
• Heterocyclic aromatic compound.• Nonbonding pair of electrons in sp2
orbital, so weak base, pK b = 8.8.
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Chapter 16 24
Pyrrole
Also aromatic, but lone pair of electrons isdelocalized, so much weaker base.
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Chapter 16 25
Basic or Nonbasic?
N N
Pyrimidine has two basic
nitrogens.
N N HImidazole has one basic
nitrogen and one nonbasic.
N
N
N
N
H
Purine?=>
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Chapter 16 26
Other Heterocyclics
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Chapter 16 27
Fused Ring Hydrocarbons• Naphthalene
• Anthracene
• Phenanthrene=>
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Chapter 16 28
Reactivity of
Polynuclear Hydrocarbons As the number of aromatic rings increases,
the resonance energy per ring decreases,
so larger PAH’s will add Br 2.
H Br
H Br H Br
Br
H
(mixture of cis and trans isomers) =>
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Chapter 16 29
Fused Heterocyclic
CompoundsCommon in nature, synthesized for drugs.
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Chapter 16 30
Allotropes of Carbon• Amorphous: small particles of graphite;
charcoal, soot, coal, carbon black.
• Diamond: a lattice of tetrahedral C’s.
• Graphite: layers of fused aromatic rings.
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Chapter 16 31
Some New Allotropes
• Fullerenes: 5- and 6-membered ringsarranged to form a “soccer ball” structure.
• Nanotubes: half of a C60 sphere fused to a
cylinder of fused aromatic rings.
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Chapter 16 32
Common Names of
Benzene DerivativesOH OCH3 NH2
CH3
phenol toluene aniline anisole
C
H
CH2 C
O
CH3
C
O
H
C
O
OH
styrene acetophenone benzaldehyde benzoic acid
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Chapter 16 33
Disubstituted Benzenes
The prefixes ortho-, meta-, and para- are
commonly used for the 1,2-, 1,3-, and 1,4-
positions, respectively.Br
Br
o-dibromobenzene o1,2-dibromobenzene
HO
NO2
p-nitrophenol or 4-nitrophenol
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Chapter 16 34
3 or More Substituents
Use the smallest possible numbers, but
the carbon with a functional group is #1.
NO2
NO2
O2 N
1,3,5-trinitrobenzen
NO2
NO2
O2 N
OH
2,4,6-trinitrophenol
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Chapter 16 35
Common Names for
Disubstituted Benzenes
CH3
CH3
CH3
CH3H3C
CH3
CO OH
OH
H3Cm-xylene mesitylene o-toluic acid p-cresol
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Chapter 16 36
Phenyl and Benzyl
Br
phenyl bromide
CH2Br
benzyl bromide
Phenyl indicates the benzene ring
attachment. The benzyl group has
an additional carbon.
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Chapter 16 37
Physical Properties
• Melting points: More symmetrical thancorresponding alkane, pack better into
crystals, so higher melting points.
• Boiling points: Dependent on dipolemoment, so ortho > meta > para, for
disubstituted benzenes.
• Density: More dense than nonaromatics,less dense than water.
• Solubility: Generally insoluble in water. =>
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Chapter 16 38
IR and NMR Spectroscopy
• C=C stretch absorption at 1600 cm-1.
• sp2 C-H stretch just above 3000 cm-1.
• 1H NMR at 7-8 for H’s on aromaticring.
• 13C NMR at 120-150, similar to alkene
carbons.
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Chapter 16 39
Mass Spectrometry
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Chapter 16 40
UV Spectroscopy
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