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KOT 222 ORGANIC CHEMISTRY II
CHAPTER 16
AROMATIC COMPOUNDS
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Benzene
An aromatic compound.
Isolated in 1825 by Michael Faraday whodetermined C:H ratio to be 1:1.
Synthesized in 1834 by Eilhard Mitscherlich
who determined molecular formula to beC6H6.
Other related compounds with low C:H ratios
had a pleasant smell, so they were classifiedas aromatic.
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Kekul Structure of Benzene
Proposed in 1866 by Friedrich Kekul, shortlyafter multiple bonds were suggested.
Showing localized double-bonds.
Failed to explain existence of only one isomer of1,2-dichlorobenzene.
Cl
Cl
Cl
ClX
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Resonance Structure of Benzene
Benzene is actually a resonance hybrid between
the two Kekul structures.The pi electrons are delocalized, with a bond
order of 1.5 between adjacent carbon atoms
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Orbital Representation of Benzene
Benzene is a flat ring of sp2 hybrid carbon atomswith their unhybridized porbitals all aligned andoverlapping.
The conjugation and delocalization of theelectrons give greater stability.
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Unusual Reactions of Benzene
Benzene as a cyclic conjugated triene is predicted toreact as polyenes.
But, its reactions are unusual.
Alkene + KMnO4 diol (addition)Benzene + KMnO4 no reaction.
Alkene + Br2/CCl4 dibromide (addition)
Benzene + Br2/CCl4
no reaction.With FeCl3 catalyst, Br2 reacts with
benzene to form bromobenzene + HBr(substitution!). Double bonds remain.
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Unusual Stabilities of Benzene
The hydrogenation of the first double bond of
benzene is endothermic.
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Annulenes
Cyclic hydrocarbons with alternating single anddouble bonds (uncharged, even no. of C atoms).
Assumptions:
All are aromatic.
Have similar stabilities asbenzene.
But, not all are aromatic.
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Six overlapping patomicorbitals form six MOs.
Three will be bonding,
three antibonding.The intermediate energy
levels are degenerate,
two MOs at each level.Lowest energy MO will
have all bonding
interactions, no nodes.As energy of MO
increases, the numberof nodes increases.
The MOs of Benzene
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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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The MOs of Cyclobutadiene
There are four MOs: the lowest-energy bonding orbital,the highest-energy antibonding orbital, and twodegenerate nonbonding orbitals.
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Energy Diagram for Cyclobutadiene
Four pi electrons fill the MOs.
Following Hunds rule, two electrons are inseparate degenerate orbitals.
This diradical with the highest-lying electrons innonbonding MOs would be very reactive.
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Polygon Rule
The energy diagram for an annulene has thesame polygonal shape as the cyclic compoundwith one vertex (all-bonding MO) at the bottom.
The nonbonding line cuts horizontally throughthe center of the polygon.
The pi electrons fill the MOs follows the aufbau
principle and Hunds rule.
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Aromatic Requirements
1. Structure must be cyclic withconjugatedpi bonds.
2. Each atom in the ring must havean unhybridized porbital.
3. The unhybridized porbitals mustoverlap to form a continuousring. (Usually planar structure.)
4. Delocalization of pi electronsover the ring must lower theelectronic energy
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Anti- and Nonaromatic
Antiaromatic compoundsare cyclic, conjugated, withoverlapping p orbitals around
the ring, but the energy of thecompound is greater than itsopen-chain counterpart.
antiaromatic
Nonaromatic compounds
do not have a continuous ring
of overlapping porbitals andmay be nonplanar.
CH3
CH3
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Hckels Rule
If the compound has a planar and continuousring of overlapping porbitals and has 4N+ 2electrons, it is aromatic.
If the compound has a planar and continuousring of overlapping porbitals and has 4Nelectrons, it is antiaromatic.
Otherwise, the system/compound isnonaromatic.
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Benzene Six pi electrons.
(4N+2) system
Aromatic
Cyclobutadiene Four pi electrons.
(4N) system
Antiromatic
Cyclooctatetraene(4N) system
AntiromaticX Nonaromatic
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Larger Annulenes
Larger 4Nannulenes are not antiaromaticbecause they are flexible enough to becomenonplanar.
[12] annulene [16] annulene
Larger 4N+2annulenes depend on whether themolecule can adopt the necessary planarconformation.
All-cis
nonaromatic
H H
Two trans
nonaromatic
naphthalene
aromatic
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MO Derivation of Hckels Rule
In a cyclic conjugated system, the lowest-energyMO is filled with two electrons.
Each of the additional shells has two degenerate
MOs, with space for four electrons.
A diradical, unstable.
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Aromatic Ions
Hckels rule also applies to systems having oddnumbers of C atoms and bearing positive ornegative charges.
Cyclopentadienyl Ions:The cation has an empty porbital, 4 electrons,
so antiaromatic.
The anion has a nonbonding pair of electrons ina porbital, 6 e-s, aromatic.
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Acidity of Cyclopentadiene
Unusually acidic (pKa of 16).
Loss of a proton converts the nonaromatic dieneto the aromatic cyclopentadienyl anion.
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Cyclopentadienyl cation
Huckels rule predicts that the cyclopentadienylcation, with four pi electrons, is antiaromatic.
The cyclopentadienyl cation is not easily formed.
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Cycloheptatrienyl Ions
The cycloheptatrienyl cation (tropylium ion) iseasily formed.
H OH
H+ , H2O
H
+
The cycloheptatrienyl anion is difficult to form.
HH
B
H
6 pi electron,(4N+2) system,aromatic
8 pi electron,
4Nsystem,antiaromatic
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Cyclooctatetraene Dianion
Cyclooctatetraene is nonaromatic.
Its dianion is easily prepared with planar, regularoctagonal structure aromatic.
Continuous overlapping
p orbitals.
(4N+2) system.
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Heterocyclic Aromatic Compounds
A heterocyclic compound is a cyclic compound in whichone or more of the ring atoms is an atom other thancarbon
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Pyridine
Aromatic nitrogen analogue of benzene. It has six delocalized electrons in its pi system.
The two non-bonding electrons on nitrogen are
in an sp2 orbital, and they do not interact with thepi electrons of the ring.
These electrons are in thesp2 orbital perpendicular to
the porbital.
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Protonation of pyridine:
Pyridine is basic, with non-bonding electronsavailable to abstract a proton.
aromatic aromatic
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Pyrrole
An aromatic five-membered heterocycle.Nonbonding electrons on N atom involve in the
pi bonding system, so much weaker base.
Nounhybridizedporbitalneeded foraromaticity
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Basic or Nonbasic?
N atom with it nonbonding electrons involve inthe pi bonding system is nonbasic.
NN Pyrimidine has two basicnitrogens.
N N H Imidazole has one basicnitrogen and one nonbasic.
N
N
N
N
H
Purine?
3 basic,1 nonbasic
Most nonbasic nitrogens
have three single bonds.
Most basic nitrogenshave a double bond in
the ring.
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Other Heterocyclics
All are aromatic
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Polynuclear Aromatic Hydrocarbons
Compounds composed of two or more fusedbenzene rings.
Naphthalene
Anthracene Phenanthrene
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Reactivity of
Polynuclear HydrocarbonsAs the number of aromatic rings increases, the
resonance energy per ring decreases.
These large PAHs can undergo addition reactions.
BrH
H Br
1
2
3
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6
7
8 9
10
Br
H
BrH
1
2
3
4
5
6
7
8
9
10
(mixture of cis and trans isomers)
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Larger Polynuclear
Aromatic HydrocarbonsFormed in combustion (tobacco smoke).
Many are carcinogenic.Epoxides form, combine with DNA base.
pyrene
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Allotropes of Carbon
Amorphous: small particles of graphite; charcoal,soot, coal, carbon black.
Diamond: a lattice of tetrahedral Cs.
Graphite: layers of fused aromatic rings.
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Some New Allotropes of Carbon
Fullerenes: 5- and 6-membered rings arrangedto form a soccer ball structure.
Nanotubes: half of a C60 sphere fused to acylinder of fused aromatic rings.
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Fused Heterocyclic Compounds
Common in nature, synthesized for drugs.
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Common Names of
Benzene Derivatives
OH OCH3NH2CH3
phenol toluene aniline anisole
C
H
CH2 C
O
CH3C
O
HC
O
OH
styrene acetophenone benzaldehyde benzoic acid
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Disubstituted Benzenes
The prefixes ortho-, meta-, and para- are commonlyused for the 1,2-, 1,3-, and 1,4- positions, respectively.
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3 or More Substituents
Use the smallest possible numbers, but
the carbon with a functional group that define thebase name is #1.
NO2
NO2
O2N
1,3,5-trinitrobenzene
NO2
NO2
O2N
OH
2,4,6-trinitrophenol
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Common Names forSubstituted Benzenes
CH3
CH3
CH3
CH3H3C
CH3
CO OH
OH
H3Cm-xylene mesitylene o-toluic acid p-cresol
Ph l d B l
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Phenyl and Benzyl
Benzene ring as a substituent on anothermolecule is called as phenyl group.
CH2 C C CH3
CH2 C C CH3Ph
1-phenyl-2-butyne
Benzyl group: seven-carbon unit consisting of abenzene ring and a methylene group.
Br
phenyl bromide
Ph sical Properties of Ben ene
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Physical Properties of Benzene
and Its DerivativesMelting points: More symmetrical than
corresponding alkane, pack better intocrystals, so higher melting points.
Boiling points: Dependent on dipolemoment, so ortho> meta> para, fordisubstituted benzenes.
Density: More dense than nonaromatics,
less dense than water.Solubility: Generally insoluble in water.
IR d NMR S t
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IR and NMR Spectroscopy
C=C stretch absorption at 1600 cm-1.
sp2 C-H stretch just above 3000 cm-1.
1
H NMR at 7-8 for Hs on aromatic ring.13C NMR at 120-150, similar to alkene
carbons.
Mass Spectrometry
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Mass Spectrometry
The most common fragmentation ofalkylbenzene derivative is the cleavage of abenzylic bond.
UV Spectroscopy
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UV Spectroscopy
Benzene has three absorptions.
max at 184 nm
Moderateband
Characteristicband
Additional conjugateddouble bond cause
bathochromic shift.
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