Download Chemical Properties of Monocyclic Aromatic Hydrocarbons(5)

有机化学 Organic Chemistry
教 学 内 容
1
5.1 芳烃的结构
2
5.2 芳烃的同分异构和命名
3
5.3 单环芳烃的物理性质
4
5.4 单环芳烃的化学性质
5
5.5 苯环上亲电取代的定位规则
6
5.6 稠环芳烃
5.7芳香性
2008.10
本章教学基本要求：
1、掌握苯、萘、蒽、菲的结构，并会用价键理论和分子轨道理论、
共振论对苯的结构进行解释；
2、掌握芳烃的命名和异构；
3、掌握单环芳烃的性质，理解亲电取代反应历程，掌握定位规则
的应用；
4、了解单环芳烃的来源和制备；
5、掌握多环芳烃的化学性质、萘的磺化反应、动力学控制和热力
学控制。
6、理解芳香性概念、芳香性的判别、休克尔规则。
7、了解非苯芳烃的类型和代表物。
本章重点和难点：
苯的结构、命名、化学性质、亲电取代反应历程和定位规则；芳香
性的判别、休克尔规则。
• Isomerism and Nomenclature of Aromatic Hydrocarbons.
• Structure and Stability of Benzene.
• Physical Properties of Monocyclic Aromatic Hydrocarbons.
• Chemical Properties of Monocyclic Aromatic Hydrocarbons.
• Chemical Properties of Polycyclic Aromatic Hydrocarbons.
• Aromaticity and the Huckel Rule.
Introduction(1)
• In 1834 the German chemist Eilhardt Mitscherlich
(University of Berlin) firstly synthesized benzene by heating
benzoic acid with calicum oxide. Using vapor density
measurements, Mitscherlich further showed that benzene
has the molecular formula C6H6:
C6H5CO2H
Benzoic Acid
+
CaO
Heat
C6H6
+
CaCO3
Benzene
• The molecular formula itself was surprising. Benzene has
only as many hydrogen atoms as it has carbon atoms, it
should be a highly unsaturated compound. Eventually,
chemists began to recognize that benzene does not show the
behavior expected of a highly unsaturated compound.
Introduction(2)
• During the latter part of the nineteenth century the Kekule –
Couper-Butlerov theory of valence was systematically applied
to all known organic compounds. Organic compounds were
classified as being either aliphatic or aromatic.
• To be classified as aliphatic meant that the chemical behavior
of a compound was “fatlike”.
• To be classified as aromatic meant that the compound had a
low hydrogen-carbon ratio and that it was “fragrant”.
Isomerism and Nomenclature of Aromatic
Hydrocarbons(2)
• Disubstituted benzenes are named using one of the prefixes
ortho(o), meta(m), or para(p).
• An ortho-disubstituted benzene has its two substituents in
a 1,2 relationship on the ring; a meta-disubstituted
benzene has its two substituents in a 1,3 relationship; and a
para-disubstituented benzene has its substituents in a 1,4
relationship. For example:
CH3
CH3
CH3
CH3
CH3
CH3
ortho- Xylene
meta - Xylene
para -Xylene
BACK
Isomerism and Nomenclature of Aromatic
Hydrocarbons(1)
• Monosubstituted benzene are systematically named in the same manner
as other hydrocarbons, with –benzene as the parent name. For example:
CH2CH2CH3
Propylbenzene
CH(CH3)2
Isopropylbenzene
• If the alkyl substituent has more than six carbons, or has carbon-carbon
double bond and triple bond, the compound is named as a phenylsubstituted alkane, alkene or alkyne. For example:
CH3
CHCH2CH2CH2CH2CH3
2 _ Phenylheptane
CH3
CHCH2CH=CHCH3
5 _Phenyl _ 2 _hexene
Structure and Stability of Benzene(1)
• In 1865, August Kekule, the originator of the structual
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, and
that one hydrogen atom is attached to each carbon atom.
• The fact that the bond angles of the carbon atoms in the
benzene ring are all 120o strongly suggests that the carbon
atoms are sp2 hydridized.
Structure and Stability of Benzene(2)
• Although benzene is clearly unsaturated, it is much more stable than other
alkenes, and it fails to undergo typical alkene reactions. For example:
+
Br2
Fe
catalyst
H
Br
+
Bromobenzene
Br
Br
HBr
H
Addition product
NOT formed
• We can get a quantitative idea of benzene’s stability from the heats of
hydrogenation.
Benzene
-----------------------------------
150kJ/mol (difference)
1,3- Cyclohexadiene
Cyclohexene
Cyclohexane
---------
-356kJ/mol --------(expected)
----------230kJ/mol
-206kJ/mol (actual)
-118kJ/mol
-------------------------------------------------------
Chemical Properties of Monocyclic AromaticHydrocarbons(1)
• Chemistry of Benzene: Electrophilic Aromatic Substitution.
• The most common reaction of aromatic compounds is electrophilic
aromatic substitution. That is, an electrophile (E+) react with an aromatic
ring and substitutes for one of the hydrogens:
E
H
+
E+
H+
+
• Many different substituents can be introduced onto the aromatic ring by
electrophilic substitution reactions. By choosing the proper reagents, it’s
possible to halogenate the aromatic ring, nitrate it, sulfonate it, alkylate it,
or acylate it.
X
•
Halogenation
•
Nitration
R
•
SO3H
NO2
Alkylation
Sulfonation
H
COR
Acylation
Chemical Properties of Monocyclic
Aromatic Hydrocarbons(2)
• Aromatic Halogenation:
• A. Bromination of Aromatic Rings
• A benzene ring , with its six πelectrons in a cyclic conjugated
system, is a site of electron density. Thus, benzene acts as an
electron donor (a Lewis base, or nucleophile) in most of its
chemistry, and most of its reactions take place with electron
acceptors (Lewis acids, or electrophiles). For example,
benzene react with Br2 in the presence of FeBr3 as catalyst to
yield the substitution product bromobenzene.
+
Br2
FeBr3
Br
+
HBr
Chemical Properties of Monocyclic
Aromatic Hydrocarbons(3)
• The mechanism of the electrophilic bromination of benzene.
Step One
.
δ
+
δ
δ
+
FeBr3
Br Br
Bromine
a weak electrophile
δ
Br
Br
FeBr3
Polarized bromine
a strong electrophile
Br
Step Two
[
.
+
+
δ
Br
Br
δ
FeBr3 Slow
H
+
Br
[+
H
+
Step Three
.
]
[
] + FeBr-4
Br
H
]
Br
H
+
+ FeBr-4
Fast
Br
+
HBr
+ FeBr3
Chemical Properties of Monocyclic
Aromatic Hydrocarbons(4)
• Aromatic Halogenation:
• B. Chlorination and Iodination of Aromatic Rings
• Chlorine and iodine can be introduced into aromatic rings by
electrophilic substitution reactions, but fluorine is too reactive,
and only poor yields of monofluoroaromatic products are
obtained by direct fluorination. For example:
H
+
Cl2
FeCl3
Cl
+
Chlorobenzene
(86%)
H
+
I2
CuCl2
I
Iodobenzene
(65%)
HCl
Chemical Properties of Monocyclic
Aromatic Hydrocarbons(5)
• Aromatic Nitration
• Aromatic rings can be nitrated by reaction with a mixture of concentrated
nitric and sulfuric acids. The electrophile in this reaction is the nitronium
ion, NO2+, which is generated from HNO3 by protonation and loss of water.
The nitronium ion react with benzene to yield a carboncation intermediate
in much the same way as Br+. Loss of H+ from this intermediate gives the
product, nitrobenzene.
...O
H
O
+
N
H
+
O
.
H+
O
H
O
N
N O
+
O N O
O
O
+O
H2O
N
O
H ] HSO4
[
+
NO2
+ H2SO4
Chemical Properties of Monocyclic
Aromatic Hydrocarbons(6)
• Aromatic Sulfonation
• Aromatic rings can be sulfonated by reaction with fuming
sulfuric acid, a mixture of H2SO4 and SO3. The reactive
electrophile is either HSO3+ or SO3, depending on reaction
conditions. Substitution occurs by the same two-step
mechanism seen previously for bromination and nitration.
O
O
S
O
+ H2SO4
O
O
O
H
+ O S
O
[
O
S
H
O
+ HSO4
O
S OH
H ] HSO4
SO3H
+ H2SO4
Chemical Properties of Monocyclic
Aromatic Hydrocarbons(7)
• Alkylation of Aromatic Rings: The Friedel-Crafts Reaction
• One of the most useful of all electrophilic aromatic substitution reactions
is alkylation, the attachment of an alkyl group to the benzene ring.
• For example:
+
Benzene
Cl
CH3CHCH3
2- Chloropropane
AlCl3
CH3
CH CH
3
+ HCl
Isopropylbenzene
• The Friedel-Crafts alkylation reaction is an electrophilic aromatic
substitution in which the electrophile is a carbocation, R+. Aluminum
chloride catalyzes the reaction by helping the alkyl halide to ionize in
much the same way that FeBr3 catalyzes aromatic brominations by
polarizing Br2 . Loss of a proton then completes the reaction.
Chemical Properties of Monocyclic AromaticHydrocarbons(8)
• The mechanism of the Friedel-Crafts alkylation reaction:
Cl
CH3CHCH3 + AlCl3
CH3CHCH3 AlCl4
CH3
CHCH3
+ CH3CHCH3 AlCl4
H ] AlCl
4
[
CH3
CH CH3
+ HCl + AlCl3
• Give the structures of the major products of the following reactions:
+ CH3CH2CH2Cl
+ CH3CH=CH2
AlCl3
HF
O
0C
• How to prepare propylbenzene by Friedel-Crafts reaction?
?
CH2CH2CH3
Chemical Properties of Monocyclic Aromatic Hydrocarbons(9)
• An acyl group, -COR, is introduced onto the ring when an aromatic
compound reacts with a carboxylic acid chloride, RCOCl, in the presence
of AlCl3. For example, reaction of benzene with acetyl chloride yields the
ketone, acetophenone.
O
+ CH3CH2CCl
O
CCH2CH3
AlCl3
+ HCl
O
80 C
• The mechanism of Friedel-Crafts acylation:
O
CH3CH2CCl
AlCl3
CH3CH2C O
CH3CH2C O
-
+ AlCl4
An acyl cation
O
C
+ CH3CH2C O
[
CH2CH3
H ]
-
AlCl4
O
CCH2CH3
+ HCl + AlCl3
Chemical Properties of Monocyclic Aromatic Hydrocarbons(10)
• How to prepare propylbenzene by Friedel-Crafts reaction?
CH2CH2CH3
?
• By contrast, the Friedel-Crafts acylation of benzene with propanoyl
chloride produces a ketone with an unrearranged carbon chain in
excellent yield.
O
+ CH3CH2CCl
AlCl3
O
O
CCH2CH3
+ HCl
80 C
• This ketone can then be reduced to propylbenzene by several methods.
One general method-called the Clemmensen reduction-consists of
refluxing the ketone with hydrochloric acid containing amalgamated
zinc.
O
C
CH2CH3
Ethyl phenyl ketone
Zn(Hg)
HCl
reflux
CH2CH2CH3
Propylbenzene( 80%)
Chemical Properties of Monocyclic Aromatic Hydrocarbons(11)
• Substituent Effects in Substituted Aromatic Rings
• Only one product can form when an electrophilic substitution occurs
on benzene, but when what would happen if we were to carry out a
reaction on an aromatic ring that already has a substituent?
• A substituent already present on the ring has two effects:
• 1. A substituent affects the reactivity of the aromatic ring. Some
substituents activate the ring, making it more reactive than benzene,
and some deactivate the ring, making it less reactive than benzene.
• For example:
OH
Reactive rate 1000
of nitration
H
1
NO2
Cl
0.033
6╳10-8
Chemical Properties of Monocyclic Aromatic Hydrocarbons(12)
• Substituent Effects in Substituted Aromatic Rings
• 2. Substituents affect the orientation of the reaction. The three possible
disubstituted products-ortho, meta, and para- are usually not formed in
equal amounts. Instead, the nature of the substituent already present on
the benzene ring determines the position of the second substitution. For
example:
Orientation of Nitration in Substitued Benzenes
•
•
•
•
•
•
•
•
•
•
•
Product (%)
Ortho Meta Para
Meta-directing deactivators
-+N(CH3)3
2
87
11
-NO2
7
91
2
-COOH
22
76
2
-CN
17
81
2
-COOCH3
28
66
6
-COCH3
26
72
2
-CHO
19
72
9
Product(%)
Ortho
Meta
Para
Ortho- and para-directing deactivators
-F
13
1
86
-Cl
35
1
64
-Br
43
1
56
-I
45
1
54
Ortho- and para-directing activators
-CH3
63
3
34
-OH
50
0
50
-NHCOCH3 19
2
79
Chemical Properties of Monocyclic Aromatic Hydrocarbons(13)
• Substituent Effects in Substituted Aromatic Rings
• Substituents can be classified into three groups:
• Ortho-and para-directing activators, ortho-and paradirecting deactivators, and meta-directing deactivators.
NH2 OCH3 CH3(alkyl)
OH NHCOCH3 Ph
•
•
Ortho-and paradirecting activators
F
Br
Reactivity
H Cl
I
O
C H
O
C OH
COCH3
O
ortho-and paradirecting
deactivators
SO3H NO2
CCH3
O
CN
NR
+ 3
Meta-directing
deactivators
Chemical Properties of Monocyclic Aromatic Hydrocarbons(14)
• An Explanation of Substituent Effects(1)
• Activation and Deactivation of Aromatic Rings
• The common feature of all activating groups is that they
donate electrons to the ring, thereby stabilizing the
carbocation intermediate from electrophilic addition and
causing it to form faster.
• The common feature of all deactivating groups is that they
withdraw electrons from the ring, thereby destabilizing the
carbocation intermediate from electrophilic addition and
causing it to form more slowly.
Chemical Properties of Monocyclic Aromatic Hydrocarbons(15)
• An Explanation of Substituent Effects(2)
• Ortho- and Para- Directing Activators: Alkyl Groups
•
Inductive and resonance effects account for the directing ability of substituents
as well as for their activating or deactivating ability. Take alkyl groups, for
example, which have an electron-donating inductive effect and behave as ortho
and para directors. The results of toluene nitration are shown as below:
CH3
Othro
CH3
H
NO2
CH3
CH3
H
NO2
Most stable
CH3
CH3
CH3
Meta
H
NO2
H
NO2
H
NO2
Para
H
NO2
CH3
CH3
CH3
H NO2
H NO2
Most stable
H NO2
Chemical Properties of Monocyclic Aromatic Hydrocarbons(15)
• An Explanation of Substituent Effects(3)
• Ortho- and Para- Directing Activators: OH and NH2
• Hydroxyl, alkoxyl, and amino groups are also ortho-para activators,
but for a different reason than for alkyl groups. Hydroxyl, alkoxyl, and
amino groups have a strong, electron-donating resonance effect that is
most pronounced at the ortho and para positions and outweighs a
weaker electron-withdrawing inductive effect. When phenol is nitrated,
only ortho and para attack is observed:
H
NO2
H
NO2
Ortho
Meta
Para
OH
H
NO2
H
NO2
Most stable
H NO2
H
NO2
H
NO2
H
NO2
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
H NO2
H NO2
H NO2
Most stable
Chemical Properties of Monocyclic Aromatic Hydrocarbons(16)
• An Explanation of Substituent Effects(4)
• Ortho- and Para- Directing Deactivators: Halogens
•
Halogens are deactivating because their stronger electron-withdrawing
inductive effect outweighs their weaker electron-donating resonance effect.
Though weak, that electron-donating resonance effect is felt only at the ortho
and para positions.
Cl
H
NO2
Ortho
Meta
Para
Cl
H NO2
Cl
H
NO2
H
NO2
H
NO2
Cl
H NO2
H
NO2
H
NO2
H
NO2
Most stable
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H NO2
Most stable
Cl
H NO2
Chemical Properties of Monocyclic Aromatic Hydrocarbons(17)
• An Explanation of Substituent Effects(5)
• Meta- Directing Deactivators
•
Meta-directing deactivators act through a combination of inductive and
resonance effects that reinforce each other. Inductively, both ortho and para
intermediates are destabilized because a resonance form places the positive
charge of the carbocation intermediate directly on the ring carbon atom that
bears the deactivating group. At the same time, resonance electron withdrawal
is also felt at the ortho and para positions. Reaction with an electrophilic
therefore occurs at the meta position.
Least stable
CHO
CHO
CHO
H
Cl
H
Cl
H
Cl
Ortho
CHO
CHO
CHO
Meta
H
Cl
H
Cl
Para
H
CHO
CHO
CHO
Cl
H Cl
Least stable
H
Cl
CHO
H
Cl
Chemical Properties of Monocyclic Aromatic Hydrocarbons(18)
• Trisubstituted Benzenes: Additivity of Effects
•
•
•
•
•
Further electrophilic substitution of a disubstituted benzene is governed by the
same resonance and inductive effects just discussed. The only difference is that
it’s necessary to consider the additive effects of two different groups. In practice,
three rules are usually sufficient:
Rule 1. If the directing effects of the two groups reinforce each other, there is no
problem.
Rule 2. If the directing effects of the two groups oppose each other, the more
powerful activating group has the dominant influence, but mixtures of products
often result.
Rule 3. Further substitution rarely occurs between the two groups in a metadisubstituted compound because this site is too hindered.
Some examples:
CH3
OH
CH3
COOH
NHCOCH3
Cl
NO2
CH3
SOH3
Chemical Properties of Monocyclic Aromatic Hydrocarbons(19)
• Synthesis of Substituted Benzenes
• One of the surest ways to learn organic chemistry is to work synthesis
problems. The ability to plan a successful multistep synthesis of a
complex molecule requires a working knowledge of the uses and
limitations of many hundreds of organic reactions. Not only must you
know which reactions to use, you must also know when to use them.
The order in which reactions are carried out often critical to the success
of the overall scheme.
• The ability to plan a sequence of reactions in the right order is
particularly valuable in the synthesis of substituted aromatic rings,
where the introduction of a new substituent is strongly affected by the
directing effects of other substituents. Planning synthesis of substituted
aromatic compounds is therefore an excellent way to gain facility with
the many reactions learned in the past few chapters. Some examples:
COOH
NO2
Br
C(CH3)3
A
B
NO2
Cl
CH2CH2CH3
C
Chemical Properties of Monocyclic Aromatic Hydrocarbons(20)
• Reduction of Aromatic Compounds
• To hydrogenate an aromatic ring, it’s necessary to use a
platinum catalyst with hydrogen gas at several hundred
atmospheres pressure. For example:
CH3
CH3
H2/Pt/ethanol
200atm, 25 oC
CH3
CH3
• Oxidation of Benzene:
O
+ O2
o
400~500 C
V2O5
O
O
Chemical Properties of Monocyclic Aromatic Hydrocarbons(21)
• Oxidation of Alkylbenzene Side Chains
• Alkyl side chains are readily attacked by oxidizing agents
and are converted into carboxyl groups, -COOH. For
example:
CH2CH3
COOH
KMnO4/H2O
C(CH3)3
C(CH3)3
KMnO4/H2O
CH3
HOOC
• Bromination of Alkylbenzene Side Chains
CH2CH3
NBS/CCl4
Br
CHCH3
hv
BACK
Chemical Properties of Polycyclic Aromatic Hydrocarbons(1)
• Polycyclic aromatic hydrocarbons have two or more benzene rings
fused together. For example:
6
5
7
8
1
6
8
4
1
2
7
2
3
3
6
3
2
4
5
9
5
10
7
9
1
4
•
Naphthalene
Anthracene
• Reactions of Naphthalene:
8
10
Phenanthrene
Cl
O
Na/NH3(l)/CH3CH2OH
O
CrO3/CH3COOH
o
10-15 C
Cl2/FeCl3
o
100~110 C
COCH3
CH3COCl/AlCl3
C6H5NO2
CH3COCl/AlCl3
COCH3 CS2 -15 oC H2SO4 165oC
NO2
HNO3/H2SO4
o
30~50 C
o
H2SO4 80 C
SO3H
SO3H
Chemical Properties of Polycyclic Aromatic Hydrocarbons(2)
• Substituent Effects in Substituted Naphthalene
Ⅱ
I
I
Ⅱ
Aromaticity and the Huckel Rule
• In 1931 the Germen physicist Erich Huckel carried out a
series of mathematical calculations based on the theory of
molecular orbital. Huckel’s rule is concerned with
compounds containing one planar ring in which each atom
has a p orbital as in benzene. His calculations show that
planar monocyclic rings containing 4n+2 πelectrons,
where n=0, 1, 2, 3,……, and so on, delocalized electrons
should be aromatic. For example:
O
Additional problems of chapter five (1)
•
•
3.1 Give IUPAC names for the following compounds:
CH3
CH3
CH3
(a)
(b)
CH2CHCH=CH2
CH2CH3
NO2
CH3
•
(c)
CH3
(d)
SO3H
•
•
•
•
3.2 Predict the major product(s) of the following reactions:
(a)
(CH3)2C=CH2
(
HF
CH3CH2Cl
AlCl3
)
(
)
K2Cr2O7
H2SO4/H2O
O
(b)
CH2CH2CCl
AlCl3
(
)
Zn/Hg/HCl
(
)
(c)
Na/NH3(l)
C2H5OH
(
)
(
)
(
)
Additional problems of chapter five (2)
•
•
3.3 At what position, and on what ring, would you expect the following
substances to undergo electrophilic substitution?
C(CH3)3
COOH
(a)
(b)
(c)
CH(CH3)2
OCH3
H3C
•
(d)
(e)
NHCOCH3
(f)
Br
(i)
H
CO
O
•
(g)
(h)
O 2N
N
C
Cl
O
•
•
H3C
Br
3.4 How would you synthesize the following substances starting from benzene?
(a)
(b)
(c)
(d)
C(CH3)3
Cl
COOH
NO2
NO2
CH2Cl
CH2CH2CH3
COOH
Br
NO2
Additional problems of chapter five (3)
•
•
3.4 Which would you expect to be aromatic compounds according to Huckel
4n+2 rule?
(a)
(b)
(c)
(d)
(e)
•
(f)
(g)
(h)
(i)
O
N