Solvation effects in the dimerization of 2-ethylprop-2-enal

Article abstract of DOI:10.1134/S1070363209030116

Rate constants for dimerization of 2-ethylprop-2-enal in different solvents cannot be approximated using a single solvent parameter. This is possible only with the aid of multiparameter equations. Polar solvents characterized by a high cohesion energy density accelerate the process.

Full text of DOI:10.1134/S1070363209030116

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 3, pp. 398–400. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © R.G. Makitra, N.M. Karpyak, G.A. Marshalok, A.Ya. Pal’chikova, I.Yu. Pyrig, 2009, published in Zhurnal Obshchei Khimii, 2009,
Vol. 79, No. 3, pp. 408–411.
Solvation Effects in the Dimerization of 2-Ethylprop-2-enal
R. G. Makitra, N. M. Karpyak, G. A. Marshalok, A. Ya. Pal’chikova, and I. Yu. Pyrig
L’vovskaya politekhnika National University, ul. S. Bandery 12, L’vov, 79013 Ukraine
e-mail: natakarpiak@mail.ru
Received June 19, 2008
Abstract—Rate constants for dimerization of 2-ethylprop-2-enal in different solvents cannot be approximated
using a single solvent parameter. This is possible only with the aid of multiparameter equations. Polar solvents
characterized by a high cohesion energy density accelerate the process.
DOI: 10.1134/S1070363209030116
Properties of acrylic polymers strongly depend on
the monomer structure. For example, extension of the
hydrocarbon chain in alkylacrylates enhances elasticity
and solubility of polymers derived therefrom. Acrylic
monomers having cyclic substituents have been
studied considerably more poorly. We examined
dimerization of 2-ethylprop-2-enal leading to 2,5-
and nonspecific solvation ability of solvents and their
structure.
2
n – 1
ε – 1
log k = a
0
+ a
1
+ a
2
3 4 T
+ a B + a E
2
n + 2
2ε + 1
2
+
a
5
δ + a
6
V
M
.
(1)
diethyl-3,4-dihydro-2H-pyran-2-carbaldehyde.
The
Equation (1) and calculations procedures were
described in detail in our previous publications [5, 6].
Correlation of the data for all 9 solvents gave six-
parameter equation with an unsatisfactorily poor
multiple correlation coefficient (R = 0.794). However,
exclusion of the most deviating data for only one
solvent, carbon tetrachloride, gave a correlation with
R = 0.999.
procedure for carrying out this reaction was described
in [1]. The resulting heterocyclic aldehyde was
proposed for use as component for copolymerization
with alkenes, dienes, and vinyl ethers. Such
copolymers attract interest as materials for polygraphy
and light-sensitive and laminating films [2–4].
CH3
CH3
The significance of particular terms of Eq. (1)
cannot be estimated from the pair correlation coef-
ficients r, for the latter were very low (r = 0.1–0.4).
Therefore, their significance was determined according
to the IUPAC recommendations [7] via successive
exclusion of particular terms and calculation of R for
the resulting equation with a smaller number of terms.
We thus revealed the solvent molar volume does not
affect the reaction rate. Exclusion of the corresponding
term reduced the R value only to 0.991. Nonspecific
solvation related to solvent polarizability also turned
out to be relatively insignificant. The relation between
the reaction rate and solvent parameters can be
described satisfactorily by four-parameter Eq. (2).
CH2
H C
CH2
O
2
H C
CH2
O
2
+
CH2
O
H
H C
2
H
CH3
H
H
O
H C
2
CH3
We a view to elucidate the mechanism of dimeriza-
tion of 2-ethylprop-2-enal we examined the kinetics of
this process at 190°C in nine solvents characterized by
different solvating abilities. The corresponding second-
order rate constants are collected in table. The reaction
was the slowest in dioxane, while the highest reaction
rate was observed in aromatic solvents. Insofar as the
available experimental data did not allow us to derive a
distinct dependence of the rate constants upon parti-
cular solvent parameters, we tried to correlate them
with the use of Koppel’–Pal’m six-parameter equa-
tion (1) which takes into consideration both specific
–3
log k = –1.578 + (4.956±0.538)f(ε) – (1.921±0.197)×10 B
–3
2
–
(0.144±0.019)E
T
+ (2.722±0.402)×10 δ ,
(2)
R 0.972, s ±0.038 N 8.
398

SOLVATION EFFECTS IN THE DIMERIZATION OF 2-ETHYLPROP-2-ENAL
399
Experimental log k values for dimerization of 2-ethylprop-2-enal (190°C) and those calculated by Eq. (2)
–
1 –1
Solvent
DMSO
Acetonitrile
,4-Dioxane
k, l mol
s
log(kexp
)
log(kcalc
)
Δ log k
–
–
–
–
–
–
–
–
–
5
5
5
5
5
5
5
5
5
2.53 × 10
3.06 × 10
1.08 × 10
3.37 × 10
3.47 × 10
2.93 × 10
2.59 × 10
2.52 × 10
1.80 × 10
–
–
–
–
–
–
–
–
–
4.5969
–4.6227
–4.4845
–4.9458
–4.4698
–4.5038
–4.5295
–4.5255
–4.6467
–4.2648
–0.0258
0.0298
0.0208
0.0026
–0.0442
0.0036
0.0612
–0.0481
0.4799
4.5143
4.9666
4.4724
4.4597
4.5331
4.5867
4.5986
4.7447
1
Toluene
Benzene
Tetrahydrofuran
Ethyl acetate
Methyl ethyl ketone
CCl
4
Exclusion of any parameters from Eq. (2)
completely destroys the correlation: the R value falls
down to 0.4–0.8; here, logk is most sensitive to solvent
basicity.
correlation with the Hammett–Taft steric constant was
obtained (R = 0.97). The correlation between logk and
σ was even poorer (R = 0.93 against a recommended
value of 0.95). On the other hand, two-parameter
correlation was characterized by an R value of 0.989.
We can conclude that n-donor solvents capable of
solvating the initial aldehyde in specific mode reduce
the reaction rate, i.e., hamper dimerization. By
contrast, increase in solvent polarity (which favors
charge separation) accelerates the process. An indirect
support for the above stated is the minimal reaction
rate in nonpolar carbon tetrachloride which is
incapable for specific solvation. The dimerization
process is also favored by increase in the cohesion
energy density. Presumably, self-association of solvent
molecules gives rise to a cage accommodating the
transition state leading to the dimeric product.
Analogous positive cage effect is also observed in
radical processes. The logk values calculated by Eq.
log k = –4.32 + (0.46±0.17)σ* + (0.62±0.14)ES.
(3)
We can state with some certainty that dimerization
of unsaturated aldehydes is determined mainly by
electronic properties of the substituents, though steric
factor is also significant. As the size of the substituent
increases, E value becomes negative, and the rate
S
constant correspondingly decreases.
Dimerization of unsaturated aldehydes is a par-
ticular case of Diels–Alder reaction. Its mechanism has
not been so far interpreted unambiguously. Three
versions of the reaction were proposed. The first of
these implies a two-step zwitterionic mechanism,
according to which heterolytic formation of new C–C
bond in the initial step gives rise to zwitterionic
intermediate. The second mechanism also includes two
steps; here, as in the first case, diradical intermediate is
formed in the first step. The third version is one-step
concerted mechanism involving cyclic transition state
with six delocalized π-electrons [8].
(
2) and their deviations from the experimental values
are given in table. It is seen that Δlogk values
generally do not exceed 2s (s = ±0.038; ethyl acetate,
methyl ethyl ketone, benzene).
Taking into account chemical aspects of the
process, the structure of initial aldehyde, specifically
the size of the substituent therein, i.e., steric factor
must be important. Therefore, we examined the
kinetics of dimerization of a series substituted
unsaturated aldehydes from acrolein to 2-butylprop-2-
enal under similar conditions. As a result, the
It is commonly believed that Diels–Alder reactions
follow one-step concerted mechanism; however, some
authors presume initial formation of one C–C bond,
followed by generation of diradical species, on the
basis of quantum-chemical calculations [9].
–
4
following rate constants k were found: 1.18× 10
–5
–5
(
acrolein), 4.33×10 (2-methylprop-2-enal), 3.45×10
In our case, considerable solvent effect on the
reaction rate, as well as the values of activation
parameters found for the dimerization process in the
–
5
–1 –1
(
2-ethylprop-2-enal), and 1.7× 10 l mol s (2-
butylprop-2-enal). An acceptable but fairly poor
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 3 2009

4
00
MAKITRA et al.
bulk (solvent-free) [1], allowed us to presume one-step
concerted dimerization mechanism. This is also
supported by the failure to accomplish photochemical
dimerization of 2-ethylprop-2-enal under UV
irradiation (80 W) in the temperature range from 20 to
0.5–1 μl; quantitation was performed by the standard
addition technique. The rate constants were calculated
using second-order equation (4).
2
k τ = 1/c – 1/c
0
.
(4)
Here, c is the initial concentration of 2-ethylprop-2-
7
0°C both in the presence and in the absence of
0
enal (1.7–2 M, depending on the solvent), c is the
current concentration of 2-ethylprop-2-enal (M), and τ
is the reaction time (s). The progress of reactions was
monitored up to 80% conversion of the initial
aldehyde. The rate constants given in table were
averaged by the least-squares procedure.
initiators (acetophenone, benzil, 2,2-dimethyl-2-
phenylacetophenone). In all cases, no dimerization
product was detected, i.e., the dimerization does not
involve intermediate formation of radical species.
Our results show that such solvents as benzene,
toluene, acetonitrile, and tetrahydrofuran accelerate
dimerization of 2-ethylprop-2-enal. From the practical
viewpoint, benzene is the best solvent (it is relatively
cheap and accessible). Furthermore, the boiling point
of benzene (80°C) approaches that of 2-ethylprop-2-
enal (92°C), so that there is no need of separating them
after isolation of the dimer, and the mixture can be
reused.
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8
9
Compounds.
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2
1
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0
–
1
gas hydrogen, flow rate 25 ml min ; sample volume
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 3 2009