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Chapter 13 Aromatic compounds
13.1 Introduction
1. How to find benzene?
2. Kekule(克库勒) was the first to recognize that these early aromatic compounds all contain a six-carbon unit.
Benzene 苯
13.2 Nomenclature of benzene Derivatives
F Cl Br NO2
Fluorobenzene Chlorobenzene Bromobenzene Nitrobenzene
(·ú±½£© £¨Âȱ½£© £¨äå±½£© Ïõ»ù±½£©
1. Monosubstituted benzenes µ¥È¡´ú±½
CH3 CH2CH3OH NH2
Methylbenzene Ethylbenzene Hedroxybenzene Aminobenzene
(¼×±½£© £¨ÒÒ±½£© £¨±½·Ó£© £¨°±»ù±½£¬ ±½°·£©
1. Monosubstituted benzenes µ¥È¡´ú±½
Toluene Phenol Aniline
Monosubstituted benzenes
SO3H COOH COCH3 OCH3
Benzenesulfonic acid Benzoic acid Acetophenone Anisole
(±½»ÇËᣩ £¨±½¼×Ëᣩ £¨±½ÒÒͪ£© £¨±½¼×ÃÑ£©
Br Br
Br
1,2-Dibromobenzene 1,3-Dibromobenzene 1,4-Dibromobenzene
(ÁÚ-¶þäå±½£©
Br
BrBr
o-Dibromobenzene m-Dibromobenzene p-Dibromobenzene
(¼ä-¶þäå±½£© (¶Ô-¶þäå±½£©
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.
Nitrobenzoic acids 硝基苯甲酸
COOH COOH
COOH
2-Nitrobenzoic acid 3-Nitrobenzic acid 4-Nitrobenzic acid
(ÁÚ-Ïõ»ù±½¼×Ëᣩ
NO2
NO2 NO2
o-Nitrobenzoic acid m-Nitrobenzoic acid p-Nitrobenzoic acid
(¼ä-Ïõ»ù±½¼×Ëᣩ (¶Ô-Ïõ»ù±½¼×Ëᣩ
The dimethylbenzenes are called xylenes (二甲苯)
CH3 CH3
CH3
1,2-Dimethylbenzene 1,3-Dimethylbenzne 1,4-Dimethylbenzene
(ÁÚ-¶þ¼×±½£©
CH3
CH3 CH3
o-xylene m-xylene p-xylene
(¼ä-¶þ¼×±½£© (¶Ô-¶þ¼×±½£©
If more than two groups are present on the benzene ring,
their positions must be indicated by the use of
numbers.CH3 Cl
Br
1,2,3-Trimethylbenzene 1,2,3-Trichlorobenzne 1,2,4-Tribromobenzene
(1,2,3-Èý¼×±½£©
CH3
ClBr
(1,2,3-ÈýÂȱ½£© (1,2,4-Èýäå±½£©
CH3
Cl
Br
(not 1,3,4-tribromobenzene)
The new parent name is used
COOHSO3H
3,5-Dinitrobenzoic acid 2,4-Difluorobenzenesulfonic acid
(3,5-¶þÏõ»ù±½¼×Ëᣩ (2,4-¶þ·ú±½»ÇËᣩ
NO2O2N
F
F
When the C6H5- group is named as a substituent, it is calle
d a phenyl group (ph-).
CH2CH2CH2CH3 CH3C=CHCH3
ph
CH3CHCH2CH2CH2CH2CH3
ph
Butylbenzene 2-Phenyl-2-butene 2-Phenylheptane
¶¡»ù±½ 2-±½»ù-2-¶¡Ï© 2-±½»ù¸ýÍé
The phenyl group is often abbreviated as C6H5-, Ph-.
The name benzyl is an alternative name for the phenylmethyl gr
oup:
CH2-
Phenyl
Benzyl CH2Cl Benzyl chloride
Cl Chlorobenzene
ÜлùÂÈ
±½»ù
Üлù
13.3 Reactions of benzene
Benzene
Br2 / CCl4
dark, 25oCNo addition of bromine
KMnO2 / H2O
25 oC
H2O, HCl
heat
H2 / Ni
No hydration
No oxidation
Slow additionat hight temperatureand pressure
Why?
Why?
Why?
Why?
Br2 FeBr3
Br Br
Br
Not observedObserved
Why?
13.4 The Kekule Structure for benzene 苯的克库勒结构
In 1865, Kekule, the originator of the structural theory, proposed the first definite structure for benzene, a structure that is still used today. Kekule suggested that the carbon atoms of benzene are in a ring, that they are bonded to each other by alternating single and double bonds, that one hydrogen atom is attach to each carbon ato
m.
The Kekule formula for benzene
苯的克库勒式
orC
C
C
C
C
CH
H
H
H
H
H
C
C
C
C
C
CH
H
H
H
H
H
C
C
C
C
C
CH
H
H
H
H
H
¿Ë¿âÀյĹ²Õñ½á¹¹Ê½
¿Ë¿âÀյĹ²Õñ½á¹¹Ê½
Br
Br
Br
Br
No isomer
Same compoundͬһÎï
13.5 The stability of benzene苯的稳定性
+
+ 2H2
+ 3H2
Pt
Pt
Pt
Benzene
1,3-Cyclohexadiene
Cyclohexene Cyclohexane
Ho = - 28.6 kcal mol -1
Ho = (2X- 28.6)= -57.2 kcal mol -1Calculated
ObservedHo = - 55.4 kcal mol -1
Ho = (3X- 28.6)= -85.8 kcal mol -1Calculated
ObservedHo = - 49.8 kcal mol -1
Difference 36.0 kcal mol -1
H2
Fig. Relative stabilities 相对稳定性
Benzene
1,3-Cyclohexadiene
Cyclohexane
Ho = - 28.6 kcal mol -1 Ho = - 55.4 kcal mol -1
Ho = (3X- 28.6)= -85.8 kcal mol -1Calculated
ObservedHo = - 49.8 kcal mol -1
Difference 36.0 kcal mol -1
(¹²ÕñÎȶ¨»¯ÄÜ£©
Pot
enti
al e
ner
gy
+ H2
+ 2H2
+ 3H2
13.6 Modern theories of the structure of benzene
13.6A the resonance explanation of the structure of benzene (苯环结构的共振理论解
释)
Benzene Cyclohexane
Ho = (3X- 28.6)= -85.8 kcal mol -1Calculated
ObservedHo = - 49.8 kcal mol -1
Difference 36.0 kcal mol -1
(¹²ÕñÎȶ¨»¯ÄÜ£©
+ 3H2
13.6B The molecular orbital explanation of the structure of benzen
e (苯结构的分子轨道理论解释)
H
H
H
H
H
H
120 o120 o
1. sp2-hybridized
2. Angel is 120o.
3. Planar (ËùÓеÄÔ ×Ó¹²Æ½Ã棩
Fig. 13.3 Overlapping p orbitals in benzene (苯的 p轨道的重
叠)
Antibonding MO
Bonding MO
·´¼ü¹ìµÀ
³É¼ü¹ìµÀ
Fig. 13.4 How six p atomic orbital combine to form six pi- molecular orbitals
Fig. 13.5 Shapes of the pi-molecular orbitals of benzene as viewed from down
Two nodes
One node
Three nodes
13.7 HUCKEL’S Rule(休克尔规则) : The (4n+2) pi-Electron Ru
le
Conditions;
1. The (4n+2) pi-electron, where n=1,2,3---
2. Planar monocyclic ring conjugate system (平面单环共轭体系)
符合这两个条件的环具有芳香性( Aromatic), 具有芳香性的物质,是稳定的。化学
性质表现出难加成,易取代。
For example
pi-electrons = 4 + 2 = 6 n=1
(Aromatic ·¼ÏãÐÔ£©
Benzene
pi-electrons = 8 didn't follow 4n+2 rule
(No Aromatic ²»¾ßÓз¼ÏãÐÔ£©
Cyclooctatetraene£¨»·ÐÁËÄÏ©£©
It is not coplanar
13.7A The Annulenes (轮烯)
The name annulene has been proposed as a general name for monocyclic compounds that can be represented by structures having alternating single and double
bonds.
Benzene Cyclooctatetraene£¨»·ÐÁËÄÏ©£©([6] annulene)
[6]-ÂÖÏ© ([8] annulene)[8]-ÂÖÏ©
14
[14] Annulene
pi-electrons is 144n+2 = 4X3+2 = 14 (·ûºÏ4n+2)(aromatic)(¾ßÓз¼ÏãÐÔ£©
18
[18] Annulenepi-electrons are 184n+2 = 4X4+2 = 18 (·ûºÏ4n+2)(aromatic)(¾ßÓз¼ÏãÐÔ£©
16
[16] Annulenepi-electrons is 164n+2 = 4X3+2 = 14(²»·ûºÏ4n+2)(not aromatic)(²»¾ßÓз¼ÏãÐÔ£©
[10] Annulene
pi-electrons is 104n+2 = 4X2+2 = 10 (·ûºÏ4n+2), Do not obey Huckel's Rule(not aromatic)(²»¾ßÓз¼ÏãÐÔ£©It is not planar (²»¹²Æ½Ã棩
HH
[10] Annulene [10] Annulene
None is aromatic because none is planar
It is not planar (²»¹²Æ½Ã棩, (²»¾ßÓз¼ÏãÐÔ£©
Cyclobutadiene is a 4n molecule not a 4n+2 molecule, and as we would expect, it is a highly unstable compound and it is not a
romatic.
Cyclobutadieneor [4] annuleneDo not obey HUCKEL's Rule(not aromatic)
13.7B Aromatic Ions 芳香离子
Cyclopentadiene is not aromatic; however, it is unusually acidic for a hydrocarbon. Because of its acidity, cyclopentadiene can be converted to its anion by treatment with moderat
ely strong bases.
H H
Cyclopentadiene»·Îì¶þÏ©
Strong
Base
H
_
Cyclopentadienyl anion»·Îì¶þÏ©¸ºÀë×Ó
+ H+
H
_
Cyclopentadienyl anion»·Îì¶þÏ©¸ºÀë×Ó
ÊÇ·ñ¾ßÓз¼ÏãÐÔÐÔ£¿
H H
Cyclopentadiene»·Îì¶þÏ©
Strong
Base _
Cyclopentadienyl anion»·Îì¶þÏ©¸ºÀë×Ó
+ H+
sp3sp2
pi-electrons are 6, follow HUCKEL'RuleIt is aromatic compoundIt is not aromatic compound
Cycloheptatriene (环庚三烯)
H H
- H-
+
Cycloheptatriene»·¸ýÈýÏ©(it is not aromatic)
Cycloheptatrienyl cation»·¸ýÈýÏ©ÑôÀë×Ó(It is aromatic)
+
13.8 Other aromatic compounds
13.8A benzenoid aromatic compounds (苯形芳香化合物)
Representatives of one broad class of aromatic compounds called polycyclic benze
noid aromatic hydrocarbons.1
2
3
45
6
7
8
NaphthaleneÝÁ
1
2
3
410
9
6
5
7
8
AnthracneÝì
7
8
9
101
2
3
4
Phenanthrene·Æ
6
5
2 5
4
10
6
3
7
8
Pyrene
ÜÅ
1
9
According to resonance theory, a molecule of naphthalene can be considered to be a hybrid of thre
e Kekule structure.
Naphthalene
Fig. 13.2 The p orbitals of naphthalene
Naphthalene
=
Naphthalene
( Aromatic ¾ßÓз¼ÏãÐÔ)
Pyrene ÜÅ
£¨·¼ÏãÐÔ£©
[14]-Annulene
( no aromatic ²» ¾ßÓз¼ÏãÐÔ) H3C CH3
trans-15,16-Dimethyldihydropyrene( aromatic ¾ßÓз¼ÏãÐÔ)
13.8B Nonbenzenoid aromatic compounds非苯型的芳香族化合
物
H3C CH3
trans-15,16-Dimethyldihydropyrene( aromatic ¾ßÓз¼ÏãÐÔ)
Azulene
°Â
The dipole moment is 1.0D
(aromatic)
13.9 Reduction of aromatic compounds: The Birch reduction 伯
奇还原
Benzene
H2 / Ni
Slow+
Cyclohexadiene
H2 / Ni
fast
H2 / Ni
fast
Cyclohexene Cyclohexane
13.9A The Birch reduction 伯奇还原
Benzene can be reduced to 1,4-cyclohexadiene by treating it with an alkali
metal(sodium (Na), lithium (Li), or potassium (K)) in a mixture of liquid ammonia and
alcohol.
Benzene
NH3, EtOH
1,4-Cyclohexadiene
Na or K
The Mechanism of Birch reduction
伯奇还原的机理
Benzene
NH3, EtOH
Benzene anion radical
Na .
:-
.
-:
.
EtOH
H
H
.
H
H
.
Cyclohexadienyl radical»·¼º¶þÏ©×ÔÓÉ»ù
Na .
H
H
:-
H
H
:-
Cyclohexadienyl aniom»·¼º¶þÏ©¸ºÀë×Ó
EtOH
H
H
H
H
1,4-Cyclohexadiene
Dissolving metal reductions of this type were developed by the Australian chemist A.J. Birch
and have come to be known as Birch reductions
Methoxybenzene±½¼×ÃÑ
NH3, EtOH
LiOCH3 OCH3
1-Methoxy-1,4-cyclohexadiene 80 %
Reduction of 1,2-dimethylbenzene (o-xylene 邻 -二甲苯 )
1,2-Dimethylbenzene
NH3, EtOH
NaCH3 CH3
1,2-Dimethyl-1,4-cyclohexadiene 77-92 %
CH3 CH3
(o-xylene)
Birch reduction of sodium benzoate
Sodium benzoate
NH3, EtOH
Na
(±½¼×ËáÄÆ)
COONa COONa
H3O+
COOH
89-52 %
13.10 Benzylic radicals and cations
Removal of a hydrogen atom from the methyl group of methylbenzene (Toluene) produces a radical called the benzyl radical
(苄基自由基)HH
H
Methylbenzene(Toluene)
HH
The benzyl radicalÜлù×ÔÓÉ»ùStable
.
-e-
H
H+
Benzylic cationÜлùÑôÀë×ÓStable
Benzylic radicals and benzylic cations are conjugated unsaturated systems and both are stable.
This exceptional stability of benzylic radicals and cations is easily e
xplained by resonance theory.
HH
The benzyl radicalÜлù×ÔÓÉ»ùStable
.
.
H H
.
H H
.
H H
Benzylic radicals are stabilized by resonance
Benzylic cations are stabilized by resonance
HH
Benzylic cation ÜлùÑôÀë×ÓStable
+
+
H H
+
H H
+
H H
13.11 Allylic and benzylic halides in nucleophilic substitution r
eactions (亲核取代反应中的烯丙基卤和苄
基卤)(不讲)
13.12 Heterocyclic aromatic compounds (杂环芳香族化合
物)Heterocyclic compounds containing nitrogen, oxygen, or sulfur are by far the most common. Four important examples are given here in their KeKule forms. These four com
pounds are all aromatic.
Pyridine
NNH:
:
1
2
3
4
5
61
2
34
5
Pyrrole
ßÁठßÁ¿©
O S:
: ::
1
2
34
5
1
2
34
5
Furan Thiophene
߻ૠàç·Ô
The nitrogen atoms in molecules of both pyridine and pyrro
le are sp2 hybridized.
Pyridine
:
ßÁà¤
NN
Fig.13.15 The orbital structure of pyridine weak base (Èõ¼î£©
NH
:
Pyrrole
ßÁ¿©
H
Fig.13.16 The orbital structure of pyrrole
13.17 The orbital structures of furan and thiophene
O::Furan
߻ૠFig.13.16 The orbital structure of Furan
S ::Thiophene
àç·ÔFig.13.16 The orbital structure of thiophene
The oxygen and sulfur atoms of furan and thiophene carry an Unshared pair of electrons in an sp2 orbital that is orthogonal to the πsystem.
13.13 Aromatic compounds in biochemistry
Two amino acids necessary for protein synthesis contain the benzene
ring:
CH2CHCOO-
NH3+
Phenylalanine±½±û°±Ëá
CH2CHCOO-
NH3+
TyrosineÀÒ°±Ëá
HO
CH2CCHOO-
NH3+
TryptophanÉ«°±Ëá
NH
N
NNH
N
PurineàÑßø
1
2
34
56 7
8
9
NH
IndoleßÅßá
N
N
Pyrimidineà×à¤
1
2
3
4
5
6
Homework P 548
Additional Problems13.22, 13.23, 13.28