有机化学 organic chemistry

有机化学 Organic Chemistry

2008.10

5.1 芳烃的结构

5.2 芳烃的同分异构和命名2

5.3 单环芳烃的物理性质3

5.4 单环芳烃的化学性质4

5.5 苯环上亲电取代的定位规则5

1

5.6 稠环芳烃 5.7 芳香性

6

教学内容

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:

• 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.

C6H5CO2H C6H6 CaCO3CaO ++Heat

Benzoic Acid Benzene

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

Xylenepara -Xylenemeta -Xyleneortho- 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:

• If the alkyl substituent has more than six carbons, or has carbon-carbon double bond and triple bond, the compound is named as a phenyl-substituted alkane, alkene or alkyne. For example:

CH2CH2CH3 CH(CH3)2

Propylbenzene Isopropylbenzene

CHCH2CH2CH2CH2CH3

CH3

CHCH2CH=CHCH3

CH3

Phenyl hexene5_ 2__Phenylheptane2_

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:

• We can get a quantitative idea of benzene’s stability from the heats of hydrogenation.

+ Br2

BrHBr

Fe

catalyst+

Bromobenzene

H

H

Br

Br

Addition product

NOT formed

-------------------------------------------------------

-----------------------------------

---------

---------

---------

Cyclohexane

Cyclohexene

Benzene

Cyclohexadiene1,3-

-118kJ/mol

-230kJ/mol

-356kJ/mol

-206kJ/mol

(expected)

(actual)

150kJ/mol

(actual)

(difference)

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:

• 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.

• Halogenation

• Nitration Sulfonation

• Alkylation Acylation

H E

+ E+ + H+

H

X

SO3H

COR

NO2

R

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

Br

HBrFeBr3

+


• The mechanism of the electrophilic bromination of benzene.

FeBr3

Polarized brominea weak electrophile a strong electrophile

Br Br FeBr3

δ +

δ -

Br Brδ -δ

+

Br Bromine

SlowBr Br FeBr3

δ +

δ -

+ + FeBr4-

+

Br

H

Br

H+

Br

H

+

[

[

[ ]

]

]

FeBr3FeBr4

-+

Br

H + FastBr

HBr+ +

Step One

Step Two

Step Three..

..

..


• 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:

FeCl3 Cl2

H

+Cl

+ HCl

CuCl2 I2

H

+

Chlorobenzene(86%)

I

Iodobenzene

(65%)


• 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.

[ ]+ +

ONO+.O.+HN

O

OH

.O... N

O

O+ +H

H

ONO

N

H

O O

+

N 2OHSO4 H2SO4

H2O


• 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 SO

OH

OS

O O

H

++OS

O OH2SO4 HSO4

O

H

SOOH

HSO4

SO3H

+ H2SO4


• 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:

• 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.

Benzene Chloropropane Isopropylbenzene2-

CH3CHCH3

Cl+ AlCl3

CHCH3

CH3+ HCl

Chemical Properties of Monocyclic AromaticHydrocarbons(8)• The mechanism of the Friedel-Crafts alkylation reaction:

• Give the structures of the major products of the following reactions:

• How to prepare propylbenzene by Friedel-Crafts reaction?

H[ ]

CHCH3

CH3

AlCl4CH3CHCH3 AlCl4+CHCH3

CH3

+ HCl + AlCl3

CH3CHCH3

Cl+ AlCl3 CH3CHCH3 AlCl4

+ CH3CH2CH2ClAlCl3

+ CH3CH=CH2HF

0 CO

CH2CH2CH3?

Chemical Properties of Monocyclic Aromatic Hydrocarbons(9)• An acyl group, -COR, is introduced onto the ring when an aromatic comp

ound reacts with a carboxylic acid chloride, RCOCl, in the presence of AlCl3. For example, reaction of benzene with acetyl chloride yields the ketone, acetophenone.

• The mechanism of Friedel-Crafts acylation:

+ CH3CH2CClO AlCl3

80 CO

CCH2CH3

O

+ HCl

CH3CH2CClO

AlCl3 CH3CH2C O CH3CH2C O + AlCl4-

An acyl cation

+CCH2CH3

O

+CH3CH2C O AlCl3

CO

CH2CH3

H[ ]AlCl4

-HCl+

Chemical Properties of Monocyclic Aromatic Hydrocarbons(10)• How to prepare propylbenzene by Friedel-Crafts reaction?

• By contrast, the Friedel-Crafts acylation of benzene with propanoyl chloride produces a ketone with an unrearranged carbon chain in excellent yield.

• This ketone can then be reduced to propylbenzene by several methods. One general method-called the Clemmensen reduction-consists of reflu

xing the ketone with hydrochloric acid containing amalgamated zinc.

CH2CH2CH3?

+ CH3CH2CClO AlCl3

80 CO

CCH2CH3

O

+ HCl

CO

CH2CH3 Zn(Hg)

HClreflux

CH2CH2CH3

Ethyl phenyl ketone Propylbenzene( )80%


• Substituent Effects in Substituted Aromatic Rings• Only one product can form when an electrophilic substitution occurs o

n 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 substit

uents activate the ring, making it more reactive than benzene, and some deactivate the ring, making it less reactive than benzene.

• For example:

Reactive rate 1000 1 0.033 6╳10-8

of nitration

OH H Cl NO2


• Substituent Effects in Substituted Aromatic Rings• 2. Substituents affect the orientation of the reaction. The three possible disubs

tituted 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 (%) Product(%)• Ortho Meta Para Ortho Meta Para • Meta-directing deactivators Ortho- and para-directing deactivators• -+N(CH3)3 2 87 11 -F 13 1 86

• -NO2 7 91 2 -Cl 35 1 64• -COOH 22 76 2 -Br 43 1 56 • -CN 17 81 2 -I 45 1 54 • -COOCH3 28 66 6 Ortho- and para-directing activators • -COCH3 26 72 2 -CH3 63 3 34 • -CHO 19 72 9 -OH 50 0 50• -NHCOCH3 19 2 79


• Substituent Effects in Substituted Aromatic Rings

• Substituents can be classified into three groups:

• Ortho-and para-directing activators, ortho-and para-directing deactivators, and meta-directing deactivators.

• Ortho-and para- ortho-and para- Meta-directing

• directing activators directing deactivators

deactivators

OH NHCOCH3 Ph H Cl I COCH3 CCH3 CN NR3

O O

NH2 OCH3 CH3(alkyl) F Br C H C OH SO3H NO2

O O

Reactivity

+


• 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.



• 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

CH3HNO2

CH3

H

NO2

CH3

H NO2

CH3HNO2

CH3HNO2

CH3

H

NO2

CH3

H

NO2

CH3

H NO2

CH3

H NO2

Othro

Meta

Para

Most stable

Most stable

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:

OH

H NO2

OH

Ortho

Meta

Para

Most stable

Most stable

OH

NO2

HOH

NO2

H

H

NO2

OH

H

NO2

OH

H

NO2

OH

H NO2

OH

H NO2

OH

H NO2

OH

OH

NO2

HOH

NO2

H



• Ortho- and Para- Directing Deactivators: Halogens• Halogens are deactivating because their stronger electron-withdrawing inductiv

e 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

Cl

Ortho

Meta

Para

Most stable

Most stableH NO2

Cl

H NO2

Cl

H NO2

Cl

H

NO2

Cl

H

NO2

Cl

H

NO2

Cl

NO2

HCl

NO2

HCl

NO2

HCl

NO2

HCl



• Meta- Directing Deactivators• Meta-directing deactivators act through a combination of inductive and resonan

ce 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.

CHO

Ortho

Meta

Para

H Cl

CHO

H

Cl

CHO

ClH

CHO

ClH

CHO

ClH

CHO

H

Cl

CHO

H

Cl

CHO

H Cl

CHO

H Cl

CHO

Least stable

Least stable


• Trisubstituted Benzenes: Additivity of Effects• Further electrophilic substitution of a disubstituted benzene is governed by the s

ame 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 meta- disubstituted compound because this site is too hindered.

• Some examples:

CH3

NO2

OH

CH3

CH3

Cl

COOH

SOH3

NHCOCH3


• 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:

Br

COOH

Cl

NO2

CH2CH2CH3

NO2

C(CH3)3

A B C


• 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:

• Oxidation of Benzene:

CH3

CH3

H2/Pt/ethanol

200atm, 25 Co

CH3

CH3

CH3

CH3

V2O5+ O2

Co

400~500O

O

O


• Oxidation of Alkylbenzene Side Chains • Alkyl side chains are readily attacked by oxidizing agents a

nd are converted into carboxyl groups, -COOH. For example:

• Bromination of Alkylbenzene Side Chains

BACK

CH2CH3KMnO4/H2O

COOH

C(CH3)3

CH3

KMnO4/H2OC(CH3)3

HOOC

CH2CH3NBS/CCl4

CHCH3

Br

hv

Chemical Properties of Polycyclic Aromatic Hydrocarbons(1)• Polycyclic aromatic hydrocarbons have two or more benzene rings fus

ed together. For example:

• Naphthalene Anthracene Phenanthrene

• Reactions of Naphthalene:

1

5 10

1

1

22 22

23 3

3

4 4

4

5

5

6 6

6

7 7

7

8 88

9

109

Cl2/FeCl3100~110 C

o

Cl

NO2

CrO3/CH3COOH

30~50 Co

HNO3/H2SO4

SO3HH2SO4 80 C

o

SO3H

165 Co

H2SO4COCH3

CH3COCl/AlCl3CS2 -15 C

o

CH3COCl/AlCl3C6H5NO2

10-15 Co

COCH3

O

O

Na/NH3(l)/CH3CH2OH

Chemical Properties of Polycyclic Aromatic Hydrocarbons(2)

• Substituent Effects in Substituted Naphthalene

III

Ⅱ 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:

• (a) (b)

• (c) (d)

• 3.2 Predict the major product(s) of the following reactions:

• (a)

• (b)

• (c)

CH3

CH2CH3

CH3

CH2CHCH=CH2

CH3

CH3

CH3

NO2

SO3H

(CH3)2C=CH2

HF( )

CH3CH2ClAlCl3

( )( )K2Cr2O7

H2SO4/H2O

CH2CH2CCl

O

AlCl3 ( ) ( )Zn/Hg/HCl

Na/NH3(l)C2H5OH

( )( )

Additional problems of chapter five (2)• 3.3 At what position, and on what ring, would you expect the following substan

ces to undergo electrophilic substitution?

• (a) (b) (c)

• (d) (e) (f)

• (g) (h) (i)

• 3.4 How would you synthesize the following substances starting from benzene?

• (a) (b) (c) (d)

COOH

CH(CH3)2

C(CH3)3

H3C

O2N

OCH3

C

O

Cl

CO

O

Br

NHCOCH3

N

H

BrH3C

C(CH3)3

COOH

NO2

Cl

CH2Cl

NO2

CH2CH2CH3

NO2

COOH

Br

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

有机化学 organic chemistry

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