Reaction of 4-chloro-3,5-dinitrobenzotrifluoride with aniline derivatives. Substituent effects

Article abstract of DOI:10.3184/030823407X240926

N-(2,6-Dinitro-4-trifluoromethylphenyl)aniline derivatives were prepared by anilino-dechlorination of 4-chloro-3, 5-dinitrobenzotrifluoride. IR, UV and ¹H NMR studies suggested an intramolecular hydrogen bond between the amino hydrogen and one o-nitro group. An addition-elimination mechanism was suggested based on the second-order kinetics and the dependence of rates on the nature and the position of the substituent in the aniline ring, as well as the high negative values of p (-3.14, -3.16, -3.01). Such values indicate a positive charge on the aniline nitrogen in the transition state and that the rate is affected by the polar effect of the substituent. The β value (0.85 at 30°C) indicates an appreciable degree of bond formation in the transition state.

Full text of DOI:10.3184/030823407X240926

JOURNAL OF CHEMICAL RESEARCH 2007
SEPTEMBER, 509–512
RESEARCH PAPER 509
Reaction of 4-chloro-3,5-dinitrobenzotrifluoride with aniline derivatives.
Substituent effects
Hamida O.M. Al-Howsawaya, Magda F. Fathallab*, Ali A. El-Bardanb and Ezzat A. Hamedb
aChemistry Department, Faculty of Applied Science, Umm Al-Qura University, Kingdom of Saudi Arabia
bChemistry Department, Faculty of Science, Alexandria University, PO 426 Ibrahimia, Alexandria 21321, Egypt
N-(2,6-Dinitro-4-trifluoromethylphenyl)aniline derivatives were prepared by anilino-dechlorination of 4-chloro-3,
5-dinitrobenzotrifluoride. IR, UV and 1H NMR studies suggested an intramolecular hydrogen bond between the
amino hydrogen and one o-nitro group. An addition-elimination mechanism was suggested based on the second-
order kinetics and the dependence of rates on the nature and the position of the substituent in the aniline ring,
as well as the high negative values of r (–3.14, –3.16, –3.01). Such values indicate a positive charge on the aniline
nitrogen in the transition state and that the rate is affected by the polar effect of the substituent. The b value (0.85 at
30°C) indicates an appreciable degree of bond formation in the transition state.
Keyword: 4-chloro-3,5-dinitrobenzotrifluoride, aniline, substituent effect
Nucleophilic substitution reactions of aromatic and
heteroaromatic derivatives (SNAr) with neutral and anionic
nucleophiles have been recognised to be considerably
affected by the properties of solvent, the type of nucleophile,
the reactivity of substrate and the nature of substituents in the
nucleophile or in the substrate as well as the possible steric
hindrance.1-13 High negative r values were reported for the
reaction of 2-chloro-1,3,5-trinitrobenzene, 2-bromo-3,5-
dinitrothiophene, 2-chloro-3,5-dinitropyridine and 2-chloro-
5-nitropyridine with substituted anilines.14-17 These values
of r indicate that (a) a positive charge is developed on the
aniline nitrogen in the transition state where no conjugation
is possible with the substituent18,19 and (b) the rates were
facilitated by electron-releasing and retarded by electron-
attracting substituents in the nucleophile.
In continuation of our previous studies in the field of
nucleophilic aromatic substitutions,16,17,20-26 this work is
planned to study the kinetics and the effect of the substituent
in the aniline ring on the reaction with 4-chloro-3,5-
dinitrobenzotrifluoride (DNCBTF). A trifluoromethyl (-CF3)
group was introduced in the substrate in an effort to gain
understanding of its effect on rate compared to the nitro and
the aza groups.
CAUTION: Methanol is toxic and causes blindness and death when
ingested, inhaled or absorbed through the skin and high exposure to
aniline derivatives can cause death, cyanosis, headaches, weakness,
eye irritation, drowsiness, and shortness of breath. Protective gloves
should be worn to avoid skin contact with aniline. Splash proof
chemical goggles and a face shield should be used when working
with aniline and its derivatives.
Kinetic runs were performed by following the appearance of the
reaction product at l = 410 nm. All reactions were carried out under
pseudo first-order conditions with various concentrations of anilines
(appropriate mol dm-3) and concentration of DNCBTF (2 × 10-4 mol
dm-3). The pseudo-first order rate constants kY were determined by
applying Eqn (1),
ln (A• – At) = – kYt + ln (A• –Ao)
(1)
where Ao, At and A∞ are the optical densities of the reaction mixture
measured at zero time, time t and infinity respectively. The second-
order rate constants, kA, were obtained from the slopes of the plots of
kY versus the concentration of substituted anilines. Rate coefficients
were reproducible to ± 2%.
Results and discussion
The reaction of 4-chloro-3,5-dinitrobenzotrifluoride (DNCBTF)
with 3- and 4- substituted anilines 2a–h gave the corresponding
substitution products 3a–h. Elemental analyses, UV, IR and
1H NMR spectra indicated the formation of N-(2,6-dinitro-
4-trifluoromethylphenyl)aniline derivatives via an anilino-
dechlorination process (see Scheme 1).
Experimental
Melting-points were uncorrected. 1H NMR spectra CDCl3 were
recorded using a DRX 400 FTNMR Bruker-400 MHz instrument.
IR spectra were taken on a Perkin-Elmer 1430 instrument. Elemental
analyses were carried out at the microanalytical laboratory of the
Faculty of Science King Abdulaziz University Jeddah, Kingdom of
Saudi Arabia.
Applying the steady state treatment to the Eqn (1), the rate
is as follows,
Rate
[1][amine]
k2k1
−1 + k2
=kA =
k
kψ
[amine]
Starting material
when k2 > k-1, kA = k1 =
determining step).
(i.e. the first step is the rate-
4-chloro-3,5-dinitrobenzotrifluoride (DNCBTF), 3- and 4- substituted
anilines and methanol were the purest available commercial samples.
The products, 3a–h, showed a shift in the N–H stretching
vibrations (n 3239–3339 cm-1) and in the vibrations of
–NO2 group, Table 1, indicating the possible formation of a
hydrogen bond between an N-H and an ortho-nitro group in
the dinitrobenzotrifluoride moiety27-29 (Fig. 1).
General procedure
A mixture of (DNCBTF) (0.5 g, 1.8 mmol) and an appropriate
substituted aniline (2.5 mmol) in 20 ml methanol were stirred at
room temperature for 30-45 min. The precipitate solid was filtered
and crystallised from methanol–water. The physical properties,
UV, IR, 1H NMR spectra and elemental analyses are given in
Tables 1–3.
O
+
N
CF3
-
O
H
Kinetic measurements
The kinetics of DNCBTF with 3- and 4-substituted anilines in
methanol were measured spectrophotometrically using a CECIL-CE
7200/Aquarius spectrophotometer and/or UV-VIS Shimadzu 160-A
at three temperatures (30-50°C).
N
NO2
R
* Corresspondent.. E-mail: mffathalla@hotmail.com
Fig. 1
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510 JOURNAL OF CHEMICAL RESEARCH 2007
R
R
Cl
+
NH2
NH2
HN
O2N
NO2
Cl
NO2
NO2
+
k1 O2N
O2N
k2
+
-
k-1
HCl
R
CF3
CF
CF3
3 a–h
I 3
1
2 a–h
DNCBTF
a, R = H; b, R = 4-OCH ; c, R = 4-CH3; d, R = 3-CH3; e, R = 3-OCH3; f, R = 4-Cl;
3
g, R =3-Cl ; h, R = 4-COCH3
Scheme 1
Table 1 Physical properties, UV, IR spectra and elemental analyses of compounds 3a–h
Compound
R
m.p./°C
Yield/% lnm/e
n/cm-1
MF
Elemental analysis
Calc’d (Found)
–NO2
–NH
asym
1537
sym
C%
H%
N%
3a
3b
3c
3d
3e
H
120–121
136–137
153–154
114–115
128–129
87
89
87
92
84
220 (13000)
254 (14660)
411 (4730)
1356
3331
C13H8F3N3O4
C14H10F3N3O5
C14H10F3N3O4
C14H10F3N3O4
C14H10F3N3O5
47.72
2.46 12.84
(47.6) (2.9) (12.7)
4-OCH3
4-CH3
3-CH3
3-OCH3
224 (16200)
250 (14500)
415 (5280)
1539
1547
1545
1533
1358
1360
1364
1360
3299
3335
3335
3339
47.07
2.82 11.76
(47.7) (3.2) (11.5)
230 (17500)
251 (19160)
414 (6540)
49.28
2.95 12.31
(49.15) (3.3) (11.85)
220 (13580)
258 (13000)
412 (5650)
49.28
2.95 12.31
(49.6) (2.5) (11.9)
223 (19660)
255 (16500)
409 (5770)
47.07
2.82 11.76
(47.6) (2.5) (11.9)
3f
4-C1
3-Cl
102–103
147–148
82
76
250 (18270)
404 (5650)
1534
1541
1350
1352
3298
3339
C13H7ClF3N3O4
C13H7ClF3N3O4
43.17
1.95 11.62
(43.0) (2.4) (11.5)
3g
220 (16700)
255 (17130)
403 (5410)
43.17 1.95 11.62
(43.0) (2.2) (11.5)
3h
4-COCH3
165–166
86
225 (18210)
301 (21140)
380 (6500)
1516
1360
3329
C15H10F3N3O5
48.79
2.73 11.38
(48.5) (2.3) (11.5)
The electronic spectra (UV-VIS) of 3a-h showed two or three
main bands at l 220–254 and 250–258 and 403–415 nm in
methanol, Table 1. DNCBTF shows one absorption band
at lmax 221 nm. The observed shift presumably arises from
the electronic transition30 between the nitro groups and the
amino one. The linear correlation of log l values in the range
380–415 nm with Hammett s-constants (gradient –3.699,
r = 0.98) indicates the polar effect on this transition except 3h
(R = 4-COCH3), Table 1. The chemical shift of the NH proton
presumably supports the existence of the intramolecular
hydrogen bond between the amino hydrogen and an o-nitro
group,16 Figure 1 and Table 2.
The pseudo first-order rate constants (kY) of the reaction of
DNCBTF with various substituted anilines 2a–h in methanol
at 30, 40 and 50°C were studied spectrophotometrically at
lmax = 410 nm, Table (3). The linear plots of kY vs. excess
and variable aniline concentrations with null intercepts
indicate that the reaction under investigation (i) obeys a clean
a second-order overall reaction (kA = kY/[amine]), (ii) is
not amine-catalysed, ruling out the dimer mechanism31
and also ruling out the possibility that the attack occurs at
an unsubstituted ring position32 (iii) has the formation of a
s-complex intermediate5-8 as the rate-determining step.
The kA values for the anilino-dechlorination of 1-chloro-
2,4-dinitrobenzene (DNCB: 4.2 × 10-4 dm3 mol-1 s-1 at 30°C),33
1-chloro-2,4,6-trinitrobenzene (TNCB:1.16 dm3mol-1s-1 at
30°C) and 2-chloro-3,5-dinitropyridine (DNCP: 4.1 × 10-1
dm3mol-1s-1 at 30°C)16 in methanol were compared with
DNCBTF (1.85 × 10-2 dm3mol-1s-1at 30°C, Table 3) to evaluate
the difference in activating effect of the –CF3 group compared
to the –NO2 and aza groups. This comparison indicated that:
(i) the introduction of the –CF3 group stabilises the developed
negative charge of the intermediate more than its absence
(kDNCBTF/kDNCB ≈ 44); (ii) the delocalisation of the negative
charge developed by the nitro group is greater than that of
–CF3 group in the intermediate (kTNClB/kDNCBTF ≈ 62); (iii) the
activation by the –CF3 group is comparable to that of the aza
one (kDNCBTF/kDNClP ≈ 0.04). These points are in agreement
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JOURNAL OF CHEMICAL RESEARCH 2007 511
Table 2 1H NMR spectra of compounds 3a–h
dppm (CDCl3)
H-2', 6’
Compound
R
H
NH
H-2, 6
H-3', 5'
H-4'
3a
9.96 (s)
8.68 (s)
7.05 (d)
(J = 7.8)
7.36 (t)
(J = 7.8, 7.4)
7.24 (t)
(J = 7.4 Hz)
a
3b
3c
3f
4-OCH3
9.95 (s)
9.96 (s)
9.88 (s)
9.90 (s)
8.43 (s)
8.45 (s)
8.48 (s)
8.52 (s)
6.99 (m)f
6.86 (m)f
(J* = 8.8 Hz)
(J* = 8.8 Hz)
b
4-CH3
7.15 (m)f
(J* = 8.2 Hz)
6.94 (m)f
(J* = 8.2 Hz)
4-Cl
6.99 (m)f
(J* = 6.8 Hz)
7.30 (m)f
(J* = 6.8 Hz)
e
3h
4-COCH3
7.09 (m)f
(J* = 8.6 Hz)
7.96 (m)f
(J* = 8.6 Hz)
H-2'g
H-4'g
H-5'g
H-6'g
c
3d
3e
3g
3-CH3
9.94 (s)
9.93 (s)
9.68 (s)
8.46 (s)
8.47 (s)
8.49 (s)
6.84(d)
7.04 (d)
7.24 (t)
7.22 (d)
(J = 2.1 Hz)
(J = 7.9 Hz)
(J = 8.3 Hz)
(J = 7.5 Hz)
d
3-OCH3
6.56 (d)
(J = 2.0 Hz)
6.61 (d)
(J = 7.8 Hz)
6.77 (t)
(J = 7.6 Hz)
7.24 (d)
(J = 7.3 Hz)
3-Cl
7.07 (s)
(J = 2.0 Hz)
7.29 (d)
(J = 8.0 Hz)
7.21 (t)
(J = 8.3 Hz)
6.93 (d)
(J = 7.8 Hz)
a4-OCH3 protons appear at d 3.81 (s) ppm; b4-CH3 protons appear at d 2.34 (s) ppm; c3-CH3 protons appear at d 2.33(s) ppm;
d3-OCH3 protons appear at d 3.79 (s) ppm; e4-COCH3 protons appear at d 2.59 (s) ppm. f AA’ XX’ system (J* = J23 = J25)g H-2', 3' 4', 5',
6' aromatic hydrogens of the phenylamino moiety.
with substituent constant values for –CF3 (sp = 0.54), and of
Consequently, the twist with respect to the ring of the nitro
group in the o-positions in 3a–h is reduced in comparison
to the twist in DNCBTF. We assume that the most likely
conformation of 3a–h has one of the o-nitro groups oriented
–NO2 (sp = 0.76).
It was found that, the rate constants for the reaction of
aniline derivatives with [TNCB],34 [DNCP],16 and [DNCBTF]
(contains two ortho nitro groups) are greater than 1-chloro-
2,4-dinitrobenzene [DNCB],33 2-nitrochlorobenzene34 and
3-nitro-2- chloropyridine17 (contains one ortho-nitro group).
This increase in rates is presumably explicable by the
electronic effect of the activating group and its proximity to
the leaving one. This is based on the steric effect of o-nitro
groups in the former compounds TNCB, DNCP and DNCBTF
and is unconvincing because the approach of the amine from
a direction orthogonal to the ring results in the chlorine atom
being pushed back from the ring plane.7,8 Another explanation
is that one of the two nitro group rotates to achieve better
hydrogen bonding with the amino group and the second
o-nitro group is then twisted more to relieve steric strain.14,31
The substitution by arylamines instead of the chloro atom
will replace a repulsive interaction with an attractive one and
thus, at least partially, restore the planar conformation.35,36
to achieve better hydrogen bonding with the amino group and
the second o-nitro group is then twisted more to relieve steric
strain.37 This concept is in agreement with the spectroscopic
data of 3a–h, Tables 1and 2.
Table 3 shows that the variation of the rate constants depend
on the nature and position of the substituents in the aniline
derivatives. The order of decreasing reactivity of substituted
anilines is found to be as follows: 4-OCH3, 4-CH3, 3-CH3,
H, 3-OCH3,4-Cl, 3-Cl, 4-COCH3. Good linear correlations
between log kA for the reaction of DNCBTF with 3- and
4-substituted anilines and the Hammett s-constants are
obtained at all temperatures with r values (–3.14, –3.16,
–3.01). These values are similar to the value given by
Chapman’s hypothesis38 and indicate that there is extensive
transfer of charge from the aniline to the dinitrobenzotrifluoride
ring in the transition state.39 The sign and magnitude of
Table 3 Rate constants and activation parameters for the reaction of 4-chloro-3,5-dinitrobenzotrifluoride 1 (2 × 10-4 mol dm-3) with
substituted anilines 2a–h (1 × 10-3-5 × 10-1 mol dm-3) in absolute MeOH
Cpd
R
102 kA mol-1 s-1
DH#/kcal mol-1
-DS #/cal mol-1 K-1
30°C
40°C
50°C
3a
3b
3c
3d
3e
3f
H
1.85 ± 0.05
16.75 ± 0.22
7.02 ± 0.10
3.09 ± 0.01
0.82 ± 0.04
0.31 ± 0.02
0.12 ± 0.05
0.04 ± 0.001
2.63 ± 0. 06
22.26 ± 0.23
10.20 ± 0.19
4.72 ± 0.07
1.33 ± 0.12
0.49 ± 0.02
0.22 ± 0.04
0.08 ± 0.001
4.08 ± 0. 05
30.80 ± 0.84
15.33 ± 0.26
6.92 ± 0.06
2.07 ± 0.16
0.74 ± 0.03
0.36 ± 0.01
0.16 ± 0. 001
29.7 ± 2.6
22.3 ± 1.4
29.3 ± 1.4
30.4 ± 0.4
35.2 ± 0.2
32.9 ± 0.4
42.3 ± 1.8
53.6 ± 0.01
180.4 ± 8.4
186.4 ± 4.4
170.5 ± 4.4
173.8 ± 1.1
168.7 ± 0.8
184.3 ± 1.3
161.3 ± 5.9
133.3 ± 2.0
4-OCH3
4-CH3
3-CH3
3-OCH3
4-Cl
3g
3h
r
3-Cl
4-COCH3
–3.14 ± 0.04
–3.16 ± 0.05
–3.01 ± 0.07
(r = 0.99)
(r = 0.99)
(r = 0.99)
b = 0.85 ± 0.1 (r = 0.97) at 30°C.
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512 JOURNAL OF CHEMICAL RESEARCH 2007
9
N.S. Nudelman, The Chemistry of Amino, Nitroso, Nitro and Related
Groups, 1996, Ch. 26, Ed. S. Patai, J. Wiley & Sons, London.
r values indicate that: (a) the negative r values are in
agreement with the donor ability of aniline derivatives;40
(b) a positive charge is developed on the aniline nitrogen
atom as the transition state is formed;41 (c) the substituent
affects the rate by its polar effect; (d) the aniline derivative
in the developed activated complex has become appreciably
bonded to the carbon of DNCBTF.
10 N.S. Nudelman, M. Savini, C.E. Silvana, V. Nicotra and J. Yankelevich,
J. Chem. Soc., Perkin Trans. 2, 1999, 1627.
11 C. Isanbor, T.A. Emokpae and M.R. Crampton, J. Chem. Soc., Perkin
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13 B.H.M. Asghar and M.R. Crampton, Org. Biomol. Chem., 2005, 3, 3971.
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16 F.M. El-Hegazy, S. Z-Abdel-Fattah, E.A. Hamed and S.M. Sharaf,
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17 A.A. El-Bardan, G.M. El-Subruiti, F.M. El-Hegazy and E.A. Hamed,
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18 T. Okamato and H.C. Brown, J. Org. Chem., 1957, 22, 485.
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27 L.J. Bellamy, The Infrared Spectra of Complex Molecules, Wiley,
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Several reports have indicated that b values are dependent
on the property of solvent and the nature of the amine as well
as the reactivity of the nitro halo derivatives.42-45 Since the
dissociation of anilinium ions39 and the reaction of DNCBTF
with substituted anilines give good Hammett plots, the latter
reaction must also give a Brønsted-like plot of the form log kA
= b pka + constant. The effect of the basicity of the nucleophile
on aromatic nucleophilic substitutions was shown by the
dependence of the rates of structurally related nucleophiles.46-48
Bordwell has pointed out that Brønsted plots reported in the
literature,37-43 for reactions in which bond formation to the
nucleophile is rate-limiting have an almost symmetric position
of the transition state along the reaction coordinate.8 The
Brønsted plot for the titled reaction at 30°C, gave a straight
line with b equal to 0.85 ± 0.1 (r = 0.97), which is comparable
with those reported.46-48 The positive sign and the relatively
high value of β is expected for a nucleophilic substitution
reaction49,50 and suggests a high extent of charge transfer
from the nucleophile to the substrate in the transition state52
(i.e. an appreciable degree of bond formation in the transition
state49,50) and that bond formation to the aniline derivatives is
rate-limiting.
The activation parameter values indicate that this reaction
is slightly enthalpically controlled, but that it is substituent
dependent, Table 3. This suggestion is corroborated by the
calculation of the isokinetic temperature (252°C) for the title
reaction (the temperature at which the substituent effect is
supposed to be reversed) and this is far from the temperatures
used in the kinetic runs. The DS# values are negative as
expected for bimolecular nucleophilic substitution.53 This is
presumably due to the transition state exhibiting much greater
charge separation than that existing in the reactants which will
be accompanied by a considerable loss of solvent freedom.
Furthermore the mechanism for the reaction series is common
for all members as indicated by the excellent straight plot of
log k30 against log k50 (gradient 1.12 ± 0.02, r = 0.99).
On the basis of no observation of an intermediate, the
effects of substituent; no amine catalysis and high negative r
values, it is evident that the reaction between DNCBTF and
substituted anilines 2a–h proceeds by a two stage mechanism
with the first stage rate-determining.
28 G. Socrates, Infrared Characteristic Group Frequencies, Wiley & Sons,
1980.
29 R.M. Silverstein, G.C. Bassler and T.C. Morrill, Spectrometric
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