Steric and electronic effects on the mechanism of nucleophilic substitution (SnAr) reactions of some phenyl 2,4,6-trinitrophenyl ethers with aniline and N-methylaniline in acetonitrile and in dimethyl sulfoxide

Article abstract of DOI:10.1039/b207997f

Rate data are reported for the reactions of a series of 3′- and 4′-substituted phenyl 2,4,6-trinitrophenyl ethers, 4, with aniline in acetonitrile, leading to 2,4,6-trinitrodiphenylamine. The main reaction flux occurs through a base catalysed pathway invo

Full text of DOI:10.1039/b207997f

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Steric and electronic effects on the mechanism of nucleophilic
substitution (S Ar) reactions of some phenyl 2,4,6-trinitrophenyl
N
ethers with aniline and N-methylaniline in acetonitrile and in
dimethyl sulfoxide
2
a
a
b
Chukwuemeka Isanbor, Thomas A. Emokpae and Michael R. Crampton
a
Department of Chemistry, University of Lagos, Lagos, Nigeria
Chemistry Department, Durham University, Durham, UK DH1 3LE
b
Received (in Cambridge, UK) 14th August 2002, Accepted 18th September 2002
First published as an Advance Article on the web 28th October 2002
Rate data are reported for the reactions of a series of 3Ј- and 4Ј-substituted phenyl 2,4,6-trinitrophenyl ethers, 4, with
aniline in acetonitrile, leading to 2,4,6-trinitrodiphenylamine. The main reaction flux occurs through a base catalysed
pathway involving formation of a zwitterionic intermediate, equilibrium constant K , and rate-limiting proton
1
transfer to base, rate constant kAn. The effects of ring substituents on values of rate and equilibrium constants are
discussed. In contrast the corresponding reactions in DMSO involve both uncatalysed and base catalysed pathways.
Reactions of 4 with N-methylaniline are extremely slow, but values of rate constants for reaction of 4-nitrophenyl
1
2
,4,6-trinitrophenyl ether were measured using H NMR spectroscopy. The value of the parameter K k is lowered
1
An
5
by a factor of 10 for N-methylaniline relative to aniline in both acetonitrile and in DMSO. This reduction is
attributed to increased steric hindrance both in formation of the intermediate and to proton transfer.
1,2
The general mechanism of S Ar reactions
when either
that for substrates carrying good leaving groups, such as phenyl
N
primary or secondary amines are the nucleophiles is given
in Scheme 1. Eqn. (1) is the steady state expression for the
ethers and phenyl sulfides, the rate-limiting proton transfer is
from zwitterion to base.
8
Although acetonitrile has been widely used as a solvent for
the examination of base catalysis in substitution reactions, the
mechanistic conclusions reached are rather less clear than those
relating to those in DMSO as solvent. A general conclusion has
been that the susceptibility to base catalysis increases as the
polarity of the solvent decreases, and is higher for secondary
9–11
than for primary amines.
There have been few studies involving substitutions in highly
12
activated substrates in acetonitrile. However it is known that
reactions of aliphatic amines with ethyl 2,4,6-trinitrophenyl
ether yield observable σ-adduct intermediates, 2, which may be
converted by general acid catalysis to substitution products
(
SB-GA mechanism) while base catalysis in reactions of the
corresponding phenyl ether is due to rate-limiting proton
transfer from zwitterions, 3, to base.
Scheme 1
observed second order rate constant, k , expressed in terms of
A
the component steps in Scheme 1.
(
1)
When kϪ1 ӷ k ϩ k [B], base catalysis may be observed and
there is continued interest in the mechanism of such catalysis
We report here kinetic studies of the reactions in acetonitrile
of some 3Ј- and 4Ј-substituted phenyl 2,4,6-trinitrophenyl
ethers, 4, with aniline catalysed by aniline itself and by
DABCO. Our aims were to compare values for the rate con-
stants for the individual steps in the reaction pathway and to
determine the electronic effects of substituents in the leaving
group on the rate and mechanism of substitution. The results
are nicely interpreted by Scheme 2 in which base catalysis is
attributed to rate-limiting proton transfer from zwitterionic
intermediates, 5, to base and is followed by rapid expulsion of
2
B
3,4
and in its importance relative to the uncatalysed, k , step. The
2
observation of general base catalysis is indicative of rate-
limiting proton transfer. This may be from the zwitterionic
intermediate, 1, to base; or, it may involve general acid catalysis
of leaving group expulsion from the deprotonated form of 1. In
dimethyl sulfoxide (DMSO) the latter, the SB-GA mechanism,
has been shown to apply to substrates, such as alkyl ethers,
5–7
carrying poor leaving groups. However there is good evidence
DOI: 10.1039/b207997f
J. Chem. Soc., Perkin Trans. 2, 2002, 2019–2024
2019
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Table 1 Kinetic results for the reactions of 4, Y-phenyl-2,4,6 trinitrophenyl ethers (Y = CH , H, Br, Cl, NO ) with aniline in acetonitrile at 25 ЊC
3
2
Ϫ2
3
Ϫ1 Ϫ1
kA/10 dm mol
s
Ϫ3
[Aniline]/mol dm
4-CH3
4-H
4-Br
4-Cl
4-NO2
3-NO2
0
0
0
0
0
0
0
0
.01
.02
.03
.04
.05
.06
.07
.08
0.47
0.95
1.42
1.90
2.25
2.85
3.23
3.49
0.47
0.93
1.38
1.71
2.22
0.83
1.58
2.41
3.10
0.92
1.60
2.55
3.40
4.00
4.60
0.93
1.98
2.78
3.62
4.26
5.02
5.68
6.24
1.10
2.12
3.14
4.06
5.00
6.10
4.40
5.02
Scheme 2
phenoxide. In the presence of excess DABCO the diphenyl-
amine product, 7, may be deprotonated in a rapid equilibrium
to give 8. In the case of 4Ј-nitrophenyl 2,4,6-trinitrophenyl ether
1
we also report rate measurements, by H NMR spectroscopy,
for reaction with N-methylaniline, a much more sterically
demanding nucleophile.
Results and discussion
Reactions with aniline in acetonitrile
The reactions of the diphenyl ethers, 4, with aniline in aceto-
nitrile proceeded to give 2,4,6-trinitrodiphenylamine, 7, in
quantitative yield in a single stage without the observation of
4
3
intermediates. UV spectra, λ
= 365 nm, ε = 1.4 × 10 dm
max
Ϫ1
Ϫ1
1
mol cm , and H NMR spectra at the completion of the
reaction were identical to those of an authentic sample of 7 in
the reaction medium. The increase in absorbance at 365 nm was
used for kinetic measurements; with aniline in large excess
of substrate a first-order process was observed whose rate
constant, kobs, was evaluated by standard methods.
Fig. 1 Plot of k versus [aniline] in acetonitrile at 25 ЊC for the reaction
A
of 4-bromophenyl 2,4,6-trinitrophenyl ether. Experimental values are
denoted by |, and the curve is generated by k
1 ϩ 2.6[aniline]).
= 0.85[aniline]/
A
(
Division, by the aniline concentration, of values of kobs
(
(
2)
3)
gave values of the second order rate constant, k , and data
A
are collected in Table 1. A typical plot, in this case for the
4
-bromophenyl substrate, of k versus aniline concentration is
A
catalysed by aniline. An equivalent form is eqn. (3).
given in Figure 1. It shows a line with some downward curvature
and with an intercept on the y-axis indistinguishable from zero.
In terms of eqn. (1) the latter indicates that the contribution
of k , the uncatalysed pathway, is negligible. Hence eqn. (1)
2
reduces to eqn. (2), where kAn represents the pathway
2
020
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The data allow the calculation of values of K1kAn with good
precision. However the rather small curvature means that
there is some uncertainty in the values obtained for the
an uncatalysed pathway, while the positive slope indicates the
presence of a base catalysed route. The data conform to eqn. (1)
with the condition kϪ1 ӷ k ϩ k [B], resulting in eqn. (4).
2
B
ratio kAn/k , and hence also in the values calculated for k1
Ϫ1
(
= K1kAnk /kAn). Values for these quantities are summarised in
= K k ϩ k kAn[aniline]
(4)
Ϫ1
k
A
1
2
1
Table 4, below.
Confirmation that the reaction is subject to general base
catalysis is given by measurements in the presence of added
DABCO. Values of K1kDABCO, where kDABCO represents the rate
constant for the DABCO catalysed route, were obtained from
the slopes of approximately linear plots of the data in Table 2.
Values were alternatively calculated from the intercepts of plots
of the data in Table 3, obtained with constant DABCO but
varying aniline concentrations.
Values obtained for K k , K kAn, and for K1kDABCO obtained in
the presence of added DABCO, are collected in Table 5.
1
2
1
Base catalysis
The data in Table 4 show that values of k , the rate constant for
1
nucleophilic attack by aniline at the 1-position of 4, are within
experimental error independent of the nature of the substituent
3
Ϫ1 Ϫ1
12
Y. The value obtained, 0.3 dm mol s , can be compared
with a value of 183 dm mol
3
Ϫ1 Ϫ1
s
for corresponding attack by
Reactions with aniline in DMSO
n-butylamine. The decrease in reactivity of aniline compared to
the butylamine is relatively small when considered, Table 6, in
relation to the difference in basicity of nearly 8 pK units. This
is compatible with an ‘early’ transition state for nucleophilic
attack with relatively little bond formation. The independence
A typical plot of data obtained in DMSO is shown in Figure 2.
The sizeable intercept on the y-axis represents reaction by
of k on the nature of the substituent Y is also in accord with
1
this idea.
The rate constant, kAn, represents proton transfer from a
zwitterionic intermediate, 5, to aniline. It has been argued pre-
12,15,16
viously
that such trinitro-activated zwitterions are con-
siderably stronger acids than the parent ammonium ion. Hence
kAn will represent a thermodynamically favourable proton
transfer between nitrogen atoms, and its value will depend more
on steric constraints than on relative basicities. This is con-
firmed by the results in Table 4 which give a value for kAn
/
kDABCO of 0.3. Despite the large difference, by 7 pK units, in the
basicities of aniline and DABCO their ability to effect the pro-
ton transfer from 5 is similar. Since the ratio <1 it is likely that
DABCO is rather less sterically demanding than aniline here.
In related fashion values of kAn are unlikely to depend on the
nature of the remote substituent, Y, since this will not affect
the steric situation at the reaction centre.
Fig. 2 Plot of kA versus [aniline] in DMSO at 25 ЊC for the reaction of
4
-chlorophenyl 2,4,6-trinitrophenyl ether.
Table 2 Kinetic results for the reactions of 4 with aniline, 0.01 mol
Ϫ3
dm , and various concentrations of DABCO in acetonitrile at 25 ЊC
A consequence of this is that changes in value of K1kAn in
Ϫ2
3
Ϫ1 Ϫ1
Table 4 will reflect variations in value of K
1
. The general trend is
kA/10 dm mol
s
that values of K increase with increasing electron withdrawal
1
Ϫ3
[DABCO]/mol dm
4-CH3
4-H
4-Cl
4-NO2
3-NO2
by the substituent Y. Since k is independent of Y the changes
1
will result largely from decreases in value of kϪ1 as the stability
0
0
0
0
0
0
.01
.02
.03
.04
.05
.06
1.92
3.36
5.36
6.90
8.67
1.94
3.30
5.27
6.85
8.66
3.19
4.90
7.61 10.0
9.79 13.5
3.81
6.84
4.41
6.41
9.60
12.1
15.2
20.0
of 5 increases. The values of kAn/kϪ1 in Table 4 similarly reflect
variations in values of k . The effects are not large, a Hammett
Ϫ1
plot would give a ρ value of ca. 0.5, since the bulk of the
negative charge can be delocalised conjugatively into the
trinitro-substituted ring.
Values in Table 5 of K1kAn measured in DMSO, similarly
show a small dependence on the nature of Y. Interestingly the
12.7
17.8
20.1
Table 3 Kinetic results for the reaction of 4 with aniline in the
presence of DABCO, 0.01 mol dm , in acetonitrile at 25 ЊC
numerical values of K1k are close in the two solvents. How-
Ϫ3
An
ever this similarlity is likely to be due to the compensation
Ϫ2
3
Ϫ1 Ϫ1
of large increases in value of K and correspondingly large
kA/10 dm mol
s
1
decreases in value of kAn as the solvent is changed from
Ϫ3
12
[Aniline]/mol dm
4-CH3
4-H
4-Cl
4-NO2
3-NO2
acetonitrile to DMSO. It has been shown previously that
values of equilibrium constants for σ-adduct formation are
several orders of magnitude higher in DMSO than in
0
0
0
0
.02
.04
.06
.08
2.01
3.23
3.57
4.65
2.09
3.27
3.73
4.61
3.94
5.48
6.85
7.52
5.19
6.60
8.62
9.81
4.61
6.80
7.93
9.70
acetonitrile. The dominant factor here is likely to be the greater
17,18
ability of DMSO to solvate charged species.
However values
of kAn in related systems are considerably lower in DMSO. It
Table 4 Summary of the rate data for the reaction of 4, Y-phenyl-2,4,6 trinitrophenyl ethers, with aniline in acetonitrile
6
Ϫ2 Ϫ1
3
Ϫ1
3
Ϫ1 Ϫ1
6
Ϫ2 Ϫ1
Reactant Y
-CH3
K1kAn/dm mol
s
kAn/kϪ1 dm mol
k /dm mol
s
K1kDABCO/dm mol
s
kAn/kDABCO
1
4
0.48 ± 0.02
0.48 ± 0.02
0.85 ± 0.03
0.95 ± 0.03
1.03 ± 0.03
1.13 ± 0.05
1 ± 0.5
2 ± 1
2.6 ± 1
3.7 ± 1
4 ± 1
0.48 ± 0.20
0.24 ± 0.10
0.33 ± 0.10
0.26 ± 0.08
0.26 ± 0.08
0.38 ± 0.10
1.5 ± 0.3
1.6 ± 0.2
2.1 ± 0.1
2.7 ± 0.4
3.4 ± 0.3
3.2 ± 0.4
0.32 ± 0.10
0.30 ± 0.05
0.40 ± 0.05
0.35 ± 0.10
0.30 ± 0.05
0.35 ± 0.10
H
4
4
4
3
-Br
-Cl
-NO2
-NO2
3 ± 1
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a
Table 5 Summary of rate data for reaction of 4, Y-phenyl 2,4,6-trinitrophenyl ethers, with aniline in DMSO
Ϫ2
3
Ϫ1 Ϫ1
6
Ϫ2 Ϫ1
6
Ϫ2 Ϫ1
3
Ϫ1
Reactant, Y
K k /10 dm mol
s
K1kAn/dm mol
s
K1kDABCO/dm mol
s
kAn/kDABCO
kAn/k dm mol
1
2
2
b
4
H
4
4
4
4
-Cb H3
5.5
5.0
12.3
12.3
22.0
32.0
0.42
0.27
0.37
0.46
0.35
0.65
1.2
1.6
1.5
1.9
1.3
0.9
0.35
0.17
0.25
0.24
0.27
0.72
7.6
5.4
3.0
3.7
1.6
2.0
c
c
-Cl
-Br
d
-CN
b
-NO2
a
b
c
d
Values quoted are ±10ꢀ. Data from ref. 13. Present work. Data from ref. 14.
Table 6 Comparison of pK values
a
c
Acid
Acetonitrile
DMSO
ϩ
a
Aniline H
10.56a
3.82
11.12
9.06
ϩ
n-Butylamine H
DABCO H
,4,6-Trinitrodiphenylamine
18.26a
18.29
ϩ
b
2
19.97
8.01
a
b
c
Ref. 21. Current work. Ref. 22.
12
has been argued that this reflects the excellent hydrogen-bond
acceptor properties of DMSO, so that the amino proton to be
transferred to base will be strongly associated with the solvent.
Before the proton transfer may occur the hydrogen bond to
solvent must be broken.
Uncatalysed pathway
The likely mechanism for the k step involves intramolecular
2
Ϫ5
Ϫ3
Fig. 3 UV/visible spectra of 7, 5 × 10 mol dm , in acetonitrile
proton transfer from nitrogen to oxygen in the zwitterions, 5,
ϩ
Ϫ3
Ϫ3
containing [DABCO H ], 1.15 × 10 mol dm and 1–7, [DABCO],
, 0.005, 0.015, 0.031, 0.069, 0.092 and 0.115 mol dm ϩ. Spectrum 8
contains [DABCO] 0.096 mol dm and zero [DABCO H ].
1,2
coupled with carbon–oxygen bond breaking. The results for
DMSO in Table 5 show that values of K k are more susceptible
Ϫ3
0
Ϫ3
1
2
than values of K1kAn to changes in the nature of Y. This is
compatible with the involvement of the expulsion of phenoxide
value of 0.021 ± 0.002 for K , the equilibrium constant relating
3
in the k step, so that values increase as the electron withdraw-
to eqn. (5).
2
ing capability of Y increases.
ϩ
In contrast the k step makes a negligible contribution to the
2
7 ϩ DABCO
8 ϩ DABCO H
(5)
reaction flux in acetonitrile, indicating a higher value of the
12
ratio kAn/k . It is known that values of kAn are considerably
2
Hence in acetonitrile the pK value of 7 is 1.7 units higher than
that of DABCO H . The data collected in Table 6 show that in
DMSO the order is reversed so that 2,4,6-trinitrodiphenylamine
is a stronger acid than DABCO H . This reflects the enhanced
ability of DMSO, compared to acetonitrile, to solvate the
delocalised aromatic anion, 7, and the DABCO H cation.
Hence the equilibrium of eqn. (5) is moved to the right.
a
ϩ
higher in acetonitrile than in DMSO. The present results show
that values of k do not experience a similar solvent depend-
2
ϩ
ence. A possible explanation is that the good hydrogen-bonding
properties of DMSO allow a solvent assisted pathway with a
transition state such as 9. There is evidence for bifurcated
hydrogen bonds in both small molecule crystal structures, and
ϩ
19,20
in protein structures.
Reactions with N-methylaniline
Substitutions involving N-methylaniline may be considerably
slower than the corresponding reactions of aniline. When
nucleophilic attack is rate-limiting this is reasonably attributed
to the greater steric requirement of the secondary amine. Thus
in reaction with 1-chloro-2,4-dinitro-6-X-benzenes in aceto-
2
3
nitrile the aniline/N-methylaniline reactivity ratio was found
4
to increase from 1.8 to 2 × 10 as the 6-substituent was
varied from H to NO . Studies of base catalysed reactions
2
24,25
have produced conflicting results.
However reactions of 4,
2
4
Y = 4-NO in methanol yielded ratios of 2 × 10 and 3 × 10
2
26
respectively for uncatalysed and catalysed pathways.
We found that studies by uv/vis spectroscopy of the reactions
Ϫ5
Ϫ3
Ϫ3
of 4, 5 × 10 mol dm , with N-methylaniline, 0.1 mol dm , in
acetonitrile were not successful. The reactions were very slow
and did not produce the expected product, N-methyl-2,4,6-
Ionisation of 2,4,6-trinitrodiphenylamine
Interestingly in the presence of DABCO, a relatively strong
3
3
base, the reaction product 2,4,6-trinitrodiphenylamine, 7, is
trinitrodiphenylamine, 10, λ
= 425 nm, ε = 6.3 × 10 dm
max
4
Ϫ1
Ϫ1
sufficiently acidic to ionise to give 8, λ = 440 nm, ε = 2.8 × 10
mol cm . This may be due to trace quantities of impurities in
the solvent or the amine, which had been re-distilled.
However H NMR measurements, with higher concen-
max
3
Ϫ1
Ϫ1
dm mol cm . The UV/visible spectra in Figure 3 show the
1
conversion of 7 to 8 as the concentration of DABCO is
increased at constant concentration of DABCO H . Measure-
ϩ
trations of reagents in CD CN solvent, were more fruitful
3
ments of absorbance at 440 nm allowed the calculation of a
and as shown in Figure 4 show the development of bands
2
022
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a
Table 7 Kinetics of reaction of 4-nitrophenyl 2,4,6-trinitrophenyl
Table 8 Rate data for the reaction of 4, Y = 4-NO with N-methyl-
2
Ϫ3
Ϫ3
2
ether, 0.04 mol dm , with N-methylaniline 0.25 mol dm in CD CN at
aniline in [ H ] DMSO at 25 ЊC
3
6
2
5 ЊC.
[
N-Methylaniline]/
a
Ϫ3
b
Ϫ6 Ϫ1
Ϫ6
3
Ϫ1 Ϫ1
Relative concentrations
mol dm
kobs /10
s
kA/10 dm ml
s
5
kobs b/10
Ϫ6 Ϫ1
Time/10 s
A
C
s
0.4
2.7
4.7
7.8
6.7
7.8
9.8
0
.6
0
1
2
3
6
6
9
2.2
3.0
4.7
.92
.74
.61
.50
.05
.93
.55
91.4
80.6
73.2
65.4
48.7
45.0
34.3
26.5
24.1
20.5
8.6
1.0
1.2
1.2
1.2
1.2
1.2
1.1
1.1
1.1
1.1
0.8
19.4
26.8
34.6
51.3
55.0
65.7
73.5
75.9
79.5
a
1
Measured by integration of H NMR bands, with substrate 0.04 mol
Ϫ3
b
dm
. Values ±10ꢀ.
1
1
1
a
1
Determined by H NMR integration of bands at δ 9.11 due to A,
4
-nitrophenyl 2,4,6-trinitrophenyl ether, and at δ 8.85 due to C,
b
N-methyl 2,4,6-trinitrodiphenylamine. Calculated as (1/t)ln(100/[A]).
The lower value for the secondary amine is likely to be the
result of decreases in both K and in kAn. In 11 there will be
1
considerable steric crowding at the 1-position resulting in a
7,12,15
reduction in the value of K . Also it is known
that rate
1
constants for proton transfer in related adducts are drastically
reduced by increasing steric hindrance to approach of the
reagents. Hence a major factor here is likely to be the much
smaller value of kN–MeAn than of kAn
.
1,2
An additional factor which may contribute to lower values
of rate constants for proton transfer in reactions of N-methyl-
aniline is intramolecular hydrogen bonding in the zwitterion
1
1 between the N–H proton and an ortho-nitro group. The
presence of such hydrogen bonding, affecting the proton to be
transferred, is often used as an argument to explain differences
1,2
in reactivity between primary and secondary amines. (In the
case of primary amines there will always be one non-hydrogen-
bonded proton available for transfer.)
Nevertheless comparison of our results for acetonitrile with
those obtained in DMSO, detailed below, do not provide
evidence for such intramolecular interaction and are better
interpreted in terms of steric effects alone.
The data in Table 8 for reaction of 4, Y = 4-NO2 with
2
N-methylaniline in [ H ]DMSO show a similar pattern to those
6
obtained with aniline with both uncatalysed and base catalysed
pathways contributing to the reaction flux. The results conform
Ϫ6
3
Ϫ1 Ϫ1
to an equation of type 4 with K k = 3 × 10 dm mol
s
and
1
2
1
Fig. 4 H NMR spectrum of the reaction of 4-nitrophenyl-2,4,6-
Ϫ6
6
Ϫ2 Ϫ1
K1kN–MeAn = 9 × 10 dm mol s . Comparison with the
Ϫ3
trinitrophenyl ether, 0.04 mol dm , with N-methylaniline, 0.25 mol
Ϫ3
results in Table 5 for reaction with aniline indicate a factor of
ca. 10 in each of these values.
dm , in CD CN after 48 hours at 25 ЊC.
3
5
Interestingly this is exactly the same aniline/N-methylaniline
reactivity ratio as found in acetonitrile. If intramolecular
hydrogen bonding to the ortho-nitro group were an important
factor in determining the reactivity of 11 then differential
effects would be expected between acetonitrile, the less polar
solvent where such bonding should be strong, and DMSO, an
excellent H-bond acceptor where hydrogen bonding should be
to the solvent. In DMSO the weaker intramolecular hydrogen
bonding would be expected to lead to a lower value of the
aniline/N-methylaniline reactivity ratio, which is not observed
experimentally. It appears that in this case, at least, where there
is severe steric crowding at the reaction centre, steric effects
alone, which should not be strongly solvent dependent, can
adequately explain the observed reactivity patterns.
attributable to the expected reaction products. Integration of
bands due to reactant and product allowed the calculation of a
value for the first order rate constant, kobs. Typical data are
in Table 7. Due to the slowness of the reactions which were
carried out over several days, measurements were limited to the
4
-nitrophenyl derivative.
Ϫ6 Ϫ1
Values obtained for k in CD CN were 1.15 × 10
s
and
obs
3
12
Ϫ5 Ϫ1
1
0
.0 × 10
s
when the amine concentrations were 0.25 and
.85 mol dm respectively. The results indicate an approxi-
Ϫ3
mately squared dependence of kobs on the amine concentration,
showing that the base catalysed pathway is dominant. In terms
of eqn. (2), the condition kϪ1 ӷ k [B] applies. Division of k
B
obs
by the square of the amine concentration leads to a value for
Ϫ5
6
Ϫ2 Ϫ1
K1k
N–MeAn
of (1.6 ± 0.2) × 10 dm mol
s
where kN-MeAn
represents the rate constant for proton transfer from the zwit-
terionic intermediate, 11, to N-methylaniline. The value, from
Table 4, of K k for the corresponding reaction of aniline is
Experimental
Diphenyl ethers, 4, were prepared by the reaction of picryl
1
An
6
Ϫ2 Ϫ1
5
13
1
.03 dm mol
s
which is ca. 10 higher than the value of
N-methylaniline.
chloride with one equivalent of base in the presence of an
J. Chem. Soc., Perkin Trans. 2, 2002, 2019–2024
2023

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1
Table 9 H NMR shifts in CD CN, and melting points for reactants and products.
3
1
a
H NMR Shifts
mp/ЊC
Compound
H3,5
H2Ј
H3Ј
H4Ј
Other
Observed
Literature
2
7
4
4
4
4
4
4
, Y = H
9.03
9.02
9.03
8.99
9.11
9.11
7.00
6.98
6.93
6.82
7.15
7.81
7.40
7.52
7.52
7.17
8.26
—
7.25
—
—
—
—
—
—
—
151
135
141
100
158
172
153
—
—
103
157
174
, Y = 4-Cl
, Y = 4-Br
, Y = 4-Me
, Y = 4-NO2
, Y = 3-NO2
2
2
2
8
8
8
2.30(Me)
—
{7.65(H5Ј)
8.05
7
.43(H6Ј)
2
2
9
3
7
0
8.96
8.85
7.15
6.86
7.33
7.26
7.25
7.00
9.96NH
3.27(Me)
179
127
178
108
1
a
Compounds 4 are numbered as 4Ј- and 3Ј-substituted phenyl 2,4,6-trinitrophenyl ethers. 7 and 10 are 2,4,6-trinitrodiphenylamine, and its N-methyl
derivative respectively. ortho-coupling, J 7–8 Hz is observed.
excess of the appropriate phenol in aqueous ethanol. Recrystal-
lisation was from ethanol. 2,4,6-Trinitrodiphenylamine and its
N-methyl derivative were prepared by reaction of picryl chlor-
ide with a four-fold excess of the appropriate amine in ethanol.
Recrystallisation was from ethanol. C,H,N analyses of all sub-
7 M. R. Crampton and P. J. Routledge, J. Chem. Soc., Perkin Trans. 2,
984, 573.
1
8
(a) R. Chamberlin and M. R. Crampton, J. Chem. Soc.,
Perkin Trans. 2, 1994, 425; (b) R. Chamberlin and M. R. Crampton,
J. Chem. Soc., Perkin Trans. 2, 1995, 1831.
9
(a) D. Ayediran, T. O. Bamkole, J. Hirst and I. Onyido, J. Chem.
Soc., Perkin Trans. 2, 1977, 597; (b) D. Ayediran, T. O. Bamkole and
J. Hirst, J. Chem. Soc., Perkin Trans. 2, 1997, 1580.
1
strates were in excellent agreement with theoretical values. H
NMR data and melting points are listed in Table 9. DABCO,
acetonitrile and DMSO were the purest available commercial
samples; Aniline, and N-methylaniline were redistilled before
use1.
1
1
0 (a) T. O. Bamkole, J. Hirst and I. Onyido, J. Chem. Soc.,
Perkin Trans. 2, 1979, 1319; (b) T. O. Bamkole, J. Hirst and
I. Onyido, J. Chem. Soc., Perkin Trans. 2, 1982, 889.
1 T. A. Emokpae, P. U. Uwakwe and J. Hirst, J. Chem. Soc., Perkin
Trans. 2, 1993, 125.
H NMR spectra were measured with Varian Mercury 200
MHz or Varian Unity 300 MHz instruments. UV/visible
spectra and kinetic measurements with aniline were made at
12 M. R. Crampton and S. D. Lord, J. Chem. Soc., Perkin Trans. 2,
1997, 369.
1
3 M. R. Crampton and I. A. Robotham, Can. J. Chem., 1998, 76,
2
5 ЊC with a Perkin-Elmer Lambda 2 or a Shimadzu UV PC
6
27.
spectrophotometer. First order rate constants were measured
with aniline concentration in large excess of substrate concen-
1
1
4 I. A. Robotham, PhD Thesis, University of Durham, UK, 1997.
5 M. R. Crampton and B. Gibson, J. Chem. Soc., Perkin Trans. 2,
Ϫ5
Ϫ3
tration, 5 × 10 mol dm , and were evaluated using standard
1
981, 533.
methods. Values are precise to ±3ꢀ. Kinetic measurements
16 C. F. Bernasconi, M. C. Muller and P. Schmid, J. Org. Chem., 1979,
44, 3189.
7 J. F. Coetzee and C. D. Ritchie, Solute Solvent Interactions, Marcel
Dekker, New York, 1969.
1
with N-methylaniline were made using H NMR spectroscopy
1
in deuteriated solvents and are precise to ±10ꢀ.
1
1
8 J. F. Coetzee, Prog. Phys. Org. Chem., 1967, 4, 45.
9 G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford
University Press, Oxford, 1997.
Acknowledgements
We thank ICSC-World Laboratory, Lausanne for a scholarship
to enable C.I. to visit Durham, UK.
20 C. B. Aakeroy and K. R. Seddon, Chem. Soc. Rev., 1993, 397.
2
2
2
1 J. F. Coetzee and G. A. Padmanabhan, J. Am. Chem. Soc., 1965, 87,
005.
2 M. R. Crampton and I. A. Robotham, J. Chem. Res., 1997, (S)
2.
3 W. Eggimann, P. Schmid and H. Zollinger, Helv. Chim. Acta, 1975,
58, 257.
5
2
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