Electronic and steric effects in the SNAr substitution reactions of substituted anilines with 2,4-dinitrophenyl 2,4,6-trinitrophenyl ether in acetonitrile

Article abstract of DOI:10.1002/poc.997

Rate measurements are reported for the reactions of 12 ring-substituted anilines with 2,4-dinitrophenyl 2,4,6-trinitrophenyl ether (1) in acetonitrile. Formation of the products, the correspondingly substituted 2,4,6- trinitrodiphenylamines, occurs without the observation of intermediates in detectable amounts by both base-catalysed and uncatalysed pathways and Hammett ρ value were determined for these processes. The results show that although substituents at the 3- or 4-positions of the anilines have only small steric effects, alkyl substituents at the 2-position may result in considerable reductions in reactivity. These effects are more pronounced for the base-catalysed pathway and in 2,6-dimethylaniline the uncatalysed pathway takes all the reaction flux. In the case of the 2-fluoro substituent the electronic effect, strong inductive electron withdrawal, is dominant over steric effects. Copyright

Full text of DOI:10.1002/poc.997

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 75–80
Published online 28 October 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.997
Electronic and steric effects in the SNAr substitution
reactions of substituted anilines with 2,4-dinitrophenyl
2,4,6-trinitrophenyl ether in acetonitrile
Michael R. Crampton,1* Thomas A. Emokpae2 and Chukwuemeka Isanbor2
1Chemistry Department, Durham University, Durham DH1 3LE, UK
2Department of Chemistry, University of Lagos, Lagos, Nigeria
Received 30 June 2005; revised 9 August 2005; accepted 7 September 2005
ABSTRACT: Rate measurements are reported for the reactions of 12 ring-substituted anilines with 2,4-dinitrophenyl
2,4,6-trinitrophenyl ether (1) in acetonitrile. Formation of the products, the correspondingly substituted 2,4,6-
trinitrodiphenylamines, occurs without the observation of intermediates in detectable amounts by both base-catalysed
and uncatalysed pathways and Hammett ꢀ value were determined for these processes. The results show that although
substituents at the 3- or 4-positions of the anilines have only small steric effects, alkyl substituents at the 2-position
may result in considerable reductions in reactivity. These effects are more pronounced for the base-catalysed pathway
and in 2,6-dimethylaniline the uncatalysed pathway takes all the reaction flux. In the case of the 2-fluoro substituent
the electronic effect, strong inductive electron withdrawal, is dominant over steric effects. Copyright # 2005
John Wiley & Sons, Ltd.
KEYWORDS: SNAr substitution reactions; anilines; 2,4-dinitrophenyl 2,4,6-trinitrophenyl ether; electronic effects; steric
effects
INTRODUCTION
primary steric effect. Here electronic effects are dominant
and Hammett ꢀ values are typically ꢀ3.5 to ꢀ4 in protic
solvents.12,13,16,19 Nevertheless, for N-substituted ani-
lines steric hindrance to nucleophilic attack may be
severe so that the rates of substitution are considerably
reduced relative to aniline.6,20
We found recently21 that in the reaction of aniline with
2,4-dinitrophenyl 2,4,6-trinitrophenyl ether (1) in aceto-
nitrile conversion of the zwitterionic intermediate to
products was rate determining and involved both unca-
talysed, k2, and base catalysed, kB[B], pathways. Here we
report a kinetic study of the reactions of anilines contain-
ing both remote- and ortho-ring substituents, allowing the
electronic and steric effects on the individual steps in
Scheme 1 to be considered.
Nucleophilic substitutions in the reactions of amines with
activated aromatic substrates generally involve the SNAr
addition–elimination mechanism,1,2 as shown in Scheme
1. When the second step is rate limiting, general base
catalysis may be observed.
The base-catalysed pathway may involve general acid
catalysis, by the conjugate acid BHþ, of leaving group
departure, the SB–GA mechanism,3 or alternatively
rate-limiting proton transfer from the zwitterionic
intermediate to base followed by rapid loss of Xꢀ, the
RLPT mechanism.4 There is now good evidence that in
acetonitrile5,6 and in dimethyl sulfoxide7,8 the displace-
ment of phenoxide ions by amines involves the latter
mechanism.
There have been several reports of substitutions invol-
ving aniline or substituted anilines.9–16 Most of these
have involved displacement of halide ions where the k1
step, nucleophilic attack by the amine, is rate limiting.
For reactions involving aniline and anilines carrying ring
substituents at the 3- or 4-positions, as for other primary
amines,17,18 there is little evidence for a significant
RESULTS AND DISCUSSION
The reactions of 1 with ring-substituted anilines, 2, gave
the expected substitution products, 4, in >95% yield.
Spectroscopic data are given in Table 1. Kinetic measure-
ments were made in acetonitrile at 25 ꢁC with amine
concentrations in large excess of concentrations of 1.
Reactions proceeded without the observation of inter-
mediates in detectable concentrations and first-order
kinetics were observed. The results are interpreted by
Scheme 2, where the zwitterion 3 may be treated as a
*Correspondence to: M. R. Crampton, Chemistry Department,
Durham University, South Road, Durham DH1 3LE, UK.
E-mail: m.r.crampton@durham.ac.uk
Contract/grant sponsor: Royal Society.
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 75–80

76
M. R. CRAMPTON, T. A. EMOKPAE AND C. ISANBOR
X
NRR'
X
NHRR'
k1
k2
+ RR'NH
+ HX
k-1
k3[B]
EWG
EWG
EWG
Scheme 1. EWG ¼ electron-withdrawing group
steady-state intermediate. Base catalysis is attributed to
rate-limiting proton transfer from 3 to base followed by
rapid expulsion of the phenoxide.5,6,21 Division of the
first-order rate constants, kobs, by the aniline concentra-
tion gave values of the second order rate constant, kA,
whose base dependence is given by Eqn (1). An alter-
For anilines 2h–l carrying ortho-substitutuents there
was no discernible curvature in plots of kA versus aniline
concentration. This indicates that here kAn/kꢀ1 < 1 and
also implies that k2/k (ꢃ kAn/kꢀ1 ꢄ k2/kAn) < 1. Hence
ꢀ1
values of K1k2 and K1kAn were obtainable from the
intercepts and slopes, respectively, of the plots.
native form of this equation is Eqn (2), where K1 ¼ k1/k
ꢀ1
is the equilibrium constant for formation of the zwitter-
ionic intermediate. Values are reported in Tables 2 and 3.
3- and 4-substituted anilines
The data in Table 4 show that for anilines carrying
substituents at the 3- or 4-positions, values of both K1k2
and K1kAn decrease strongly with increasing electron
withdrawal by the ring-substituents. Plots (not shown)
versus Hammett ꢁ values22 gave excellent straight lines
with slopes, ꢀ, of ꢀ5.5 for K1k2 and ꢀ5.4 for K1kAn.
The rate constant kAn represents a proton transfer from
the zwitterionic intermediate 3 to the corresponding ani-
line. It has been shown previously5,23,24 that such trinitro-
activated zwitterions are considerably stronger acids than
the parent anilinium ions. Hence kAn represents a thermo-
dynamically favourable proton transfer between nitrogen
atoms so that its value will depend on steric constraints
rather than basicity considerations. For anilines 2a–g
carrying remote substituents the steric hindrance at the
reaction centre for proton transfer should not change, so
that values of kAn will be constant. This leads to a ꢀ value
ꢀ5.4 for K1, consistent with the positive charge devel-
oped on nitrogen.
kobs
k1ðk2 þ kAn½AnꢂÞ
kA ¼
¼
ð1Þ
ð2Þ
½Anꢂ kꢀ1 þ k2 þ kAn½Anꢂ
K1k2 þ K1kAn½Anꢂ
kA ¼
kAn½Anꢂ
k2
kꢀ1
1 þ
þ
kꢀ1
For anilines 2a–g carrying substituents at the 3- or 4-
positions, plots (not shown) of kA versus aniline concen-
tration had positive intercepts on the y-axis, representing
the uncatalysed pathway and curved with decreasing
slope as the aniline concentration increased. Values
calculated using Eqn (2) with the parameters given in
Table 4 gave good fits to the experimental data. The
values of K1k2 and of K1kAn have low error limits
although, because of the small curvature, values of kAn/
k
ꢀ1 are less precise. Similarly values of k1, calculated as
K1kAnkꢀ1/kAn have fairly high error limits.
Table 1. Spectroscopic data for reaction products
Compound
1H NMR shifts (ppm)a
Picryl ring
UV maximumb
NH
Other aromatic
Alkyl
ꢂ (nm) Log[" (dm3 molꢀ1 cmꢀ1)]
4a, R ¼ 4-OMe
4b, R ¼ 4-Me
4c, R ¼ 3-Me
4d, R ¼ H
10.21
10.18
10.10
9.96
10.21
10.19
10.19
10.01
9.95
8.90
8.90
8.90
8.96
8.93
8.94
8.97
8.97
8.90
7.11, 6.87
7.11, 7.04
7.1
7.33, 7.25, 7.15
7.2
3.73
2.26
2.30
—
—
—
370
355
355
365
360
360
350
350
350
1.6
1.5
1.5
1.4
1.5
1.6
1.4
1.9
1.6
4e, R ¼ 4-F
4f, R ¼ 4-Cl
4g, R ¼ 3-Cl
4h, R ¼ 2,4-Me2
4i, R ¼ 2-Me
7.36, 7.17
7.3
—
2.33, 2.34
2.30
7.19, 7.0
7.28, 7.17
7.09, 7.03
7.32, 7.22
7.10, 7.02
7.27, 713
7.12, 7.08
4j, R ¼ 2-Et
9.95
8.91
1.24
2.69
—
350
1.6
4k, R ¼ 2-F
10.09
10.10
8.94
8.87
350
347
1.6
1.6
4l, R ¼ 2,6-Me2
2.09
aIn [2H6]DMSO.
bIn acetonitrile.
Copyright # 2005 John Wiley & Sons, Ltd.
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SNAR REACTIONS OF ANILINES WITH 2,4-DINITROPHENYL 2,4,6-TRINITROPHENYL ETHER
77
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 75–80

78
M. R. CRAMPTON, T. A. EMOKPAE AND C. ISANBOR
Table 2. Kinetic resultsa for the reaction of 1 with 2a–g in acetonitrile at 25 ꢁC
kA (10ꢀ2 dm3 molꢀ1 sꢀ1
)
[Aniline] (mol dmꢀ3
)
4-OMe
4-Me
3-Me
4–F
4-Cl
3-Cl
0.002
0.004
0.005
0.008
0.010
0.015
0.020
0.030
0.040
0.050
0.060
0.070
0.080
0.10
4.3 (4.3)
17.1 (17.5)
19.4 (19.4)
280 (280)
320 (320)
66 (65)
70 (71)
4.8 (5.0)
5.1 (5.2)
0.75 (0.70)
0.09 (0.10)
0.13 (0.13)
380 (380)
430 (430)
480 (480)
530 (530)
80 (81)
90 (92)
100 (98)
110 (112)
22.0 (22.4)
25.3 (25.3)
28.8 (28.1)
6.1 (6.1)
7.5 (7.1)
0.86 (0.89)
1.03 (1.09)
1.2 (1.3)
1.4 (1.5)
1.6 (1.6)
0.16 (0.17)
0.19 (0.19)
33.1 (33.3)
0.20 (0.22)
0.25 (0.26)
0.31 (0.30)
138 (137)
153 (152)
2.0 (2.0)
2.4 (2.3)
2.9 (2.9)
0.14
0.15
0.20
0.40 (0.40)
0.50 (0.49)
aValues in parentheses were calculated using Eqn (2) with the values collected in Table 4.
Since values of K1k2 show an almost identical depen-
dence on the substituents, ꢀ value of ꢀ5.4, values of k2 for
intramolecular proton transfer within the zwitterions are
essentially constant with these anilines. This is also
apparent from the constancy, at ꢅ30 dm3 molꢀ1, of the
ratio kAn/k2 in Table 4. Values of k2 will, of course, be
expected to show a strong dependence on the nucleofug-
ality of the phenolic leaving group,2,21 which is constant
in the present measurements.
A Hammett plot of values of k1, the rate constant for
nucleophilic attack by the anilines, gives a ꢀ value of
ꢀ4.2. This indicates a strong dependence on the nature of
the substituent and infers substantial development of
positive charge on nitrogen in the transition state. Corre-
spondingly, there will be considerable development of
negative charge on the other groups at the 1-position. Our
previous measurements6,21 showed that the nature of the
substituents in the phenolic leaving group had only a
small effect on values of k1 for nucleophilic attack by
aniline. The data in Table 3 in Ref. 21 give a ꢀ value of ca
þ0.5. The conclusion drawn was that there was little
charge development and hence an ‘early’ transition state.
Nevertheless, consideration of the structure of the zwit-
terion 3 shows that the bulk of the negative charge will be
delocalised into the trinitrophenyl ring so that substitu-
ents in the incipient leaving group have only a small
influence on charge delocalisation. Hence our current
results and those given previously accord better with a
‘late’ transition state for nucleophilic attack with con-
siderable charge development.
Table 3. Kinetic resultsa for the reaction of 1 with 2h–l in
acetonitrile at 25 ꢁC
kA (10ꢀ2 dm3 molꢀ1 sꢀ1
)
[Anilineꢀ] 3
(mol dm
2-Substituted anilines
)
2,4-Me2
2-Me 2-Et
2-F
2,6-Me2
0.005
0.010
0.015
0.020
0.025
0.030
0.040
0.050
0.060
0.080
0.10
2.5
2.6
2.7
2.8
Whereas the effects of remote substituents, at the 3- or 4-
position, in the nucleophile are dominated by electronic
effects, in 2-substituted anilines there is evidence for
significant steric interactions. Thus the effect of a 2-
methyl substituent is to reduce the value of K1kAn by a
factor of 55 in aniline and 80 in 4-methylaniline. The
corresponding factor for 2-ethylaniline is 200. Our results
do not allow the separation of steric effects on values of
K1 and kAn, although both terms are expected to be
reduced by steric congestion at the reaction centre.
Comparison of the values of K1k2 for aniline and 2-
methylaniline and for 4-methylaniline and 2,4-dimethy-
laniline yields ratios of 17 and 28, respectively. These
ratios are 3–4 times smaller than those for the similar
0.48
0.50
0.53
0.55
0.56
0.20
2.9
0.21
0.22
0.24
0.25
0.042
0.70
0.79
0.83
0.054
0.071
0.087
0.105
0.007
0.12
0.15
0.20
0.30
0.006
0.006
0.006
aLinear plots of kA versus [aniline] yield the values given in Table 4.
Copyright # 2005 John Wiley & Sons, Ltd.
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SNAR REACTIONS OF ANILINES WITH 2,4-DINITROPHENYL 2,4,6-TRINITROPHENYL ETHER
79
Table 4. Summary of rate data for the reaction of 1 with ring-substituted anilines 2a–l in acetonitrile at 25 ꢁC
Substituent(s),
R
K1k2
K1kAn
kAn/k
k1
kAn/k2
ꢀ1
(dm3 molꢀ1 sꢀ1
)
(dm6 molꢀ2 sꢀ1
)
(dm3 molꢀ1
)
(dm3 molꢀ1 sꢀ1
)
(dm3 molꢀ1
)
a, 4-OMe
b, 4-Me
c, 3-Me
d, Ha
e, 4-F
f, 4-Cl
g, 3-Cl
h, 2,4-Me2
i, 2-Me
j, 2-Et
k, 2-F
l, 2,6-Me2
2.9 ꢆ 0.3
0.68 ꢆ 0.05
0.18 ꢆ 0.05
0.08 ꢆ 0.01
0.045 ꢆ 0.01
90 ꢆ 10
16 ꢆ 1
5 ꢆ 1
3.5 ꢆ 1
2.5 ꢆ 1
2.7 ꢆ 0.5
2.5 ꢆ 0.5
1.8 ꢆ 0.5
1 ꢆ 0.5
<1
18 ꢆ 5
4.5 ꢆ 1.5
31
24
22
28
28
46
33
8
4 ꢆ 0.5
1.6 ꢆ 0.5
2.2 ꢆ 0.2
0.8 ꢆ 0.3
1.25 ꢆ 0.1
0.23 ꢆ 0.02
0.026 ꢆ 0.004
0.2 ꢆ 0.05
0.04 ꢆ 0.005
0.011 ꢆ 0.003
(8 ꢆ 1) ꢄ 10ꢀ3
0.5 ꢆ 0.2
ꢀ3
ꢀ4
(5 ꢆ 1) ꢄ 10
0.12 ꢆ 0.06
0.026 ꢆ 0.01
(8 ꢆ 1) ꢄ 10
0.024 ꢆ 0.004
ꢀ3
(4.5 ꢆ 0.5) ꢄ 10
<1
<1
<1
9
6
80
ꢀ3
(1.8 ꢆ 0.4) ꢄ 10
ꢀ4
ꢀ5
(1 ꢆ 0.2) ꢄ 10
(6 ꢆ 1) ꢄ 10
aData from Ref. 21.
comparisons of K1kAn values, indicating that steric effects
on k2 are less severe than on kAn. This effect is also seen
in the reduction in value of kAn/k2 in Table 4 from ꢅ30 to
ꢅ8 for the ortho-alkyl-substituted anilines.
Table 5 the pKa values, measured in acetonitrile, for some
20-substituted-2,4,6-trinitrodiphenylamines which are the
reaction products. These relate to the process shown in
Scheme 3 and show that 20-alkyl substituents have only
small effects on the acidity of 4 whereas a 20-fluoro-
substituent increases the acidity by about one pKa unit.
The latter effect is consistent with the expected stabilisa-
tion of the anionic centre in 5 by the inductive electron
withdrawal of the fluorine substituent. The small effect of
the alkyl substituents argues against an increase in the
steric interactions between the aromatic rings, which
would be expected to make more difficult ionisation to
give the delocalised anion 5. The second piece of evi-
dence comes from previously reported21 crystal struc-
tures of the related 2,6-disubstituted-phenoxy 2,4,6-
trinitrophenyl ethers. Although relating to diphenyl
ethers rather than diphenylamines, these show that steric
interactions between the rings increase only slightly in
the presence of the 2,6-substituents.
In conclusion, the results from this and our previous
work21 show that all phenyl trinitrophenyl ethers are
sterically strained structures. However, the steric hin-
drance to the steps involved in nucleophilic substitution
by aniline and ring-substituted anilines is only signifi-
cantly increased when either the entering or leaving
groups contains two ortho-substituents or when both
entering and leaving groups carry an ortho-substituent.
It might be argued that since k2 involves an intramo-
lecular proton transfer, steric effects should be small.
Hence the major steric effect on K1k2 values is likely to be
in the equilibrium formation of the zwitterionic inter-
mediates yielding reduced values of K1. A reduction in
the value of kAn represents hindrance of approach to the
reaction centre carrying an ortho-substituted aniline by a
second ortho-substituted aniline molecule. It is worth
noting that in the reaction of 2,6-dimethylaniline (Table
4), this hindrance is so severe as to reduce the catalysed
pathway below its observable limit. The value of K1k2 is
reduced by a factor of 75 relative to its value for 2-
methylaniline.
The effect of a 2-fluoro substituent is to reduce con-
siderably the values of both K1kAn and K1k2 relative to
aniline. However, in contrast to the behaviour of the 2-
alkyl derivatives, the ratio kAn/k2 is somewhat higher than
for aniline itself. Values of Es, the Taft steric factor,25 are
H 1.24, F 0.78, Me 0.00 and Et ꢀ0.07, so that steric
effects are likely to be less important for the 2-fluoro
derivative. Here the results indicate the dominance of
electronic effects. It is known26 that owing to its strongly
electron-withdrawing inductive effect, an ortho-fluorine
substituent will have a strongly destabilising effect on the
adjacent cationic nitrogen centre in the zwitterion 3k.
This is expected to result in a large reduction in the value
of K1, which is likely to be the major effect with only
relatively small steric effects on kAn or k2.
Table 5. pKa values of 4i–k in acetonitrile
UV maximuma (nm)
Substituent
4
5
K
pKa
The product-forming steps involve loss of the phenolic
leaving group and rotation of the aniline moiety towards
the ring-plane of the trinitro-substituted ring. It is worth
considering whether steric effects due to the presence of
an ortho-substituent in the aniline might reduce values of
rate constants for this process. Evidence that such effects
are not severe comes from two sources. First, we report in
i, 2-Me
j, 2-Et
k, 2-F
d, H
360, (4.15)
360, (4.15)
353, (4.14)
445, (4.37)
445, (4.37)
436, (4.40)
0.025
0.023
0.19
19.89
19.93
19.02
19.97b
aValues of logꢃ are given in parentheses.
bRef. 6.
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 75–80

80
M. R. CRAMPTON, T. A. EMOKPAE AND C. ISANBOR
R
R
R
N
NH
N
O2N
NO2
O2N
NO2
O2N
NO2
+ H
+ H
NO2
NO2
NO2
5
4
Scheme 3
The main steric effects are thought to result in the
Acknowledgement
reduction in values of K1 for formation of the zwitterionic
intermediates and in values of kAn for the base-catalysed
pathway.
We thank the Royal Society, London, for financial
assistance to allow C. Isanbor to visit Durham.
N-Substitution in the aniline does have, as shown
previously,6,20 a dramatic steric effect on the substitution
process.
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22. Barlin GB, Perrin DP. Q. Rev. Chem. Soc. 1966; 20: 75–102.
23. Bernasconi CF, Muller MC, Schmid P. J. Org. Chem. 1979; 44:
3189–3196.
mined using Eqn (4) with the known value29 for
DABCOHþ
pKa
of 18.29. Results are given in Table 5.
24. Crampton MR, Gibson B. J. Chem. Soc., PerkinTrans. 2 1981; 533–
539.
25. Gallo R. Prog. Phys. Org. Chem. 1983; 14: 115–163.
26. Chambers RD. Fluorine in Organic Chemistry. Blackwell: Oxford,
2004.
27. Crampton MR, Robotham IA. J. Chem. Res. (S) 1997; 22–23.
28. Crampton MR, Gold V. J. Chem. Soc. B 1966; 893–900.
29. Coetzee JF, Padmanabhan. J. Am. Chem. Soc. 1965; 87: 5005–5010.
K
4 þ DABCO Ð 5 þ DABCOHþ
ð3Þ
ð4Þ
þ
pKa ¼ pKaDABCOH ꢀ logK
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 75–80