Kinetics of the reaction of 2-chloro-3,5-dinitrobenzotriflouride with aniline in toluene and methanol-toluene mixed solvents

Article abstract of DOI:10.1002/cjoc.201180467

Kinetics of the reaction of 2-chloro-3,5-dinitrobenzotriflouride with aniline were studied in toluene, methanol-toluene binary solvents, benzene and chloroform. The reaction in toluene exhibits third-order kinetics consistent with aggregates of aniline. Thermodynamic parameters ΔH^#, ΔS^# and ΔG^#are calculated and discussed for the reaction of 2-chloro-3,5-dinitrobenzotriflouride with aniline in methanol-toluene. Molecular complexes between aniline and the substrate are rejected spectrophotometricaly. The mechanism is studied and compared with the reaction in presence of pyridine. It shows an amine dependence and formation of homo and/or hetero mixed aggregates between aniline and pyridine i.e. dimer mechanism. Copyright

Full text of DOI:10.1002/cjoc.201180467

FULL PAPER
DOI: 10.1002/cjoc.201180467
Kinetics of the Reaction of 2-Chloro-3,5-dinitrobenzotriflouride
with Aniline in Toluene and Methanol-Toluene Mixed Solvents
Fathalla, Magda F.
Chemistry Department, Faculty of Science, Alexandria University, P.O. 426 Ibrahimia, Alexandria 21321, Egypt
Kinetics of the reaction of 2-chloro-3,5-dinitrobenzotriflouride with aniline were studied in toluene, metha-
nol-toluene binary solvents, benzene and chloroform. The reaction in toluene exhibits third-order kinetics consistent
with aggregates of aniline. Thermodynamic parameters ΔH#, ΔS# and ΔG# are calculated and discussed for the
reaction of 2-chloro-3,5-dinitrobenzotriflouride with aniline in methanol-toluene. Molecular complexes between
aniline and the substrate are rejected spectrophotometricaly. The mechanism is studied and compared with the reac-
tion in presence of pyridine. It shows an amine dependence and formation of homo and/or hetero mixed aggregates
between aniline and pyridine i.e. dimer mechanism.
Keywords kinetics, 2-chloro-3,5-dinitrobenzotriflouride, methanol-toluene
Introduction
and the determination of the kinetic data in binary mix-
tures.[17] In continuation to the previous work carried
out in our laboratory in the field of aromatic nucleo-
philic substitutions,[19-27] the present work investigates
the solvent effect on the reaction of 2-chloro-3,5-dinitro-
benzotriflouride with aniline in pure toluene, benzene,
chloroform and in mixed toluene-methanol. Further, the
effect of external base such as pyridine is examined.
2-Chloro-3,5-dinitrobenzotrifluoride, in vitro micro-
tubule inhibitor of several Leishmania species, has been
synthesized from 2-halo-5-(trifluoromethyl)-benzene-
sulfonyl chlorides.[1] The analogues exhibited moderate
to excellent activity when tested against Leishmania
donovani amastigotes.[1-3] Fluorine-containing aromatics
have been incorporated into drugs (hypnotics,
tranquilizers, anti-inflammatory agents and analgesics,
antibacterial) and into crop-protection chemicals
(herbicides, insecticides, fungicides).[4]
Experimental
Reagent and solvent
Aryl halides are common substrates to react with
nucleophilic aromatic substitution. In recent years, the
photo dissociation dynamics of aryl halides has been a
subject of intensive studies, which is closely related to
the atmospheric chemistry.[5]
Many aromatic nucleophilic substititution reactions,
SNAr, have been subject to general base catalysis.[6,7]
The effect of basicity of nucleophile on this reaction has
recently been shown by the dependence of the rate on
structurally related nucleophiles.[8] Pervious kinetic
studies on aromatic nucleophilic substitutions SNAr of
dinitro substituted benzene in aprotic solvent[9-11] have
reported the existence of molecular complexes such as
amine dimer, substrate-nucleophile or substrate-product
complexes and the possible formation of mixed aggre-
gates aniline-HBA additive (solvent as hydrogen-bond
acceptor).[12-15]
2-Chloro-3,5-dinitrobenzotriflouride was purest
available material (Sigma-Aldrich D 89555 Steinheim),
methanol (MeOH), toluene (Tol), benzene and pyridine
were purified as reported previously.[28] Aniline and
pyridine (BDH) were kept overnight over KOH and
then distilled (b.p. 184 and 115 ℃, respectively). Cau-
tion: 2-chloro-3,5-dinitrobenzotriflouride: irritating to
eyes, respiratory system and to skin. In case of contact
with eyes, rinse immediately with plenty of water and
seek medical. Also suitable gloves, eye face protection
should be used
General procedures
A methanol solution of 1 (1.8 mmol, 5 mL methanol)
was treated with the aniline (18 mmol, 5 mL methanol).
The reaction was refluxed for approximately 1 h, and
the solid precipitate was filtered and crystallized from
benzene-petroleum ether to give N-(2,4-dinitro-
6-trifluoromethylphenyl)aniline, which was reported
previously.[19]
Most studies of the solvent effects have been per-
formed in pure solvents.[16,17] Nevertheless, different
studies aimed at the characterization of mixed solvents
*
E-mail: mffathalla@hotmail.com
Received February 15, 2011; accepted September 1, 2011.
Chin. J. Chem. 2012, 30, 109—114
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
109

Fathalla
FULL PAPER
Kinetic measurements
base-general acid to the final product (SB-GA). It has
been widely observed in catalysed SNAr, the deprotona-
tion of the zwitterionic intermediate is the rate deter-
mine step.[22] The overall reaction comes from the con-
ventional addition-elimination mechanism which is
given by Eq. 1.[29,30]
Kinetics of 2-chloro-3,5-dinitrobenzotriflouride, 1
－
1
(4×10－ mol•L ) with different concentrations of ani-
line in toluene, benzene chloroform and methanol-
toluene mixed solvent at 40 ℃ were measured spectro-
photometrically using a Shimadzu 160 A spectrophoto-
meter in conjunction with thermo-bath (TB-85) tem-
perature control (±0.2 ℃). The reaction in presence of
methanol-toluene was measured at different tempera-
tures ranging from 25 to 45 ℃. The kinetic runs were
performed by following the appearance of the product at
λ＝370 nm. In all cases the pseudo first order values, kΨ
were calculated from the slope of the plot of ln(A∞－At)
vs. time which exhibited straight line over nearly 80%
of the reaction, where At and A∞ are the optical density
of the reaction mixture at time t and at the end of the
reaction.
4
kk +kk [Am]
1 2 1 3
kA＝
(1)
k－1+k2 +k3[Am]
where, Am＝PhNH2, kA represents the apparent second
order rate constant obtained by dividing the pseudo-first
order constant, kψ, by aniline concentration. Eq. 1 de-
scribes generally a dependence of kA on [Aniline],
which is linear at low concentration but changes to a
plateau as the base concentration is increased. In fact at
low [Am] value k－ >>(k2＋k3[Am]), then, kA＝Kk2＋
1
Kk3[Am] where K＝k1/k－ .[30-33] In contrast, when (k2＋
1
k3[Am]>>k－ , Eq. 1 reduces to kA＝k1 and the reaction
is not base catalyzed.
1
Results and Discussion
Effect of aniline concentrations
Table 1 Reaction of 1 with aniline in toluene, pseudo-first (kΨ)
and second kA order rate constant at 40 ℃
The rate of the reaction of 1 with aniline in toluene
was measured using different aniline concentrations
－
1
－
[Aniline]/(mol•L )
－
－
1
104kΨ/s－
0.24
0.72
1.32
2.00
2.46
3.93
501
104kA/(L•mol •s )
1
1
1
ranging from 0.5 to 3 mol•L . The pseudo first order
rate constant, kΨ values are given in Table 1. kA values
calculated from the kΨ/[Am] were found to increase
rapidly with increasing aniline concentrations. Actually
the kA values are dependent of the aniline concentrations.
Figure 1 indicated that the mechanism is suggested to
proceed through either, (i) specific base [Scheme 1,
pathway a or c] or (ii) a dimer process [Scheme 1,
pathway b]. The proposed mechanism does not preclude
attack by the monomer which straight forwardly would
form intermediate S (Am), which undergoes specific
0.5
1.0
1.4
1.8
2.0
2.5
2.8
3.0
0.48
0.72
0.94
1.11
1.23
1.57
1.79
2.02
6.06
Scheme 1
Am
O2N
NO2
Cl
O2N
NO2
+
NH2Ph
Cl
k1
O2N
NO2
-
+
ka
PhNH2
k-1
+ AmHCl
fast
CF3
NHPh
pathway a
CF3
CF3
O2N
NO2
Cl
k3 [Am]
O2N
NO2
+
NHPh
O2N
NO2
CF3
k2
-
+ AmHCl
NHPh
kb
PhNH2 + PhNH2
PhNH2
NH2Ph
Cl
k-2
CF3
CF3
H
NH2Ph
pathway b
O2N
NO2
Cl
O2N
NO2
O2N
-
NO2
+
NHPh
Am (P)
+ PHCl
+ AmHCl
CF3
PhNH2-P
NHPh
PhNH2 + P
kc
Cl
CF3
pathway c
pathway d
CF3
H
P
110
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, 30, 109—114

Kinetics of the Reaction of 2-Chloro-3,5-dinitrobenzotriflouride with Aniline
showed linear behaviour for kA, similar to that observed
for kA values for the reaction of 1 with different pyridine
concentrations at fixed aniline concentration. Accord-
ingly a dimer mechanism is suggested involving the
formation of homo and hetero aggregate between ani-
line-aniline and aniline-pyridine, respectively (Scheme
1, pathway d).
－
－
1
1
Table 2 Second order rate constant [104 kA/(L•mol •s )] when
1 reacted with aniline in presence of pyridine in toluene at 40 ℃
－
[Aniline]/(mol•L )
1
－
[Pyridine]/(mol•L )
1
0.5
1.0
2.0
1.23
5.23
6.60
7.75
9.76
11.56
0
0.47
1.50
1.80
2.50
3.30
3.90
0.73
3.15
3.40
4.01
5.10
6.23
Figure 1 Second order rate constant, kA for the reaction of 1
with aniline in toluene at 40 ℃ as a function of [Aniline].
0.1
0.2
0.3
0.4
0.5
It is of interest to examine the possibility of the for-
mation of EDA complexes because aniline is known as
better donor amines.[34] The absence of formation of the
molecular complex between the substrate and aniline
rejects this mechanism. This is because there is no ab-
sorbance of the adduct complex at λ＝370 nm (at zero
time) for the reaction carried out at several aniline con-
centrations up to 3 mol•L . Other alternative mecha-
nism of the title reaction is presumably proceeding
through a dimer process (Scheme 1). In pathway b,
(PhNH2•PhNH2) attacks the substrate forming a zwitter-
ionic intermediate involving the substrate and two
amine molecules. The overall rate expression for the
dimer mechanism is given by the application of the
steady state approximation on all intermediates involved
in Scheme 1, pathway b (Eq. 2).
The overall reaction picture involves the monomer,
the dimer and mixed aggregate thus the rate constant is
represented by Eq. 3. Pathway a represents the uncata-
lyzed reaction, it is first order in amine and its attack is
rate limiting. When the rate constants show a parabolic
dependence on the amine concentration, it can be ex-
plained by a dimer mechanism[22] (pathway b). Pathway
c represents the third order by assistance of external
base.
－
1
kA a[Aniline] b[Aniline]2 c[Aniline][Pyridine]
＝
＋
＋
d[pyridine]
(3)
＋
k1k2K [Am] + k3k1K[Am]2
The four constants a, b, c and d could be determined
by linear regression which is expressed by Eq. 4.
kA ＝
(2)
k－1 + k2 +k3 [Am]
kA (0.76 1.4)[Aniline] (0.144 0.5)[Aniline]2
＋
＝
±
＋
±
The dimer mechanism can be confirmed by many
features such as: (a) the dimer amines are more reactive
than amine monomer because of its higher basicity, (b)
abnormal solvent effect such as effects of HBD (solvent
as hydrogen bond donor) and HBA (solvent as hydrogen
bond acceptor) additives. In this report the non-polar
character of toluene (π*＝0.54) possibly aggregates the
aniline and thus a dimer mechanism is possible.[13]
Accordingly the mechanism for the reaction of 1
with aniline in toluene can proceed either by dimer
mechanism pathway b or as in Scheme 1 (pathway b
and c), in which kA is linearly at low aniline concentra-
tion and shows a plateau at high aniline concentrations.
(8.23±1.1)[Aniline][Pyridine]＋(2.02±1.5)×
[pyridine] (n 18, r 0.988) (4)
＝
＝
The coefficients of Eq. 4 show that the contribution
of [Aniline][Pyridine] is the highest value indicating
that the pathway involving hetero aggregates is pre-
dominant. This is presumably because aggregation by
stronger pyridine (pKa＝5.2) is more important than that
by aniline (pKa＝4.6) i.e. the mixed pyridine-aniline
(c ＝ 8.23±1.1) is more important than the
self-associated aniline molecules (b＝0.144±0.5).
Solvent effect
Effect of pyridine additive
The effect of solvent on the reaction of 1 with ani-
line was examined in benzene and chloroform (Table 3).
It is of interest to note that the reaction in benzene and
chloroform shows dependence of kA on [Aniline]. This
means that the overall order of the reaction in toluene is
third order. Accordingly the following order was ob-
served, kA(CHCl3) ＞ kA(toluene) ＞ kA(benzene). This
The reaction of 1 with aniline in toluene was studied
in presence of pyridine as hydrogen bond acceptor
catalyst (HBA) which is intrinsically involved in dimer
mechanism.[35] The second order rate constants, kA for
the reaction in presence of fixed concentration of pyri-
dine and different aniline concentrations (Table 2),
Chin. J. Chem. 2012, 30, 109—114
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111

Fathalla
FULL PAPER
－
－
1
1
order agrees with the polarity of the solvent [ε(CHCl3)
＝4.8; ε(benzene)＝2.3; ε(toluene)＝2.0] and is in har-
mony with the ET values [preferential solvation,
ET(CHCl3)＝39.1; ET(benzene)＝34.5, ET(toluene)＝
Table 4 Second order rate constant, 103kA/(L•mol •s ) when
－
1
1 reacted with aniline (1.8 mol•L ) in methanol-toluene binary
solvents at 40 ℃
w(MeOH)
0
φ(MeOH)/% ET
104kΨ
103kA
0.11
0.51
0.77
1.17
1.34
1.49
1.58
1.85
1.95
2.62
33.9] and ETNN values [ET (CHCl3)＝0.259; ET (benzene)
＝0.111; ET (toluene)＝0.099].[36] This points out that
the mechanism in benzene and chloroform can proceed
by a dimer mechanism in toluene.
N
N
0
10
20
30
40
50
60
70
80
100
33.9
47.9
49.4
—
2.00
0.226
0.396
0.529
0.636
0.724
0.798
0.860
0.913
1.0
9.23
13.83
21.00
24.17
26.93
28.53
33.32
35.10
47.15
－
－
1
Table 3 Second order rate constant 104kA/(L•mol •s ) for the
reaction of 1 with aniline in different solvents at 40 ℃
1
51.2
—
－
[Aniline]/(mol•L )
1
Benzene
0.23
Chloroform
1.63
52.4
—
0.5
1.0
1.5
1.8
2.0
2.5
3.0
0.32
2.05
53.1
55.5
0.43
2.31
0.50
2.66
0.58
2.91
0.73
3.25
0.79
—
This concept directs the investigation of the reaction
to study the effects of changing polarities of the medium,
e.g. addition of methanol to toluene as well as the tem-
peratures.
Effect of methanol addition
The effect of progressive addition of various
amounts of methanol to toluene (0—100% φmethanol) was
－
1
studied at constant aniline concentrations (1.8 mol•L )
at 40 ℃. Table 4 collects the values of kA for the reac-
tion of 1 with aniline in methanol-toluene binary sol-
vents, and then the change in kA should increase more
than linear with increasing MeOH. Solvent effects on
the reaction rate constants are generally correlated with
one of the solvent parameters that represent polarity,
hydrogen bonding and other properties. The only sol-
vent polarity parameter derived from measurements of
solvatochromic effects such as ET (Table 4). To insure
these explanations several correlations were made such
as (i) plot of lg kA versus mole fractions of methanol
(not shown) to indicate what is the preferential solvent,
(ii) a plot of lg kA against ET values[30] and 1/ε [ε, di-
electric constant of the medium (not shown)] for various
methanol-toluene mixed solvents to decide which effect
controls the specific or non specific solvation. Table 4
shows an increase in rate upon going from methanol to
toluene suggesting that adding methanol is probably
associated with the hydrogen bond of the medium (spe-
cific solvation). It is found that non-linear relations exist
between lg kA values and ET (preferential solvation)
(Figure 2). Therefore, this non-linear correlation is due
to specific solvation effect of the methanol-toluene
mixed solvent. According to the discussion with respect
to dimer mechanism the reactant-toluene are not reac-
tive whereas the addition of small amount methanol
affects the reaction than aniline-aniline aggregates.[37]
Figure 2 Second order rate constant for the reaction of 1 with
1.8 mol•L－ aniline in toluene-methanol as a function of ET val-
1
ues at 40 ℃.
These observations could be due to the interaction of
methanol with (i) the non-polar aprotic solvent, (ii) the
substrate, (iii) intermediates through the reaction path-
way or (iv) the amine.
Addition of methanol to non-polar aprotic solvent
should increase the dielectric constant of the medium,
thereby increasing the rate of SNAr reactions due to the
extra stabilization of the intermediate first formed from
the reaction of the amine with substrate.
Complex formation between the substrate on addi-
tion of methanol could be a probable factor. Observa-
tions have shown, however, that a particular substrate in
non-polar aprotic medium such as toluene could display
rate acceleration in its reaction with one amine and a
continuous rate diminish in its reaction with another
amine both in progressive addition of methanol. For
example, the reaction of 1 with aniline (present study)
showed
a
rate increase similar to phenyl
2,4,6-trinitrophenyl ether with the same amine.[34] Also
the reaction of 2,4-dinitrofluorobenzene with
piperidine[35] in benzene displayed a rate increase with
added methanol while the reaction of the same substrate
produced the opposite effect in its reaction with cis and
trans 1,2-dinitro cyclohexylamine.[38] It can therefore be
112
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Chin. J. Chem. 2012, 30, 109—114

Kinetics of the Reaction of 2-Chloro-3,5-dinitrobenzotriflouride with Aniline
reasonably assumed that a methanol-substrate interac-
tion is not responsible for the observed increase in rate
in present case.
Interaction of added methanol with zwitterionic in-
termediate first formed would assist the simultaneous
abstraction of a proton and expulsion of the leaving
group through hydrogen bonding in the transition state
which in turn increase the rate of reaction.
The formation of aggregate via hydrogen-bonding
between aniline and hydrogen-bond donor methanol has
been widely studied.[39-41] The methanol molecule acts
as a proton donor to amine resulting in the formation of
an aggregate as shown in Eq. 5.[18]
equation, ln kA/T＝aa＋bb(1/T), where aa＝ln(kB/h)＋
ΔS/R and bb＝－(ΔH/R). The computed values of the
thermodynamic parameters ΔH#, ΔS# and ΔG# show that
both ΔH# and ΔS# curves pass through a maximum or
minimum at x≈0.396 and another at 0.860. While the
ΔH# and ΔS# did not varied with mole fraction of sol-
vent. Figure 3 is a criterion of specific solvation. This
behaviour can be visualized in light of change of
methanol structure in presence of toluene. Furthermore
the highly negative values of ΔS# for the entire range of
solvent composition support the formation of strongly
ordered transition state. Moreover, the small change of
ΔG# with the change of composition of MeOH is attrib-
uted to the compensation between ΔH# and ΔS#. The
plot of ΔH# versus ΔS# gives straight line (r＝0.99)
where the slope β (isokinetic temperature) value was
214 K which is lower than experimental temperature,
suggesting that the titled reactions are entropic control.
ROH RNH ROH NH R
(5)
＋
→
…
2
2
The nitrogen atom, having thus used its lone pair of
electrons partially for hydrogen-bond formation be-
comes less nucleophilic compared with the free amine.
The amine-methanol aggregate of reduced nucleophilic-
ity can either react with the substrate in the first step of
the SNAr reaction or the catalyzing entity in the decom-
position of the zwitherionic intermediate complex in the
second step. The second assumption is obviously more
likely, since a catalysed reaction in the present study
displayed rate increase on addition of methanol to tolu-
ene. It is therefore proposed that, the mechanism for the
reaction of 1 with aniline on progressive addition of
methanol to toluene medium is presumably becoming
specific base or specific base-general acid mechanism[29]
than dimer one suggested in the aprotic toluene.
Figure 3 Variation of activation parameters with mole fraction
－
#
－
1
of methanol at 25 ℃. ♦ ΔG# in kJ•mol , ● ΔH in kJ•mol ,
1
－
－
1
and ▲ －ΔS# in J•mol •K .
1
Effect of temperature
The kA values for the reaction of 1 with aniline in
methanol-toluene binary mixtures (0—100%, φ) carried
out at different temperatures ranging from 25 to 45 ℃
(interval temperature is 5 ℃) are listed in Table 5.
Thermodynamic parameters were calculated by least
square fit using the equation ln kA＝a＋b(1/T), where,
a＝ln A and b*＝－E/R and least square fit using the
The base catalysis has previously been reported for a
few reactions of dinitrochlorobenzene in non polar
aprotic solvents.[42-46] This explanation agrees with
Scheme 1, pathway a and b, where the proposed reac-
tion route is a reversible nucleophilic attack by the
amine dimer followed by the base-catalyzed transfor-
mation of intermediate to product.
－
－
－
1
1
Table 5 Second order rate constant 104kA/(L•mol •s ) of the reaction of 1 with aniline (1.8 mol•L 1) in binary methanol-toluene mix-
tures at different temperatures, and activation parameters at 25 ℃
Temperature/℃
#
#
#
－
－
－
－
1
1
1
1
w(MeOH) φ(MeOH)/%
ΔG /(kJ•mol ) ΔH /(kJ•mol ) －ΔS /(J•mol •K )
25
35
40
45
0.226
0.396
0.529
0.636
0.724
0.798
0.860
0.913
1.0
10
20
30
40
50
60
70
80
100
2.10
4.43
3.83
6.41
5.10
7.64
6.73
9.05
93.5±2.1
92.2±0.1
91.6±3.1
90.9±3.8
90.7±3.7
90.4±4.0
90.1±4.8
90.0±1.6
89.8±6.1
41.0±1.1
25.6±0.1
35.4±1.6
28.1±1.9
29.8±1.9
27.8±2.1
26.1±2.4
30.2±0.8
40.0±3.1
176.3±3.5
233.1±0.1
188.4±5.1
213.2±6.3
204.3±6.1
210.1±6.6
214.1±8.0
200.4±2.6
167.1±10.1
5.40
8.84
11.67
13.43
14.96
15.85
18.51
19.50
26.19
13.85
15.42
18.31
19.41
20.09
24.44
33.76
7.23
10.33
11.61
12.52
14.68
16.31
18.29
8.10
9.00
10.00
10.56
11.60
Chin. J. Chem. 2012, 30, 109—114
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
113

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FULL PAPER
[16] Harati, M.; Gholami, M. R. Int. J. Chem. Kinet. 2005, 37, 90.
[17] Headley, A. D.; Strarness, S. D.; Cheung, E. T.; Malone, P. L. J.
Phys. Org. Chem. 1995, 8, 26.
Conclusions
In this work kinetics of 2-chloro-3,5-dinitrobenzo-
triflourid, 1 with aniline of different concentrations were
investigated spectrophotometrically in toluene. The re-
action in aprotic solvents exhibits a third-order kinetics.
The effect of pyridine was studied. The reaction is ex-
plained by a dimer mechanism which involves the for-
mation of homo and hetero aggregate between ani-
line-aniline and aniline-pyridine respectively (Scheme
1). The effect of solvent on the reaction of 1 with aniline
in toluene, benzene and chloroform follows the order of
kA(CHCl3)＞kA(Toluene)＞kA(Benzene). The effect of
adding amounts of methanol to toluene till 100%
methanol is studied at constant aniline concentrations.
Thermodynamic parameters were calculated in case of
methanol-toluene binary solvent at different tempera-
tures showing the non linear variation of ΔH# and ΔS#
with mole fraction of solvent. This behaviour can be
visualized in light of change of methanol structure in
presence of toluene.
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