Electrophilic aromatic substitution in substituted anilines; kinetics of the reaction with 4,6-dinitrobenzofuroxan

Article abstract of DOI:10.1139/v99-033

Kinetic results are reported for reaction of aniline and six of its N- and ring-substituted derivatives with 4,6-dinitrobenzofuroxan (DNBF) in water - dimethyl sulfoxide mixtures. In acidic solution σ-adducts, the result of electrophilic substitution in the anilines, are formed with bonding between a ring-carbon atom of the anilines and the 7-position of DNBF. Reaction normally occurs at the 4-position of the aniline unless this carries a substituent, when reaction occurs at the 2-position. A value of 2.0 for k(H)/k(D), the kinetic isotope effect, indicates that in the reaction with aniline bond formation is largely rate determining in the substitution pathway. The results allow estimates to be made for the pK(a) values relating to carbon protonation of the anilines.

Full text of DOI:10.1139/v99-033

639
Electrophilic aromatic substitution in
substituted anilines; kinetics of the reaction
with 4,6-dinitrobenzofuroxan
Michael R. Crampton, Lynsey C. Rabbitt, and François Terrier
Abstract: Kinetic results are reported for reaction of aniline and six of its N- and ring-substituted derivatives with 4,6-
dinitrobenzofuroxan (DNBF) in water – dimethyl sulfoxide mixtures. In acidic solution σ-adducts, the result of
electrophilic substitution in the anilines, are formed with bonding between a ring-carbon atom of the anilines and the
7-position of DNBF. Reaction normally occurs at the 4-position of the aniline unless this carries a substituent, when
reaction occurs at the 2-position. A value of 2.0 for kH/kD, the kinetic isotope effect, indicates that in the reaction with
aniline bond formation is largely rate determining in the substitution pathway. The results allow estimates to be made
for the pKa values relating to carbon protonation of the anilines.
Key words: electrophilic substitution, σ-adducts, 4,6-dinitrobenzofuroxan, ambident reactivity of anilines.
Résumé : On rapporte les données cinétiques relatives à la réaction de l’aniline (et six de ses dérivés substitués sur
l’azote et sur le noyau) avec le 4,6-dinitrobenzofuroxane (DNBF) dans des mélanges formés d’eau et de
diméthylsulfoxyde. En solutions acides, il se forme des adduits-σ, résultant d’une substitution électrophile sur les
anilines, qui lient un atome de carbone du noyau des anilines et la position 7 du DNBF. La réaction se fait
généralement en position 4 de l’aniline, à moins que cette position porte un substituant; la réaction se fait alors en
position 2. Une valeur de 2,0 pour l’effet isotopique cinétique, kH/kD, indique que, dans cette réaction la formation de
la liaison avec l’aniline constitue l’étape cinétiquement déterminante de la réaction de substitution. Les résultats
permettent d’évaluer les valeurs de pKa associées à la protonation du carbone des anilines.
Mots clés : substitution électrophile, adduits-σ, 4,6-dinitrobenzofuroxane, réactivité ambidente des anilines.
[Traduit par la Rédaction] Crampton et al. 646
Aniline and its substituted derivatives usually act as nitro-
gen nucleophiles. Thus, reaction with 1,3,5-trinitrobenzene
(1, 2) in dimethyl sulfoxide (DMSO) in the presence of a
strong base gives N-bonded σ-adducts, 1; reaction with sub-
strates carrying good leaving groups may result in substitu-
tion leading to diphenylamine derivatives (3–5).
Nevertheless, the ambident reactivity of aniline and its
derivatives has been shown in reactions with 4,6-dinitro-
benzofuroxan, DNBF. Here σ-adducts have been observed
involving nucleophilic attack either through nitrogen or
through a ring carbon atom para- or ortho- to the amino
function (6–10).
The high electrophilic reactivity of DNBF is well known
(5, 11, 12). In fact, it has been shown to be a more powerful
electrophile than the 4-nitrobenzenediazonium ion (13) or
the proton (14). Previously kinetic studies have been re-
ported of electrophilic substitutions involving reactions of
DNBF with pyrroles (14), 5-substituted indoles (13), 3,4-
diaminothiophene (15), 3-methoxythiophene (16), and
hydroxy- and methoxy-substituted benzenes (17). In the
present work, we report kinetic studies of the reactions of
aniline and some N-substituted or ring-substituted deriva-
tives with DNBF in water–DMSO mixtures. Measurements
were made in acidic solutions where the anilines are very
largely protonated. Here the results are in accord with the re-
actions, shown in Scheme 1, leading to the formation of car-
bon bonded σ-adducts. In the case of anilines 2a–e, reaction
was found to occur para to the amino position. However,
Received November 22, 1998.
This paper is dedicated to Professor Jerry Kresge in recognition of his outstanding contributions to the study of physical organic
chemistry.
M.R. Crampton1 and L.C. Rabbitt. Chemistry Department, Durham University, Durham DH1 3LE, U.K.
F. Terrier. Université de Versailles, SIRCOB, Bâtiment Lavoisier, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France.
1Author to whom correspondence may be addressed. Telephone: +44 191 374 3130. Fax: +44 191 384 4737.
e-mail: M.R.Crampton@durham.ac.uk
Can. J. Chem. 77: 639–646 (1999)
© 1999 NRC Canada

640
Can. J. Chem. Vol. 77, 1999
Scheme 1.
when the 4-position is blocked, reaction occurs ortho to the
amino function to yield products 5f and 5g.
at the 7′ and 5′ positions of DNBF, are typical of C-adduct
formation (5, 13, 14, 17), consistent with the production of
4a–e. In 4-substituted anilines, the band due to the 7′ proton
is at δ 5.8, consistent with the formation of 5f and g. In solu-
tions containing higher mole ratios of aniline to DNBF,
there was NMR evidence for the kinetically controlled for-
mation of N-bonded adducts followed by conversion to the
thermodynamically more stable C-bonded adducts. In the
case of aniline, 2a, bands due to the N-bonded adduct were
at δ 6.04 and 8.64, at positions similar to those reported pre-
viously (10).
Kinetic measurements were conveniently made in water–
DMSO mixtures. Most data relate to a solvent composition
of 30/70 (v/v) water/DMSO, although for comparison some
measurements using aniline were made in 70/30 and 50/50
(v/v) water/DMSO mixtures. All measurements were made
in the presence of excess hydrochloric acid. This had the ef-
fect of reducing the concentrations of free amines to very
low levels, and under these conditions there was no evidence
for nucleophilic attack via the nitrogen centre of the anilines.
These reactions may be regarded as neutral nucleophile–
electrophile combination reactions (18), and are examples of
electrophilic substitution in the aniline ring. The powerful
activating effect of the amino group towards electrophilic
substitution in aromatic systems has been noted previously
(19, 20). There are many examples of halogenation (21, 22)
and of hydrogen exchange (23) in aniline derivatives. One of
the aims of the present work was to use the reaction with
DNBF to compare the reactivity of aniline with other aro-
matic systems.
Results and discussion
1H NMR spectra have been reported previously for the
products of reaction of DNBF and several amines (6–10).
Our results relating to 1:1 mole ratio mixtures of DNBF and
amines 2a–g in [2H6] DMSO are given in Table 1. The spec-
tra, showing bands at δ 5.3–5.6 and δ 8.7 for the hydrogens
© 1999 NRC Canada

Crampton et al.
Table 1. H NMR shifts for σ-adducts, 4 or 5, in [2H6] DMSO.
641
1
Compound
H7′
H5′
Aromatica
Other
DNBF
4a
4b
4c
4d
4e
5f
5g
8.94
5.36
5.38
5.39
5.60
5.56
5.84
5.84
9.27
8.74
8.74
8.74
8.72
8.66
8.74
8.74
7.19 (d), 7.33 (d), J 8
7.30 (d), 7.37 (d), J 8.5
7.40 (m)
2.90 (NMe)
3.12 (NMe2)
2.58 (Me)
3.68 (OMe)
2.20 (Me)
7.08 (m)
6.89 (s), 6.87 (d), 7.34 (d), J 7.4
6.92 (s), 7.10 (d), 7.16 (d), J 8
6.53 (s), 6.95 (d), 7.18 (d), J 8.5
3.67 (OMe)
aJ in Hz.
[1]
Fig. 1. UV-vis spectra for the reaction in 30/70 (v/v)
water/DMSO of DNBF, 4 × 10–5 mol dm–3, with aniline
hydrochloride 0.05 mol dm–3 and hydrochloric acid 0.03
mol dm–3. The absorbance at 489 nm increases with time.
A further beneficial effect of the acidic media was to reduce
the equilibrium concentrations of the hydroxy adduct, 6, of
DNBF which is known to form in neutral media, as shown
in eq. [1].
Spectrophotometric measurements, using the absorbance
of 6 at 470 nm, gave values for KH O, defined in eq. [1], of
2
0.0196 ± 0.0008 mol dm–3 in 70% DMSO and 0.0045 ±
0.0002 mol dm–3 in 50% DMSO. These values are in excel-
lent agreement with literature values of 0.019, 0.0040, and
0.00080 mol dm–3 in media containing 70, 50, and 30%
DMSO, respectively (24).
Reaction with aniline
The equilibrium between DNBF and 6 is established rap-
idly. Similarly, any adduct formation resulting from attack of
aniline as a nitrogen nucleophile on DNBF will be a rapid
process (3, 5). The UV/visible spectra in Fig. 1 show that in
acidic solutions there is a slow reaction between DNBF and
aniline, giving a band at 489 nm. In view of the NMR data,
this is attributed to the C-bonded adduct 4a. It has been re-
ported (9, 10) that di-adducts may be formed by reaction of
both N- and C-centres of an aniline molecule with two mole-
cules of DNBF. There was no evidence for such 1:2 interac-
tion in our systems, which contain very low, 4 × 10–5
mol dm–3, concentrations of DNBF. Kinetic data, with con-
centrations of aniline hydrochloride and free hydrochloric
acid in large excess of the DNBF concentration, indicated
that adduct formation was an accurately first-order process.
Values of kobs, the measured rate constant, are in Table 2.
The results show that at constant concentration of free acid,
values increase linearly with the concentration of aniline hy-
drochloride. However, values decrease with increasing acid
concentration. This observation indicates that reaction is
likely to involve free aniline rather than its protonated form.
Hence, we define by eq. [2] a rate constant k, which is re-
lated by eq. [3] to the constants for individual steps shown
in the scheme. There is no evidence from NMR or UV/visi-
ble spectra of the accumulation of 3, the intermediate in the
substitution pathway. It may be shown that k is related to
kobs by eq. [4], where Ka is the acid dissociation constant of
the anilinium ion. This equation allows for the small concen-
tration of the hydroxy-adduct 6 in equilibrium with DNBF.
In 70/30 (v/v) DMSO/water, the pKa value for the anilinium
ion is known to be 3.41 (25). Hence, at the acid concentra-
tions used [H+] >> Ka, allowing eq. [4] to be simplified to
give eq. [5].
−d[DNBF]
[2]
[3]
= k[DNBF][aniline]
dt
k1k2
k−1 + k2
k =
© 1999 NRC Canada

642
Can. J. Chem. Vol. 77, 1999
Table 2. Rate data for reaction of DNBFa with aniline in acidic solutions in 30/70 (v/v) water/DMSO at 25°C.
[Aniline H+Cl–]/mol dm–3
[HCl]/mol dm–3
104 kobs/s–1b
k/dm3 mol–1 s–1c
0.050
0.050
0.050
0.050
0.050
0.050
0.020
0.040
0.10
0.059
0.117
0.146
0.200
0.242
0.30
0.033
0.033
0.033
0.31
8.28
5.15
4.30
3.58
3.10
2.44
4.76
9.60
23.1
5.12
3.3
3.6
3.7
4.0
4.2
4.0
3.2
3.2
3.1
4.3
0.10
aDNBF concentration is 4 × 10–5 mol dm–3.
bMeasured at 489 nm.
cCalculated from eq. [5] with Ka 3.89 × 10–4 mol dm–3 and KH 0.0196 mol dm–3.
2O
Table 3. Isotope and solvent effects on reaction of DNBFa and aniline.
Vol.%
DMSO
[C6D5NH3 Cl–]/mol dm–3 [C6H5NH3 Cl–]/mol dm–3
104 kobs/s–1
kH/kD
+
+
[HCl]/mol dm–3
k/dm3 mol–1 s–1b
70
0.050
0.100
—
—
—
0.050
0.100
—
0.30
0.30
0.30
0.30
0.030
0.030
0.030
0.030
0.030
0.030
0.030
0.030
1.31
2.22
2.51
5.12
1.90
3.10
3.87
5.71
0.45
0.70
0.91
1.54
2.13
1.85
4.10
4.27
1.17
1.15
2.39
2.12
0.55
0.52
1.13
1.14
2.1 ± 0.1
—
50
30
0.030
0.050
—
—
2.0 ± 0.1
2.1 ± 0.1
0.030
0.050
—
—
0.030
0.050
—
0.030
0.050
—
—
aAll measurements were made with DNBF 4 × 10–5 mol dm–3 at 25°C.
bCalculated from eq. [5] with values for Ka 3.89 × 10–4 mol dm–3, 70% DMSO, ref. 25; 1.86 × 10–4, 50% DMSO, ref. 15; 8.3 × 10–5 mol dm–3, 30%
DMSO, ref. 15; and values of KH given in the text.
2O
Table 4. Rate data for reaction of DNBFa with 2b, N-methylaniline, in acidic solution in 30/70 (v/v) water/DMSO
at 25°C.
[N-Methylaniline H+Cl–]/mol dm–3
[HCl]/mol dm–3
104 kobs/s–1b
k/dm3 mol–1 s–1c
0.050
0.050
0.050
0.050
0.050
0.050
0.014
0.025
0.036
0.102
0.150
0.200
136
104
85
42
26
10.6
10.6
10.8
11.4
9.9
19.6
9.8
aDNBF concentration is 4 × 10–5 mol dm–3.
bMeasured at 494 nm.
cCalculated from eq. [5] with Ka 8.7 × 10–4 mol dm–3 and KH 0.0196 mol dm–3.
2O
that reaction involves free aniline rather than the anilinium
ion.
Ka
Ka + [H+ ] [H+ ] + K
[H+ ]
[4]
[5]
kobs = k[aniline H+]
H2 O
The availability of pentadeuterioaniline, C6D5NH2, allowed
the evaluation of the kinetic isotope effect, kH/kD, on the
substitution reaction. Measurements were made in media
containing 70, 50, and 30% DMSO by volume and are re-
ported in Table 3. For accuracy of comparison, measure-
ments were made simultaneously on deuteriated and
nondeuteriated systems using the multicell facility in the
(KH O + [H+ ])
Ka [aniline H+]
kobs
2
k =
The invariance of values, in Table 2, of k with acid con-
centration or aniline hydrochloride concentration confirms
© 1999 NRC Canada

Crampton et al.
Table 5. Rate data for reaction of DNBFa with 2c, N,N-dimethylaniline, in acidic solution in 30/70 (v/v)
643
water/DMSO at 25°C.
[N,N-Dimethylaniline H+Cl–]/mol dm–3
[HCl]/mol dm–3
104 kobs/s–1b
k/dm3 mol–1 s–1c
0.020
0.020
0.020
0.020
0.020
0.020
0.024
0.044
0.055
0.108
0.200
0.230
30.6
20.5
15.8
9.3
5.3
5.0
6.5
6.4
5.8
5.8
5.8
6.2
aDNBF concentration is 4 × 10–5 mol dm–3.
bMeasured at 486 nm.
cCalculated from eq. [5] with Ka 1.02 × 10–3 mol dm–3 and KH 0.0196 mol dm–3.
2O
Table 6. Rate data for reaction of DNBFa with 2d, 3-methylaniline, in acidic solution in 30/70 (v/v) water/DMSO
at 25°C.
[3-Methylaniline H+Cl–]/mol dm–3
[HCl]/mol dm–3
104 kobs/s–1b
k/dm3 mol–1 s–1c
0.050
0.050
0.050
0.050
0.050
0.050
0.036
0.070
0.153
0.200
0.241
0.300
56
34
18.5
14.2
11.1
9.3
28
27
28
28
27
26
aDNBF concentration is 4 × 10–5 mol dm–3.
bMeasured at 485 nm.
cCalculated from eq. [5] with Ka 2.24 × 10–4 mol dm–3, and KH 0.0196 mol dm–3.
2O
Table 7. Rate data for reaction of DNBFa with 2e, 3-methoxyaniline, in acidic solution in 30/70 (v/v) water/DMSO
at 25°C.
[3-Methoxyaniline H+Cl–]/mol dm–3
[HCl]/mol dm–3
104 kobs/s–1b
k/dm3 mol–1 s–1c
0.010
0.030
0.030
0.050
0.090
0.070
0.17
1100
3700
1660
7170
1470
1350
1220
1280
0.050
aDNBF concentration is 4 × 10–5 mol dm–3.
bMeasured by stopped-flow spectrophotometry at 487 nm.
cCalculated from eq. [5] with Ka 8.1 × 10–4 mol dm–3 and KH 0.0196 mol dm–3.
2O
Table 8. Rate data for reaction of DNBFa with 2f, 4-methylaniline, in acidic solution in 30/70 (v/v) water/DMSO at
25°C.
[4-Methylaniline]/mol dm–3
[HCl]/mol dm–3
104 kobs/s–1b
k/dm3 mol–1 s–1c
0.050
0.050
0.050
0.050
0.0142
0.0249
0.0356
0.0570
1.63
1.28
1.06
0.75
1.10
1.14
1.17
1.15
aDNBF concentration is 4 × 10–5 mol dm–3.
bMeasured at 485 nm.
cCalculated from eq. [5] with Ka 1.0 × 10–4 mol dm–3 and KH 0.0196 mol dm–3.
2O
spectrophotometer. These values, rather than the value of
3.7 dm3 mol–1 s–1 for kH in 70% DMSO, were used in the
calculation of the isotope effect. It is known that ring-deu-
teriation has only a small effect on the acidity of anilinium
and k–1 will show only small changes with isotopic substitu-
tion, since these involve secondary isotope effects. However,
the value of k2 that involves carbon–hydrogen bond cleavage
is likely to show a primary isotope effect of ca. 7. These as-
sumptions, together with the observed value of kH/kD = 2.1,
lead to a value for k–1/k2 of 0.2 for aniline with k1 = 1.2k.
This indicates that, as with most electrophilic substitution
reactions (19), the k1 step, the initial attack, is largely rate
determining. A similar situation was reported in reactions of
+
ions (26), thus in water values are 4.71 for C6H5NH3 and
+
4.74 for C6D5NH3 . We have assumed that values for Ka are
unaffected by deuteriation in the DMSO–water mixtures.
The overall value of k contains contributions from k1, k2, and
k–1 as indicated in eq. [3]. It is likely that the values of k1
© 1999 NRC Canada

644
Can. J. Chem. Vol. 77, 1999
Table 9. Rate data for reaction of DNBFa with 2g, 4-methoxyaniline, in acidic solution in 30/70 (v/v) water/DMSO
at 25°C.
[4-Methoxyaniline H+Cl–]/mol dm–3
[HCl]/mol dm–3
104 kobs/s–1b
k/dm3 mol–1 s–1c
0.050
0.050
0.050
0.0142
0.0356
0.0570
0.62
0.47
0.38
1.24
1.54
1.72
aDNBF concentration is 4 × 10–5 mol dm–3.
bMeasured at 490 nm.
cCalculated from eq. [5] with Ka 3.39 × 10–5 mol dm–3 and KH 0.0196 mol dm–3.
2O
Table 10. Summary of kinetic data for reaction of DNBF with substituted anilines in 30/70 (v/v) water/DMSO at 25°C.
λmax/nmb
k/dm3 mol–1 s–1
a
Aniline
pKa
2a
3.41
489
4.0 ± 0.7
2b
2c
2d
3.06
2.99
3.65
494
486
485
10.3 ± 0.6
6.0 ± 0.8
27 ± 1
2e
2f
3.09
4.00
4.47
487
485
490
1300 ± 100
1.14 ± 0.03
1.5 ± 0.2
2g
apKa values of the anilinium ions in 30/70 (v/v) water/DMSO from ref. 25.
bAbsorption maxima of adducts.
DNBF with substituted indoles (13, 14). However, in reac-
tions with 1,3,5-trimethoxybenzene, a larger isotope effect
was observed showing that here proton loss from the inter-
mediate is more kinetically significant (17).
Our results show that for substitution in aniline, the value
of the isotope effect is independent of the solvent composi-
tion in the range 30–70% DMSO. The small increase in k
from 1.1 dm3 mol–1 s–1 in 30% DMSO to 4.2 dm3 mol–1 s–1
in 70% DMSO is thus likely to reflect a similar increase in
value for k1. This may indicate slightly better solvation by
DMSO than by water of the charged polarizable intermedi-
ate 3. The value of the rate constant for attack of water on
DNBF shown in eq. [1] also increases with DMSO content
of the solvent (24).
© 1999 NRC Canada

Crampton et al.
645
Table 11. Carbon nucleophilicity of some π-excessive aromatics
towards DNBF in 50/50 (v/v) water/DMSO.
though steric inhibition of solvation in the intermediates
leading to the adducts 5 is a possibility.
Rate constants are available (17) for reaction, in 50/50
(v/v) water/DMSO, of DNBF at the 4-position of 1,3-
dimethoxybenzene (0.52 dm3 mol–1 s–1) and with 1,3,5-
trimethoxybenzene (205 dm3 mol–1 s–1). Comparison of
these values with data in Table 10 shows that the 4-amino
group is considerably more activating, in terms of carbon
nucleophilicity, than the 4-methoxy group.
Compounda
k/dm3 mol–1 s–1
pKa
a,b
3-Methoxythiophene
5-Cyanoindole
Aniline
N,N-Dimethylanilinec
N-Methylanilinec
3-Methylanilinec
5-Bromoindole
5-Chloroindole
Pyrrole
0.5
1.1
2.3
3.5
6.0
16
46
52
520
560
750
2100
2600
3500
24 000
9 × 105
–6.5
–6.0
(–6 to ~–5)
(–5 to ~–4.5)
–4.4
–4.5
–3.8
–3.5
(–3)
–3.3
–2.9
–2.3
Comparison with other π-excessive
aromatics
Indole
It is of interest to compare our results for the nucleo-
philicity of aniline derivatives towards DNBF with those for
other arenes. Most of the literature data refer to a solvent
system of 50/50 (v/v) water/DMSO, whilst our data gener-
ally refer to 30/70 (v/v) water/DMSO. However, for aniline
we have data in both media and have assumed that the sol-
vent effect will be similar for substituted derivatives. Data
are in Table 11. It has been shown previously (16) that there
is an approximately linear relationship between carbon
nucleophilicities towards DNBF, and pKa values in water for
carbon protonation. On the assumption that the anilines obey
the same relationship, the pKa values for C-protonation may
be estimated. For aniline and its N-methylated derivative
values of ca. –6 to –5 are obtained, while values for 3-
methylaniline and 3-methoxyaniline are ca. –4.7 and –3,
respectively. The results clearly show that in water N-pro-
tonation of the anilines, with pKa ca. 4, is strongly favoured
over ring protonation.
3-Methoxyanilinec
5-Methylindole
5-Methoxyindole
N-Methylindole
1,2,5-Trimethylpyrrole
3,4-Diaminothiophene
–0.5
1.5–2
aData from ref. 16 or from present work.
bpKa values in water for protonation on carbon
cValues of k in 30/70 water/DMSO were multiplied by 2.3/4.0.
Reaction with substituted anilines
Data analogous to that reported in Table 2 were obtained
for reaction of DNBF with the substituted anilines 2b–g. Re-
sults are given in Tables 4–9. Values of k calculated from
eq. [5] using the value of Ka appropriate to the particular
substituted anilinium ion are summarized in Table 10.
The results show that the value of k, the rate constant for
adduct formation, increases on N-methylation. However, fur-
ther methylation to give the N,N-dimethyl derivative results
in a reduction in reactivity. Successive N-methylations might
be expected to increase reactivity by increasing the electron
releasing ability of the amino group. Thus σ+ values (27, 28)
are –1.30 for para-NH2 and –1.70 for para-NMe2, with the
NHMe group showing intermediate behaviour (29). How-
ever these σ+ values relate to largely aqueous media, while
our results were obtained in media rich in DMSO. It is
known that DMSO is an extremely good hydrogen-bond ac-
ceptor (30, 31), and in this solvent the stabilization of the
cationic moiety of the intermediate, 3, will be expected to
increase as the number of NH bonds available for hydrogen-
bonding increases. This effect may temper the increase in
electron donation expected on N-methylation. A similar ar-
gument may be used (25) to explain the reduced acidity of
the anilinium ion, pKa = 3.41, in DMSO–water relative to
the N,N-dimethyl derivative, pKa = 2.99.
Experimental
4,6-Dinitrobenzofuroxan was prepared by the Drost
method (33); mp 172°C (lit. (16, 33) 172–174°C). All other
reagents and solvents were the purest available commercial
products.
1H NMR spectra were recorded using a Varian Mercury
200 MHz spectrometer. UV-vis spectra and kinetic measure-
ments were made at 25°C with a Perkin–Elmer Lambda 2
spectrophotometer, a Shimadzu UV-2101 PC spectrometer,
or an Applied Photophysics SX-17 MV stopped-flow
spectrophotometer. Reported rate constants are the means of
several determinations and are precise to ±5%. Potassium
chloride was used to maintain ionic strength I (ϵ (1/2)Σcizi2)
0.1 mol dm–3 in those solutions with lower ionic concentra-
tions.
Acknowledgements
The values of k for reaction at the 4-positions of 2d and
2e are considerably higher than that for aniline. Here the
electron releasing effect of the 3-substituent is dominant
with σ+ values reported as –0.31 for methyl and –0.78 for
methoxy (32). There may be some steric retardation here,
since the substituents are ortho to the reaction centre.
When the 4-position is blocked, reaction occurs at the 2-
position, ortho to the amino function. Here the values for k
are reduced relative to that for aniline. These reductions
probably represent a steric effect to nucleophilic attack, al-
Financial assistance for this project from Zeneca Fine
Chemicals, Huddersfield, is gratefully acknowledged.
References
1. E. Buncel and J.G.K. Webb. Can. J. Chem. 50, 129 (1972).
2. E. Buncel and W. Eggimann. J. Am. Chem. Soc. 99, 5958
(1977).
3. M.R. Crampton and I.A. Robotham. Can. J. Chem. 76, 627
(1998).
© 1999 NRC Canada

646
4. O. Banjoko and P. Otiono. J. Chem. Soc. Perkin Trans. 2, 399
Can. J. Chem. Vol. 77, 1999
19. R.O.C. Norman and R. Taylor. Electrophilic substitution in
benzenoid compounds. Elsevier, Amsterdam. 1965; R. Taylor.
Electrophilic aromatic substitution. Wiley, New York. 1990.
20. P.W. Robertson, P.B.D. de la Mare, and B.E. Swedlund. J.
Chem. Soc. 782 (1953).
21. E. Berliner. J. Am. Chem. Soc. 72, 4003 (1950).
22. J. Berthelot, C. Guette, P.-L. Desbene, J.-J. Basselier, P.
Chaquin, and D. Masure. Can. J. Chem. 67, 2061 (1989).
23. S. Clementi and A.R. Katritzky. J. Chem. Soc. Perkin Trans. 2,
1076 (1973).
24. F. Terrier, H.A. Sorkhabi, F. Millot, J.-C. Hallé, and R. Schaal.
Can. J. Chem. 58, 1155 (1980).
25. K. Yates and G. Welch. Can. J. Chem. 50, 474 (1972).
26. C.F. Bernasconi, W. Koch, and H. Zollinger. Helv. Chim. Acta,
46, 1184 (1963).
(1981).
5. F. Terrier. Nucleophilic aromatic displacement. VCH, New York.
1991.
6. R.J. Spear, W.P. Norris, and R.W. Read. Tetrahedron Lett. 24,
1555 (1983).
7. R.W. Read, R.J. Spear, and W.P. Norris. Aust. J. Chem. 37,
985 (1984).
8. R.W. Read and W.P. Norris. Aust. J. Chem. 38, 435 (1985).
9. M.J. Stauss, R.A. Renfrow, and E. Buncel. J. Am. Chem. Soc.
105, 2473 (1983).
10. E. Buncel, R.A. Renfrow, and M.J. Strauss. J. Org. Chem. 52,
488 (1987).
11. E. Buncel, R.A. Manderville, and J.M. Dust. J. Chem. Soc.
Perkin Trans. 2, 1019 (1997).
12. J.-C. Hallé, D. Vichard, M.-J. Pouet, and F. Terrier. J. Org.
Chem. 62, 7178 (1997).
27. H.C. Brown and Y. Okomoto. J. Am. Chem. Soc. 80, 4979
(1958).
13. F. Terrier, M.-J. Pouet, J.-C. Hallé, S. Hunt, J.R. Jones, and E.
Buncel. J. Chem. Soc. Perkin Trans. 2, 1665 (1993).
14. F. Terrier, E. Kizilian, J.-C. Hallé, and E. Buncel. J. Am.
Chem. Soc. 114, 1740 (1992).
15. F. Terrier, M.-J. Pouet, E. Kizilian, J.-C. Hallé, F. Outurquin,
and C. Paulmier. J. Org. Chem. 58, 4696 (1993).
16. E. Kizilian, F. Terrier, A.-P. Chatrousse, K. Gzoudi, and J.-C.
Hallé. J. Chem. Soc. Perkin Trans. 2, 2667 (1997).
17. F. Terrier, M.-J. Pouet, J.-C. Hallé, E. Kizilian, and E. Buncel.
J. Phys. Org. Chem. 11, 707 (1998).
28. S. Clementi and A.R. Katritzky. J. Chem. Soc. Perkin Trans. 2,
1077 (1973).
29. M. Charton. Prog. Phys. Org. Chem. 13, 211 (1981).
30. M.J. Kamlet and R.W. Taft. J. Am. Chem. Soc. 98, 377 (1976)
31. M.R. Crampton and I.A. Robotham. J. Chem. Res. Synop. 22
(1997).
32. L.M. Stock and H.C. Brown. Adv. Phys. Org. Chem. 1, 35
(1963).
33. P. Drost. Justus Liebigs Ann. Chem. 307, 49 (1899).
18. H. Mayr. Angew. Chem. Int. Ed. 33, 938 (1994).
© 1999 NRC Canada