Solvent and substituent effects on the reaction of 2- and 4-chloro-3,5-dinitrobenzotrifluorides with substituted anilines

Article abstract of DOI:10.1002/kin.20455

The solvent effect on a nucleophilic substitution reaction of 2- and 4-chloro-3,5- dinitrobenzotrifluoride with substituted anilines was studied in methanol, acetonitrile, and toluene at 25°C. This reaction is of second order, except 2-chloro-3,5-dinitrobenzotrifluoride in toluene shows third order. The k_A values are found to be dependent on the substituent in aniline and give good Hammett correlations. The obtained ρ values are -4.07 and -4.62, for the reaction of anilines with 2-chloro-3,5- dinitrobenzotrifluoride in methanol and acetonitrile, respectively. The ρ values for the reaction of the anilines with 4-chloro-3,5- dinitrobenzotrifluoride are-3.38,-4.11, and-4.34 in methanol, acetonitrile, and toluene, respectively. The reaction of the former compound with anilines in toluene shows a second order in aniline. The dependence of the reaction on the external base such as DABCO suggests a proton transfer controlling step.

Full text of DOI:10.1002/kin.20455

Solvent and Substituent
Effects on the Reaction of
2- and 4-Chloro-3,5-
dinitrobenzotrifluorides
with Substituted Anilines
BASIM H. M. ASGHAR,1 MAGDA F. FATHALLA,2 EZZAT A. HAMED2
1Chemistry Department, Faculty of Applied Science, Umm Al-Qura University, Makkah, Kingdom of Saudi Arabia
2Chemistry Department, Faculty of Science, Alexandria University, P.O. 426 Ibrahimia, Alexandria 21321, Egypt
Received 19 January 2009; revised 14 March 2009, 14 April 2009, 17 June 2009; accepted 18 June 2009
DOI 10.1002/kin.20455
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The solvent effect on a nucleophilic substitution reaction of 2- and 4-chloro-3,5-
dinitrobenzotrifluoride with substituted anilines was studied in methanol, acetonitrile, and
toluene at 25◦C. This reaction is of second order, except 2-chloro-3,5-dinitrobenzotrifluoride in
toluene shows third order. The kA values are found to be dependent on the substituent in ani-
line and give good Hammett correlations. The obtained ρ values are −4.07 and −4.62, for the
reaction of anilines with 2-chloro-3,5-dinitrobenzotrifluoride in methanol and acetonitrile, re-
spectively. The ρ values for the reaction of the anilines with 4-chloro-3,5-dinitrobenzotrifluoride
are −3.38, −4.11, and −4.34 in methanol, acetonitrile, and toluene, respectively. The reaction of
the former compound with anilines in toluene shows a second order in aniline. The dependence
of the reaction on the external base such as DABCO suggests a proton transfer controlling step.
ꢀ
C
2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 777–786, 2009
aromatic ring substituents, amine, as well as the nature
of the solvent on the reactivity and mechanism of nu-
cleophilic aromatic substitution reactions [12–16]. The
solvent effect includes the interaction of the substrate
and/or the intermediate(s) with the solvent molecules,
which may be extensive and complex [17–19], whereas
electronic and hydrogen bonding effects have been
identified as important factors that influenced reac-
tivity [7–11,16–23]. Different mechanisms have been
shown to apply for the nucleophilic aromatic substi-
tution reaction [12–16] of several rings activated aryl
halides, alkyl aryl ethers, and thio-ethers with primary
or secondary amines in various solvents as given in
INTRODUCTION
The mechanism and reactivity for the reaction of
nitroaryl derivatives with amines have interested
chemists for some years and continue to attract their
attention [1–11]. Many studies showed the effect of
Correspondence to: Magda F. Fathalla; e-mail: mffathalla@
hotmail.com
Values of the first-order rate constant, kψ (in Tables S1 and
S2) are available as Supporting Information in the online issue at
www.interscience.wiley.com.
c
ꢀ
2009 Wiley Periodicals, Inc.

778
ASGHAR, FATHALLA, AND HAMED
HN<
+
L
NH<
_
k/AmAm
Pathway (b)
k4
NH<
>NH
(NO2)n
k
k5
III
+
L
N<
L
NH<
pathway (a)
+
k1
, H
L
_
+ HN<
HL
+
k
k2
(NO2)n
(NO2)n
(NO2)n
I
n = 1, 2, 3
Zwitterion intermediate
k3
k
k6
kAm
H+
L
N<
Pathway (c)
_
AmH+
N<
L
+
NH<
L
((NO2)n
-
-
H
(NO2)n
IV
Zwitterion intermediate
H
(NO2)n
II
V
Meisenheimer intermediate
Meisenheimer intermediate
Scheme 1
pathways (a)–(c) in Scheme 1. Pathway (a) represents
the uncatalyzed reaction, first order in amine and its
attack is rate limiting [13–16,20–23]. However, this
does not exclude the case where the reaction under-
goes catalysis either by (i) specific base-general acid
(SB-GA) when the k3 step, i.e., departure of the leav-
ing group, is the rate limiting [24–30] or (ii) general
base catalysis (SB) is likely to involve the rate-limiting
deprotonation of the initially formed zwitterion inter-
mediate I, kAm > k−1, followed by rapid loss of the
leaving group [31–38].
of the substrate with adducts IV–V formed by reaction
at the unsubstituted position [38].
Here we report the effect of the solvents, such
as methanol (MeOH), acetonitrile (AN), and toluene
(Tol), the electronic and steric effects of the trifluo-
romethyl group, as well as the substituent in the nucle-
ophile on the reactivity and mechanism for the reaction
2- and 4-chloro-3,5-dinitrobenzotrifluorides (1 and 2,
respectively) with aniline derivatives 3a–g.
When the rate constants show a parabolic de-
pendence on the amine concentration, it can be ex-
plained by a dimer mechanism [39–42], pathway (b).
In this mechanism, the amine dimer acts as a nu-
cleophile forming an intermediate complex III while
the third molecule of amine assists in the decomposi-
tion. Pathway (c) represents the substitution of leaving
group L as a slow step followed by rapid equilibration
CF3
CF3
Cl
O2N
NO2
O2N
NO2
Cl
1
2
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REACTION OF 2- AND 4-CHLORO-3,5-DINITROBENZOTRIFLUORIDES WITH ANILINES
779
Table I Physical Properties, IR, UV Spectra, and Elemental Analysis of N-(2,4-Dinitro-6-trifluromethylphenyl)aniline
Derivatives, 4 a–g
Elemental Analysis,
ν(cm−1), NO2
Calcd (Found)
Compound
R
H
MP Yield (%) λ(nm) (ε) Asymmetric Symmetric NH
MF
C
H
N
4a
121.0
86
90
95
90
89
96
95
360 (19369)
240 (23119)
370 (18479)
240 (21897)
370 (20023)
240 (23975)
365 (18631)
240 (24436)
360 (17009)
240 (19520)
360 (19525)
240 (22256)
360 (17076)
240 (23118)
1595
1612
1600
1600
1596
1601
1595
1340
1343
1338
1338
1338
1337
1337
3349 C13H8F3N3O4 47.72 2.46 12.84
(47.50) (2.13) (12.64)
4b
4c
4d
4e
4f
4-OCH3 105.6
4-CH3 122.0
4-Cl 116.7
3-OCH3 90.0
3-CH3 110.0
3-Cl 108.9
3384 C14H10F3N3O5 47.07 2.82 11.76
(46.88) (2.63) (11.60)
3398 C14H10F3N3O4 49.28 2.95 13.31
(49.50) (2.53) (13.64)
3396 C13H7ClF3N3O4 43.17 1.95 11.62
(43.51) (2.03) (11.44)
3351 C14H10F3N3O5 47.07 2.82 11.76
(47.45) (2.44) (11.64)
3365 C14H10F3N3O4 49.28 2.95 13.31
(49.58) (2.73) (13.74)
4g
3360 C13H7ClF3N3O4 43.17 1.95 11.62
(43.57) (1.93) (11.69)
EXPERIMENTAL
Reaction Product
General Procedure. A methanol solution of 1 (1.8
mmol, 5 mL methanol) was treated with the appropri-
ate aniline derivatives 3a–g (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-trifluoromethyl-phenyl)aniline derivatives 4a–g.
Tables I and II show the physical properties, IR, UV,
and 1H NMR characteristics of compounds 4a–g,
The purification of AN, MeOH, Tol, and aniline
was reported previously [43]. 4-Methyl-, 4-methoxy-,
4-chloro-, 3-chloro-, 3-methyl-, and 3-methoxy-
anilines used were of commercial grade and were
purified by crystallization or vacuum distillation.
2- and 4-Chloro-3,5-dinitrobenzotrifluorides were the
purest available materials (Sigma-Aldrich, Steinheim,
Germany).
Table II 1HNMRa Spectra of N-(2,4-Dinitro-6-trifluromethylphenyl)aniline Derivatives, 4a–g
R
H
NH
H-4
H-2
H 2ꢁ, 6ꢁ
H3ꢁ, 5ꢁ
H-4ꢁ
4a
7.63 8.91 (J2,4 = 2.3 Hz 8.69 (J2,4 = 2.3 Hz)
7.01 (d)
(J = 7.65 Hz)
6.98 (m)
7.35 (t)
(J = 7.7; 7.7 Hz)
6.85 (m)
7.24 (t)
(J = 7.65 Hz)
b
4b 4-OCH3 7.45 8.88 (J2,4 = 2.3 Hz) 8.65 (J2,4 = 2.3 Hz)
4c 4-CHc3 7.59 8.89(J2,4 = 2.3 Hz) 8.67 (J2,4 = 2.3 Hz)
(J∗ = 8.4 Hz)
(J∗ = 8.4 Hz)
7.14 (m)
6.91 (m)
(J∗ = 8.4 Hz)
7.31(m)
(J∗ = 8.4 Hz)
6.95 (m)
4d 4-Cl
7.64 8,89 (J2,4 = 2.3 Hz) 8.7 (J2,4 = 2.3 Hz
(J∗ = 6.8 Hz)
(J∗ = 6.8 Hz)
H- 2ꢁ
H4ꢁ
6.56 (d)
H-5ꢁ
6.75 (t)
H-6ꢁ
7.24 (d)
d
4e 3-OCH3 7.5 8.89 (J2,4 = 2.3 Hz) 8.69 (J2,4 = 2.3 Hz)
6.51 (d)
(J = 2.3 Hz) (J = 7.5 Hz) (J = 7.5 Hz) (J = 7.3 Hz)
6.8 (d) 7.03 (d) 7.21 (t) 7.25 (d)
(J = 2.3 Hz) (J = 7.65 Hz) (J = 7.65 Hz) (J = 7.65 Hz)
7.01 (d) 7.19 (d) 7.27 (t) 6.88 (d)
(J = 2.3 Hz) (J = 8.0 Hz) (J = 8.3 Hz) (J = 8.2 Hz)
e
4f 3-CH3
4g 3-Cl
7.53 8.93 (J2,4 = 2.3 Hz) 8.68 (J2,4 = 2.3 Hz)
7.59 8.94 (J2,4 = 2.3 Hz) 8.72 (J2,4 = 2.3 Hz)
aIn CDCl3/TMS.
b4-OCH3 proton appears at δ 3.80 (s) ppm.
c4-CH3 protons appear at δ 2.33 (s) ppm.
d 3-OCH3 appears at δ 3.77(s) ppm.
e3-CH3 protons appear at δ 2.41(s); (e)AAꢁ XXꢁ system (J∗ = 2.3 = J = 2.5).
H-2ꢁ, 4ꢁ,5ꢁ,6ꢁ aromatic hydrogens of the phenylamino moiety.
International Journal of Chemical Kinetics DOI 10.1002/kin

780
ASGHAR, FATHALLA, AND HAMED
whereas compounds 5a–g were prepared and identi-
fied earlier by us [13]. IR spectra were recorded on
a Perkin–Elmer 1430 ratio, recording IR spectropho-
tometer using potassium bromide pellets. The 1H
NMR spectra were measured in CDCl3 and recorded
on a JEOL 500 MHz spectrometer. Elemental analyses
were performed at the microanalytical laboratory in
the Faculty of Science, Cairo University, Egypt.
Table I shows a remarkable shift for the N H and
−NO2 stretching vibration of compounds 4a–g. It is
observed that the frequency of NH for 4a–g is shifted
by about 2–49 cm−1 compared to that of the unsub-
stituted aniline moiety and depending on the nature
and position of the substituent in the aniline moiety.
Table II indicates that H-3 and H-5 appear as highly
deshielded doublets because of their location between
two nitro groups in the former and between nitro and
trifluoromethyl groups in the latter [44].
Kinetic results show that the substitution is first or-
der in the aniline concentrations for the reaction with
1 in MeOH and AN and 2 in MeOH [13], AN, and
Tol. This indicates that the reaction proceeds by path-
way (a) of Scheme 1, where the nucleophilic attack to
form zwitterion intermediates I-1 and I-2 is the rate-
determining step.
Kinetic Measurements
The kinetics of 1 and 2 with substituted anilines
3(a–g) was measured spectrophotometrically using
Shimadzu UV-160A spectrophotometer in conjunction
with Shimadzu thermo bath (TB-85) temperature con-
trol ( 0.1). The kinetic runs were carried out at differ-
ent concentrations of substituted anilines ranging from
2 × 10−3 to 1.6 mol dm−3 at λ ranging from 360 to
370 nm for compound 1 and from 400 to 410 nm for
compound 2, depending on the nature and position of
the substituent in aniline. All reactions were carried
out under pseudo-first-order kinetic conditions, utiliz-
ing 4 × 10−4 mol dm−3 for both 1 and 2. The observed
rate constants kꢀ were calculated from the slope of
the plot of ln(A∞ − At ) versus time. The second-order
rate constants kA were calculated from the slope of the
plot of kꢀ versus amine concentrations.
CF3
CF3
+
NH2-Ar
-
-
Cl
O2N
NO2
+
NH2-Ar
O2N
NO2
Cl
I-2
I-1
Unexpectedly, the reaction of 1 with aniline derivatives
3a–g in Tol shows a second-order dependence on the
aniline concentrations, indicating that this reaction in-
volves catalysis, pathway (a) of Scheme 1, where either
SB-GA or SB catalysis is the rate-determining step. It
can be represented by the following equation:
RESULTS AND DISCUSSION
Attack by substituted anilines on 1 and 2 results in dis-
k1(k2 + kAm[aniline])
kA =
(2)
placement of a chlorine substituent in quantitative yield
k
−1 + k2 + kAm[aniline]
1
(Eq. (1)). Elemental analyses, IR, UV, and H NMR
spectra indicated the formation of N-(2,4-dinitro-
6-trifluoromethyl-phenyl)aniline derivatives 4a–g via
an anilino-dechlorination process (see the Experimen-
tal section), whereas N-(2,6-dinitro-4-trifluoromethyl-
phenyl) aniline derivatives 5a–g were prepared previ-
ously [13].
If k ꢂ k2 + kAm [aniline], Eq. (2) is reduced to
−1
Eq. (3), where k1 represents the rate-determining step
of the uncatalyzed reaction:
kA = k1
(3)
whereas when k ꢃ k2 + kAm [aniline], Eq. (2) is
−1
converted to Eq. (4), representing that the decompo-
sition of intermediate is the rate-limiting step.
CF3
+
NH2-C6H4-X
3(a g)
1 , 2
(1)
kA = K1k2 + K1 kAm [aniline]
(4)
O2N
NO2
NH-C6H4-X
-
The catalytic role in deprotonation of the initially
formed zwitterion intermediate I to the Meisenheimer
complex II is shown by Eq. (5) through pathway (a),
where k2 the uncatalyzed pathway is negligible, kAm
2-Anilino, 5 (a
-g) = 4-Anilino
=
4(a-g)
derivatives
X = a, H; b, 4-OMe; c, 4-Me; d, 4-Cl; e, 3-OMe; f, 3-Me; g, 3-Cl
International Journal of Chemical Kinetics DOI 10.1002/kin

REACTION OF 2- AND 4-CHLORO-3,5-DINITROBENZOTRIFLUORIDES WITH ANILINES
781
Table III Second-Order Rate Constants for the
represents the catalyzed step by aniline. An equivalent
form is given by Eq. (6).
Reaction of 1 with Substituted Aniline, 3a–g in MeOH
and AN at 25◦C
103 kA (dm3mol−1 s−1
)
k1kAm[aniline]
kA =
kA =
(5)
(6)
k
−1 + kAm[aniline]
Compound Substituent
MeOH
AN
K1kAm[aniline]
kAm
3a
3b
3c
3d
3e
3f
H
2.41 0.06
0.395 0.008
10.6 0.5
1 +
[aniline]
4-OMe
4-Me
4-Cl
3-OMe
3-Me
3-Cl
42
1
k
−1
11.3 0.3
2.48 0.06
0.286 0.003 0.040 0.02
This equation indicates that the breakdown of the zwit-
terion intermediate containing poor nucleofuge occurs
by rate-limiting proton transfer [44].
1.580 0.02
3.08 0.05
0.192 0.007
0.63 0.02
3g
0.070 0.001 0.007 0.0002
Reaction of 1 and 2 with Anilines 3 a–g
(Uncatalyzed Pathway)
Meisenheimer intermediate through intermolecular hy-
drogen bonding. Furthermore, this specific reaction be-
comes slower in media of high dielectric constant ε,
because kH(MeOH)/kH(AN) is 6 and 3 for compounds 1
and 2, respectively. Also, the higher rate constant of 2
(ortho-like, contains an ortho nitro group with respect
to the leaving Cl) over 1 (para-like, contains a para ni-
tro group with respect to the leaving Cl) [kA (4-Cl)/kA
(2-Cl)] is 6- and 12-folds in MeOH and AN, respec-
tively, presumably indicating that the intermediate de-
veloping from the reaction of 2 is more polar than that
developing from 1.
Generally, the nucleophilic substitution in 2,4,6-
triactivated is faster at position between two or-
tho nitro groups than at positions ortho and para
to two nitro groups, e.g., there is a kinetic prefer-
ence for reaction at 4-position in chloro-1-cyano-3,5-
dinitrobenzene [11] and in chloro-1-trifluromethyl-
3,5-dinitrobenzene [26]. However, in compounds that
lack a good leaving group the adduct formed by reac-
tion at the 2-position has higher thermodynamic sta-
bility [13,14]. Accordingly, the observation that com-
pound 2 is more reactive than 1 is in agreement with
literature results [47,48].
Solvent Effects. The kinetics of the reaction of various
concentrations of aniline derivatives with compound 1
in MeOH and AN and compound 2 in MeOH [13], AN,
and Tol at 25◦C was studied under pseudo-first-order
condition. Values of the first-order rate constant, kψ ,
are available as supporting information (in Tables S1
and S2), and the second-order rate constants, kA, are
presented in Tables III and IV. It is found that kA val-
ues vary linearly with amine concentrations. We inter-
preted these results in terms of Scheme 1 (pathway (a)),
assuming that aniline attack is rate limiting (Eq. (3)). In
all the cases, 1 and 2 were shown to give the expected
substitution product. However, intermediates II-1 and
II-2 were not detected spectroscopically.
CF3
-
CF3
-
Cl
NH-Ar
NO2
O2N
NO2
NH-Ar
O2N
Cl
For compounds
1
and 2, 2-chloro-3,5-
II-1
II-2
dinitropyridine, 1-chloro-2,4-dinitro-benzene, and
2-chloro-1,3,5-trinitrobenzene [23], the contribution
of the bulky ortho substituents to the reaction center
as well as its activating effect clearly has to be
considered. The comparison shows that the reactivity
order is NO2 > ring N > CF3 > H, and in general
the ratios do not depend significantly on the nature of
amine. We concluded that an o-NO2 group is more
activating than the ring nitrogen, whereas the o-CF3
group has more profound steric effect. The steric
argument of o-NO2 may be unconvincing because the
approach of the amine from a direction orthogonal
to the ring in an o-substituted derivative will allow
chlorine to be pushed out of the benzene plane [26].
Tables III and IV show that the order of decreasing
reactivity for the reaction of a given nucleophile with
compound 1 or 2 as follows:
kA(MeOH) > kA(AN) > kA(Tol)
This is in agreement with the formation of a dipo-
lar intermediate as seen from the effect of hydrogen
bond donor α compared to hydrogen bond acceptor
β, polarity polarizability π*, and Dimroth–Rechardt’s
ET solvatochromic parameters [45,46]. This reac-
tivity order is due to the strong solvation of the
International Journal of Chemical Kinetics DOI 10.1002/kin

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ASGHAR, FATHALLA, AND HAMED
Table IV Second-Order Rate Constants, for the Reaction of 2 with Substituted Aniline 3a–g in MeOH, AN, and Tol at
25◦C
103 kA (dm3 mol−1 s−1
)
Compound
Substituent
MeOHa
AN
Tol
3a
3b
3c
3d
3e
3f
H
15
140
52
2.4 0.2
6.4 0.4
2
2
1
4.9 0.2
3.94 0.05
4-OMe
4-Me
4-Cl
3-OMe
3-Me
3-Cl
78
2
67
3
27.8 0.8
0.77 0.02
2.24 0.09
7.2 0.2
18.4 0.8
0.523 0.008
2.040 0.001
3.98 0.07
24.8 0.1
0.90 0.01
3g
0.14 0.05
0.076 0.001
aExtrapolated data from [13].
Other activating factor is presumably attributable to
the efficient delocalization of the developed charge
with the NO2 more than the aza group [19,22].
destroyed in the Meisenheimer complex-type interme-
diate formed, and (ii) the electron pair on the nitrogen
atom of aniline is transferred to the nitrochlorobenzene
transition states.
A significant feature of variable solvents and sub-
stituents in the anilines 3a–g is shown by the plot of
log(k/ko) for the reaction of 1 against log(k/ko) for
the reaction of 2 in the same solvent, which gives a
linear relationship with the slope close to unity (gra-
dient = 0.81, r = 0.98)MeOH and (gradient = 0.93,
r = 0.99)AN. Interestingly, this type of plot has the
same behavior for 2 in toluene (gradient = 0.81,
r = 0.99) Also, the ρ values for these reactions are
quite the same (within experimental errors); the iden-
tity of these ρ values shows that the structure of the
transition state does not alter significantly with the
change in the nature and position of the substituents
in aniline, the properties of solvent, and the position of
the chlorine atom in compounds 1 and 2.
Effect of Substituent on the Nucleophile. The reac-
tivity of the substituted anilines 3a–g was found to
decrease in the order 4-OMe > 4-Me > 3-Me > H >
3-OMe > 4-Cl > 3-Cl depending on the electronic
nature and position of the substituent. The reactivity
differences between compounds 1 and 2 with aniline
derivatives may be ascribed to the electronic effect of
the substituent on the aniline. This may be quantified
by the use of a Hammett equation [49].
A good linear correlation between log kA for the re-
actions of compounds 1 and 2 with aniline derivatives
versus σ-Hammett in all solvents at 25◦C is estab-
lished. The obtained ρ values are −4.07 and −4.62
for compound 1 in MeOH and AN, respectively, and
−3.38, −4.11, and −4.34 for compound 2 in MeOH
[13], AN, and Tol, respectively. The negative ρ val-
ues are comparatively in agreement with those of the
Chapman et al. hypothesis of ρ (−3.1 and −3.5), which
are characteristic of the displacement of the chlorine
atom from an aromatic carbon by aromatic amines and
suggesting the formation of an intermediate in the rate-
determining step [50]. The sign and magnitude of ρ
indicates that (a) the substituent affects the rate of the
reaction by the polar effect; (b) the negative ρ val-
ues are in agreement with the donor ability of aniline
derivatives, and the positive charge is developed on the
aniline nitrogen atom in the transition state; and (c) an
anilino moiety participates in the bond formation in the
transition state.
Reaction of 1 with Anilines 3a–c,e,f
(Amine Catalysis)
The reaction of 1 with substituted anilines 3a–c,e,f
in toluene was studied in different amine concentra-
tions at 25◦C. The data of kA values (kꢀ/[aniline]), in
Table V, show an amine concentration dependence,
indicating that the proton transfer step kAm is rate lim-
iting or a dimer mechanism, pathway (a) or (c) of
Scheme 1, respectively. This is another example of
the base-catalyzed reaction for substrate that contains
a chloro atom that is known as a good leaving group
[52–57]. The dimer mechanism remains unproved and
is difficult to distinguish it from mechanisms involving
a consecutive reaction with two amine molecules. This
mechanism is rejected on the ground that it does not
show a parabolic dependence on the amine concentra-
tion [39–43].
On the other hand, our ρ values are similar to the
values given by Wepster et al (−3.19) for the ioniza-
tion of substituted anilinium ions [51]. The similarity of
these ρ values can be explained on the basis of the fol-
lowing facts: (i) The conjugation of the amino group is
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REACTION OF 2- AND 4-CHLORO-3,5-DINITROBENZOTRIFLUORIDES WITH ANILINES
783
Table V Reaction of 2-Chloro-3,5-dinitrobenzotrifluoride, 1 with Substituted Aniline 3a–g in Toluene at 25◦C; Second
Order (kA)
104 kA (dm3 mol−1s−1
4-Me (3c)
)
Substituent [Am]
H (3a)
4-OMe (3b)
3-OMe (3e)
3-Me (3f)
0.20
0.25
0.3
4.80
6.88
8.96
10.94
12.82
1.63
1.77
1.87
1.95
2.20
0.35
0.40
0.45
0.50
0.60
0.70
0.80
0.90
1.00
1.20
1.40
1.60
0.46
0.50
0.58
0.70
0.77
0.41
0.47
0.22
–
0.55
0.63
0.79
0.28
0.30
0.36
0.43
protonated amine (AmH+) to the leaving group (the
SB-GA mechanism).
It is found that a plot of kA values from Table V
versus aniline concentrations shows linear correlations
with negligible intercepts (Fig. 1). In terms of Eq. (2),
the contribution of k2 is negligible; hence Eq. (5) or
the alternative Eq. (6) is applied, where kAm represents
The zwitterion I-1 can undergo proton transfer in
the rate-controlling step forming Meisenheimer inter-
mediate II-1 (general base catalysis) or the leaving
group departure is the rate-limiting step (specific base
general acid SB-GA). Since chloride is an extremely
good leaving group, this is likely to be from the zwit-
terionic intermediate I-1 to amine rather than from the
0.0014
0.0012
0.0010
0.0008
H
4-OMe
4-Me
3-OMe
3-Me
k
0.0006
0.0004
0.0002
0.0000
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
[Aniline]
Figure 1 Variation of kA for the reaction of 1 with substituted aniline concentration in toluene at 25◦C.
International Journal of Chemical Kinetics DOI 10.1002/kin

784
ASGHAR, FATHALLA, AND HAMED
Table VI Kinetic Results for the Reactions of 1 with Substituted Aniline (1.0 mol dm−3 except 4-OMe, 0.1 mol dm−3
,
and 4-Me, 0.3 mol dm−3) at Various DABCO Concentrations in Toluene at 25◦C
103 kA (dm3 mol−1 s−1
)
[DABCO]
(mol dm−3
)
H (3a)
4-Me (3b)
4-OMe (3c)
4-Cl (3d)
3-OMe (3e)
3-Me (3f)
3-Cl (3g)
0.0
0.1
0.2
0.3
0.4
0.5
0.05
0.29
0.47
0.73
0.99
1.09
0.896
1.56
2.95
3.81
4.93
1.00
7.15
12.65
17.71
21.41
0.03
0.16
0.28
0.37
0.46
1.06
0.37
0.71
0.94
1.15
0.06
0.09
0.11
0.02
0.03
0.04
the catalyzed pathway. Figure 1 shows that kAm/k
substituents. This agrees with the relatively high ba-
sicity values of 4-OMe and 4-Me aniline in toluene,
presumably suggesting that the proton transfer process
is the rate-controlling step.
−1
has lower values and then K1kAm is calculated from
the slopes. To examine such catalysis, the reaction is
subjected to general base catalysis by adding DABCO
as an external base. The kA values for the reaction of 1
with substituted anilines 3a–c,e,f (1.0 mol dm−3 except
4-OMe, 0.1 mol dm−3 and 4-Me, 0.3 mol dm−3) with
various DABCO concentrations in toluene are given
in Table VI. The values of K1kDABCO (where kDABCO
represents the rate constant for the DABCO-catalyzed
route) were obtained from the slopes of the linear plots
of the data in Table VI. The reactions of 1 with dif-
ferent substituted aniline concentrations in the pres-
ence of constant DABCO, 0.1 mol dm−3, in toluene
were performed, and the values of kA are presented
Comparison of 1 and 2 with 3a–g in MeOH,
AN, and Toluene
The possibility of hydrogen bonding between the am-
monium hydrogen atom and two o-nitro groups I-2
causes the 4-chloro isomer to be less susceptible to
catalysis than the 2-chloro one. Moreover, the hyper-
conjugation of the CF3 group may pronounce a partial
negative on the carbon atom bearing the leaving chlo-
rine atom at 2-position of 1 more than the 4-position
of 2 causing the former to undergo catalysis with ease.
The important factor is that in toluene the value of
in Table VII. The values K1km, K1kDABCO, and kAm
kDABCO are summarized in Table VIII, which reflects
variations in the values of K1kAm and in general kAm
kDABCO increased with increasing electron-donating
/
/
k
will be relatively high because the polar interme-
−1
diate will be poorly solvated, and kAm will be reduced
Table VII Kinetic Results for the Reactions of 1 with Substituted Aniline, in the Presence of DABCO, 0.1 mol dm−3, in
Toluene at 25◦C
103 kA (dm3 mol−1 s−1
4-OMe (3c)
)
[Aniline] (mol dm−3
)
H (3a)
4-Me (3b)
3-OMe (3e)
3-Me (3f)
0.05
2.83
5.10
7.15
9.21
11.26
0.075
0.100
0.125
0.150
0.300
0.350
0.400
0.500
0.800
0.900
1.000
1.200
1.400
4.68
5.50
6.57
8.32
3.05
3.32
3.67
5.15
1.4
2.48
2.95
3.35
3.63
1.63
1.98
3.30
International Journal of Chemical Kinetics DOI 10.1002/kin

REACTION OF 2- AND 4-CHLORO-3,5-DINITROBENZOTRIFLUORIDES WITH ANILINES
785
Table VIII Summary of the Rate Constant Data for the Reaction of 1 with Substituted Anilines in Toluene
Substituent
103K1kAm (dm6 mol−2 s−1
)
103K1kDABCO (dm6 mol−2 s−1
)
kAm/kDABCO
H
0.06 0.01
3.01 0.1
0.47 0.01
2.1 0.1
11.0 0.6
4.79 0.3
0.2 0.02
1.1 0.05
2.7 0.01
0.08 0.0
0.03
0.27
0.09
4-OMe
4-Me
4-Cl
3-OME
3-Me
3-Cl
0.02
0.08
0
0
0.02
0.01
by steric and electrostatic repulsion between the ap-
proaching amine catalyst and the reaction center of
compound 1. This concept may be acceptable because
the pervious work has shown that the CF3 group is ster-
ically demanding and is greater in size than the nitro
group [10,11]. This is unlikely to affect values of k1
since nucleophilic attack occurs orthogonal to the ring.
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