KOT 222 ORGANIC CHEMISTRY II
CHAPTER 16
AROMATIC COMPOUNDS
1
Benzene
¾ An aromatic compound.
¾ Isolated in 1825 by Michael Faraday who
determined C:H ratio to be 1:1.
¾ Synthesized in 1834 by Eilhard Mitscherlich
who determined molecular formula to be
C6H6.
¾ Other related compounds with low C:H ratios
had a pleasant smell, so they were classified
as aromatic.
2
Kekulé Structure of Benzene
¾ Proposed in 1866 by Friedrich Kekulé, shortly
after multiple bonds were suggested.
¾ Showing localized double-bonds.
¾ Failed to explain existence of only one isomer of
1,2-dichlorobenzene.
Cl
Cl
X
Cl
Cl
3
Resonance Structure of Benzene
¾ Benzene is actually a resonance hybrid between
the two Kekulé structures.
¾ The pi electrons are delocalized, with a bond
order of 1.5 between adjacent carbon atoms
4
Orbital Representation of Benzene
¾ Benzene is a flat ring of sp2 hybrid carbon atoms
with their unhybridized p orbitals all aligned and
overlapping.
¾ The conjugation and delocalization of the
electrons give greater stability.
5
Unusual Reactions of Benzene
¾ Benzene as a cyclic conjugated triene is predicted to
react as polyenes.
¾ 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.
6
Unusual Stabilities of Benzene
¾ The hydrogenation of the first double bond of
benzene is endothermic.
7
Annulenes
¾ Cyclic hydrocarbons with alternating single and
double bonds (uncharged, even no. of C atoms).
Assumptions:
™All are aromatic.
™Have similar stabilities as
benzene.
¾ But, not all are aromatic.
8
The MOs of Benzene
¾ Six overlapping p atomic
orbitals form six MOs.
¾ Three will be bonding,
three antibonding.
¾ The intermediate energy
levels are degenerate,
two MOs at each level.
¾ Lowest energy MO will
have all bonding
interactions, no nodes.
¾ As energy of MO
increases, the number
of nodes increases.
9
10
11
Energy Diagram for Benzene
¾ The six electrons fill three bonding pi orbitals.
¾ All bonding orbitals are filled (“closed shell”), an
extremely stable arrangement.
12
The MOs of Cyclobutadiene
¾ There are four MOs: the lowest-energy bonding orbital,
the highest-energy antibonding orbital, and two
degenerate nonbonding orbitals.
13
Energy Diagram for Cyclobutadiene
¾ Four pi electrons fill the MOs.
¾ Following Hund’s rule, two electrons are in
separate degenerate orbitals.
¾ This diradical with the highest-lying electrons in
nonbonding MOs would be very reactive.
14
Polygon Rule
¾ The energy diagram for an annulene has the
same polygonal shape as the cyclic compound
with one vertex (all-bonding MO) at the bottom.
¾ The nonbonding line cuts horizontally through
the center of the polygon.
¾ The pi electrons fill the MOs follows the aufbau
principle and Hund’s rule.
15
Aromatic Requirements
1. Structure must be cyclic with
conjugated
pi bonds.
2. Each atom in the ring must have
an unhybridized p orbital.
3. The unhybridized p orbitals must
overlap to form a continuous
ring. (Usually planar structure.)
4. Delocalization of pi electrons
over the ring must lower the
electronic energy
16
Anti- and Nonaromatic
¾ 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.
antiaromatic
¾ Nonaromatic compounds
do not have a continuous ring
of overlapping p orbitals and
may be nonplanar.
CH3
CH3
17
Hückel’s Rule
¾ If the compound has a planar and continuous
ring of overlapping p orbitals and has 4N + 2
electrons, it is aromatic.
¾ If the compound has a planar and continuous
ring of overlapping p orbitals and has 4N
electrons, it is antiaromatic.
¾ Otherwise, the system/compound is
nonaromatic.
18
Benzene
Six pi electrons.
(4N+2) system
Aromatic
Cyclobutadiene
Four pi electrons.
(4N) system
Antiromatic
Cyclooctatetraene
(4N) system
X
Antiromatic
√ Nonaromatic
19
Larger Annulenes
¾ Larger 4N annulenes are not antiaromatic
because they are flexible enough to become
nonplanar.
[12] annulene
[16] annulene
¾ Larger 4N+2 annulenes depend on whether the
molecule can adopt the necessary planar
conformation.
H H
All-cis
nonaromatic
Two trans
nonaromatic
naphthalene
aromatic
20
MO Derivation of Hückel’s Rule
¾ In a cyclic conjugated system, the lowest-energy
MO is filled with two electrons.
¾ Each of the additional shells has two degenerate
MOs, with space for four electrons.
A diradical, unstable.
21
Aromatic Ions
¾ Hückel’s rule also applies to systems having odd
numbers of C atoms and bearing positive or
negative charges.
Cyclopentadienyl Ions:
¾ The cation has an empty p orbital, 4 electrons,
so antiaromatic.
¾ The anion has a nonbonding pair of electrons in
a p orbital, 6 e-’s, aromatic.
22
Acidity of Cyclopentadiene
¾ Unusually acidic (pKa of 16).
¾ Loss of a proton converts the nonaromatic diene
to the aromatic cyclopentadienyl anion.
23
Cyclopentadienyl cation
¾ Huckel’s rule predicts that the cyclopentadienyl
cation, with four pi electrons, is antiaromatic.
¾ The cyclopentadienyl cation is not easily formed.
24
Cycloheptatrienyl Ions
¾ The cycloheptatrienyl cation (tropylium ion) is
easily formed.
H
H
OH
+
H , H2O
+
6 pi electron,
(4N+2) system,
aromatic
¾ The cycloheptatrienyl anion is difficult to form.
H
H
H
B
8 pi electron,
4N system,
antiaromatic
25
Cyclooctatetraene Dianion
¾ Cyclooctatetraene is nonaromatic.
¾ Its dianion is easily prepared with planar, regular
octagonal structure – aromatic.
Continuous overlapping
p orbitals.
(4N+2) system.
26
Heterocyclic Aromatic Compounds
A heterocyclic compound is a cyclic compound in which
one or more of the ring atoms is an atom other than
carbon
27
Pyridine
¾ Aromatic nitrogen analogue of benzene.
¾ It has six delocalized electrons in its pi system.
¾ The two non-bonding electrons on nitrogen are
in an sp2 orbital, and they do not interact with the
pi electrons of the ring.
These electrons are in the
sp2 orbital perpendicular to
28
the p orbital.
Protonation of pyridine:
¾ Pyridine is basic, with non-bonding electrons
available to abstract a proton.
aromatic
aromatic
29
Pyrrole
¾ An aromatic five-membered heterocycle.
¾ Nonbonding electrons on N atom involve in the
pi bonding system, so much weaker base.
No
unhybridized
p orbital
needed for
aromaticity
30
Basic or Nonbasic?
¾ N atom with it nonbonding electrons involve in
the pi bonding system is nonbasic.
N
N
Pyrimidine has two basic
nitrogens.
N
N H
Imidazole has one basic
nitrogen and one nonbasic.
N
N
H
N
N
Purine?
3 basic,
1 nonbasic
Most nonbasic nitrogens
have three single bonds.
Most basic nitrogens
have a double bond in
31
the ring.
Other Heterocyclics
¾ All are aromatic
32
Polynuclear Aromatic Hydrocarbons
¾ Compounds composed of two or more fused
benzene rings.
Naphthalene
Anthracene
Phenanthrene
33
Reactivity of
Polynuclear Hydrocarbons
¾ As the number of aromatic rings increases, the
resonance energy per ring decreases.
¾ These large PAHs can undergo addition reactions.
6
7
5
8
H
Br
9
1
4
7
2
6
3
5
H
10
Br
4
8
3
9
Br
2
10
1
H
H
Br
(mixture of cis and trans isomers)
34
Larger Polynuclear
Aromatic Hydrocarbons
¾ Formed in combustion (tobacco smoke).
¾ Many are carcinogenic.
¾ Epoxides form, combine with DNA base.
pyrene
35
Allotropes of Carbon
¾ Amorphous: small particles of graphite; charcoal,
soot, coal, carbon black.
¾ Diamond: a lattice of tetrahedral C’s.
¾ Graphite: layers of fused aromatic rings.
36
Some New Allotropes of Carbon
¾ Fullerenes: 5- and 6-membered rings arranged
to form a “soccer ball” structure.
¾ Nanotubes: half of a C60 sphere fused to a
cylinder of fused aromatic rings.
37
Fused Heterocyclic Compounds
Common in nature, synthesized for drugs.
38
Common Names of
Benzene Derivatives
OH
CH3
phenol
toluene
H
C
styrene
CH2
OCH3
NH2
aniline
anisole
O
O
O
C
C
C
acetophenone
CH3
benzaldehyde
H
OH
benzoic acid
39
Disubstituted Benzenes
The prefixes ortho-, meta-, and para- are commonly
used for the 1,2-, 1,3-, and 1,4- positions, respectively.
40
3 or More Substituents
Use the smallest possible numbers, but
the carbon with a functional group that define the
base name is #1.
OH
O 2N
NO 2
NO 2
1,3,5-trinitrobenzene
O2N
NO2
NO2
2,4,6-trinitrophenol
41
Common Names for
Substituted Benzenes
CH3
CH3
O
OH
C
OH
CH3
CH3
m-xylene
H3C
CH3
mesitylene
o-toluic acid
H3C
p-cresol
42
Phenyl and Benzyl
¾ Benzene ring as a substituent on another
molecule is called as phenyl group.
CH2 C
C
CH3
Br
Ph
CH 2 C
C CH 3
1-phenyl-2-butyne
phenyl bromide
¾ Benzyl group: seven-carbon unit consisting of a
benzene ring and a methylene group.
43
Physical Properties of Benzene
and Its Derivatives
¾Melting points: More symmetrical than
corresponding alkane, pack better into
crystals, so higher melting points.
¾Boiling points: Dependent on dipole
moment, so ortho > meta > para, for
disubstituted benzenes.
¾Density: More dense than nonaromatics,
less dense than water.
¾Solubility: Generally insoluble in water.
44
IR and NMR Spectroscopy
¾C=C stretch absorption at 1600 cm-1.
¾sp2 C-H stretch just above 3000 cm-1.
¾ 1H NMR at δ7-δ8 for H’s on aromatic ring.
¾ 13C NMR at δ120-δ150, similar to alkene
carbons.
45
Mass Spectrometry
¾ The most common fragmentation of
alkylbenzene derivative is the cleavage of a
benzylic bond.
46
UV Spectroscopy
¾ Benzene has three absorptions.
λmax at 184 nm
Additional conjugated
double bond cause
bathochromic shift.
Moderate
band
Characteristic
band
47
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