Aromatic Chemistry

171 Pages·2003·28.9 MB·English

by  John D. Hepworth

Cover images 0M urray Robertsonhisual elements 1998-99, taken from the
109 Visual Elements Periodic Table, available at www.chemsoc.org/viselements
ISBN 0-85404-662-3
A catalogue record for this book is available from the British Library
0 The Royal Society of Chemistry 2002
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Preface
Aromatic chemistry, in terms of the production of derivatives of benzene
and, to a less extent, other carbocyclic aromatic compounds, is of
immense industrial importance and is the mainstay of many chemical
companies. Derived products are in general use across such diverse indus-
tries as pharmaceuticals, dyestuffs, and polymers.
The aromatic chemistry required by an undergraduate in chemistry,
biochemistry, materials science and related disciplines is assembled in this
text, which also provides a link to other aspects of organic chemistry and
a platform for further study. In line with the series style, a number of
worked problems and a selection of questions designed to help the stu-
dent to understand the principles described are included.
The first chapter discusses the concept of aromaticity, after which there
is a description of aromatic substitution reactions. Chapters covering the
chemistry of the major functionalized derivatives of benzene follow. A
chapter on the use of metals in aromatic chemistry discusses not only the
chemistry of Grignard reagents and aryllithium compounds but also the
more recent uses of transition metals in the synthesis of aromatic com-
pounds. The penultimate chapter discusses the oxidation and reduction of
the benzene ring and the text concludes with the chemistry of some poly-
cyclic compounds.
We have chosen to use the names of chemicals that are in common
usage on the basis that students should then be able to read and make use
of the chemicd literature and also to locate chemicals in the laboratory.
Systematic names are given in parentheses at the first appropriate oppor-
tunity. Ideally, a student should be able to use both systems interchange-
ably without difficulty. The RSC website has an Appendix of Common
and Systematic Names (http://www.chemsoc.org/pdf/tct/functionalap-
pendix.pdf) to which students are referred. A Further Reading list is also
available at (http://www.chemsoc.org/pdf/tct/functionalreading.pd~.
We are grateful to Dr. Mark Heron for his valuable comments on the
draft manuscript and to Dr. Alan Jones and Ms. Beryl Newel1 for their
help in preparation of the final manuscript. Mr. Martyn Berry and
Professor Alwyn Davies FRS offered advice, encouragement and criti-
cism throughout the preparation of the text which were most appreciated.
Mrs. Janet Freshwater of the Royal Society of Chemistry was involved in
the project from start to finish and we thank her for her efficiency and
guidance. We thank our wives, Annabelle, Margaret and Anita, for their
help, patience and understanding during the writing of this book.
J. D. Hepworth, University qf Central Lancashire
D. R. Waring, formerly of Kodak Ltd., Kirkby, Liverpool
M. J. Waring, AstraZenecu, Alderley Park, Cheshire
L U I1O R - I N -C ti I t t rxr c uTivt LDITOKS L: D U C AT I0 N A L CONSC I LTA N T
Professor E W Ahel Profl.ssor A G Davirs Mr M Berry
Prqfl.ssor D Phillips
Professor J D Woollins
This series of books consists of short, single-topic or modular texts, concentrating on the funda-
mental areas of chemistry taught in undergraduate science courses. Each book provides a
concise account of the basic principles underlying a given subject, embodying an independent-
learning philosophy and including worked examples. The one topic, one book approach ensures
that the series is adaptable to chemistry courses across a variety of institutions.
TITLES IN THE SERIES FO RT H COM I N Ci T I T L ES
Stereochemistry D G Morris Mechanisms in Organic Reactions
Reactions and Characterization of Solids Molecular Interactions
S E Dann Reaction Kinetics
Main Group Chemistry W Henderson X-ray Crystallography
d- and f-Block Chemistry C J Joncs Lanthanide and Actinide Elements
Structure and Bonding J Burvc.fr Maths for Chemists
Functional Group Chemistry J R Himson Bioinorganic Chemistry
Organotransition Metal Chemistry A F Hill Chemistry of Solid Surfaces
Heterocyclic Chemistry M Sriinshurj9 Biology for Chemists
Atomic Structure and Periodicity J Barrett Multi-element NMR
Thermodynamics and Statistical Mechanics Peptides and Proteins
J M Soddon & J D Gale Biophysical Chemistry
Basic Atomic and Molecular Spectroscopy Natural Product: The Secondary
J A4 Hollas Metabolites
Organic Synthetic Methods J R Hunson
Aromatic Chemistry
J D Hepivorth, D R Wuring & M J Waring
Quantum Mechanics for Chemists
D 0 Hay\t’ard
Furtlier inforniution about this series is uvailahle at \vivtr. cliernsoc.o rgltct
Orders und cnyuiries should he sent to:
Sales and Customer Care, Royal Society of Chemistry, Thomas Graham House,
Science Park, Milton Road, Cambridge CB4 OWF, UK
Tel: [phone]; Fax: [phone]; Email: [email protected]
Contents
1
I. 1 Introduction
2
1.2 Structure of Benzene
1.3 Stability of the Benzene Ring 2
5
1.4 The Huckel Rule
I. 5 Nomenclature 11
15
2.1 Introduction
16
2.2 Electrophilic Aromatic Substitution (SEAr)
2.3 Reactivity and Orientation in Electrophilic Aromatic
Substitution 20
2.4 The Hammett Equation 31
2.5 Nucleophilic Aromatic Substitution 33
35
2.6 ips0 Substitution
3.1 Introduction 38
3.2 Source of Alkylbenzenes 38
39
3.3 Introduction of Alkyl Groups
42
3.4 Reactions of Alkylbenzenes
44
3.5 Aryl Derivatives of Benzene
V
vi Contents
4.1 Introduction 47
4.2 Industrial Synthetic Methods 47
4.3 Laboratory Syntheses 48
4.4 The Acidity of Phenols 50
4.5 Reactions of the Hydroxy Group 51
4.6 Reactions of the Ring 53
4.7 Dihydroxybenzenes 55
5.1 Introduction 58
5.2 Introduction of Acidic Groups 58
5.3 Reactions of Aromatic Acids 60
5.4 Acidity of Aromatic Acids 63
5.5 Compounds with More Than One Acidic Group 64
5.6 Side-chain Acids 65
6.1 Introduction 67
6.2 Aromatic Alcohols 67
6.3 Aromatic Aldehydes 68
6.4 Aromatic Ketones 76
7.1 Introduction 79
7.2 Introduction of the Nitro Group 79
7.3 Charge Transfer Complexes 83
7.4 Reactions of Nitro Compounds 83
7.5 Nitrosobenzene and Phenylhydroxylamine 85
8.1 Introduction 88
8.2 Introduction of the Amino Group 89
8.3 Reactions of Aromatic Amines 91
8.4 Related Compounds 93
8.5 Basicity of Amines 94
8.6 Diazonium Salts 95
Contents vii
9.1 Introduction 104
9.2 Synthesis of Aryl Halides 105
9.3 Reactions of Aryl Halides 108
9.4 Aromatic Halogen Compounds Substituted in the Side
Chain 111
10.1 Grignard and Organolithium Reagents 114
10.2 Electrophilic Metallation 118
10.3 Transition Metal Mediated Processes 119
10.4 Aryl Coupling Reactions 121
10 .5 Arene-Chromium Tricarbonyl Complexes 125
11.1 Introduction 129
1 1.2 Reduction of the Benzene Ring 129
I 1.3 Oxidation of the Benzene Ring 131
12.1 Introduction 135
12.2 Chemistry of Naphthalene 135
12.3 Chemistry of Anthracene 141
12.4 Chemistry of Phenanthrene 143

Aromaticit y
I.I Introduction
0
The classification of organic compounds is based on the structure of the
molecules. compounds have open-chain structures such as
hexane (1) and can contain single (C-C), double (C=C) and triple (C=C) 1 2
0 0
bonds. In molecules, the carbon atoms form a cyclic structure,
as in cyclohexane (2) and cyclohexene (3).
compounds are unsaturated cyclic molecules that possess
3 4
additional stability as a result of the arrangement of .Tc-electrons
associated with the unsaturation of the ring system. This book will
concentrate on the chemistry of benzene (4) and its derivatives and
related polynuclear hydrocarbons. Aromatic compounds are also known
as ; they can be , indicating that the ring skeleton con-
tains only carbon atoms, or , with at least one atom other
than carbon in the ring. These heteroatoms are typically N, 0 or S.
Heterocyclic compounds, which can be aromatic or alicyclic, are covered
in another book in this series.
Initially, we will look at what distinguishes aromatic compounds from
other cyclic molecules and how chemists’ understanding of aromaticity
has developed up to the present day.
2 Aromatic Chemistry
1.2 Structure of Benzene
Based on elemental composition and relative molecular mass determi-
nations, the formula of benzene was found to be C,H,. The saturated
hydrocarbon hexane has the molecular formula C,H,, and therefore it
was concluded that benzene was unsaturated. Kekule in 1865 proposed
the cyclic structure 4 for benzene in which the carbon atoms were joined
by alternate single and double bonds. Certain reactions of benzene,
such as the catalytic hydrogenation to cyclohexane, which involves the
addition of six hydrogen atoms, confirmed that benzene was a ring
compound and that it contained three double bonds. However, since
benzene did not undergo addition reactions with HCl and HBr, it was
concluded that these double bonds were different from those in ethene
and other unsaturated aliphatic compounds.
In 1867, Dewar proposed several possible structures for benzene, one
of which was 5. However, in 1874, Ladenburg proved experimentally
that all the hydrogen atoms of benzene were equivalent and suggested
the prismatic structure 6.
Kekule’s proposed structure 4 looks more in keeping with our current
M knowledge of benzene, although it does not explain how the double
bonds differ from the aliphatic type. Furthermore, although the two
6
structures 7 and 8 can be drawn for a 1,2-disubstituted benzene, only
5
one such compound exists. Kekule proposed that the equivalent struc-
R R
I tures 7 and 8 oscillated between each other, averaging out the single and
double bonds so that the compounds were indistinguishable.
1.3 Stability of the Benzene Ring
7 8
Kekule’s proposals gained wide acceptance and were supported by the
experimental work of Baeyer in the late 19th century, but these ideas did
not explain the unusual stability of benzene. This is typified by its chem-
ical reactions, which are almost exclusively substitution rather than the
expected addition. Throughout this book there will be many examples
of this property. In addition, physical properties such as enthalpies
of hydrogenation and combustion are significantly lower than would be
expected for the cyclohexatriene structure of Kekule. The enthalpy
of hydrogenation (AH) of the double bond in cyclohexene is -120 kJ
mol-I and that of cyclohexa-1,3-diene with two double bonds is almost
twice that at -232 kJ mol I. Cyclohexatriene, if it existed, would be
expected to have an enthalpy of hydrogenation of three times the value
of cyclohexene, a AH of approximately -360 kJ mol -I. However, the value
for benzene is less exothermic than this comparison suggests, being only
-209 kJ mol I. Thus benzene is 151 kJ mol-’ more stable than cyclo-
hexatriene (Figure 1.1). This is known as the of ben-