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Created byProfessor William Tam & Dr. Phillis Chang
Chapter 14
Aromatic Compounds
Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.
In the 19th century, organic compounds were classified as being either aliphatic or aromatic
Aliphatic● The chemical behavior of a
compound was “fatlike” Aromatic
● The compound had a low hydrogen-to-carbon ratio and was “fragrant”
© 2014 by John Wiley & Sons, Inc. All rights reserved.
© 2014 Pearson Education, Inc.
The Structure of Benzene
© 2014 Pearson Education, Inc.
Resonance ContributorsResonance Hybrid
2
2. Nomenclature of BenzeneDerivatives
Naming monosubstituted benzenes● In many simple compounds, benzene is
the parent name and the substituent is simply indicated by a prefixF Cl Br NO2
Fluorobenzene Chlorobenzene Bromobenzene Nitrobenzene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● For other simple and common compounds, the substituent and the benzene ring taken together may form a commonly accepted parent name
CH3 O N SO3H
Toluene Phenol Aniline Benzene-sulfonic acid
H H H
OH
O O
O
Anisole
CH3
Benzoic acid Acetophenone© 2014 by John Wiley & Sons, Inc. All rights reserved.
Naming disubstituted benzenes● When two substituents are present,
their relative positions are indicated by the prefixes ortho-, meta-, and para-(abbreviated o-, m-, and p-) or by the use of numbers
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Other examples
2-Nitrobenzoic acid(o-Nitrobenzoic acid)
NO2
CH3
OH
3-Methylphenol(m-Methylphenol)
4-Chlorotoluene(p-Chlorotoluene)
(1-Chloro-4-methyl-benzene)
CH3
Cl
COOH
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3
● The dimethylbenzenes are often called xylenes
1,2-Dimethylbenzene(o-xylene)
CH3
CH3
CH3
1,3-Dimethylbenzene(m-xylene)
1,4-Dimethylbenzene(p-xylene)
CH3
H3C
CH3
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Naming benzene rings with more than two groups● If more than two groups are present on
the benzene ring, their positions must be indicated by the use of numbers
● The benzene ring is numbered so as to give the lowest possible numbers to the substituents
1,2,3-Trichlorobenzene
1,2,4-Tribromobenzene(not 1,3,4-Tribromobenzene)
ClCl
Cl
12
3
4
5
6
BrBr1
2
3
45
6
Br© 2014 by John Wiley & Sons, Inc. All rights reserved.
● When more than two substituents are present and the substituents are different, they are listed in alphabetical order
ClF1
2
3
45
6
Br
4-Bromo-1-chloro-2-fluorobenzene© 2014 by John Wiley & Sons, Inc. All rights reserved.
● When a substituent is one that, together with the benzene ring gives a new base name, that substituent is assumed to be in position 1 and the new parent name is usedCl
32
1
6
5
4
OHCl3,5-Dichlorophenol
COOH1
6
54
3
2
Br5-Bromo-2-methylbenzoic acid
H3C
© 2014 by John Wiley & Sons, Inc. All rights reserved.
4
● When the C6H5 group is named as a substituent, it is called a phenyl group
● A hydrocarbon composed of one saturated chain and one benzene ring is usually named as a derivative of the larger structural unit. However, if the chain is unsaturated, the compound may be named as a derivative of that chain, regardless of ring size
© 2014 by John Wiley & Sons, Inc. All rights reserved.
● Examples
Butylbenzene Isopropylbenzene
3
2
1 6 8
4
trans-1-Phenyl-1-butene (R)-3-Phenyloctane
31 752 4
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● Benzyl is an alternative name for the phenylmethyl group. It is sometimes abbreviated Bn.
The benzyl group(the phenylmethyl group)
Benzyl chloride(phenylmethyl chloride
or BnCl)
Cl
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3. Reactions of Benzene
Br2CCl4
Br
Br
Br2CCl4
No Reaction
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5
OH
OH
No Reaction
1. OsO4
2. NaHSO3
1. OsO4
2. NaHSO3
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OH
No Reaction
H+
H2O
H+
H2O
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H2/Ni
25oC, 1 atm
H2/Ni
high temperatureand pressure
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Benzene undergoes substitution, but not addition
Br2CCl4
Br
Br(C6H10) (C6H10Br2)
FeBr3(a Lewis acid)
Br2
(C6H6)
H Br
(C6H5Br)
(an addition)
(a substitution)
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6
4. The Kekulé Structure for Benzene
CC
CCC
CH
HH
H
HH
or
The Kekulé formula for benzene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
and
Br
Br
Br
Br
1
23
4
56
1
23
4
56
Br
Br
Br
Br
1
23
4
56
1
23
4
56
X
These 1,2-dibromobenzenes do not exist as isomers
There is no such equilibrium between benzene ring bond isomers
© 2014 by John Wiley & Sons, Inc. All rights reserved.
No ReactionBr2
Br2Br
Br
© 2014 by John Wiley & Sons, Inc. All rights reserved.
5. The Thermodynamic Stabilityof Benzene
Since bonds are formed from side-way overlap of orbitals, electron clouds are above & below the plane of the double bond
-electrons aboveand below ring
© 2014 by John Wiley & Sons, Inc. All rights reserved.
7
© 2014 by John Wiley & Sons, Inc. All rights reserved.
6. Modern Theories of the Structure of Benzene
All C C bond lengths the same (1.39 Å) (compare with C—C single bond 1.54 Å, C=C double bond 1.34 Å)
Extra stabilization due to resonance aromatic
6A. The Resonance Explanation of theStructure of Benzene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
3-D structure
-electrons aboveand below ring
● Planar structure● All carbons sp2 hybridized
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6B. The Molecular Orbital Explanationof the Structure of Benzene
© 2014 by John Wiley & Sons, Inc. All rights reserved.
8
© 2014 by John Wiley & Sons, Inc. All rights reserved.
7. Hückel’s Rule: The (4n + 2) Electron Rule
Hückel’s rule is concerned with compounds containing one planar ring in which each atom has a orbital as in benzene
Planar monocyclic rings containing (4n + 2) electrons, where n = 0, 1, 2, 3, and so on (i.e., rings containing 2, 6, 10, 14 . . . etc. electrons), have closed shells of delocalized electrons like benzene and have substantial resonance energies
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Hückel’s rule states that planar monocyclic rings with 2, 6, 10, 14 . . . delocalized electrons should be aromatic
© 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 Pearson Education, Inc.
Examples of Compounds That are Not Aromatic
Cyclobutadiene has an even number of pairs of π electrons.
Cyclooctatetraene has an even number of pairs of π electrons andit is not planar.
9
The bonds of cyclooctatetraene are known to be alternately long and short; X-ray studies indicate that they are 1.48 and 1.34 Å, respectively, and that the molecule has a tub-like shape
© 2014 by John Wiley & Sons, Inc. All rights reserved.
7B. The Annulenes Hückel’s rule predicts that annulenes
will be aromatic if their molecules have (4n + 2) electrons and have a planar carbon skeleton
© 2014 by John Wiley & Sons, Inc. All rights reserved.
(4n + 2) planar annulenes:
[14]Annulene(aromatic)
Benzene[6]Annulene
[18]Annulene(aromatic)
All these (4n + 2) , planar annulenes are aromatic
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Non-planar (4n + 2) annulenes are non-aromatic
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10
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11
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non anti
aromatic
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7D. Aromatic Ions
H HH H
pka = 16pka = 36
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Bu Li(a strong base)
H H H
H
HH
strong
base
sp3 sp2
6 electrons aromatic
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H H
- H
6 electrons(aromatic)
© 2014 by John Wiley & Sons, Inc. All rights reserved.
Based on sound calculations or experiments● If the ring has lower -electron
energy, then the ring is aromatic● If the ring and the chain have the
same -electron energy, then the ring is nonaromatic
● If the ring has greater -electron energy than the open chain, then the ring is antiaromatic
© 2014 by John Wiley & Sons, Inc. All rights reserved.
13
Cyclobutadiene-electron
energy increases+ H2
1,3-Butadiene4 electrons
Cyclobutadiene4 electrons(antiaromatic)
-electronenergy decreases
+ H2
1,3,5-Hexatriene6 electrons
Benzene6 electrons(aromatic)
Benzene
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1. must have a ring2. every atom must have a p orbital. otherwise --> nonaromatic 3. ring must be flat. otherwise --> nonaromatic4. 4n+2. if 4n=antiaromatic
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8. Other Aromatic Compounds
Benzenoid polycyclic aromatic hydrocarbons consist of molecules having two or more benzene rings fused together
8A. Benzenoid Aromatic Compounds
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8B. Nonbenzenoid AromaticCompounds
(Azulene)
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8C. Fullerenes
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9. Heterocyclic Aromatic Compounds
Cyclic compounds that include an element other than carbon are called heterocyclic compounds
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Examples of useful heterocyclic aromatic compounds
NH
HO
NH2
Serotonin(neurotransmitter)
S
N
N
S
HOOC
COOHO
H
O
Penicillin(antibiotic)
OO2NN N N
O
H
ONitrofurantoin(urinary antibacterial)
O
N
NN
N
S N
OH
OO "Viagra"
N
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Aromaticity
X
X
NH
X = O, S
N H
6 e: aromatic
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Aromaticity● Evidence: 1H NMR shift
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Basicity of nitrogen-containing heterocycles
N
N
N NNH HH
Order of Basicity: >> >
pKa of theconjugate acid: 11.2 7 5.2 0.4
(c.f. Et3N, pKa of the conjugate acid = 9.7)
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17
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Basicity of nitrogen-containing heterocycles
(still aromatic)Imidazole
(a very common basein organic synthesis)
N
NH
+ H+N
N
N
NH
H
H
HN
N
H
H
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NN H1
23
45 (aromatic)
NN H XX H
NN H
H
+
(aromatic)
NN H XX H
NN
H+
(aromatic)6 electrons
H
(non-aromatic)4 electronsbasic
nitrogen
Non-basic nitrogen
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10. Aromatic Compounds in Biochemistry
Two amino acids necessary for protein synthesis contain the benzene ring
Phenylalanine
O
O
NH3
Tyrosine
O
O
NH3HO
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N
N1
2 3 4
5N
NH
N
N
6 7
8
9
5
6 1 2
34
PyrimidinePurine
Derivatives of purine and pyrimidine are essential parts of DNA and RNA
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•The bases with the carbonyl groups (uracil, thymine, cytosine and guanine) are not aromatic as drawn.•However, if we consider the important resonance contributor from the amide systems, then we can see their aromatic character as shown for cytosine below:
Nicotinamide adenine dinucleotide, one of the most important coenzymes in biological oxidations and reductions, includes both a pyridine derivative (nicotinamide) and a purine derivative (adenine) in its structure
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11. Spectroscopy of AromaticCompounds
The ring hydrogens of benzene derivatives absorb downfield in the region between 6.0 and 9.5 ppm
11A. 1H NMR Spectra
The carbon atoms of benzene rings generally absorb in the 100–170 ppmregion of 13C NMR spectra
11B. 13C NMR Spectra
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7C. NMR Spectroscopy: Evidence forElectron Delocalization inAromatic Compounds
The 1H NMR spectrum of benzene consists of a single unsplit signal at 7.27
The signal occurs at relatively high frequency, which is compelling evidence for the assertion that the electrons of benzene are delocalized
© 2014 by John Wiley & Sons, Inc. All rights reserved.
The circulation of electrons in benzene creates an induced magnetic field that, at the position of the protons, reinforces the applied magnetic field. This reinforcement causes the protons to be strongly deshielded and to have a relatively high frequency ( ~ 7) absorption
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HH
HH
H
H
H H
H
H
H
H
H H
H
H
H
H
( -3.0)
( 9.3)
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N
H O
(c)
(d)
N
H O
N
H O
N
H O
A B C D
(c)
(d)
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11C. Infrared Spectra of SubstitutedBenzenes
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11D. Ultraviolet–Visible Spectra ofAromatic Compounds
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11E. Mass Spectra of AromaticCompounds
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