Aromatic Compounds

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Chapter 14

• Aromatic Compounds

Modified from sides of William Tam Phillis Chang
Ch. 14 - 1
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Nomenclature
benzene
Naming monosubstituted benzenes Most benzene is
the parent name and the substituent is
a prefix
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Other simple, common benzenes, have accepted
parent name (for substituent
and ring)
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Disubstituted benzenes With two substituents
Their relative positions are indicated by
prefixes or numbers ortho- (abbreviated o-, or
1,2-) meta-, (m- , 1,3-)
para-, (p- , 1,4-)
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examples
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Dimethylbenzenes xylenes
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More than two groups 1. Positions must be
indicated by numbers
2. Number the benzene ring to give Substituents
the lowest possible numbers
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More than two different substituents list in
alphabetical order
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A substituent gives special base name
(aniline, anisole, etc.) that
substituent is position 1
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Benzene as a substituent phenyl group
(C6H5)
hydrocarbon with saturated chain and
1 benzene ring Base/parent is the
larger structural unit.
(S)-2-phenylheptane
t-butylbenzene
butylbenzene
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Unsaturated chains, the parent/base name is of
that chain, (regardless of ring size)
trans-1-phenyl-2-butene
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Benzyl (Bn) is a common name for the phenylmethyl
group
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recall
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(No Transcript)
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Reactions of Benzene
substitution not addition
HBr substitution
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The Kekulé Structure for Benzene
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These 1,2-dibromobenzenes are not isomers
or an equilibrium
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However
aromatic character?
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3-D structure
Note Planar structure All carbons sp2
hybridized
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Hückels Rule The 4n 2 p Electron Rule
(1) Planar monocyclic rings (2) containing 4n
2 p electrons, where n 0 or an integer (2, 6,
10, 14 . . .etc.) have substantial resonance
energies, aromatic
i.e. a planar ring containing 6 p electrons is
aromatic
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Hückels rule states that planar monocyclic rings
with 2, 6, 10, 14 . . . delocalized electrons
should be aromatic
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How To Diagram the Relative Energies of p
Molecular Orbitals in Monocyclics Based on
Hückels Rule
circled polygon
type of ? orbitals
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p molecular orbitals of cyclooctatetraene, if
planar Predicted to have 2 nonbonding orbitals
and an unpaired electron in each nonbonding
orbital
?Not be expected to be aromatic
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System not planar

• The bonds alternately long and short
• (1.48 and 1.34 Å)

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The Annulenes
Hückels rule predicts that annulenes will be
aromatic
if the molecule has 4n 2 p electrons and have a
planar carbon skeleton
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All these (4n 2)p, planar annulenes are aromatic
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Non-planar (4n 2)p annulenes are antiaromatic
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(4n)p non-planar annulenes are antiaromatic
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NMR Spectroscopy Evidence for Electron
Delocalization in Aromatic Compounds
1H NMR spectrum 1H occurs at relatively high
frequency Is compelling evidence for aromaticity
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(d 9.3)
(d -3.0)
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Aromatic Ions
pKa unsaturated and saturated hydrocarbon 44-53
pka 36
pka 16
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6 p electrons aromatic
sp3
sp2
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strong base
LA
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Aromatic, Antiaromatic, and Nonaromatic Compounds
An aromatic compound has its p electrons
delocalized over the entire ring and
It is stabilized by the p-electron delocalization
Evaluation compare cyclic compound vs acyclic
with same number of electrons.
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Based on sound calculations or experiments
Ring is aromatic if the ring has lower ?-electron
energy then the acyclic chain
Nonaromatic if the ring and the chain have the
same ?-electron energy non-planar
Antiaromatic if the ring has greater p-electron
energy than the open chain 4n ? es
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Cyclobutadiene
Benzene
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Other Aromatic Compounds
Benzenoid Aromatic Compounds
Benzenoid polycyclic aromatic hydrocarbons having
two or more fused benzene rings.
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Nonbenzenoid Aromatic Compounds
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Fullerenes
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Heterocyclic Aromatic Compounds
heterocyclic compounds cyclic compounds with an
element(s) other than carbon, e.g. piperidine
aromatic heterocyclic
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Examples of useful heterocyclic aromatic compounds
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Aromaticity
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Basicity of nitrogen-containing heterocycles
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Basicity of nitrogen-containing heterocycles
poor base
loss of aromaticity
still aromaticity
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Aromatic Compounds in Biochemistry
Two amino acids necessary for protein synthesis
contain the benzene ring
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Derivatives of purine and pyrimidine are
essential parts of DNA and RNA
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Nicotinamide adenine dinucleotide important
coenzymes in oxidations and reductions
-pyridine derivative (nicotinamide)
-purine derivative (adenine)