Study of a novel reaction between N,N-diphenylthiourea and p-chloranil through a charge-transfer intermediate

Article abstract of DOI:10.1039/a810008j

The well-known electron acceptor, p-chloranil (A) and donor, N,N'-diphenylthiourea (B) have been observed to form a labile charge-transfer (CT) complex (AB), which decays slowly into two products viz. 2,3,5-trichloro-6-(N,A'-diphenylthioureido)benzo-1,4-quinone (P) and 5,6-dichloro-2,3-(N,N'-diphenylthioureyIene)benzo-1,4-quinone (Q). The final products have been isolated in pure form and one of them, P, has been shown to exhibit sensitive solvatochromism. A detailed kinetic study of the decay process of the CT complex has been carried out and based on kinetic data, the following steps have been suggested for the decay process.

Full text of DOI:10.1039/a810008j

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Study of a novel reaction between N,NЈ-diphenylthiourea and
p-chloranil through a charge-transfer intermediate
Tridibranjan Roy, Kakali Datta, Mrinal K. Nayek, Asok K. Mukherjee, Manas Banerjee
and Bejoy K. Seal*
Department of Chemistry, Burdwan University, Golapbag, Burdwan 713104, India
Received (in Cambridge, UK) 24th December 1998, Accepted 23rd July 1999
The well-known electron acceptor, p-chloranil (A) and donor, N,NЈ-diphenylthiourea (B) have been observed to form
a labile charge-transfer (CT) complex (AB), which decays slowly into two products viz. 2,3,5-trichloro-6-(N,NЈ-
diphenylthioureido)benzo-1,4-quinone (P) and 5,6-dichloro-2,3-(N,NЈ-diphenylthioureylene)benzo-1,4-quinone (Q).
The final products have been isolated in pure form and one of them, P, has been shown to exhibit sensitive
solvatochromism. A detailed kinetic study of the decay process of the CT complex has been carried out and based on
kinetic data, the following steps have been suggested for the decay process.
k1
A ϩ B ⇐⇒ AB (fast)
AB
(AB)Ј (slow)
k2
k3
(AB)Ј
P ϩ HCl
P ϩ B
Q ϩ HCl; k3 Ӷ k2
Electron donor–acceptor (EDA) or charge-transfer (CT) com-
plexes are of current interest for their potential non-linear
optical activity.1 Formation equilibria of such complexes are
currently being studied in micelles and micro-emulsions.2–4
Although a vast literature exists on the spectroscopic and other
physicochemical aspects5–8 of these complexes, reports on
detailed kinetic studies of chemical reactions proceeding
through the ground states of such complexes are few in
number.9–12 Several such reactions have been carried out fairly
recently.13–20 In the present paper a detailed kinetic study
(including isolation and characterisation of the products) on
the reaction between N,NЈ-diphenylthiourea (a well-known
electron donor) and p-chloranil† (an electron acceptor2) has
been reported. Solvatochromism of one of the products has
also been studied.
(EtAc) in Bz. The blue compound (P) was collected in the
benzene fraction and the brown compound (Q) in the 10%
EtAc ϩ Bz fraction. 2,3,5-Trichloro-6-(N,NЈ-diphenylthioure-
ido)benzo-1,4-quinone (P) and 5,6-dichloro-2,3-(N,NЈ-diphen-
ylthioureylene)benzo-1,4-quinone (Q) are isolated in pure form
by repeated chromatography in the dark.
Characterisation of the final products
At first the stoichiometric ratio DPTU–p-chloranil required for
the formation of P and Q was determined by Job’s method of
continuous variation. Mixtures containing the same total con-
centration of DPTU and p-chloranil but different molar ratios
(in CHCl3) were kept in the dark for 12 h and their absorbances
were measured at 620 nm (the λmax of isolated P). Absorbances
of the same solutions were measured at 479 nm (the λmax of
isolated Q where P has no absorption) after keeping them for
24 h in the dark. Experimental data are shown in Table 1 and
Job curves at these two wavelengths are shown in Fig. 1. Results
indicate a 1:1 ratio for P and a 1:2 (p-chloranil–DPTU) ratio
for Q. A Shimadzu UV-VIS 160A spectrophotometer fitted
with a temperature-regulated cell holder was used.
IR spectra (KBr pellet) of the starting materials and of P and
Q show that the 3150 cmϪ1 N–H stretching observed in DPTU
is weak in P and shifted to a lower ν(2900 cmϪ1). There is
enough evidence, particularly with p-chloranil as the acceptor,23
that N–H stretching is much weakened by complexation. This,
together with the fact that P readily eliminates HCl in the
presence of an excess of DPTU (or other alkalis) to form Q,
leads to the proposition of structure I (Fig. 2) for P. In the IR
spectrum of Q the N–H band is totally absent.
Experimental
Materials
p-Chloranil (Fluka AG) was purified by repeated sublimation
to get bright yellow crystals (mp = 290 ЊC). N,NЈ-Diphenyl-
thiourea (DPTU) was purified by recrystallisation from dried
toluene. The solvents chloroform, carbon tetrachloride,
dichloromethane and acetonitrile (for the kinetic study) and
the other solvents (for the study of solvatochromism of P) were
purified by standard methods21,22 just before use.
Isolation of the final products
A mixture of DPTU and p-chloranil in a mole ratio slightly
greater than 2:1 in chloroform was refluxed for 8 h in the dark.
The blackish blue solution, after filtration, was extracted with
aqueous 2% solution of NaHCO3 to remove unreacted p-
chloranil and then subjected to chromatographic separation on
silica gel; eluents used are successively petroleum ether (PE),
50% mixture of benzene (Bz) and PE, Bz and 10% ethyl acetate
The proposed structures are consistent with the results of
elemental analysis: P: C19H11N2O2Cl3S requires C, 52.11; H,
2.51; N, 6.4. Found C, 52.09; H, 2.49; N, 6.37%. Q:
C19H10N2O2Cl2S requires C, 56.86; H, 2.49; N, 6.98. Found C,
56.90; H, 2.48; N, 6.95%. Further support for these structures is
1
provided by H NMR (300 MHz) and 13C NMR (75.5 MHz)
spectra recorded with a Bruker AM-300L superconducting
1
magnet NMR spectrometer using a 5 mm H–13C dual probe
and operating with Bruker DISR 861 software (solvent CDCl3,
† p-Chloranil is 2,3,5,6-tetrachlorobenzo-1,4-quinone.
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Table 1 Results of Job type experiment in CHCl3 at λ = 620 nm after
12 h and at λ = 479 nm after 24 h. A = p-chloranil, B = DPTU. Total
conc. [A]ϩ[B] = 1.0006 × 10Ϫ2 mol dmϪ3. Temp = 297 K
Table 2 Variation of absorbance of A ϩ B mixture in chloroform
medium with time at 297 K. [A]0 = 7.805 × 10Ϫ3 mol dmϪ3
[B]0 = 1.2324 × 10Ϫ2mol dmϪ3
,
Absorbancea
Absorbancea
[B]/10Ϫ2 mol
[A]/10Ϫ2 mol
Time/min
475 nm
392 nm
620 nm
dmϪ3
dmϪ3
[B]/([A] ϩ [B])
620 nm
479 nm
2
5
8
0.131
0.147
0.158
0.172
0.177
0.180
0.183
0.186
0.191
0.195
0.198
0.116
0.163
0.205
0.269
0.323
0.373
0.417
0.458
0.530
0.564
0.617
0.031
0.044
0.052
0.061
0.067
0.073
0.080
0.086
0.096
0.102
0.112
0.9339
0.8672
0.8005
0.7338
0.6671
0.6004
0.5003
0.4003
0.3335
0.0667
0.1334
0.2001
0.2668
0.3335
0.4002
0.5003
0.6003
0.6671
0.9333
0.8666
0.8000
0.7333
0.6666
0.6000
0.5000
0.4000
0.3333
0.116
0.222
0.303
0.398
0.483
0.527
0.532
0.491
0.466
0.173
0.307
0.492
0.654
0.732
0.727
0.660
0.571
0.516
13
18
23
28
33
41
46
53
a Against chloroform as reference.
a Against chloroform as reference.
Fig. 1 Job curves: (a) at λ = 620 nm after 12 h and (b) at λ = 479 nm
after 24 h.
internal standard TMS): 1H NMR of P shows a broad peak at
δ 4.35 corresponding to the N–H proton and an aromatic
multiplet in the δ 6.5 to 8 region, the ratio of the signal inten-
sities being nearly 1:10. DEPT in the aromatic region reveals
four distinct branches, viz., δ 6.92 (dd, J 8.0, 0.3 Hz), 7.11 (dd,
J 8.0, 8.0 Hz), 7.28–7.40 (m, J 9.2, 9.0, 2.7, 2.1 Hz) and 7.58 (dd,
J 6.0, 0.17 Hz). The 1H NMR spectrum of Q shows only
an aromatic multiplet in the δ 6.8 to 8.0 region, with DEPT
analysis, δ 6.84 (dd, J 7.3, 1.05 Hz), 6.99 (dd, J 7.5, 7.5 Hz),
7.19–7.3 (m, J 4.5, 7.8, 6.0, 5.4 and 2.1 Hz) and 7.46 (m, J 7.5,
4.0, 2.0, 1.8 Hz). 13C chemical shift assignments of P and Q are
shown in Fig. 2.
Owing to the very low solubility of p-chloranil (A) in the
solvent used, determination of the pseudo order of the reaction
with respect to DPTU (B) requires a very low concentration of
the latter. Reaction mixtures with such low concentrations
(~10Ϫ2 mol dmϪ3 in A and ~10Ϫ3 to 10Ϫ4 mol dmϪ3 in B) did not
show appreciable spectral change within about two hours after
mixing. For determination of the pseudo order with respect to
A, the reverse was attempted, i.e., B was used in large excess,
but it was found that the nature of variation of the spectrum of
the A ϩ B mixture was the same as that when they were taken
in equimolar quantities (~10Ϫ2 mol dmϪ3 each); indeed it was
observed that the rate depends on the product of the concentra-
tions of A and B, which suggests the involvement of an AB
adduct as intermediate. This was further supported by the
appearance of a broad absorption band (different from those of
p-chloranil and DPTU) centered at 475 nm, within a few min-
utes of mixing the reactant solutions. Owing to the non-
Fig. 2 Chemical shift assignment for 13C of the compounds P (struc-
ture I) and Q (structure II).
availability of a stopped flow set up the time evolution of the
system in this short period could not be studied. However, the
change in the spectrum in the subsequent 1 h is slow and inter-
esting and the kinetics of the reaction in this period have been
followed and reported in this paper. A typical case showing the
variation of absorbance, D, of the A ϩ B mixture in chloro-
form at λ = 392 nm and 475 nm is shown in Table 2. The
variation of the whole spectrum of the mixture at 294 K is
shown in Fig. 3. Similar variation was observed in acetonitrile,
dichloromethane and 2:1 mixture (by volume) of CH2Cl2 and
CCl4. A kinetic study was carried out at four temperatures in
CHCl3 medium. The effect of the solvent on the kinetics was
studied at 297 K.
Data treatment
Since the conventional method of isolation for the determin-
ation of the reaction order could not be applied, we tried a
number of schemes based on the following observations: (1)
elimination of HCl was confirmed by classically testing the
presence of HCl in the vapour over the reaction mixture while
the reaction was on. (2) The intensity of the broad absorption
band centered at 475 nm increases with time and simul-
taneously a new band at 390–400 nm (for the various solvents
used) appears and intensifies with time (Fig. 3). (3) Two end
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Table 3 Enthalpy of formation (∆Hf) of AB and enthalpy (∆H*) of
activation for the isomerisation step
Thermodynamic parameters
obtained
Temp/K
k1r/10Ϫ3 minϪ1 Kε3
294
297
304
309.3
7.500
5.834
10.200
31.150
1676
∆Hf = (Ϫ7.07 0.15) kcal molϪ1
∆H* = (Ϫ17.19 5.9)kcalmolϪ1
1495
1147
912
Table 4 Effect of relative permittivity of the medium on the rate of
the isomerisation step (at 297 K)
Solvent
Relative permittivity (d) k1r/10Ϫ3 minϪ1
Fig. 3 Variation of the whole spectrum of the A ϩ B mixture in
chloroform at 294 K. [A]0 = 0.8317 × 10Ϫ2 mol dmϪ3, [B]0 = 1.0274 ×
10Ϫ2 mol dmϪ3, (1) t = 1 min, (2) t = 3 min, (3) t = 5 min, (4) t = 7 min,
(5) t = 9 min, (6) t = 25 min.
CHCl3
4.6
5.1
8.1
5.8
6.8
15.1
37.0
CH2Cl2 ϩ CCl4(2:1 by vol)
CH2Cl2
CH3CN
36.0
products P and Q were obtained and isolated. P could be con-
verted into Q by adding DPTU or NaOH solution. (4) The
intensities of the absorption bands were found to increase more
rapidly when the product of the initial concentrations of the
A
and B increased. Out of various tentative schemes,
experimental data were found to fit the following well:
k1
A ϩ B ⇐⇒ AB (fast; formation const. = K) AB (AB)Ј(slow)
k2
k3
(AB)Ј
P ϩ HCl
P ϩ B
Q ϩ HCl
The molecular complex, AB, has a broad absorption band
with λmax = 475 nm while its isomer, (AB)Ј absorbs both at 390–
400 nm and at 475 nm as revealed by Fig. 3. An initial 5 to 6
minutes are required for the establishment of the formation
equilibrium of AB. After that the rate of formation of (AB)Ј is
given by eqn. (1), which yields eqn. (2) on integration, where
Fig. 4 Plots showing the validity of eqn. (5). ᭺ for [A]0[ B]0 =
1.0993 × 10Ϫ4 (mol dmϪ3)2, and ∆ for [A]0[B]0 = 0.9619 × 10Ϫ4 (mol
dmϪ3)2.
d[(AB)Ј]/ dt = k1K [A]0 [B]0 Ϫ k2 [(AB)Ј]
(1)
(2)
[(AB)Ј] = k1K[A]0[B]0 (1 Ϫ eϪk t)/k2
2
DЈ Ϫ DЈ1Ј = k1K[A]0[B]0 ε1 (t Ϫt1)
D475 Ϫ D1475 = k1K[A]0[B]0 ε2 (t Ϫt1)
(8)
(9)
square brackets denote molar concentrations of the species en-
closed and the subscript 0 indicates values at t = 0. Absorbance
(DЈ) in the region 390–400 nm in CHCl3, which is solely due to
(AB)Ј, is then given by eqn. (3) and that at 475 nm by eqn. (4),
Results of kinetic study
In all the cases studied, eqns. (5), (8) and (9) were found to be
valid. One typical plot for each of these eqns. is shown in Figs.
(4) and (5). From the intercept of the plot of D475/[A]0[B]0
against DЈ/[A]0[B]0, Kε3 was determined. The slope of the plot
gave k1K[A]0[B]0ε2 according to eqn. (9). Thus an effective rate
constant k1r, where r = ε2/ε3, was determined. Assuming molar
absorptivities in a particular solvent do not vary with tempera-
ture, the enthalpy of formation of the complex AB was esti-
mated from a plot of ln(Kε3) against 1/T (Fig. 6a) and the
enthalpy of activation for the isomerisation step was obtained
from a plot of ln(k1r) against 1/T (Fig. 6b). Results are summar-
ised in Table 3. The effect of solvent on k1r is given in Table 4. It
was found that k1r increases with increase in relative permittiv-
ity (d) of the solvent and the plot of ln(k1r) against (d Ϫ 1)/
(2d ϩ 1) is fairly linear, as shown in Fig. 7, the correlation being
given by eqn. (10) with a correlation coefficient of 0.84. This
DЈ = k1K[A]0[B]0ε1 (1 Ϫ exp(Ϫk2t))/k2
(3)
D475 = k1K[A]0[B]0ε2 (1 Ϫ exp(Ϫk2t))/k2 ϩ K[A]0[B]0ε3 (4)
where ε1 and ε2 are the molar absorptivities of (AB)Ј at some
λ in the region 390–400 nm and at 475 nm respectively. A
combination of eqns. (3) and (4) gives eqn. (5).
D475/[A]0[B]0 = (ε2/ε1) DЈ/[A]0[B]0 ϩ Kε3
(5)
A plot of D475/[A]0[B]0 against DЈ/[A]0[B]0 is therefore expected
to be linear with intercept Kε3. During the first hour of study,
absorbance at 390–400 nm increases appreciably with time but
the development of the 620 nm absorption band (which corre-
sponds to the formation of P) is too slow to be detected. Hence
k2 is very small and we can approximate eqns. (3) and (4) to the
forms (6) and (7). With a suitable initial time t1 these equations
can be simplified to eqns. (8) and (9).
ln(k1r) = 22.93 (d Ϫ 1)/(2d ϩ 1) Ϫ 14.2
(10)
suggests that the isomerisation step AB→(AB)Ј involves a
charge separated species AϪ ؒ ؒ ؒ Bϩ as an intermediate (activ-
ated state) so that an increase in solvent polarity decreases the
activation energy for this step, as proposed by Laidler and
DЈ = k1K[A]0[B]0 ε1 t
(6)
(7)
D475 = k1K[A]0[B]0ε2 t ϩ K[A]0[B]0 ε3
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Fig. 5 Plots showing the validity of eqns. (8) and (9) in chloroform at 297 K. (A) at λ = 475 nm with (i) [A]0[ B]0 = 1.0993 × 10Ϫ4 (mol dmϪ3)2, and (ii)
[A]0[ B]0 = 0.9619 × 10Ϫ4 (mol dmϪ3)2. (B) at λ = 392 nm with the same set of concentrations as in (A).
Fig. 7 Effect of solvent polarity on the rate of the isomerisation step.
varies linearly with the ET(30) and Z-polarity indices25–31 of
solvents. Nine different solvents were studied and the results are
shown in Table 5 and Fig. 8. Except for benzene and toluene the
plot of ZЈ vs. Z and ET(30) are fairly linear, the correlation
being given by eqn. (11).
Fig. 6 (a) Plot of ln(Kε3) against 1/T for evaluation of ∆Hf of AB. (b)
Plot of ln(k1r) against 1/T for evaluation of activational parameters.
ZЈ = 0.154 Z ϩ 35.28, corrln. coeff. = 0.99
Eyring.24 This is similar to the observation by Karmakar and
Basu16 in the reaction of halanils with procaine, benzocaine
and lignocaine.
ZЈ = 0.174 ET(30) ϩ 38.43, corrln. coeff. = 0.89 (11)
The deviation of benzene and toluene may be attributed
to the π-donor ability of the solvent molecules to form an
intermolecular CT complex with the solute P whose molecule
has a π-acceptor moiety. These results indicate that P may well
be used as a probe material for measuring polarity of non-
aromatic solvents. The electronic absorption spectra of Q,
however, did not show such sensitive solvatochromism.
Solvatochromism of the blue compound P
The broad, long wavelength absorption band of the compound
P (having molar absorptivity 710 at λ = 620 nm in CHCl3) was
found to be appreciably sensitive to solvent polarity. The transi-
tion energy (ZЈ, eV) calculated from the λmax of this band,
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Table 5 Solvatochromism of the blue compound P. The ET(30) and Z
values were collected from refs. 25–31
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Solvent
λmax/nm of P Transition energy (Z)Ј)/ev ET(30)
Z
CCl4
C6H6
650
611.5
611
620
613
598
582
578
42.72
45.37
45.40
44.78
45.29
46.43
47.71
48.04
46.74
32.5
34.5
33.9
39.1
41.1
46.0
51.9
55.4
48.6
C6H5CH3
CHCl3
CH2Cl2
CH3CN
C2H5OH
CH3OH
63.2
64.2
71.3
79.6
83.6
76.3
Propan-2-ol 594
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of a number of solvents.
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react via a 1:1 molecular complex as an intermediate giving two
products, P and Q. In the 1:2 stoichiometry as obtained by
Job’s method, the second molecule of DPTU is required
for abstraction of HCl from P. The reaction path has been
established by kinetic study. One product, P, shows solvato-
chromism sensitive enough to be used as a probe material for
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Acknowledgements
The authors thank the learned referees for their valuable
suggestions and also Professor S. Thakur, Department of
Chemistry, Burdwan University, for his help in assigning
IUPAC names to the end products. Financial assistance by the
UGC, New Delhi, extended through the DSA project, is also
gratefully acknowledged.
Paper 8/10008J
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