Energy and volume activation parameters of the retro-Diels-Alder reaction in different solvents

Article abstract of DOI:10.1007/s11172-009-0003-9

The temperature and pressure effects on the decay rate of an adduct obtained from 9-chloroanthracene and tetracyanoethene by a Diels-Alder reaction were studied. The rate constants (298.2 K) and the enthalpies, entropies, and volumes of activation were determined for the retro-Diels-Alder reaction in different solvents. The data obtained confirm a possible decrease in the molar volume of the solvated adduct upon partial bond cleavage on the way to the transition state. The reversal of the sign in front of the activation volume cannot be indicative of a changed reaction mechanism.

Full text of DOI:10.1007/s11172-009-0003-9

Russian Chemical Bulletin, International Edition, Vol. 58, No. 1, pp. 21—24, January, 2009
21
Energy and volume activation parameters of the retroꢀDiels—Alder reaction
in different solvents
V. D. Kiselev, E. A. Kashaeva, A. V. Bolotov, I. I. Shakirova, and A. I. Konovalov
A. M. Butlerov Chemical Institute, Kazan State University,
1
8 ul. Kremlevskaya, 420008 Kazan, Russian Federation.
Fax: +7 (843 2) 92 7278. Eꢀmail: Vladimir.Kiselev@ksu.ru
The temperature and pressure effects on the decay rate of an adduct obtained from
ꢀchloroanthracene and tetracyanoethene by a Diels—Alder reaction were studied. The rate
9
constants (298.2 K) and the enthalpies, entropies, and volumes of activation were determined
for the retroꢀDiels—Alder reaction in different solvents. The data obtained confirm a possible
decrease in the molar volume of the solvated adduct upon partial bond cleavage on the way to
the transition state. The reversal of the sign in front of the activation volume cannot be
indicative of a changed reaction mechanism.
Key words: retroꢀDiels—Alder reaction, kinetic parameters.
was focused on the accuracy of measurements.7,8 Accordꢀ
ing to the accumulated experimental data, the negative
activation volumes appreciably exceed in absolute value
the volumes for a large group of Diels—Alder reactions
The kinetics of the forward Diels—Alder reaction is
studied much better than that of its reverse. The retroꢀ
1
Diels—Alder reaction is characterized by high activation
energies and usually low activation entropies. From
data for the pressure effect on the rate of the forward
Diels—Alder reaction it follows that the activation
volume of the forward reaction can be more negative
than the volume of the reaction itself. This requires the
assumption that the transition state is more compact than
the adduct.2 Such a change in the volume profile of
the reaction is difficult to explain for the Diels—Alder
reaction, which is an isopolar process. It has been asꢀ
sumed1 that the solvent effects on the rate constant and
activation parameters of the forward and retroꢀDiels—
Alder reactions are due to a subtle differentiation between
the solvation effects in the transition state.
2
—8
but do not for another group that is not smaller.
Nevertheless, the reliability of those data should be
confirmed by independent direct measurements of actiꢀ
vation volumes in retroꢀDiels—Alder reactions. Only in
two studies5 devoted to the baric kinetics of the retroꢀ
reaction was a slight volume reduction observed on the
way from the adduct to the transition state. Because
Diels—Alder adducts are sufficiently stable, the stock of
suitable objects of investigation is limited. Considerable
differences in the bond conjugation energy for reagents
,9
—4
,5
1
distinctly correlate with the reaction enthalpy, which
allows selection of convenient objects regarding rates
and equilibrium.
Here we obtained additional data on the enthalpy,
entropy, and activation volume in the decay of adduct 1
synthesized from 9ꢀchloroanthracene (2) and tetracyanoꢀ
ethene (3) (Scheme 1).
Subtle distinctions between the solvation effects
are difficult to explain in terms of common solvent
6
parameters. One can assume that the transition state
is differently accessible to different solvents during the
solvation. If such unusual features of the solvation deterꢀ
mine changes in the activation entropy and affect the
partial molar volumes, then the activation volume should
be expected to change in a nonmonotonic way. This volꢀ
ume is defined as a difference between the partial molar
volumes of the transition state (V ) and the adduct (V ):
The heat of a forward reaction between compounds 3
and 2 (–66 kJ mol– in 1,2ꢀdichloroethane) is noticeably
lower than those of reactions with other substituted
1
1
anthracenes; because of this, the decay rate of adduct 1 is
sufficiently high even at room temperature. Since the
TS
1
equilibrium constant is very high (~103 L mol ),
–1
5
≠
∂
lnk/∂P = –ΔV /(RT) = –(V – V )/(RT).
(1)
the complete decomposition of adduct 1 can be effected
only in the presence of an appropriate reagent for irreꢀ
versible trapping of diene 2 or dienophile 3. It was found
that cyclopentadiene (4) rapidly and irreversibly
combines with compound 3, which is released in the
TS
1
A decreased volume of an adduct upon its partial
decomposition in the transition state is usually difficult to
explain for an isopolar process. Considerable attention
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 21—24, January, 2009.
066ꢀ5285/09/5801ꢀ0021 © 2009 Springer Science+Business Media, Inc.
1

22
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 1, January, 2009
Kiselev et al.
Scheme 1
than that in inert solvents.11 According to the data
obtained (see Table 1), the rate constants and the parameꢀ
ters of activation of the decay of adduct 1 in benzene and
toluene do not differ from those in other solvents. The
activation enthalpies of the decay of the adduct are higher
by 20—30 kJ mol– than the balance between the energies
of bond cleavage and bond formation in this reaction
retroreaction, to give a more stable adduct (5). Insofar as
the rate constant of the reaction of compound 3 with
diene 4 is three orders of magnitude higher than that of its
1
reaction with diene 2, the observed accumulation rate of
diene 2 corresponds to the rate of the reaction under
study. Our data on the rate constants and the parameters
of activation of the retroreaction are given in Table 1.
In πꢀdonating aromatic solvents, the rate of the Diels—
Alder reaction with compound 3 is nearly 100 times lower
1
–
1
(66 kJ mol ), which suggests a dominant contribution
from the energy of σꢀbond cleavage in the adduct to the
Table 1. Rate constants (k–1/s–1) and the enthalpies (ΔH /kJ mol ), entropies (ΔS /J mol K–1), and volumes (ΔV /cm mol–1) of
≠
–1
≠
–1
≠
3
activation of the decay of adduct 1 at different temperatures (T/K) in some solvents
4
ΔH≠
ΔS≠a
ΔV≠b
Bс
с
D •104
Solvent
k–1•10 /T
Acetonitrile (1)
3.26/293.2
87.9±2.5
–12±8
–7.9±1.0
1.287±0.0194
5.28±0.107
6
1
1
.05/298.2
0.8/303.2
9.6/308.2
1
,2ꢀDichloroethane (2) 2.48/288.2
87.0±1.3
91.2±2.5
–11±4
+2±4
–6.5±0.5d
–2.2±1.0
1.557±0.0892
2.202±0.0488
2.79±0.199
3.28±0.086
4
8
1
2
.51/293.2
.72/298.2
5.0/303.2
8.2/308.2
Chlorobenzene (3)
2.45/288.2
5
9
1
.38/293.2
.64/298.2
7.4/303.2
Benzene (4)
Toluene (5)
2.67/288.2
8
1
83.3±0.4
85.4±1.3
–12±2
–18±4
—e
1.205±0.0809
1.630±0.0825
1.70±0.166
2.49±0.160
.95/298.2
5.8/303.2
2.28/288.2
–7.2±1.5
4
7
1
2
.14/293.2
.92/298.2
3.7/303.2
4.5/308.2
Ethyl acetate (6)
Butyl acetate (7)
0.927/293.2
97.5±0.4
+10±2
+20±7
+1.5±1.0
+3.0±1.0
1.456±0.0181
1.328±0.0252
7.02±0.110
5.29±0.134
1
6
2
.89/298.2
.82/308.2
3.2/318.2
0.854/293.2
100.4±2.2
1
3
7
1
2
.86/298.2
.70/303.2
.11/308.2
4.1/313.2
2.8/318.2
a
For 298.2 K.
For 293.2 K.
b
c
The coefficients of Eq. (3).
Data from Ref. 5.
At 293.2 K, benzene crystallizes under a pressure of ~550 bar.
d
e
10

Activation volume of the retroꢀDiels—Alder reaction Russ.Chem.Bull., Int.Ed., Vol. 58, No. 1, January, 2009
23
≠
3
–1
ΔV /cm mol
matography on alumina with hexane—benzene (6 : 1) as an eluꢀ
ent, yellowish green needles, m.p. 373—374 K. Adduct 1 preꢀ
cipitated in a nearly 100% yield from an equimolar mixture of
the starting reagents in benzene. The resulting white crystals
of adduct 1 were dried, m.p. 433—436 K (decomp.; the formaꢀ
tion of a dark green melt). Found (%): C, 70.80; H, 2.78.
C H ClN . Calculated: C, 70.49; H, 2.66. Cyclopentadiene 4
4
2
0
7
6
3
2
0
9
4
–
2
4
6
8
was prepared by cracking of dicyclopentadiene (420—430 K),
dried, and distilled before making measurements.
–
–
–
The course of the reaction was monitored by observing
–1
2
1
changes in the absorption of diene 2 at λ = 394 nm (25 380 cm ),
5
where compounds 4 and 5 do not absorb. The calculation of the
rate constant of the retroreaction (Eq. (2)) was complicated by
superposition of the fluorescence of diene 2 on its absorption,
which made it necessary to determine the optical density
of diene 2 as a function of its concentration in solution
(ε = F(C )).
ΔS /J mol K–1
≠
–1
–
20
–10
0
10
Рис. 1. Correlation between values of activation enthropy
ΔS ) and activation volume (ΔV ) in the reaction of decay of
≠
≠
(
2
2
adduct 1. Point numbers correspond to the solvents in Table 1.
k–1t = ln(C0,1/Ct,1) = ln[(A∞,2/ε )/(A /ε – A /ε )] (2)
2
∞,2
2
t,2
2
formation of an activation barrier. The activation entroꢀ
pies are positive only in ethyl acetate and butyl acetate.
Using the dependence of the reaction rate in solution
on the external pressure (see Eq. (1)), we calculated
activation volumes (see Table 1). During the activation
of adduct 1, the volume decreases in acetonitrile,
Such calibrations were performed for all solutions at 394 nm
and can be written as
A = Bln[(C + D)/D].
(3)
2
where A is the optical density of diene 2, C is its molar concenꢀ
2
–4
–1
tration (up to 7•10 mol L ), and B and D are coefficients
specified in Table 1. The coefficients B and D in Eq. (3) were
found to remain constant (within the accuracy of the experiꢀ
ment) with an increase in the temperature to 320 K or in the
pressure to 1000 bar. It should be noted that the relation like (3)
for the absorption of diene 2 is true for other spectral regions
1
,2ꢀdichloroethane, chlorobenzene, and toluene. In conꢀ
trast, the activation volumes are positive in ethyl acetate
and butyl acetate. We found a correlation (r = 0.96)
between the changes in the activation volumes and in
the activation entropies (Fig. 1).
(
390—400 nm). The rate constants were calculated (see Eq. (2))
Since we have no reasons to suggest solvent electroꢀ
striction for the isopolar Diels—Alder reaction, slight and
irregular changes in the molar volumes of the initial
and transition states can be attributed to small differences
in the compactness of the solvation shells of these states
and to possible flattening of the transition state upon
with consideration for an increased absorption diene during (At,2
)
and after the completion of the reaction (A∞,2). By using linear
regression and taking into account Eqs (2) and (3), one can
calculate A∞,2 from experimental At,2 values. The results
obtained agree well with data from direct measurements. The
total error in the rate constant determination is ±5%.
8
The kinetics of the highꢀpressure reaction was measured on
partial bond loosening in the adduct. It is known that
5
a setup described earlier. Because the change in the rate
the van der Waals activation volumes and the volumes
of the forward Diels—Alder reaction do not exceed
constant of the retroreaction under a pressure up to 1000 bar did
not exceed 40%, the activation volumes (see Table 1) were
calculated from the rate constants at a normal pressure and at
3
–1
–
10 cm mol . The same values were obtained by us
1
2
for the solidꢀstate reaction. Therefore, the activation
volumes and the volumes of the reaction itself are mainly
contributed by a change in the intermolecular packing
during the reaction in solution.
1
000 bar (see Eq. (1)).
We are grateful to the officers of the Collective Spectral
Analysis Center of the Kazan Research Center of the
Russian Academy of Sciences for performing the analyses.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 08ꢀ03ꢀ00219).
The data obtained (see Table 1) provide evidence that
the molar volume of the solvated adduct can decrease
even when the bonds between the diene and dienophile
fragments become somewhat longer on the way to the
transition state. The reversal of the sign in front of
the activation volume cannot be indicative of a changed
reaction mechanism.
References
1
. A. I. Konovalov, V. D. Kiselev, Izv. Akad. Nauk, Ser. Khim.,
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2
Experimental
3
. R. van Eldik, T. Asano, W. J. le Noble, Chem. Rev., 1989,
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8
Tetracyanoethene 3 (Merck) was purified by sublimation
in vacuo (~50 Pa, 380 K), white crystals, m.p. 473—474 K.
4
. A. Drljaca, C. D. Hubbard, R. van Eldik, T. Asano, M. A.
Basilevsky, W. J. le Noble, Chem. Rev., 1998, 98, 2167.
9
ꢀChloroanthracene 2 (Aldrich) was purified by column chroꢀ

24
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 1, January, 2009
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6
7
8
. V. D. Kiselev, E. A. Kashaeva, A. I. Konovalov, Tetrahedron,
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Received June 23, 2008;
in revised form December 22, 2008