Effect of external pressure and solvent on the equilibrium constant of the Diels-Alder reaction of 9-chloroanthracene with tetracyanoethylene

Article abstract of DOI:10.1023/A:1013946802615

The effect of external pressure and solvent on the equilibrium constant of the Diels-Alder reaction of tetracyanoethylene with 9-chloroanthracene at 25°C was studied. The molar reaction volume is strongly solvent-dependent, cm³/mol: -11.3±1.0 in ?-xylene, -14.9±1.0 in toluene, -20.6±1.5 in 1,2-dichloroethane, -22.6±1.5 in ethyl acetate, and -24.2±1.5 in acetonitrile.

Full text of DOI:10.1023/A:1013946802615

Russian Journal of General Chemistry, Vol. 71, No. 10, 2001, pp. 1565 1569. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 10, 2001,
pp. 1652 1657.
Original Russian Text Copyright
2001 by Kiselev, Iskhakova, Shikhab, Konovalov.
Effect of External Pressure and Solvent
on the Equilibrium Constant of the Diels Alder Reaction
of 9-Chloroanthracene with Tetracyanoethylene
V. D. Kiselev, G. G. Iskhakova, M. S. Shikhab, and A. I. Konovalov
Butlerov Research Chemical Institute, Kazan State University, Kazan, Tatarstan, Russia
Received February 16, 2000
Abstract The effect of external pressure and solvent on the equilibrium constant of the Diels Alder
reaction of tetracyanoethylene with 9-chloroanthracene at 25 C was studied. The molar reaction volume is
strongly solvent-dependent, cm3/mol: 11.3 1.0 in o-xylene, 14.9 1.0 in toluene, 20.6 1.5 in 1,2-di-
chloroethane, 22.6 1.5 in ethyl acetate, and 24.2 1.5 in acetonitrile.
The effect of increased external pressure on the rate
and equilibrium of chemical reactions at constant
temperature fits the pressure dependences of the free
energies of activation and reaction [1 3]. If pressure-
assisted changes in solvent properties exert no rate or
equilibrium effects, which can also be simulated at
atmospheric pressure, then the pressure dependences
of the free energies of activation or reaction at
constant temperature allow the molar volumes of
activation or reaction to be estimated by Eqs. (1)
and (2), respectively.
ficient, and this may show up in that the experimental
entropy of activation would be pressuredependent.
The problem of equilibrium between initial and
transition states has been actively discussed [1 5].
There are some grounds to state that when solvent has
moderate viscosity (<10 cP) and half-conversion time
2
(>10 s), the probability of such equilibrium is rather
high. For faster reactions in high-viscosity media, the
rate at increased pressure is already diffusion-control-
led, and, therefore, not adequately described by the
transition state theory, while the molar activation
volume is already not the only factor controlling the
rate pressure dependence [7, 8]. Polar and especially
ionic activated complexes can strongly interact with
solvent; as a result, the partial molar volumes of these
states decrease considerably, thus increasing the
absolute molar activation volume [9]. The largest
molar activation volumes in such processes are ob-
served in nonpolar media, which is associated with
increased compressibility of nonpolar solvents
[1 5, 9]. However, the allowance for the pressure-
assisted solvent effect on the rate of heterolytic reac-
tions results in a much weaker solvent dependence of
the activation volume [6]. This may be accounted for
by increased energy of interaction of the polar acti-
vated complex with more polar solvents whose com-
pressibilies are frequently, while not always, higher
that those of nonpolar media [5].
ln kP/k0
=
V*P/RT,
VP/RT.
(1)
(2)
ln KP/K0
=
Increased pressure affects such solvent properties
as density, dielectric constant, and viscosity [4, 5],
which may occasionally produce rate and equilibrium
effects, even if the states in hand have different
volumes.
Recently we showed [6], with heterolytic dipolar
(2+2)-cycloaddition reactions and the Menshutkin
reaction as examples, that the rate effect of external
pressure is associated not only with change in the
volume [Eq. (1), V*], but also with a considerable
(up to 30%) additional change in the free energy of
activation, which results from pressure-assisted
changes in the dielectric constant of solvent. One of
the reasons why relationships (1) and (2) for hetero-
lytic reactions deviate from linearity lies in the non-
linearity of the dependence of the dielectric constant
(and its reciprocal) of solvent on external pressure.
The contribution of pressure-assisted solvent effect
into experimental molar volumes of reactions between
polar, nonpolar, and ionic structures [Eq. (2)] can be
estimated
independently via the partial molar
volumes of reagents and products, while the effect of
electrostriction of nonpolar and polar solvents, via the
partial molar volumes of polar or ionic compounds in
It terms of the transition state theory, change in
solvent viscosity may affect the transmission coef-
1070-3632/01/7110-1565$25.00 2001 MAIK Nauka/Interperiodica

1566
KISELEV et al.
these media. Unfortunately, because of the poor
solubility of ionic compounds in nonpolar media, their
partial molar volumes cannot be measured with suf-
ficient reliability. For the highly polar Reichardt’s
zwitter ion ( 12 D), no correlation between partial
molar volume and solvent polarity was found [10].
ing reagent absorbances at increased pressure, main-
taining constant temperature, and ensuring observation
time sufficient for the system to attain equilibrium. In
preliminary experiments we found that the sharp tem-
perature rise with pressure ceases within 3 5 min,
while the optical density of the solution at 25 C no
longer varies within 2 h in o-xylene and toluene and
within 1 h in acetonitrile and ethyl acetate. For more
precise equilibrium constants all measurements were
performed with a large excess of one of the reagents.
Recently we showed [11, 12] that in cases where
one of the states enters strong specific interactions
with solvent, a strong effect of the medium of the
molar reaction volume can be observed with nonpolar
compounds, too. The partial molar volumes of tetra-
cyanoethylene, a strong -acceptor (EA 2.88 eV [13]),
cyclopentadiene, and their adduct in the Diels Alder
reaction in 11 solvents we estimated molar reaction
volumes, cm3/mol, which were found to sharply
decrease in going from -donor ( 21.8 in mesitylene,
24.3 in o-xylene, 28.6 in toluene, 32.0 in benzene,
and 32.0 in chlorobenzene) to n-donor and inert
solvents ( 34.3 in cyclohexane, 35.9 in 1,4-dioxane,
36.8 in 1,2-dichloroethane, 37.2 in dichloromethane,
40.7 in acetonitrile, and 45.6 in ethyl acetate). As
noted in [12], the partial molar volume of cyclo-
pentadiene varies along this series no more than
2 cm3/mol, whereas the partial molar volumes of
tetracyanoethylene and the adduct are comparable
vary almost equally but in different direction, which
For all the solutions we preliminarily determined
optical density pressure dependences (Table 1).
It should be noted that Tait’s equation which
reliably describes the dependence of solvent volume
on pressure fails to predict trends in the absorbance
of tetracyanoethylene and 9-chloroanthracene solu-
tions, since increasing pressure produces batho-
chromic shifts of the absorption bands. Therefore, as
the pressure increases, the optical density of the solu-
tions may increase, remain invariable, or decrease,
depending on whether it is measured at the descending
or ascending branch of the absorption bands, or near
its maximum.
The equilibrium constants were calculated by
Eqs. (3) or (4), depending on whether the absorbance
results in a large difference in the partial molar reac- of diene I or dienophile II is followed, respectively.
tion volumes (in going from mesitylene to ethyl
acetate, V = 23.8 cm3/mol). To assess the possible
pressure-assisted solvent effect on molar reaction
volume, we estimated Diels Alder reaction volumes
for tetracyanoethylene in a series of solvents from the
experimental dependence of the equilibrium constant
on external pressure [Eq. (2)]. The Diels Alder reac-
tion of tetracyanoethylene with cyclopentadiene is
virtually nonequilibrium; therefore, we studied the
effect of external pressure on the equilibrium of the
reaction between 9-chloroantharacene (I) with tetra-
cyanoethylene (II).
KP = c3/(c01
KP = c3/(c02
c3)c02 = (D01
at c02 >> c01,
c3)c01 = (D02
at c01 >> c02.
D1)/(D1)c02
D2)/(D2)c01
(3)
(4)
Here D01 and D1 are the initial optical density of
diene I before reaction and in equilibrium, and D02
and D2, the same for dienophile II. Obviously, all
optical densities should be reduced to the same pres-
sure. Since D01 and D02 are more convenient to
measure at atmospheric pressure, the equilibrium
optical densities at increased pressure (D1 or D2) were
reduced, using the preliminarily established DP/DP=1
P dependence (Table 1), to P = 1. Reducing the
optical densities to a pressure (P), at which KP is
determined, would give the same result, since the
Cl
NC
NC
CN
CN
+
I
II
NC
NC
(D01
kPD1)/(kPD1) ratio for P = 1 is equal to
D1)/(D1) for increased pressure P, where
CN
CN
(D01/kP
k2
k1
kP = DP=1/DP. Thus estimated equilibrium constants
are listed in Table 2.
Cl
III
The molar volumes of the Diels Alder reactions
with substituted anthracenes are always larger in
absolute value than those of the reactions with cyclo-
pentadiene and, especially, butadiene, even with the
When measuring equilibrium constants at increased
pressure, one deals with problems of directly measur-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

EFFECT OF EXTERNAL PRESSURE AND SOLVENT
1567
same dienophile [1 3,6]. The possible reason for this
difference is, in our opinion, reduced accessibility of
shielded atoms of adduct III for interaction with
solvent. In this case, the latter adduct has more
cavities inaccessible for solvent, and, therefore, its
molar volume increases to a greater extent than those
of the adducts with cyclopentadiene and butadiene.
The variation in the equilibrium constants with increas-
ing pressure for all the reactions (Table 2) is decribed
by Eq. (5), from which follows Eq. (6):
Table 1. Dependence of measured optical density (D) on
external pressure
P, kg/cm2
DP/D1
P, kg/cm2
DP/D1
1
Solvent o-xylene,
c2 1.59 10 M, DP=1 0.564
19050 cm ,
3
1
200
395
595
800
990
890
710
520
320
1
1
355
550
780
1000
865
660
460
300
150
1
1.172
1.269
1.381
1.474
1.418
1.319
1.222
1.140
1.072
0.999
1.090
1.186
1.278
1.370
1.453
1.413
1.333
1.238
1.140
1.001
ln KP/K0 = a + bP,
V = bRT.
(5)
(6)
For the pressure expressed in kg/cm2, R =
1
84.78 kg cm K 1 mol . As seen from Table 2, the
molar volume, cm3/mol, of the reaction in study is
11.3 1 in o-xylene, 14.9 1 in toluene, 20.6 1.5
in 1,2-dichloroethane [6], 24.2 1.5 in acetonitrile,
and 22.6 1.5 in ethyl acetate. Thus, the strong varia-
tion in the molar volume of the Diels Alder reaction
of tetracyanoethylene with cyclopentadiene, we
deduced earlier from partial molar volumes [12],
could now be revealed for the reaction of tetracyano-
ethylene with 9-chloroanthracene from the depen-
dence of equilibrium constants on external pressure.
We failed to determine the partial molar volume of
adduct III and the molar volumes of the reactions in
toluene and o-xylene because of the poor solubility
and the high degree of dissociation of compound III
into the initial reagents. The molar volume of the re-
action in 1,2-dichloroethane has been determined in
[6] from the pressure dependence of the equilibrium
constant ( 20.6 1.5 cm3/mol), from the difference in
the activation volumes of the direct ( 28.5
1.5 cm3/mol) and reverse ( 6.5 0.5 cm3/mol) reac-
tions, and from the difference in the volumes of
adduct III (255.5 1.5 cm3/mol), diene I (170.7
0.5 cm3/mol), and dienophile II (107.8 0.2 cm3/mol),
from which it follows that the molar reaction volumes
estimated by the three independent methods fairly
agree with each other.
4
DP/D1 = (0.999 0.003) + (4.720 0.050) 10 P;
n 21, r 0.9989, s 0.0071.
1
Solvent toluene,
22730 cm ,
4
c2 1.90 10 M, DP=1 0.491
1
245
495
755
590
410
1
215
380
680
850
1
1000
710
400
270
120
1
1010
830
1
1.262
1.175
1.095
1.053
1.016
0.998
1.281
1.205
0.997
1.188
1.062
1.119
1.181
1.142
1.104
0.999
1.050
1.095
1.178
1.251
800
1
Solvent acetonitrile,
25770 cm ,
5
c1 6.30 10 M, DP=1 0.295
1
250
530
1270
1100
910
1
760
600
515
410
255
1
0.888
0.905
0.919
0.936
0.959
0.959
0.919
0.841
0.861
0.875
1.001
4
DP/D1 = (1.000 0.001)
(1.789 0.058) 10 P
+ (4.350 0.458) 10 P2; n 12, r 0.9982, s 0.002.
8
1
Solvent ethyl acetate,
25770 cm ,
5
c1 8.03 10 M, DP=1 0.708
1
265
440
625
820
1030
1
180
360
570
1
775
1015
1
230
425
645
820
1010
1
0.955
0.933
1.001
0.994
0.985
0.971
0.953
0.935
1.001
Since the acceptor properties of tetracyanoethylene
are much reduced not only in going to the adduct, but
also in going to the activation complex of the Diels
Alder reaction [14], one can expect a similar solvent
effect on the molar activation volume of the reaction
involving this dienophile.
0.994
0.981
0.966
0.950
0.937
0.999
0.997
0.984
0.972
EXPERIMENTAL
5
Tetracyanoethylene, 9-chloroanthracene, and sol-
vents were purified as described in [6, 12, 15]. The
design of the barostat with direct spectrophotometric
DP/D1 = (1.001 0.001)
(2.813 0.526) 10 P
(3.703 0.508) 10 P2; n 19, r 0.9916, s 0.002.
8
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

1568
KISELEV et al.
Table 2. Optical densities of solutions at equilibrium
reagent concentrations (DP) at increased pressure, their
values reduced from the (DP/D1 P) dependence to P = 1
(Dcalc), and equilibrium rate constants (K) for the Diels
Alder reaction of tetracyanoethylene with 9-chloroan-
thracene at 25 C
Table 2. (Contd.)
P, kg/cm2
D2
Dcalc
KP
ln KP/K1
Acetonitrile
5
3
D01 0.663, c01 6.32 10 , c02 1.10 10 M,
1
P, kg/cm2
D2
Dcalc
KP
ln KP/K1
25770 cm
1
0.422
0.422
519
0
o-Xylene
5
3
2
3
D01 0.663, c01 6.32 10 , c02 1.10 10 M,
D
02 0.600, c01 2.04 10 , c02 1.59 10 M,
1
1
25770 cm
19050 cm
1
200
250
450
520
710
910
0.504
0.541
0.548
0.578
0.605
0.636
0.662
0.504
0.494
0.490
0.485
0.485
0.476
0.464
9.5
10.7
11.2
11.8
11.8
13.0
14.6
0
980
770
565
215
0.234
0.273
0.317
0.370
0.270
0.307
0.347
0.383
1323
1054
828
0.935
0.708
0.467
0.246
0.119
0.165
0.217
0.217
0.314
0.430
664
5
3
D01 0.660, c01 6.32 10 , c02 1.25 10 M,
1
25770 cm
2
D
02 0.556, c01 2.04 10 , c02 1.50 10 3 M,
1
250
505
775
1015
645
0.399
0.347
0.295
0.242
0.205
0.271
0.399
0.362
0.320
0.272
0.238
0.300
523
658
850
1140
1420
960
0
1
19050 cm
0.230
0.485
0.779
0.999
0.607
1
460
960
0.479
0.564
0.648
0.479
0.463
0.447
8.7
10.8
13.1
0
0.216
0.409
2
3
D02 0.566, c01 2.04 10 , c02 1.55 10 M,
4
1
lnKP/K1 = ( 0.00154 0.0174) + (9.566 0.084) 10 P;
n 11, r 0.9958, s0 0.003.
19050 cm
1
530
990
0.487
0.586
0.658
0.487
0.468
0.449
8.7
11.3
14.0
0
V
24.2 1.5 cm3/mol
0.258
0.475
Ethyl acetate
4
lnKP/K1 = (0.0113 0.0082) + (4.484 0.0427) 10 P
n 14, r 0.9918, s 0.021.
5
3
D01 0.688, c01 8.03 10 , c02 5.66 10 M,
1
V
11.3 1.0 cm3/mol
25770 cm
Toluene
1
245
350
410
700
0.415
0.375
0.364
0.345
0.296
0.249
0.415
0.378
0.369
0.351
0.308
0.266
116
145
153
170
218
280
0
2
4
D02 0.642, c01 2.94 10 , c02 2.0 10 M,
0.223
0.277
0.382
0.631
0.881
1
22730 cm
1
0.397
0.385
0.386
0.396
0.401
0.398
0.397
0.307
0.313
0.333
0.376
0.398
20.9
37.1
35.7
31.6
24.0
20.9
0
1000
0.574
0.535
0.413
0.138
0
910
750
280
1
1000
5
3
D01 0.688, c01 8.03 10 , c02 5.66 10 M,
1
25770 cm
2
4
D02 0.642, c01 2.94 10 , c02 2.0 10 M,
1
1
300
620
990
0.415
0.375
0.313
0.249
0.415
0.379
0.323
0.266
116
144
200
280
0
22730 cm
0.216
0.545
0.881
1
260
525
760
0.398
0.396
0.391
0.380
0.398
0.374
0.346
0.319
20.9
24.4
29.1
34.4
0
0.155
0.331
0.498
4
lnKP/K1 = ( 0.00953 0.0108) + (8.952 0.060) 10 P;
4
lnKP/K1 = ( 0.00111 0.0114) + (5.920 0.062) 10 P;
n 10, r 0.9951, s 0.023.
n 10, r 0.9980, s0 0.003.
V
14.9 1.0 cm3/mol
V
22.6 1.5 cm3/mol
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

EFFECT OF EXTERNAL PRESSURE AND SOLVENT
1569
Measurements in benzene and mesitylene were not
performed, since in the former solvent the absorption
bands of the reagents appreciably overlap, while in
the second, a very low equilibrium constant is
observed.
ln KP/K1
4
3
1.4
1.2
1.0
0.8
0.6
0.4
0.2
The concentration of the reagent taken in excess
was considered to be independent of pressure.
2
ACKNOWLEDGMENTS
The work was financially supported by the Russian
Foundation for Basic Research (project no. 98-03-
33053a) and the Research Department of Kazan State
University.
1
The authors are grateful to E.A. Kashaeva and
M.D. Medvedev for help in certain experiments.
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0
200 400 600 800 1000
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1
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001