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Ch. 14 - 1
Chapter 14Chapter 14
Aromatic CompoundsAromatic Compounds
Modified from sides of William Tam & Phillis Chang
Nomenclature
Naming monosubstituted benzenesMost: benzene is the parent name and the substituent is a prefix
benzene
Other simple, common benzenes, have accepted parent name (for substituent and ring)
Disubstituted benzenesWith two substituents:
Their relative positions are indicated by prefixesor numbers : ortho- (abbreviated o-, or 1,2-) meta-, (m- , 1,3-) para-, (p- , 1,4-)
examples
Dimethylbenzenes = xylenes
More than two groups1. Positions must be indicated by numbers
2. Number the benzene ring to giveSubstituents the lowest possible numbers
More than two different substituents list in alphabetical order
A substituent gives “special” base name (aniline, anisole, etc.) that substituent is position 1
Benzene as a substituent = phenyl group (C6H5)
hydrocarbon with saturated chain and 1 benzene ring Base/parent is the larger structural unit.
butylbenzene t-butylbenzene (S)-2-phenylheptane
Unsaturated chains, the parent/base name is of that chain, (regardless of ring size)
trans-1-phenyl-2-butene
Benzyl (Bn) is a common name for the phenylmethyl group
recall
substitution not additionReactions of Benzene
[+ HBr] substitution
The Kekulé Structure for Benzene
These 1,2-dibromobenzenes are not isomers
RESONANCE
or an equilibrium
X
X
+ other Br2 additons
However
aromatic character?
3-D structure
π-electrons above and below ring
Note: Planar structure All carbons sp2 hybridized
Hückel’s Rule: The 4n + 2 π Electron Rule
(1) Planar monocyclic rings (2) containing 4n + 2 π electrons, where n = 0
or an integer (2, 6, 10, 14 . . .etc.) have substantial resonance energies, “aromatic”
i.e. a planar ring containing 6 π electrons is “aromatic”
Hückel’s rule states that planar monocyclic rings with 2, 6, 10, 14 . . . delocalized electrons should be aromatic
How To Diagram the Relative Energies of p How To Diagram the Relative Energies of p Molecular Orbitals in Monocyclics Molecular Orbitals in Monocyclics Based on HückelBased on Hückel’’s Rules Rule
circledpolygon
antibonding orbitals
nonbonding orbitals
bonding orbitals
type of orbitals
orbitalenergy levels
π 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
System not planar
The bonds alternately long and short; (1.48 and 1.34 Å)
The AnnulenesThe Annulenes
Hückel’s rule predicts that annulenes will be aromatic
if the molecule has 4n + 2 π electrons and have a planar carbon skeleton
All these (4n + 2)π, planar annulenes are aromatic
Non-planar (4n + 2)π annulenes are antiaromatic
(4n) non-planar annulenes are antiaromatic
NMR Spectroscopy: Evidence for ElectronNMR Spectroscopy: Evidence for Electron Delocalization in Aromatic CompoundsDelocalization in Aromatic Compounds
1H NMR spectrum 1H occurs at relatively high frequencyIs compelling evidence for aromaticity
(δ -3.0)
(δ 9.3)
Aromatic Ions
pka = 16 pka = 36
pKa unsaturated and saturated hydrocarbon 44-53
sp3 sp2
6 π electrons aromatic
strong base
LA
Aromatic, Antiaromatic, and Nonaromatic Aromatic, Antiaromatic, and Nonaromatic CompoundsCompounds
An aromatic compound has its π electrons delocalized over the entire ring and
It is stabilized by the π-electron delocalization
Evaluation: compare cyclic compound vs acyclic with same number of electrons.
Ring is aromatic if the ring has lower -electron energy then the acyclic chain
Based on sound calculations or experiments
Nonaromatic if the ring and the chain have the same -electron energy [non-planar]
Antiaromatic if the ring has greater π-electron energy than the open chain [4n e’s]
Cyclobutadiene
Benzene
Other Aromatic Compounds
Benzenoid polycyclic aromatic hydrocarbons having two or more fused benzene rings.
Benzenoid Aromatic CompoundsBenzenoid Aromatic Compounds
Nonbenzenoid Aromatic CompoundsNonbenzenoid Aromatic Compounds
FullerenesFullerenes
Heterocyclic Aromatic Compoundsheterocyclic compounds cyclic compounds with an
element(s) other than carbon, e.g. piperidine
aromatic heterocyclic:
Examples of useful heterocyclic aromatic compounds
Aromaticity
Basicity of nitrogen-containing heterocycles
Basicity of nitrogen-containing heterocycles
poor base:
loss of aromaticity
still aromaticityimidazolearomatic weak base
Aromatic Compounds in Biochemistry
Two amino acids necessary for protein synthesis contain the benzene ring
Derivatives of purine and pyrimidine are essential parts of DNA and RNA
Nicotinamide adenine dinucleotide important coenzymes in oxidations and reductions
-pyridine derivative (nicotinamide) -purine derivative (adenine)
O O