Solvent effect on the volume of activation and volume of the Diels-Alder reaction

Article abstract of DOI:10.1002/poc.398

Volumes of activation, ΔV, and reaction, ΔV, partial molar volumes, V, and enthalpies of solution, Δ_solH, were determined for tetracyanoethylene, cyclopentadiene, 1,3-butadiene, trans, trans-1,4-diphenyl-1,3-butadiene and their Diels

Full text of DOI:10.1002/poc.398

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2001; 14: 636–643
DOI:10.1002/poc.398
Solvent effect on the volume of activation and volume of
the Diels±Alder reaction
1
2
3
1
Vladimir D. Kiselev, * Alexandr I. Konovalov, Tsutomu Asano, Elena A. Kashaeva, Gulnara G.
1
1
1
Iskhakova, Mahdi S. Shihab and Margarita D. Medvedeva
1
Butlerov' Institute of Chemistry, Kazan State University, Kremlevskaya Str. 18, Kazan 420008, Russian Federation
Arbuzov' Institute of Organic and Physical Chemistry, Russian Academy of Science, Arbuzov str. 8, Kazan 420088, Russian Federation
Department of Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1192, Japan
2
3
Received 5 September 2000; revised 23 January 2001; accepted 4 April 2001
≠
ꢀ
ABSTRACT: Volumes of activation, DV , and reaction, DV, partial molar volumes, V, and enthalpies of solution,
epoc
D H, were determined for tetracyanoethylene, cyclopentadiene, 1,3-butadiene, trans, trans-1,4-diphenyl-1,3-
sol
butadiene and their Diels–Alder adducts at 25°C in some solvents of p- and n-donor type. The values of the activation
and reaction volumes were exceptionally small in the former type of solvents. Large solvent effects on D H
(up
sol TCNE
À1
≠
3
À1
ꢀ
to 26 kJ mol ), V
, DV and DV (up to 11 cm mol ) were observed in aromatic solvents and these values are
TCNE
linearly correlated with each other. Poor correlations were found for n-donor solvents, but a linear dependence
between DV and ꢀ H (enthalpy of reaction) was obtained for all solvents. Copyright  2001 John Wiley & Sons,
rÀn
Ltd.
Additional material for this paper is available from the epoc website at http://www.wiley.com/epoc
KEYWORDS: volume of activation; volume of reaction; solvent effect; tetracyanoethylene
SYMBOLS
be obtained from the pressure dependence of the
equilibrium constant at a fixed temperature:
K , equilibrium constant at pressure P; k , rate constant at
p
p
≠
pressure P; DG, free energy of process; DG , free energy
of activation; DV, volume of reaction; DV , volume of
activation; f, apparent molar volume; V, partial molar
volume; V*, partial molar volume of transition state;
ꢀ@ÁG=@P†
T
≠
ꢀ@ ln K=@P† ˆ À
ˆ ÀÁV=RT
ꢀ1†
T
RT
ꢀ
ꢀ
Since thermodynamic equilibrium between the initial and
transition states is assumed in the transition state theory,
the same conclusion is applicable to a kinetic pressure
effect provided that the solvent thermal motions are fast
enough:
D
D
H, enthalpy of reaction; D H, enthalpy of solution;
H, enthalpy of solvation; DsublH, enthalpy of
rÀn
solv
sol
sublimation; IP, ionization potential; T, temperature
(
K); R, gas constant; c , concentration of substance N
N
at equilibrium; c0,N, initial concentration of N; DN,
optical density of N at equilibrium; D0,N, initial optical
density of N; r, correlation coefficient; s, standard
deviation; n, number of measurements; M, molar mass;
d0, density of solvent; d, density of solution.
6
ꢀ@ÁG =@P†
T
6
ꢀ
@ ln k=@P† ˆ À
ˆ ÀÁV =RT ꢀ2†
T
RT
Pressure affects the free energy of reaction through the
PDV term and the various changes in the solute–solvent
and solvent–solvent interactions. As an example, high
pressure affects solvent properties (i.e. dielectric con-
stant, viscosity, etc.) and for this reason additional
changes of the free activation or reaction energies may
occur. These effects may reflect on the validity of the
INTRODUCTION
Information on the volume change during a reaction can
1
volumes of activation and reaction.
Another factor in the pressure effect is the intermol-
ecular distance. Intermolecular distances decrease under
pressure, leading to an increase in the potential energy.
The molecules try to adjust to this new situation by taking
a more crowded conformation, which occupies a smaller
volume. Shifts of the conformational equilibrium of 1,2-
*
Correspondence to: V. D. Kiselev, Butlerov’ Institute of Chemistry,
Kazan State University, Kremlevskaya str. 18, Kazan 420008, Russian
Federation.
E-mail: vkiselev@kzn.ru
Contract/grant sponsor: Russian Foundation for Basic Research;
Contract/grant number: 98-03-33053.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 636–643

SOLVENT EFFECT IN THE DIELS–ALDER REACTION
637
À
Figure 1. Correlations of the values of the partial molar volume of tetracyanoethylene ꢀV ) with ꢀa) the ionization potentials of
1
the solvent, ꢀb) the heat of solution of TCNE, ꢀc) the free energy of the formation of the complex between TCNE and the solvent
and ꢀd) the rate constant in the Diels±Alder reaction of TCNE and anthracene using the data in Table 1. &, Aromatic solvents; ^,
non-aromatic solvents
dichloroethane from an anti to a gauche conformation
determined from Eqns (1) and (2). Therefore, it is useful
to compare the values of the volume parameters from
pressure effects with pressure-independent measure-
ments.
3
À1
2
ꢀ
ꢀ
(
V
À V
= À3.8 cm mol in hexane ) may be
gauche
anti
understood as a result of a combination of these two
effects. In the case of fast reactions and in a medium with
high viscosity the reaction rate can be controlled by
diffusion and the pressure dependence of the rate
In contrast to the activation volume, the value of the
reaction volume can be independently determined via the
difference between the partial molar volumes of the
3–5
constant can be complicated by this effect.
ꢀ
Only when the free energy changes under pressure are
caused by the contribution of the PDV term the values of
reaction and activation volumes can be correctly
products and of the reagents. Partial molar volumes (V)
of solutes at infinite dilution can be estimated by
extrapolation of the apparent molar volumes (') obtained
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 636–643

6
38
V. D. KISELEV ET AL.
À
Table 1. Ionization potentials of solvents ꢀIP), partial molar volume of tetracyanoethylene ꢀV ), enthalpy of solution ꢀD H) and
solvation ꢀDsolvH), free energy ꢀDG) of complex formation of TCNE with alkylbenzenes and the rate constants ꢀk) of the Diels±
Alder reaction of TCNE with anthracene at 25°C
1
sol
b
ÀD
H
solv
a
3
À1
À1
À1
a
À1
c
À1 À1
ꢀ
No. Solvent
IP (eV)
V (cm mol
)
D H (kJ mol
sol
)
(kJ mol
)
ÀDG (kJ mol
)
k (1 mol
s )
1
1
2
3
4
5
6
7
8
9
0
1
2
3
Chlorobenzene
9.10
9.25
109.2
108.4
104.6
102.1
101.5
98.1
108.7
112.1
110.4
105.7
107.8
23.1
14.9
9.7
58.1
66.3
71.5
79.8
81.2
83.9
66.0
72.0
73.6
76.9
59.9
57.8
56.7
À0.65
1.72
3.24
4.81
5.04
7.07
—
1.82
0.38
0.13
0.061
—
0.010
2.18
0.24
0.20
0.34
3.82
4.28
5.47
Benzene
Toluene
8.82
o-Xylene
8.58
1.4
p-Xylene
8.48
0
Mesitylene
8.14
À2.7
15.2
9.2
Acetonitrile
Ethyl acetate
Cyclohexanone
1,4-Dioxane
1,2-Dichloroethane
Dichloromethane
Chloroform
12.12
9.54
—
9.14
9.13
11.12
11.35
11.42
7.6
4.3
21.3
23.4
24.5
—
1
1
1
1
—
—
d
107.5
108.9
—
—
a
b
c
d
For alkylbenzenes from Ref. 9, and for other solvents from Ref. 10.
À1
Calculated with the value of Dsub1H 81.2 kJ mol from Ref. 11.
From Ref. 12.
From Ref. 13.
by measuring the solvent and solution densities:
Waals spheres, the voids around them and the volume
change of the solvent in the process of shell formation
around the solute. The last contribution depends on the
6
'A ˆ 1000ꢀd À d†=c d ‡ M =d
ꢀ3†
ꢀ4†
0
A
0
A
0
difference between solute–solvent and solvent–solvent
interaction energies. In the case of strong ion–solvent
interactions the contraction of the solvent by electro-
striction can exceed the value of the structural volume of
the solute and the value of the partial molar volume can
'A ˆ 1000ꢀd À d†=m d d ‡ M =d
0
A
0
A
'A ˆ ꢀd À d†M ꢀ1 À x †=x d d ‡ M =d ꢀ5†
0
S
A
A
0
A
7
where M and M denote the molar mass of solute and
be negative. From this standpoint the partial molar
A
S
solvent, respectively, d and d0 are the densities of
solution and solvent, respectively, and c , m and x are
volume is not the volume parameter of a solute only, but
can be assigned to the volume change of the system as a
whole, which involves the solute concentration. Never-
theless, the difference between these experimental values
of partial molar volumes of the products and of the
A
A
A
the concentration scales in molarity, molality and mole
ꢀ
fraction, respectively. For a solute the value of VA
includes the structural volume, i.e. the volume of van der
Scheme 1
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 636–643

SOLVENT EFFECT IN THE DIELS–ALDER REACTION
639
À
3
À1
Table 2. Partial molar volumes ꢀV , cm mol ) of dienes 2 and 4 and adducts 6, 7 and 8 and volumes of reactions I±IV ꢀDV,
3
À1
cm mol ) with tetracyanoethylene in various solvents at 25°C
ꢀ
V
ÀDV
No.
Solvent
2
4
6
7
8
I
II
III
IV
1
2
3
4
5
6
7
8
9
Benzene
83.2
82.4
82.4
83.1
82.4
82.6
83.8
81.4
81.8
83.5
83.6
—
199.3
199.8
199.8
199.8
—
200.0
195.2
201.7
200.4
—
159.6
158.3
160.2
159.4
159.6
150.6
150.3
157.5
151.6
153.8
154.7
—
161.9
159.7
160.8
162.3
—
157.6
155.4
158.6
156.1
—
280.8
279.9
280.7
281.0
—
275.5
272.8
278.1
279.8
—
32.0
28.6
24.3
21.8
32.0
40.7
45.5
34.3
35.9
37.2
36.7
—
34.7
33.1
29.5
24.0
—
39.3
44.9
40.0
37.8
—
26.9
24.5
21.2
16.9
—
33.2
34.5
34.0
26.3
—
—
14.9
11.3
—
Toluene
o-Xylene
Mesitylene
Chlorobenzene
Acetonitrile
Ethyl acetate
Cyclohexanone
1,4-Dioxane
Dichloromethane
1,2-Dichloroethane
Chloroform
—
24.2
22.6
—
—
—
20.6
—
1
1
1
0
1
2
202.7
199.8
154.4
154.2
278.8
273.8
41.6
42.9
31.7
34.9
reagents properly equals the reaction volume of the given
reaction system. Similarly, the change of a heat of
reaction in different solvents is determined by the
difference in the heats of solution of products and of
reagents. The heat of solution and the partial molar
volume both reflect the changes in the intermolecular
interactions.
between the energetics and the volumetrics of chemical
reactions. In a series of carefully selected solvents, it is
reasonably expected that the solvent dependence of
reaction volume is strongly correlated with the difference
in the solvation energies of the reagents and of the
products.
For ionic processes, the large solvent effect on the
activation volumes is conditioned by the solvent electro-
The interpretation of the thermodynamic pressure
effects in terms of molar volume changes has been6
proved for some reactions, e.g. the ionization of water.
The reaction volume derived from the pressure depen-
dence of the ionization constant was in good agreement
with that obtained from acid–base reactions. For a non-
polar Diels–Alder reaction the values of the volume of
reaction calculated from the difference in the activation
volumes of forward and backward processes and from
pressure effects on equilibrium were in good agreement
3
,6,7
striction.
In this work the large solvent effects on
≠
activation and reaction volumes (DV and DV) of non-
polar Diels–Alder reactions will be analyzed.
RESULTS AND DISCUSSION
Tetracyanoethylene (1) is known to be a good dienophile
in the Diels–Alder reaction, and, owing to its strong p-
1
8
with those obtained from partial molar volumes. There-
acceptor tendency (lit. E = 2.88 eV), its partial molar
A
fore, the pressure effect can be considered as a bridge
volume is expected to depend on the p-donor properties
À1
Table 3. Enthalpy of solution ꢀD H, kJ mol ) of tetracyanoethylene ꢀ1), dienes 2 and 4 and adducts 6, 7 and 8 and relative
sol
À1
change of the heat of reaction ꢀꢀ H, kJ mol ) in various solvents at 25°C
rÀn
D H
sol
ꢀ
H
rÀn
a
No.
Solvent
2
4
6
7
8
I
II
III
1
2
3
4
5
6
7
8
9
Benzene
0
24.1
23.9
24.4
25.4
32.0
22.5
20.5
21.4
24.4
22.8
—
22.9
22.7
23
22.9
22.5
20.7
22.0
20.2
16.1
15.2
14.8
21.6
21.1
—
20.3
18.3
17.3
18.8
14.4
6.9
13.6
18.4
26.4
30.1
1.3
8.3
14.0
17.4
24.2
28.8
À0.1
7.9
Toluene
0.2
0.8
1.2
2.4
0.5
0.5
0.1
0.6
—
12.0
18.2
23.8
2.9
o-Xylene
Mesitylene
23
Acetonitrile
13.3
7.5
10.3
4.1
16.3
—
16.0
22.9
Ethyl acetate
Cyclohexanone
1,4-Dioxane
1,2-Dichloroethane
Chloroform
3.4
6.7
7.4
2.7
7.8
7.3
9.1
5.3
10.7
16.1
b
b
b
13.0
11.4
—
0
0
0
10
11
12
—
À1.6
5.4
À2.6
À3.2
Dichloromethane
Chlorobenzene
À0.2
—
—
0
—
—
—
—
—
a
Data on the heat of solution of benzene from Ref. 14 (see text).
b
1
,2-Dichloroethane was used as reference. The values of the heats of reactions I, II, III and IV in 1,2-dichloroethane are À113, À154, À97 and
À1
À66 kJ mol , respectively, from Ref. 15.
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 636–643

6
40
V. D. KISELEV ET AL.
Figure 3. Correlation of solvent effect on the volumes of
reactions ꢀ^) I, ꢀ&) II and ꢀ*) III with the solvent effect on
the heats of these reactions from the data in Tables 2 and 3.
The numbers of the solvents correspond to those in Table 2.
The ordinate values of reactions I and II are shifted up by 10
Figure 2. Correlation of solvent effect on the volumes of
reactions ꢀ^) II, ꢀ&) III and ꢀ~) IV with the volume of
reaction I from the data in Table 2. The numbers of the
solvents correspond to those in Table 2
3
À1
and 20 cm mol , respectively
of the solvent. Table 1 lists some effects of specific
interactions of acceptor 1 with the solvent. The results for
p-donor solvents (Nos 1–6, Table 1) demonstrate strong
this possibility, the partial molar volumes and the
enthalpies of solution of cyclopentadiene (2), 1,3-
butadiene (3) and trans, trans-1,4-diphenyl-1,3-buta-
diene (4) and those of their adducts with TCNE (Scheme
1) were determined. For reactions I–III the results are
given in Tables 2 and 3.
ꢀ
correlations between V and other magnitudes, as shown
1
in Fig. 1. From these correlations one can conclude that
p,p-complex formation plays a major role in the
stabilization of 1 and this stabilization results in an
approximately proportional decrease in the partial molar
volume. On the other hand, however, poor correlations
were found for n-donor solvents despite the large
The tetracyanoethanoic part in an adduct no longer has
p-acceptor properties and this is reflected in the
ꢀ
differences in D H. The increase in the solute–solvent
constancy of the values of V (6–8) (Table 2) and D H
sol
sol
interaction energy does not result in a regular decrease in
(6–8) in aromatic solvents (Table 3). The heats of
solution of the adducts are more endothermic in the
aromatic solvents than in the others. This may be one of
the reasons for their very low solubility in aromatic
solvents. As expected, the reaction volumes tend to be
less negative in the aromatic solvents than in the others,
as a result of the smaller partial molar volume of TCNE,
caused by p–p interactions. From the enthalpy cycle
ꢀ
V , probably because the n-p interaction does not restrict
1
the thermal motions of the solvent molecules as much as
the p–p interaction, does.
From the results in Table 1 we can propose that the
volume of reaction, DV, and the heat of reaction, D H,
rÀn
for the Diels–Alder reaction of TCNE will change
systematically in p-donor solvents. In order to examine
Table 4. Experimental dependences of the optical densities ꢀD or D ) and the equilibrium constants ꢀK) on pressure ꢀ1±1000
1
5
À2
3
À1
kg cm ) and volume ꢀcm mol ) of reaction IV in various solvents at 25°C
Solvent Experimental relations
Toluene
À4
À1
D /D = 0.992 Æ 0.004 ‡ (2.632 Æ 0.0132)  10 P; n = 21; r = 0.9937; s = 0.0070; n = 22730 cm ; absorption of
p
1
À4
complex (1–solvent) lnK /K = (À0.00111 Æ 0.0114) ‡ (5.920 Æ 0.062)  10 P; n = 10, r = 0.9951, s = 0.023;
p
1
DV = À14.9 Æ 1.0
D /D = (0.999 Æ 0.003) ‡ (4.720 Æ 0.050)  10 P; n = 21; r = 0.9989; s = 0.0071; n = 19050 cm ; absorption
À4
À1
o-Xylene
p
1
À4
of complex (1–solvent) lnK /K = (0.0113 Æ 0.0082) ‡ (4.484 Æ 0.0427)  10 P; n = 14; r = 0.9918; s = 0.021;
p
1
DV = À11.3 Æ 1.0
À5
À8
2
Ethyl acetate
Acetonitrile
D /D = (1.001 Æ 0.001) À(2.813 Æ 0.526) Â 10 P À(3.703 Æ 0.508) Â 10 P ; n = 19; r = 0.9916; s = 0.002;
p
1
À1
À4
n = 25770 cm ; absorption of diene 5. lnK /K = (À0.00953 Æ 0.0108) ‡ (8.952 Æ 0.060)  10 P; n = 10;
p
1
r = 0.9980; s = 0.003; DV = À22.6 Æ 1.5
À4
À8
2
D /D = (1.000 Æ 0.001) À(1.789 Æ 0.058)  10 P ‡ (4.350 Æ 0.458)  10 P ; n = 12; r = 0.9982; s = 0.002;
p
1
À1
À4
n = 25770 cm ; absorption of diene 5. lnK /K = (À0.00154 Æ 0.0174) ‡ (9.566 Æ 0.084)  10 P; n = 11;
p
1
r = 0.9958; s = 0.003; DV = À24.2 Æ 1.5
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 636–643

SOLVENT EFFECT IN THE DIELS–ALDER REACTION
641
À2
À1 À1 À1
3
Table 5. Effect of pressure ꢀ1±1000 kg cm ) on the rate constant ꢀk, l mol
s ) and volume of activation ꢀcm mol ) of
reaction III in various solvents at 25°C
Solvent
Experimental relations
À3
≠
≠
Benzene
ln k = (À4.940 Æ 0.015) ‡ (1.298 Æ 0.014)  10 P; n = 8; r = 0.9970; s = 0.0237; DV = À32.7 Æ 0.4
À3
Toluene
ln k = (À5.972 Æ 0.021) ‡ (1.175 Æ 0.014)  10 P; n = 7; r = 0.9970; s = 0.0345; DV = À29.6 Æ 0.4
À3 ≠
o-Xylene
Mesitylene
ln k = (À6.818 Æ 0.018) ‡ (1.067 Æ 0.010)  10 P; n = 10; r = 0.9962; s = 0.0314; DV = À26.9 Æ 0.3
À4 ≠
ln k = (À8.476 Æ 0.037) ‡ (9.180 Æ 0.235)  10 P; n = 6; r = 0.9903; s = 0.0453; DV = À23.2 Æ 0.6
À3
≠
1
,2-Dichloroethane
ln k = (À2.344 Æ 0.018) ‡ (1.260 Æ 0.011)  10 P; n = 10; r = 0.9972; s = 0.0303; DV = À31.7 Æ 0.3
À3 ≠
Acetonitrile
ln k = (À2.522 Æ 0.020) ‡ (1.312 Æ 0.013)  10 P; n = 8; r = 0.9971; s = 0.0297; DV = À33.1 Æ 0.3
À3 ≠
Cyclohexanone
ln k = (À4.260 Æ 0.026) ‡ (1.346 Æ 0.018)  10 P; n = 9; r = 0.9938; s = 0.0510; DV = À34.1 Æ 0.5
≠
3
À1
Table 6. Volume of activation of the forward ꢀDV ) and
and from that of reaction III by 10 cm mol . The
crowding in the transition state of the non-polar Diels–
Alder reaction is less than that in the adduct and, because
f
≠
backward ꢀDV ) Diels±Alder reaction III, partial molar
b
volume of the transition state of this reaction ꢀV*) and the
≠
3
À1
)
ratio ꢀDV /DV ) in various solvents at 25°C ꢀcm mol
f
III
≠
of this, the value of the DV /DV ratio can be >1.
≠
The heat of solution and the partial molar volume both
depend on the solute–solvent interactions. When the
volumes of reactions I–III are compared with the
changes in the heat of these reactions (Fig. 3), there is a
clear tendency for a more negative volume of reaction
with a larger exothermic heat of reaction in the solvents
under consideration.
The volumes of activation were determined here only
for the relatively sluggish reaction III (Table 5). It is
interesting that the effect of aromatic solvents on the
DV /
DVIII
f
DV≠
DV≠
ꢀ
V*
No.
Solvent
f
b
1
2
3
4
5
6
7
Acetonitrile
275.6 À33.1
1.4
1.00
1.00
1.00
1.21
1.21
1.27
1.37
Cyclohexanone
278.0 À34.1 À0.1
1,2-Dichloroethane 278.8 À31.7
0.0
Benzene
Toluene
275.0 À32.7 À5.8
274.8 À29.6 À5.1
275.0 À26.9 À5.7
274.7 À23.2 À6.3
o-Xylene
Mesitylene
values of activation volume is related to that on the
≠
[
Eqn. (6)] the relative reaction enthalpy, ꢀ H, with
rÀn
reaction rate: DV = À46.1 À 2.75lnk , r = 0.996. The
2
respect to 1,2-dichloroethane (S ) was calculated:
0
values of the partial molar volume of the transition state
ꢀ
(
V*) (Table 6) were calculated from the experimental
values of the activation volume of the forward reaction
ꢀ
H ˆ Á HꢀS † À Á HꢀS †
rÀn
rÀn
sol
i
rÀn
sol
0
≠
III (DV ) (from Table 5) and the partial molar volumes
ˆ
Á H ꢀS † À Á H ꢀS †
f
6
i
6
0
of reagents 1–4 (from Table 2). The values of the
À Á H
ꢀS † ‡ Á Hꢀ1‡2†ꢀS0† ꢀ6†
sol ꢀ1‡2†
i
sol
≠
activation volume of the backward reaction III (DV )
b
ꢀ
ꢀ
were calculated from the difference between V* and V .
8
For reactions I–III all the data are summarized in Table
From the data in Table 6 it can be seen that the activation
volumes of the backward reaction in non-aromatic
solvents (Nos 1–3) are close to zero, and appreciably
3
.
For reaction IV, which is reversible at room tempera-
ture, the values of the reaction volume can be obtained
only from the pressure effect on the equilibrium constants
Table 4). It is obvious that the change in partial molar
3
À1
negative (À5 to À6 cm mol ) in aromatic solvents (Nos
≠
4
–7). To a first approximation, the ratio DV /DV reflects
(
the progress of the transition state along the reaction
coordinate. In this work the ratio of the volumes of
activation and reaction (Table 6) is very close to unity in
non-aromatic solvents and appreciably longer in aromatic
solvents. This means that in aromatic solvents the
transition state is more accessible for solvation, com-
pared with the branched structure of the adduct 8.
The changes of partial molar volume of tetracya-
noethylene in aromatic solvents correlate strongly with
the volume of activation of the forward reaction III [Eqn.
volume of tetracyanoethylene in aromatic solvents makes
an essential contribution to the change in reaction
volume. When the volumes of reactions II, III and IV
are compared with the volume of reaction I (Fig. 2),
nearly the same solvent effect on the reaction volumes is
observed. It is interesting to note (Table 2, Fig. 2) that for
the same process of breaking and forming bonds of non-
polar Diels–Alder reactions I–IV, the values of the
reaction volume differ sharply, depending on the adduct
structure. In solution the voids in the crowded structure of
adducts 8 and especially 9 are more or less inaccessible to
solvent molecules. For this reason, the values of the
partial molar volumes of these adducts are larger than
those of the adducts with a structure completely
accessible for solvation. The volume of reaction IV
(
[
7)] and with the volumes of reactions I [Eqn. (8)], II
Eqn. (9)] and III [Eqn. (10)]:
6ˆ
III
ÁV
ˆ ꢀÀ0:89 Æ 0:01†V ‡ 64:4; r ˆ 0:997;
1
ꢀ7†
3
À1
n ˆ 7
differs from that of reactions I and II by 20 cm mol
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 636–643

6
42
V. D. KISELEV ET AL.
ÁV ˆ ꢀÀ1:03 Æ 0:05†V ‡ 79:7; r ˆ 0:985; n ˆ 4 ꢀ8†
hexane–benzene (5:1), m.p. 100–101°C. The adducts 6–8
16
I
1
were obtained as described with a yield of about 90%
1
6
ÁV ˆ ꢀÀ1:06 Æ 0:07†V ‡ 79:6; r ˆ 0:971; n ˆ 4 ꢀ9†
II
1
and dried in vacuo, m.p. adduct 6 211–213°C (lit. 206–
16
2
09°C), adduct 7 202–203°C (lit. 197–199°C) and
ÁVIII ˆ ꢀÀ0:99 Æ 0:04†V ‡ 80:1; r ˆ 0:990; n ˆ 4
16
1
adduct 8 211–212°C (lit. 211–212°C). The adduct 9
was prepared as described previously. All the solvents
1
ꢀ10†
17
were purified by known methods, stored over molecular
sieves 4A and distilled before the measurements. The
water content was ꢂ0.002% (w/w) in aromatic solvents
and ꢂ0.02% (w/w) in all the other solvents.
From the values of the slopes, it can be definitely
concluded that the p-acceptor properties of tetracya-
noethylene are entirely lost in adducts 6–8 and that they
are still partly retained in the transition state of reaction
III (within 10%).
The heats of solution were measured at 25°C using a
differential calorimeter with a volume of solvent in each
3
18
vessel of 180 cm , as reported previously. The accuracy
was within 2%. The heat of solution of cyclopentadiene
was very low in the usual range of concentrations [(1–
From these results, a strong solvent effect would be
expected on the activation and reaction volumes not only
of ionic and polar reactions, as is well known from the
À3
À1
À1
5)Â10 mol l ]. When the concentration increased to
3,7
literature, but also of non-polar processes, where one of
the states of the process under consideration interacts
with solvent specifically.
0.1 mol l , the heat effect of the diene (2) dimerization
was overlapping. For this reason, the heat of solution
14
of benzene was used as an alternative to that of
cyclopentadiene. The same changes in the heat of
solution applied to 1,3-butadiene.
Apparent molar volumes were determined at 25 Æ
CONCLUSION
À3
1
 10 °C by means of a Paar digital vibrating-tube
densimeter (DMA 602) with accuracy ꢂ0.5%. The
apparent molar volumes of compounds 1, 2, 4 and 6–8
for each of the solutions were invariable in the
The strong dependence of the heat of solution, the partial
molar volume of TCNE, the volume of activation and the
volume and the heat of reaction in aromatic solvents
unequivocally demonstrated that p–p interactions play a
vital role in the Diels–Alder reaction of TCNE. This is the
À1
concentration range 0.01–0.05 mol l . No changes in
the densities of solutions were observed within a few
hours. The molar volume of 1,3-butadiene was assumed
first example of large systematic solvent effects on DV,
3
À1
to be 82.2 cm mol in all the solvents under considera-
tion.
≠
DV and ꢀ H of non-polar Diels–Alder reactions. In
rÀn
contrast to p-donor aromatic solvents, n–p and n–n
interactions between n-donor solvents and the reagents
are possible. Moreover, relatively strong n–n interactions
with the transition state and the reaction product can
occur. This is the reason for the very sharp difference in
the solubilities of adducts 6–9 in p- and n-donor solvents.
The partial molar volumes should be larger for the
branched structure in comparison with the less branched,
more solvated structure. This difference may be the
reason for the change in the volumes of activation and
reaction: DV ꢁ DV < DV < DV . From the same
Pressure effects on the equilibrium constants were
determined only for reaction IV of tetracyanoethylene
with 9-chloroanthracene in some solvents (Table 4). The
adducts 6–8 are very stable to decomposition under these
conditions. The values of the equilibrium constants were
determined from the equations
K ˆ c =ꢀc À c †c ˆ ꢀD0;5 À D †=D c
ꢀ11†
ꢀ12†
9
0;5
9
0;1
5
5
0;1
0;5
K ˆ c =ꢀc À c †c ˆ ꢀD0;1 À D †=D c
9
0;1
9
0;5
1
1
I
II
III
IV
standpoint, the structure of the transition state of non-
polar Diels-Alder reactions is less branched than that of
This depends on the monitoring of the absorption of diene
[in non-aromatic solvents when c0,1 >> c , Eqn. (11)]
5
≠
0,5
the adduct and because of this the value of the DV /DV
or of dienophile 1 [in aromatic solvents when
c0,5 >> c , Eqn. (12)]. In Eqns (11) and (12), D and
ratio can be >1.
0,1
0,5
D are the optical densities of diene 5 corresponding to
5
the concentrations c0,5 before the reaction and c at
5
EXPERIMENTAL
equilibrium. D0,1, D , c0,1 and c are the corresponding
1 1
characteristics for dienophile 1. It is clear that all the
values of D should be recalculated to the same pressure.
The optical densities as a function of pressure were
determined for all solutions of reactants 1 and 5 (Table 4).
The Tait equation can never be used because of a minor
bathochromic shift of the absorption bands under
pressure. Optical densities can increase or decrease with
the pressure, depending on whether monitoring is
Tetracyanoethylene (Merck) was sublimed in vacuo
(
50 Pa) at 110°C as white crystals, m.p. 200–201°C.
Cyclopentadiene, after cracking of the dimer, was dried
with CaCl and distilled before the measurements. trans,
2
trans-1,4-Diphenyl-1,3-butadiene was recrystallized
from ethanol, m.p. 150–151°C. 9-Chloroanthracene
was purified on an alumina column, eluted with n-
Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 636–643

SOLVENT EFFECT IN THE DIELS–ALDER REACTION
643
performed on the descending or ascending side of the
absorption bands. It was checked that the ratio of the
optical densities of solutions with different concentra-
tions at each value of pressure equals the ratio of these
concentrations. The change in optical density of tetra-
cyanoethylene (reaction IV) in o-xylene and in toluene
solutions under pressure is not related to the increase in
the fraction of the molecular complex (MC) 1–solvent.
From the values of the equilibrium constants of the
N u¨ rnberg) for their interest in this work. This work was
supported by the Russian Foundation for Basic Research
(grant No. 98-03-33053).
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Copyright  2001 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2001; 14: 636–643