Nonlinear bronsted and hammett correlations associated with reactions of 4-chloro-7-nitrobenzofurazan with anilines in dimethyl sulfoxide solution

Article abstract of DOI:10.1002/kin.20751

The kinetics and mechanism of nucleophilic aromatic substitution reactions of 4-chloro-7-nitrobenzofurazan 1 with 4-X-substituted anilines 2a-g (X = OH, OCH₃, CH₃, H, I, Cl, and CN) are investigated in a dimethyl sulfoxide (Me_2<

Full text of DOI:10.1002/kin.20751

Nonlinear Brønsted and
Hammett Correlations
Associated with Reactions
of 4-Chloro-7-
nitrobenzofurazan with
Anilines in Dimethyl
Sulfoxide Solution
I. JMAOUI,1 T. BOUBAKER,1 R. GOUMONT2
1Laboratoire C. H. P. N. R, Faculte´ des Sciences de Monastir, Universite´ de Monastir, 5019 Monastir, Tunisia
2Institut Lavoisier de Versailles, UMR 8180, Universite´ de Versailles, 78035 Versailles Cedex, France
Received 28 February 2012; revised 26 June 2012; accepted 27 June 2012
DOI 10.1002/kin.20751
Published online 15 January 2013 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The kinetics and mechanism of nucleophilic aromatic substitution reactions of
4-chloro-7-nitrobenzofurazan 1 with 4-X-substituted anilines 2a–g (X = OH, OCH3, CH3, H, I,
Cl, and CN) are investigated in a dimethyl sulfoxide (Me2SO) solution at 25◦C. The Hammett
plot of log k1 versus σ is nonlinear for all the anilines studied due to positive deviations of
the electron-donating substituents. However, the corresponding Yukawa–Tsuno plot resulted
in a good linear correlation with σ + r (σ+ − σ). The corresponding Brønsted-type plot is also
nonlinear, i.e., the slope (βnuc) changes from 1.60 to 0.56 as the basicity of anilines decreases.
These results indicate a change in a mechanism from a polar SNAr process for less basic
nucleophiles (X = I, Cl, and CN) to a single electron transfer for more basic nucleophiles (X =
OH, OCH3, and CH3). The satisfactory log k1 versus Eo correlation obtained for the reactions
of 1 with anilines 2a–d in the present system is consistent with the proposed mechanism.
Interestingly, the βnuc = 1.60 value measured for 1 in Me2SO reflects one of the highest
ꢀ
C
coefficients Brønsted ever observed for SNAr reactions.
Chem Kinet 45: 152–160, 2013
2013 Wiley Periodicals, Inc. Int J
INTRODUCTION
Correspondence to: T. Boubaker; e-mail: boubaker taoufik@
yahoo.fr.
Nucleophilic substitutions of 4-chloro-7-nitrobenzofu-
ranzan 1 with a series of 4-X-substituted anilines have
been investigated in 70–30 (v/v) and 20–80 (v/v)
Supporting Information is available in the online issue at
www.wileyonlinelibrary.com.
c
ꢀ
2013 Wiley Periodicals, Inc.

KINETICS OF REACTIONS OF 4-CHLORO-7-NITROBENZOFURAZAN WITH ANILINES
H2O-Me2SO mixtures at 25◦C. The single electron
153
EXPERIMENTAL
transfer (SET) mechanisms were proposed on the basis
of the large Brønsted coefficients (βnuc = 0.93–0.96)
[1].
The main interest in these reactions is that it consti-
tutes a rare example in which the magnitudes of βnuc
values are substantially higher than that reported for
polar SNAr reactions [2–9].
4-Chloro-7-nitrobenzofurazan 1 (Fluka BioChemica)
was used as received. Anilines 2a–g (Aldrich) were
recrystallized or redistilled before use whenever nec-
essary. Me2SO was distilled over calcium hydride at a
reduced pressure and was stored under nitrogen.
The kinetic study was performed using a Shimadzu
UV–vis model 1650 PC spectrophotometer. All the re-
actions were carried out under pseudo–first-order con-
ditions in the presence of excess aniline. Generally, the
aniline concentration was varied over the range 1 ×
10−3 to 8 × 10−1 mol dm−3, whereas the 4-chloro-
7-nitrobenzofurazan concentration was 3 × 10−5 mol
As an extension of this work and to obtain fur-
ther information on reactivity and on the electron-
transfer mechanism in SNAr reactions, exhaustive ki-
netic studies of the same reactions in dimethyl sul-
foxide (Me2SO) at 25◦C with a wider range of 4-X-
substitued anilines (X = OH, OCH3, CH3, H, I, Cl,
and CN) have been investigated. The second-order
rate constants have been obtained and analyzed in
terms of aniline basicity and of the electronic effect
of the substituents X borne by these aniline nucleo-
philes.
Surprisingly, we found that the structure–reactivity
relationships displayed later in Figs. 2 and 4 show
clear evidence of nonlinear behaviors. As will be seen,
the nonlinear Brønsted (log k1 vs. pKa) and Hammett
(log k1 vs. σ) plots were interpreted as a change in
mechanism from a polar SNAr process for electron-
withdrawing substituents to a SET for electron donat-
ing according to Scheme 1 and support the view that
values of βnuc close to or greater than 1 may be an
indicator of electron transfer in nucleophilic aromatic
substitution reactions.
dm−3
.
RESULTS AND DISCUSSION
Kinetic Studies
The SNAr reactions of a number of 4-X-substituted
anilines 2a–g by 4-chloro-7-nitrobenzofurazan 1 have
been kinetically studied in dimethyl sulfoxide at 25◦C.
In all experiments, only one relaxation time corre-
sponding to the quantitative formation of the substi-
tution products 4a–g (482 nm < λmax < 498 nm)
was observed. A typical experimental is illustrated
in Fig. 1, which shows the set of UV–vis absorption
spectra, describing the progressive conversion of the
Scheme 1 A detailed possible mechanistic interpretation for the reaction of 4-X-substituted anilines 2a–g with 4-chloro-7-
nitrobenzofurazan 1 in Me2SO at 25◦C.
International Journal of Chemical Kinetics DOI 10.1002/kin.20751

154
JMAOUI, BOUBAKER, AND GOUMONT
Figure 1 Reactions of 4-chloro-7-nitrobenzofurazan 1 (3 × 10−5 mol dm−3) and aniline 2d (10−2 mol dm−3) by absorption
spectroscopy in Me2SO at 25◦C. Scan interval = 25 min. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
1 to the product 4d. The process was found to be
very clean, confirmed by the presence of an isobestic
point at λ = 400 nm. Pseudo–first-order rate con-
stants (kobsd) were determined from the equation
ln (A∞ − At ) = −kobsdt + ln(A∞ − Ao). The corre-
lation coefficient for the linear regression was usually
higher than 0.9970 (see Table I). The plots of kobsd
Brønsted Behavior and Mechanistic
Interpretation
When the logarithm of the second-order rate constants,
k1, measured in dimethyl sulfoxide for the reactions of
4-chloro-7-nitrobenzofurazan 1 with 4-X-substituted
anilines 2a–g were plotted against the pKa values of
the conjugate acids of the anilines 2a–g in the dimethyl
sulfoxide solution [10,11] (see Table II), a plot, show-
ing two straight lines exhibiting two different slopes,
is obtained with a break observed for the unsubstituted
aniline 2d (Fig. 2).
As can be seen, the Brønsted-type plot for the re-
actions of 4-chloro-7-nitrobenzofurazan 1 is described
by two linear relationships: The first line observed for
electron-acceptor X-substituents (βnuc = 0.56, pKa <
3.82) is defined by the following equation:
versus the aniline concentration were linear (R2
>
0.9954) passing through the origin, indicating that gen-
eral base catalysis by the second aniline molecule is
absent and confirming the first order of the reactions of
1 with 2a–g. Moreover, the plots of log (kobsd) versus
log [2a–g]0 were linear (R2 > 0.9968) having slopes
close to unity, indicating a first-order dependence of the
reaction rate on [2a–g]0 (see Fig. S1 in the Supporting
Information).
All the reactions studied in this work followed the
simple kinetic law given by Eq (1), where k1 is the
second-order rate constant for the aniline addition to
the 4-chloro-7-nitrobenzofurazan 1.
ꢀ
ꢁ
log k1 = −4.07 + 0.56 pKa
R2 = 0.9961 (2)
and the second straight line, for the electron-donating
X-substituents (βnuc = 1.60, pKa > 3.82), is defined
kobsd = k1 [2a − g]0
(1)
by
log k1 = −8.04 + 1.60 pKa (R2 = 0.9982) (3)
The k1values were determined from the slope of
these linear plots. Generally six different aniline con-
centrations were used to determine k1 values. The rate
constants measured were reproducible to within 5%.
Kinetic data are summarized in Table I.
A careful examination of the literature showed that
Brønsted plots highlighting two straight lines with two
different slopes are rarely reported in SNAr reactions.
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KINETICS OF REACTIONS OF 4-CHLORO-7-NITROBENZOFURAZAN WITH ANILINES
155
Table I kobsd and k1 Values for the Reactions of 4-Chloro-7-nitrobenzofurazan 1 with 4-X-Substituted Anilines 2a–g in
Me2SO at 25◦C
X
[p-X-Aniline] (mol dm−3
)
kobsd(s−1
)
k1(mol−1 dm3 s−1
)
Correlation coefficient
0.9954
OH
1 × 10−3
2 × 10−3
4 × 10−3
6 × 10−3
8 × 10−3
1 × 10−2
5 × 10−3
8 × 10−3
1 × 10−2
2 × 10−2
3 × 10−2
5 × 10−2
5 × 10−3
8 × 10−3
1 × 10−2
2 × 10−2
4 × 10−2
6 × 10−2
1 × 10−2
2 × 10−2
4 × 10−2
6 × 10−2
8 × 10−2
2 × 10−1
1 × 10−2
2 × 10−2
4 × 10−2
5 × 10−2
6 × 10−2
8 × 10−2
5 × 10−2
1 × 10−1
2 × 10−1
4 × 10−1
5 × 10−1
6 × 10−1
1 × 10−2
2 × 10−2
4 × 10−2
6 × 10−2
8 × 10−2
1 × 10−1
1.39 × 10−3
3.19 × 10−3
7.16 × 10−3
1.01 × 10−2
1.32 × 10−2
1.53 × 10−2
1.94 × 10−3
3.17 × 10−3
4.45 × 10−3
8.31 × 10−3
1.37 × 10−2
2.16 × 10−2
4.13 × 10−4
6.48 × 10−4
8.10 × 10−4
1.54 × 10−3
2.89 × 10−3
4.44 × 10−3
1.12 × 10−4
2.20 × 10−4
4.47 × 10−4
6.80 × 10−4
9.26 × 10−4
2.50 × 10−3
3.58 × 10−5
6.87 × 10−5
1.40 × 10−4
1.86 × 10−4
2.33 × 10−4
3.21 × 10−4
2.32 × 10−4
4.64 × 10−4
8.19 × 10−4
1.70 × 10−3
2.01 × 10−3
2.30 × 10−3
3.50 × 10−6
8.20 × 10−6
1.50 × 10−5
2.63 × 10−5
3.15 × 10−5
4.20 × 10−5
1.57
OCH3
CH3
H
4.40 × 10−1
7.22 × 10−2
1.27 × 10−2
3.61 × 10−3
3.59 × 10−3
4.16 × 10−4
0.9986
0.9998
0.9995
0.9973
0.9972
0.9967
I
Cl
CN
Table II pKa for Anilinium Ions in Dimethyl Sulfoxide Solution and Oxidation Potentials, Eo, Values of the
4-X-Substituted Anilines 2a–g
Aniline (X)
2a (OH)
2b (OCH3)
2c (CH3)
2d (H)
2e (I)
2f (Cl)
2g (CN)
pK
E◦ a(V)c
5.15a
0.76
4.75a
0.79
4.34b
0.92
3.82b
1.02
3.02a
1.02
2.79b
1.01
1.22a
–
a
Values were determined by extrapolation using a relationship between pKa and Hammett σgaluesg (similar pKa vs. σ correlations are well
documented in other systems [11] p.
bpKa values of anilines are taken from [10].
cIn aqueous solution, from [29].
International Journal of Chemical Kinetics DOI 10.1002/kin.20751

156
JMAOUI, BOUBAKER, AND GOUMONT
1
OH
0
-1
-2
-3
OCH3
β
= 1.60
nuc
CH3
β
nuc = 0.56
H
I
Cl
CN
-4
0
1
2
3
4
5
6
pKa
Figure 2 Brønsted-type plot for the reactions of 4-chloro-7-nitrobenzofurazan 1 with 4-X-substituted anilines 2a–g in Me2SO
at 25◦C.
However, nonlinear Brønsted-type plots have been re-
ported for many nucleophilic substitution reactions
[12–18]. Koh and coworkers, for example, have ob-
served a curved Bro¨nsted-type plot with βnuc decreas-
ing from 0.71 0.03 to 0.21 0.06 as the pyridine
basicity increases for the reaction of methyl chlorofor-
mate with X-pyridines in acetonitrile, at 25◦C [13b].
Um and coworkers investigated the reactions of O-4-
nitrophenyl X-substituted thionobenzoates with substi-
tuted X-pyridines in 20 mol% Me2SO at 25◦C. Curved
Brønsted plots are obtained with a change in slope
from a large value (βx = 1.38 − 1.41) to a small value
(βx = 0.36 − 0.39) at pKa = 9.30, corresponding to
the unsubstituted pyridine [15a].
The coefficient of Brønsted βnuc (0.56), obtained for
this series of anilines 2e–g, is very close to those ob-
tained by several authors in dimethyl sulfoxide [3,19–
21]. Based on the fact that the magnitude of βnuc values
has been suggested to represent the degree of electron
transfer at the rate-determining transition state [3,19–
24], the measured βnuc value of 0.56 (e.g., for reactions
with electron-withdrawing substituents) suggests that
at the transition state the degree of carbon–nitrogen
bond formation is approximately half complete in the
transition state and falling in the typical domain of
polar nucleophilic aromatic substitutions mechanism
(pathway A, Scheme 1) in which the formation of the
zwitterionic σ-adduct (ZH ) is the rate-determining
step with fast decomposition of this intermediate into
the substituted product 4.
However, the βnuc value of 1.60 (for reactions with
electron-donating substituents) is greater than unity,
definitively larger than value observed in the range of
polar SNAr reactions (from 0.50 to 0.70) [2–8]. It has
to be noted that βnuc values close to or greater than 1
have been observed for nucleophilic aromatic substi-
tutions reactions, for example, βnuc = 1.43 and 1.30
for a reaction of 3-methyl-1-(4-nitrobenzofurazanyl)-
imidazolium ions with a series of 4-X-substituted
anilines (X = NH2, OCH3, CH3, and H) in H2O-
Me2SO mixtures containing 30% and 80% Me2SO,
respectively [1]. βnuc = 0.95 for the reaction of 2-(4-
nitrophenoxy)-4,6-dimethoxy-1,3,5-triazine with phe-
nolate ions in aqueous solution has also been reported
[25].
Terrier and coworkers have recently discussed the
originof the abnormallyhigh βnuc values and suggested
that these values are the reflection of a possible con-
tribution of a SET process [1]. On the other hand, it is
well known that σ-complexation and SNAr processes,
involving strongly electron-deficient aromatic or het-
eroaromatic substrates, may proceed via the electron-
transfer mechanism [3, 26–28].
Accordingly, the larger βnuc value (1.60) obtained
in the present work for electron-donor X-substituents
(X = 4-OH, 4-OCH3, and 4-CH3) suggests a SET
mechanism (pathway B, Scheme 1) in which the forma-
tion of the substitution products 4a–c will then proceed
through initial (fast) electron transfer from the ani-
line donor 2a–c to the 4-chloro-7-nitrobenzofurazan 1
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KINETICS OF REACTIONS OF 4-CHLORO-7-NITROBENZOFURAZAN WITH ANILINES
157
2
1
OH
OCH3
0
CH
3
–1
H
–2
Cl I
–3
–4
0.7
0.75
0.8
0.85
0.9
0.95
1
1.05
1.1
E o, V
Figure 3 Influence of the oxidation potential, Eo, of anilines on the rate constants of reactions of 4-chloro-7-nitrobenzofurazan
1 with anilines 2a–f in Me2SO at 25◦C. The identity of points is given in Tables I and II.
acceptor and subsequent (slow) coupling of the result-
ing cation and anion radicals within the solvent cage
via the transition state 3.
Substituent Effects and Origin of the
Nonlinear Hammett Correlation
The susceptibility of the reaction of 4-chloro-7-
nitrobenzofurazan 1 with para-X-substituted anilines
2a–g can be highlighted by the study of the slopes of
the Hammett plots (log k1 vs. σ) [30]. In fact, the effect
of substituent X on reactivity has been graphically il-
lustrated in Fig. 4. As can be seen, the Hammett plot is
nonlinear but consists of two intersecting straight lines,
with a negative slope, ρ = −5.75 when σ < 0 and with
a larger negative slope, ρ = −1.97 when σ > 0. We
note that the kinetics of the same reactions performed
in 20–80 and 70–30 (v/v) H2O-Me2SO mixtures have
been reported by Terrier and coworkers, and in this
latter case linear Hammett plots are obtained [1].
The magnitudes of ρ obtained in this present system
are clearly consistent with the development of a posi-
tive charge at the nitrogen atom of the aniline moiety
in the transition state for formation of the zwitteri-
onic intermediate ZH and appears to be highly de-
pendent on the nature of the substituent X. In fact, the
transmission of electronic effects is more significant
for the reactions with electron-donating X-substituents
(ρ = −5.75) than for electron-acceptor X-substituents
(ρ = −1.97), clearly indicating a mechanistic change
at σ ≈ 0.
To support the above argument of a system pro-
ceeding through a SET mechanism, the measured
rate constants, k1, for the reactions of 4-chloro-7-
nitrobenzofurazan 1 with anilines 2 have been plot-
ted against the oxidation potential E◦ values of the
4-substituted aniline radical cations, as measured by
Jonsson and coworkers in aqueous solution [29] (see
Table II). As seen from Fig. 3, the plot of log k1 versus
E◦ is quite good and the corresponding relationship is
log k1 = 5.76 − 7.51 E◦ (R2 = 0.9935) (4)
The points for 4-iodoaniline 2e and 4-chloroaniline
2f, deviating from the linear plot, were not included
in the determination of Eq. (4). This is a very sig-
nificant result because this result is consistent with
the preceding argument that the reaction of 2e and
2f proceeds through a different mechanism, for ex-
ample, through a polar SNAr mechanism. The same
conclusion has been drawn from reactions of 4-chloro-
7-nitrobenzofuranzan 1 with a series of 4-X-substituted
in 30–70 (v/v) and 20–80 (v/v) H2O-Me2SO mixtures
at 25◦C [1].
Thus, it can reasonably be concluded that the lin-
ear log k1 versus Eo plot associated with reactions of
4-chloro-7-nitrobenzofurazan 1 with 4-X-substituted
anilines 2a–d is more consistent with a SET than for a
polar mechanism.
The ρ value of −1.97 shown in Fig. 4 is comparable
to that reported for reactions that have been suggested
to proceed through a polar nucleophilic aromatic
substitution mechanism, for example, ρ = −1.84
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JMAOUI, BOUBAKER, AND GOUMONT
1
OH
0
OCH
ρ = -5.75
3
–1
CH
3
–2
–3
–4
ρ = -1.97
H
I
Cl
CN
–0.6
–0.4
–0.2
0
0.2
0.4
0.6
0.8
σ
Figure 4 Hammett plot for the reactions of 4-chloro-7-nitrobenzofurazan 1 with 4-X-substituted anilines 2a–g in dimethyl
sulfoxide at 25◦C. The identity of points is given in Table I.
for a reaction of 2,6-bis(trifluoromethanesulfonyl)-
4-nitroanisole with anilines in acetonitrile [4] and
ρ = −1.70 for a reaction of 2-chloro-5-nitropyridine
with various arenethiolates in methanol [31]. How-
ever, the ρ = −5.75 value obtained with anilines
2a–c appears to be a huge value for reactions
that proceed through a polar nucleophilic aromatic
substitution mechanism. Nevertheless, a similar ρ
value has also been reported (ρ = −5.27) by Ter-
rier and coworkers for substitution of 3-methyl-1-(4-
nitrobenzofurazanyl)-imidazonium ions with a series
of 4-X-substituted anilines (X = NH2, OCH3, CH3
and H) in the 20–80 (v/v) H2O-Me2SO mixture [1].
It is interesting to note that our value of ρ = −5.75
obtained in Me2SO is more negative than those mea-
sured for the same reaction in the H2O-Me2SO mix-
ture: ρ = −3.82 and ρ = −2.68 in 80% and 30%
Me2SO, respectively [1]. This result agrees with the
fact that the reaction must proceed through a dipolar
transition state. On the other hand, the decrease in the
ρ value from the H2O-Me2SO mixture to pure Me2SO
provides evidence that the solvation by H2O through
hydrogen bonding reduces the positive charge on the
nitrogen atom in the transition state [32].
of a change in the rate-determining step from break-
down to formation of a tetrahedral intermediate [33a].
Recently, Um and coworkers have reported nonlinear
Hammett plots for nucleophilic substitution reactions
of o-4-nitrophenyl X-substituted thionobenzoates with
benzylamine in 80% H2O-20% Me2SO at 25◦C, which
is interpreted as a stabilization of the groundstate of the
substrate through a resonance interaction between the
electron-donating substituent X and the thionocarbonyl
moiety [34a].
The nonlinear Hammett plot shown in Fig. 4 has
been analyzed using a Yukawa–Tsuno equation (Eq.
(5)) in which r and (σ+ − σ) represent the relative
extended value of the resonance contribution and the
resonance substituent constant measuring the capac-
ity for π-delocalization of a given π-electron donor
substituent, respectively [35].
log k1X/k1H = ρ(σ + r(σ+ − σ))
(5)
As can be seen, the Yukawa–Tsuno plot (Fig. 5) for
the reactions of 4-chloro-7-nitrobenzofurazan 1 with
anilines 2a–g exhibits a straight line (R2 = 0.9930)
with ρ = −2.19 and r = 1.05. Such a linear plot
clearly indicates that the mesomeric stabilization is
responsible for the nonlinear Hammett plot shown in
Fig. 4. These results suggest that the structure of the
aniline moiety in the transition state +sh•ould be closely
Deviations from Hammett plots based on standard
σgalues have been rarely reported in SNAr reactions,
but have been commonly observed in the SN2 pro-
cess [33, 34]. Koh and coworkers, for example, in-
vestigated the reactions of substituted phenacyl bro-
mides with pyridines in acetonitrile at 45◦C and the
curved Hammett plots have been interpreted in terms
related to the radical cation (XC6H4 H ).
N
2
It has to be noted that radical cations R1R2 +•H,
N
have also been obtained in the reactions of picryl
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KINETICS OF REACTIONS OF 4-CHLORO-7-NITROBENZOFURAZAN WITH ANILINES
159
3
2
1
OH
OCH3
0
CH3
ρ = –2.19
–1
r = 1.05
R2 = 0.9930
H
I
Cl
–2
–3
CN
–1.5
–1
–0.5
0
0.5
1
σ + r (σ+ – σ)
Figure 5 Yukawa–Tsuno correlation for reactions of 4-chloro-7-nitrobenzofurazan 1 with anilines 2a–g in Me2SO at 25◦C.
The σ and σ+ values are taken from [30].
chloride with primary and secondary amines in
dimethyl sulfoxide [20].
may be an indicator of electron transfer in nucleophilic
aromatic substitution reactions.
Thus, the results of the nonlinear Hammett plot in
Fig. 4 reinforce the conclusions drawn above on the
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