Electronic effects of substituents in aromatic nitration

216 Pages·2013·6.67 MB·English

by  Graeme John Wright

ELECTRONIC EFFECTS OF
SUBSTITUF~TS
IN AROMATIC NITRATION
A thesis presented for the degree
of Doctor of Philosophy in Chemistry
in the University of Canterbury,
Christchurch, New Zealand.
by
G. J. Wright
1965
ABSTRACT
Product distributions from the nitration of toluene,
m-xylene, hemimellitene and pseudocumene have been
~~xylene,
determined in nitric acid-acetic anhydride mixtures,
nitronium tetrafluoroborate-sulpholane mixtures and in the
nitric acid-sulphuric acid-nitromethane system. The
Additivity Principle is applied to the toluene results to
calculate product ratios for the other hydrocarbons.
Comparison of calculated and experimental results shows this
to be an unsatisfactory method of calculating product
distributions for these reactions. The differences between
calculated and experimental results are examined quanti
tatively in terms of modified ortho substituent constants.
Mixtures of nitric acid in acetic anhydride are shown
to bring about direct acetoxylation of the aromatic nucleus
when activated by more than one methyl group. The acetoxy
lation reaction has the characteristics of an electrophilic
substitution reaction with large steric requirements and
high selectivity. An estimate of the Hammett rho for the
reaction is made.
Product distributions from the nitration of indan and
tetralin in the three nitrating systems have also been
determined. These results are discussed in terms of the
Mills-Nixon effect. An explanation for these and the results
of other investigations of indan and tetralin is presented.
CONTENTS
. . . . . . .
INTRODUCTION • • 1
• • • • • • • • • • If • •
The Mechanism of Electrophilic Aromatic Substitution 2
. . . .
Formation of ~-complexes • • • • • • • 5
. . . . .
Position of the transition state 11
. . .
The Nitrating Systems • • • • • • • • • • . • • 14
Nitronium tetrafluoroborate-sulpholane mixtures 15
Nitric acid-nitromethane mixtures ••• 18
• a • • • •
Nitric acid-acetic anhydride mixtures 21
Additivity in Electrophilic Aromatic Substitution •• 34
The Mills-Nixon Effect • • • • • • • • • • • •• 50
. .
EXPERIMENTAL • 56
o e • e • o • e o • o • • • • 0 •
Preparation and Purification of Materials • 56
• •
Hydrocarbons 56
• • • • • • • • • • • • • 0 • • • • •
. . . .
Reagents and solvents • • • • • • • • • 58
. .
Reaction products •••••••••••• 60
• • •
Identification of Reaction Products • • • 74
Reaction Procedure 82
• • • • • • & • • • • • • • • • •
Effect of changing reaction conditions 83
• • • • • •
Test of extraction procedure 87
• • • • • • e o • • •
Gas Chromatography • • • • • • 89
• u • • • • • • • • •
. . . . . . .
The Megachrom • • .. 89
• • • • • • •
Gas density chromatograph • • • • • • • • • • • • • 91
Calibration of gas density detector • • • • • ••• 93
Measurement of peak areas • • • • . . • • 97
. .
Column conditions for chromatography •• 97
Determination of nitrite formed in nitric acid-
. . .
acetic anhydride mixtures • • • • • • • 1 00
. .
RESULTS • • • • • 1 04
e o o e o o o e o e • o o
. . . . . . . . .
Tables of Results •• • 1 OLj.
• •
. . . . . . . . . . . . . .
Precision of results • 127
Chromatograms • • • • • • 129
• • • • • 8 • • • •
. . . .
DISCUSSION • • • • • • • • • 1 35
o o o e a e o o o o
Composition of Reaction Mixtures • • • • 135
. . . . . .
The Acetoxylation Reaction • • . 138
. . . . . .
Reaction products ••• • 138
. . . .
Nature of the reaction • 140
• • • • •
. . . . . . . . . . . . . .
Kinetic evidence • • • 143
. . . . .
Hammett rho for acetoxylation • • . • . 145
Structure of the protonated acetyl nitrate • • . • 149
The ortho:para ratio in nitric acid-acetic
anhydride mixtures • • • . • • • • • • • • • • • • 153
The Additivity Principle •••••••••••• 156
Prod1J.ct distributions in nitric acid-acetic
anhydride mixtures ••••••••••••••• 156
Toluene • • • • • • • • • • • • • • • • • • • • • • 157
Partial rate factors for the polymethylbenzenes .• 161
Calculated product distributions • • • 162
• • 0 • • •
. . . . . . .
Modified substituent constants • 170
. . . . . . . . . . . . . . . . . .
Conclusion .• 177
.
Page
. . . . . . . . . .
The Mills-Nixon Effect • 180
Bromination results 180
• • • • • • • • • • • • • • •
Nitration results •••••••••••••••• 186
Quantum mechanical and other treatments • • • • • 189
Other results • • • • • • • • • • • • • • • • • • 191
REFERENCES • • • • • • • • • • • • • • • • • • • • • • 193
1
INTRODUCTION
The Additivity Principle has been used successfully
to predict the rates of electrophilic substitution of the
in a number of reactions. This thesis
~ethylbenzenes
examines the usefulness of the Principle in predicting the
product distributions arising from the nitration of these·
hydrocarbons, by comparing calculated and experimental
product ratios. Product distributions from the hydro
carbons indan and tetralin are also discussed as evidence
for the nature of the Mills-Nixon effect. During the
course of the work, mixtures of nitric acid ·and acetic
anhydride were found to bring about direct acetoxylation
of the aromatic nucleus, and a study of this reaction is
made.
The work forming the background to the investi
gation is described in this Introduction. A brief outline
of the mechanism of electrophilic aromatic substitution
with special reference to nitration is given, and the
nature of the nitrating species in the three reaction
systems used is discussed. The Additivity Principle is
reviewed and the history of the Mills-Nixon effect
described.
2
The em of electrophilic aromatic substitution.
mecha;~,,
It is now generally accepted that the majority of
aromatic substitution reactions proceed via a
e~ectrophilic
1
-complex, or Wheland intermediate (I) and that the
0
reaction may be represented as follows:
X
©
+
H
+
k.,
I
Such a mechanism implies that while the energy profile for
attack by different reagents may vary somewhat, the general
2
shape and the reaction coordinates are very similar.
The a -complex is not a transition state; it is better
regarded as a more or less stable intermediate which for
certain reactions at least, appears to have been success-
. fully isolated. Objections to an intermediate of this type
were raised on the grounds that loss of the full resonance
energy of the benzene ring would make the a -comple.x very
unstable, but it has been pointed out3 that stabilisation
is in part restored bY- the energy of formation of the Ar-X
and by hyperconjugation of the hydrogen, and possibly
bond~
the electrophile, with the residual conjugated system.
Such a reaction scheme would present different kinetic
behaviour and substituent effects depending on the relative
3
magnitudes of the various k values.
The profile for such is generally
reactio~ reactio~s
drawn with the a -complex occupying a valley between two
transition states--one for the formation of and the other
for the destruction of the o -complex. The relative
heights of the two transition states determines whether
formation of the Ar-X bond or rupture of the Ar·-H bond is
rate-determining. Hammond4 has postulated that the o -
complex is a good model for both transition states, and
that conversion between any of these requires only very
small changes in molecular parameters. This postulate is
assumed to be correct in most discussions of the
~enerally
nature of the transition state.
A considerable body of experimental evidence is
available to support this mechanism. The kinetic isotope
5
work pioneered by Melander has proved capable of delicate
investigation into the nature of the reaction profiles •
2 1
. Substitution of H or H3 for the departing H
produces a significant change in rate (generally by a
factor of four and greater) if the second transition state
is appreciably higher than the first. When no such primary
isotope effect is observed, smaller secondary effects can
give useful information about the reaction pathway, though
6
these effects may be difficult to interpret. As an
indication of the validity of Hammond's postulate, it has
been pointed out7 that practically all electrophilic
4
substitution reactions with the exception of nitration.may
proceed with an experimentally measurable isotope effect
provided the right condi tiona ar•e chosen.
Considerable progress has been made in the isolation
of o -complexes, sometimes under reaction conditions.
8
This work, which is well reviewed by Berliner, lends
support to the intermediacy of such complexes in these
reactions. Evidence is also available to show that
HC1-A1Cl and HF-BF systems form_ o -complexes with
3 3
aromatic systems, while HCl forms 1C-complexes in which
interaction occurs between the HCl and the whole -
~
electron cloud of the aromatic system.9 Comparisons of
reactivity and 11 a -complexn basicity show striking corre
lations, which are not exhibited by comparisons with
8
11 1C -complex" basicity.
Nitration of aromatic substrates has been
extensively studied in many nitrating systems. This work,
10
. reviewed by de la Mare and Ridd, has shown that the
reaction profile below (Fig. 1), with a transition state
T very close to the a.'-complex, is a very satisfactory
model for aromatic nitration.
T
E
X
@
+
+ H
reaction coordinate
Fig, 1
The formation of -complexes.
1t
The existence of charge-transfer complexes, or
1t -
·complexes, between aromatic hydrocarbons and electron
8
acceptors is well-established. The interaction between
the aromatic 1C -electron cloud and the acceptor molecule
leads to a complex of low stability which cannot normally
be isolated, and the existence of such compounds is
generally inf.erred from physical measurements made on
solutions. A few ~ -complexes like picrates and tr
nitrobenzene derivatives are particularly stable and can