Transcript UNIT 2 AROMATIC MATERIALS AND THEIR REACTIONS

UNIT 2
AROMATIC MATERIALS AND THEIR
REACTIONS
• Structure of aromatics (Kekulé structure)
• Properties of aromatics (chemical/physical/spectroscopy)
• Aromatic/anti-aromatic/non-aromatic
• Huckel’s rule
• Electrophilic Aromatic Substitution
• Nucleophilic Aromatic Substitution
• Reactions on aromatic side chains
Aromatic compounds
• Benzene in the simplest of the aromatic compounds it was first
isolated in 1825 by Michael Faraday.
• Benzene has a carbon to hydrogen ratio of 1:1 and a molecular
formula of C6H6.
• Other related compounds with low C:H ratios had a pleasant
smell, so they were classified as aromatic.
Aromatic compounds
Kekulé proposed a cyclic structure for benzene that consisted of
alternating single and double bonds in 1866.
There are a number problems with this structure when we consider..the
chemistry of benzene.
There is only a single 1,2-dichlorobenzene, not two which is possible
with the above structure.
The C-C bond length would be expected to be different, but all of the
bonds are the same.
Aromatic compounds
Benzene is a planar resonance hybrid of the two Kekulé structures. The carboncarbon bonds are all equivalent with a bond length of 1.397 A. Which is about
half way between a C-C single (1.48 A) and double bond (1.34 A).
The pi bonds (electrons) are delocalized over the ring. Since the bonds are
delocalized over the ring it is common to draw a circle in an aromatic ring.
However, we will commonly use the Kekulé structures in drawing reaction
mechanisms where we are showing the movement of individual pairs of
electrons.
Aromatic compounds
Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular
to the ring which overlaps around the ring. This results in a continuous cloud
of delocalized electrons on both faces of the ring.
Aromatic compounds
Unusual reactivity of benzene.
Benzene is a cyclic conjugated triene. With three double bonds we would
expect benzene to undergo the same reactions as alkenes.
Alkene + KMnO4  diol (addition)
Benzene + KMnO4  no reaction.
Alkene + Br2/CCl4  dibromide (addition)
Benzene + Br2/CCl4  no reaction.
Aromatic compounds
Unusual reactivity of benzene.
Benzene does not undergo simple addition reactions like alkene, except
under extreme conditions.
Interestingly, addition of a Lewis acid catalyst to a solution of benzene and
bromine does result in a reaction. But the product of this reaction is not the
addition of bromine across a double bond.
The product that is obtained is a substitution product where a hydrogen atom
has been replaced by a bromine atom.
Aromatic compounds
Unusual stability of benzene.
The extra stability of benzene is due to the “resonance energy” extra
stability associated with the resonance/delocalization of the pi electrons.
This is illustrated by the heats of hydrogenation shown below.
Aromatic compounds
Annulenes
Annulenes are similar to benzene in that they are
cyclic hydrocarbons with an analogous conjugated
system of single and double bonds.
Aromatic compounds
Annulenes
Aromatic compounds
Annulenes
Kekulé-like resonance structures can be drawn for
annulenes similar to what we saw with benzene. However,
many of these materials do not exhibit unusual stability
like benzene.
Hückel’s Rule: If the number of pi electrons in the cyclic
system is:
(4N+2), the system is aromatic;
(4N) the system is antiaromatic, where N is an integer.
This rule applies to planar cyclic compounds with atoms
other than carbon, positive/negative charges and odd number
ring sizes.
Aromatic compounds
MO model 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. (bonding orbital)
•As energy of MO increases, the number of nodes
increases. (antibonding orbital)
Aromatic compounds
MO model benzene
Aromatic compounds
Polygon Rule
The energy diagram for an annulene has the same shape
as the cyclic compound with one vertex at the bottom.
Aromatic compounds
MO model for benzene
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The six electrons fill three bonding pi orbitals.
All bonding orbitals are filled (“closed shell”), an extremely stable
arrangement.
Aromatic compounds
MO model cyclobutadiene
Aromatic compounds
MO model cyclobutadiene
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Following Hund’s
rule, two electrons are
in separate orbitals.
This diradical would
be very reactive.
Aromatic compounds
Aromatic Requirements
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Structure must be cyclic with some conjugated
pi bonds.
Each atom in the ring must have an unhybridized p
orbital. (no sp3 hybridized atoms)
The p orbitals must overlap continuously around the
ring. (Planar structure or close to planar.)
Compound is more stable than its open-chain
counterpart. (lower electronic energy)
Aromatic compounds
Anti- and Nonaromatic
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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.
Nonaromatic compounds do not have a continuous
ring of overlapping p orbitals and may be
nonplanar.
Aromatic compounds
Anti- and Non-aromatic
Hückel’s Rule: If the number of pi electrons in the cyclic
system is:
(4N+2), the system is aromatic;
(4N) the system is antiaromatic, where N is an integer.
This rule applies to planar cyclic compounds with atoms
other than carbon, positive/negative charges and odd number
ring sizes.
Aromatic compounds
Cyclopentadienyl anion
The anion has a nonbonding pair of electrons in a p
orbital, 6 e-’s, aromatic.
What about the cyclpentadienyl cation?
Aromatic compounds
Cyclopentadienyl cation
Aromatic compounds
Cycloheptatrienyl cation
H
H
OH
+
H , H2O
+
How many pi electrons are in the above ion?
How many p orbitals are in the above?
Aromatic or antiaromatic?
What about the cycloheptantrienyl anion?
Aromatic compounds
Cyclooctatetraene
How many pi electrons are in the above anion?
Aromatic compounds
Cyclooctatetraene
Huckel’s rule does not apply because there is not continuous
over lap of p orbitals around the ring.
Aromatic compounds
+
H
N
H
H
-
H
O
N
N
+
H H
N+
See page 722 for additional examples.
-
Aromatic compounds
Which of the following are aromatic?
Aromatic compounds
Which of the following are aromatic, antiaromatic,
nonaromatic?
N+
H
H
: N :-
O
O
N
H
..-
Heterocyclic Aromatic compounds
nitrogen heterocyclics
Pyridine : Aromatic heterocycle that is a weak base
pKb = 8.8, often used in organic synthesis as a base.
Heterocyclic Aromatic compounds
nitrogen heterocyclic
Pyrrole: Weaker base than pyridine because the lone
pair of electrons on the nitrogen are delocalized over
the ring.
Aromatic compounds
Fused ring aromatics
Aromatic compounds
Fused ring aromatics
As the number of aromatic rings increases, there is a
decrease in the resonance energy per ring. As a result
these materials can undergo reactions that are more
characteristic of nonaromatic polyenes.
Br
Br2
CCl4
Br
Allotropes of Carbon
Allotropes are pure elemental substances that can exist
with different crystalline structures.
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Amorphous: small particles of graphite; charcoal, soot,
coal, carbon black.
Diamond: a lattice of tetrahedral C’s.
Graphite: layers of fused aromatic rings.
Fullerenes (1985) contain closed structures with alternating
6- and 5- membered rings
Diamond
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One giant molecule.
Tetrahedral carbons.
Sigma bonds, 1.54 Å.
Electrical insulator.
Graphite
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Planar layered structure.
Layer of fused benzene
rings, bonds: 1.415 Å.
Only van der Waals
forces between layers.
Conducts electrical
current parallel to layers.
New Allotropes of Carbon
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Fullerenes: 5- and 6-membered rings arranged to form a
“soccer ball” structure.
Nanotubes: half of a C60 sphere fused to a cylinder of fused
aromatic rings.
Aromatic compounds
Nomenclature polynuclear aromatic hydrocarbons
naphthalene
8
1
2
7
3
6
5
4
anthracene
phenanthrene
..-
Heterocyclic Aromatic compounds
Heterocyclic aromatics
O
S
N
furan
thiophene
H
indole
N H
N
pyridine
pyrrole
Heterocyclic Aromatic compounds
nitrogen heterocyclic
N
N
N
N H
Pyrimidine has two basic
nitrogens.
Imidazole has one basic
nitrogen and one nonbasic.
Aromatic compounds
Common aromatic materials (IUPAC)
OH
CH3
OCH3
NH2
phenol
toluene
aniline
anisole
(benzenol)
(methylbenzene)
(benzenamine)
(methoxybenzene)
O
CH CH2
styrene
(vinylbenzene)
C CH3
acetophenone
CHO
benzaldehyde
CO2H
benzoic acid
Aromatic compounds
Common aromatic ions
=
+
cycloheptatrienyl cation
tropylium ion
=
cyclopentadienyl anion
-
Aromatic compounds
Nomenclature of substituted benzene
Aromatic compounds
Nomenclature of substituted benzene
Number the ring so that you have the smallest possible numbers. Note:
you can only use ortho, meta and para on disubstituted benzenes.
OH
O2N
NO2
NO2
1,3,5-trinitrobenzene
O2N
NO2
NO2
2,4,6-trinitrophenol
Aromatic compounds
Nomenclature of substituted benzene
Name the following benzene derivatives.
Cl
CH3
CO2H
H2N
NO2
OH
OH
Br
CO2H
Cl
Cl
Cl
Cl
Cl
Aromatic compounds
Nomenclature of benzene derivatives
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If the benzene ring is attached to a complex structure it can be difficult to
name the compound as a benzene derivative. In these case the benzene ring is
named as a phenyl substituent. ( in drawings it is common for the phenyl group
to be abbreviated as Ph or Φ.
Ph
Aromatic compounds
Nomenclature of benzene derivatives
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An aryl group (Ar) is used to refer to a phenyl substituent that has
substituents on the ring.
Ar-OH = an aryl alcohol (a phenol)
Ar-NH2 = an aryl amine
}
X = any group other than H and can be ortho, meta or para.
X
CH2
Benzyl group
Aromatic compounds
Physical Properties
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Physical properties of benzene and derivatives
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Benzene has a boiling point and melting point comparable cyclohexane.
b.p. = 80 C for benzene vs. 81 C for cyclohexane m.p. = ~6 C for
both benzene and cyclohexane
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Substitutions that make stacking of molecules more difficult in the solid
state dramatically reduce melting points.
CH3
m.p. = 6 C
m.p. = 95 C
Aromatic compounds
Physical Properties
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Physical properties of benzene and derivatives
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Asymmetrical derivatives with polar groups tend to have higher boiling
points. (Dipole moment allows for stronger dipole-dipole interactions in
liquid state.)
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Symmetrical derivatives tend to stack better in the solid state and have
higher melting points as a result.
Cl
Cl
Cl
Cl
=0
Cl
b.p. = 181 C
b.p. = 173 C
m.p. = 17 C
m.p. = 25 C
Cl
b.p. = 170 C
m.p. = 54 C