Aromatic Chemistry. Organic Chemistry for General Degree Students

299 Pages·1969·5.95 MB·English

by  P. W. G. Smith and A. R. Tatchell (Auth.)

ORGANIC CHEMISTRY FOR
GENERAL DEGREE STUDENTS
VOLUME 2: AROMATIC CHEMISTRY
by
P. W .G. SMITH,
D.i.c,
PH.D., A.R.C.S., A.R.I.C.
A. R. TATCHELL,
M . S C, PH.D., F.R.I.C.
Senior Lecturers in Organic Chemistry
The Woolwich Polytechnic
PERGAMON PRESS
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Copyright © 1969 Pergamon Press Ltd.
First edition 1969
Library of Congress Catalog Card No. 64-66138
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PREFACE
THE scope and the manner of treatment of the subject-matter of
this text book of Organic Chemistry are outHned in the preface to
Volume 1. In this volume we have endeavoured to cover the im­
portant fundamental aspects of aromatic chemistry, including
simple heteroaromatic systems, to a level which, broadly speaking,
is adequate to meet the requirements of students reading for a B.Sc.
General Degree and for examinations of a similar standing. The
decision to separate the aromatic section from the aliphatic was
intentional rather than one of convenience. We felt that in a course
of this level the interests of the student are best served by a
systematic study in the first instance of the chemistry of functional
groups, based on their structural characteristics, in aliphatic
systems. The study of aromatic compounds can then begin with
the properties of the aromatic nucleus followed by a consideration
of the manner in which interaction with the aromatic system may
modify the reactivity of functional groups. On the basis that the
student has already acquired some familiarity with the basic
concepts of reaction mechanisms from the material presented in
Volume 1, we have taken the opportunity, in this volume, of
extending the treatment of selected mechanistic topics to a some­
what greater depth.
Woolwich Polytechnic P. W. G. S.
London, S.EA8 A. R. T.
VII
CHAPTER 1
BENZENE-
STRUCTURE AND REACTIVITY
Introduction
As has been mentioned in the introduction to Volume 1, organic
chemistry has from an early date been broadly divided into ali­
phatic and aromatic branches. This classification was largely one
of convenience since aromatic compounds (so named because
many of the earliest examples isolated from plant extracts had
pleasant aromas) were found to possess chemical properties which
were characteristically different from those of the aliphatic group.
Many of the aromatic compounds isolated from these sources
were recognised as derivatives of benzene (QH^) into which they
could be converted by suitable reaction sequences; the following
are illustrative.
[O]
C6H5CHO -> C6H5CO2H ^ • Q Ha
Benzaldehyde Benzoic acid Benzene
(from oil of bitter almonds)
C,H4(OH)CO,H-^vC,H50H
Salicylic acid Phenol Benzene
(from oil of wintergreen)
A detailed study of the reactions of benzene and its derivatives
therefore formed the basis of aromatic chemistry as it is now known
and aromatic chemistry may be defined as the chemistry of com-
1
ORGANIC CHEMISTRY FOR GENERAL DEGREE STUDENTS
pounds having the characteristic chemical properties of benzenoid
compounds.
The high carbon: hydrogen ratio found in benzene (CaH^^
C„H2,,_6, compare the alkanes, C,,H2„+2) might lead one to expect
that it should react as a highly unsaturated compound. In fact the
most significant feature of the chemistry of benzene (and of aro­
matic compounds in general) is its reluctance to enter into the
addition reactions which unsaturated compounds so easily under­
go. For example, benzene does not decolorise a solution of
bromine in carbon tetrachloride, a reagent commonly used for the
detection of unsaturation. Rather does the action of bromine in the
presence of a suitable catalyst lead to the formation of bromo-
benzene by a substitution process.
C6H6 + Br2 —•CóHsBr + HBr
Bromobenzene
Benzene is, furthermore, not affected by alkaline potassium per­
manganate solution, and in fact the benzene nucleus remains
intact throughout many reactions involving relatively strongly
oxidising conditions. Nevertheless, that benzene has a degree of
unsaturation is shown by the fact that with dry chlorine, in the
presence of light of a suitably short wavelength, it yields a hexa-
chloride (C6H6C16), and that with hydrogen in the presence of
catalysts it yields cyclohexane (C6H12).
CI2 CM 6^12
CHCl
CIHC^ ^<^HC1
C1HC\ /CHCl
CHCl
Benzene hexachloride Benzene Cyclohexane
I
This relationship of benzene to cyclohexane confirms the cyclic
structure of benzene originally proposed by Kekulé (1865) but
BENZENE—STRUCTURE AND REACTIVITY 3
precisely how the necessary degree of unsaturation can be incor­
porated into the six-membered cycHc system constitutes what may
be termed the "problem" of the structure of benzene. Kekulé's
original formulation of benzene as cyclohexatriene (I) was the best
acceptable solution for many years and is supported by the ob­
servation that, although reaction is slow, benzene yields with ozone
a triozonide (II) which is decomposed by water to give glyoxal.
CHO
OCH
Hp HO
HO
OCH
CHO
No really satisfactory solution of the problem of the structure of
benzene was possible, however, until the application of physico-
chemical methods established the planar symmetrical hexagonal
nature of the benzene molecule and allowed the structure to be
interpreted upon the basis of the two complementary theoretical
treatments, namely the molecular orbital (M.O.) and valence bond
(V.B.) methods.!
The Structure of the Benzene Molecule
The geometry of the benzene molecule, which has a planar,
regular hexagonal carbon skeleton with a carbon-carbon bond
distance of 1-39 A, is established by X-ray and electron diffraction
measurements. The σ-bonds which unite the carbon atoms in the
ring arise from the mutual overlapping of two of the sp^ hybrid
orbitals of each carbon atom with those of its neighbours. The six
remaining hybrid orbitals are involved in the formation of the six
t A preliminary discussion of the structure of benzene has already been
presented in Volume 1.
4 ORGANIC CHEMISTRY FOR GENERAL DEGREE STUDENTS
carbon-hydrogen bonds. These carbon-carbon and carbon-hydro­
gen σ-bonds accommodate three of the valence electrons on each
carbon atom, leaving the six electrons originally occupying the
unhybridised /?-orbital on each of the carbon atoms to be accounted
for. It is these electrons (commonly referred to as the ''aromatic
sextet") which are responsible for the special properties of benzene
and a completed description of benzene incorporating these
electrons can be arrived at by either the M.O. or the V.B. method.
In the molecular orbital treatment, the six discrete unhybridised
/7-orbitals are replaced by six molecular orbitals encompassing the
entire ring system. The energy level of three of these molecular
orbitals is less than that of an original unhybridised /?-orbital and
these orbitals are therefore bonding orbitals, A pictorial representa­
tion of the molecular orbital of lowest energy, which has a nodal
plane of zero electron density in the plane of the ring, has already
been given (Vol. 1). Viewed from above this may be alternatively
represented as III; each of the remaining two bonding orbitals has
an additional nodal plane at right angles to the plane of the ring
and may similarly be represented as IV and V. The energy levels of
IV and V are identical (but higher than that of III); orbitals having
equal energy levels are said to be degenerate. In the ground state of
the molecule, each of these bonding orbitals is occupied by a pair
of electrons of opposite spin; the completed description of the
molecule in the ground state is then arrived at by superimposing
these three occupied molecular orbitals.
Π IS
The remaining three molecular orbitals have further nodal
planes at right angles to the plane of the ring (VI, VII and VIII).
BENZENE—STRUCTURE AND REACTIVITY 5
As these all have a higher energy than the unhybridised p-orbitals
they are anti-bonding orbitals', they are only occupied when the
molecule assumes an excited state following the promotion of an
electron from a bonding orbital as the result of the absorption of
energy.
Vlli
In the valence bond treatment the unhybridised /^-orbitals are
considered to overlap in pairs to form π-orbitals, each of which is
occupied by a shared electron pair. Structures IX (i) to (v) illus­
trate the possible ways in which such overlapping may be achieved;
IX (i) and (ii) represent the classical Kekulé structures of benzene;
IX (iii), (iv) and (v) are additional (Dewar) forms.
(lii) (V)
The Kekulé forms have the lowest energy since overlapping be­
tween adjacent /7-orbitals is more effective than the overlapping
between the diametrically opposite p-orbitals which is necessary to
6 ORGANIC CHEMISTRY FOR GENERAL DEGREE STUDENTS
form the elongated ("formal") bond in the Dewar structures. The
non-localised character of the π-electrons in benzene is thus ex­
pressed in this valence bond approach by regarding the molecule
as a resonance hybrid of all these possible forms, the equivalent
Kekulé structures making the greater contribution.
Both these theoretical treatments lead to a similar picture of
benzene as a symmetrical hexagonal molecule which possesses a
lower energy (as a result of electron delocalisation) than that ex­
pected from a molecule represented by a classical cyclohexatriene
structure. The extent to which benzene is thus stabilised can be
estimated from measurements of its heat of combustion, but a
more accurate value is obtainable by comparing the heat of hydro-
genation of benzene with that of cyclohexene. Since the value for
benzene (49-8 kcal/mole) is less than three times that of cyclo­
hexene (3x28-8 kcal/mole), the difference (37 kcal/mole) is a
measure of the energy required to offset the stabilisation conferred
upon the molecule by electron delocalisation. It is for this reason
that the benzene molecule not only possesses exceptional chemical
stability but also, as explained in the following section, exhibits its
marked preference to undergo reaction by substitution rather
than addition. In formulating the reactions of benzene (and of
other aromatic compounds in general) the molecule is as a matter
of convenience usually represented by either of the Kekulé
formulations, but the true structure of the molecule and the hybrid
character of the carbon-carbon bonds must always be borne in
mind.
Electrophilic Substitution in Benzene
From a consideration of the electronic description of the
benzene molecule it is to be expected that benzene will be pre­
ferentially attacked by reagents of the electrophilic type, as indeed
is the case with the olefins. Whereas, however, with the latter
electrophilic addition results, benzene undergoes substitution.
The mechanism for electrophilic substitution in benzene by an
electrophile X® may be presented in the following way:
BENZENE—STRUCTURE AND REACTIVITY
(iii) Χ
Φ
Η
Mesomeric cation
X
X
XIII XV
The preferred hne of approach of the attacking electrophile is
approximately perpendicular to the plane of the ring. Initially an
association of X® with the π-electron system results in the forma­
tion of an ill-defined 1:1 complex (XII; a π-complex). True
chemical union of the reacting species necessitates the formation of
a σ-bond between X® and one of the ring carbons giving the
charged intermediate (X). This must involve the rehybridisation of
the appropriate carbon atom to an sp'^ state thereby disrupting the
closed delocalised system. Most of the resonance stabilisation of
the system is thereby lost, although the carbonium ion X is
stabilised to some extent by delocalisation of the remaining four π