Kinetic study of the reaction of triphenylphosphine with acrylic acid in alcohol media

Article abstract of DOI:10.1134/S1070363210090033

Kinetic study of the reaction of triphenylphosphine with acrylic acid in alcohol media was carried out by spectrophotometric method. Use of alcohols as solvents is shown to introduce a specific feature in the kinetic picture of the reaction due to the app

Full text of DOI:10.1134/S1070363210090033

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 9, pp. 1738–1742. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.V. Salin, A.A. Sobanov, Yu.V. Bakhtiyarova, A.A. Khabibullin, V.I. Galkin, 2010, published in Zhurnal Obshchei Khimii, 2010,
Vol. 80, No. 9, pp. 1418–1422.
Kinetic Study of the Reaction of Triphenylphosphine
with Acrylic Acid in Alcohol Media
A. V. Salin, A. A. Sobanov, Yu. V. Bakhtiyarova, A. A. Khabibullin, and V. I. Galkin
Kazan State University, Kremlyevskaya ul, 18, Kazan, 420008 Tatarstan, Russia
e-mail: salin555@mail.ru
Received November 10, 2009
Abstract—Kinetic study of the reaction of triphenylphosphine with acrylic acid in alcohol media was carried
out by spectrophotometric method. Use of alcohols as solvents is shown to introduce a specific feature in the
kinetic picture of the reaction due to the appearance of parallel channels of proton transfer: from alcohol and
from another molecule of acrylic acid in the solution.
DOI: 10.1134/S1070363210090033
The unique reactivity of the tertiary phosphines as
compared to their closest analogs, amines, led in recent
decades to the discovery of a huge number of new
reactions of activated alkenes, alkynes and allenes in
which phosphines act as nucleophilic catalysts [1].
Most of these transformations include the processes of
proton transfer, which, according to the latest
experimental and theoretical studies [2, 3], may limit
the whole reaction rate, or require the participation of a
third molecule, the proton donor. While studying the
mechanism of the quaternization of tertiary phosphines
with unsaturated carboxylic acids, we recently reported
the results of kinetic studies of these reactions in acetic
acid [4] and also concluded that the process of proton
transfer in the reaction course takes place not
spontaneously, but only with the participation of
solvent molecules, and it limits the whole process rate.
The substrate carboxy group shows no specific effect
on the reaction course. Quaternization of phosphine in
all cases is described by the third order kinetic
equation (1), which, in addition to the concentration of
reactants, includes the concentration of acetic acid as
the protonating reagent.
acetic acid, by an alcohol, which is less prone to
release proton, will affect the reaction mechanism.
Here we report on the results of kinetic study of the
reaction of triphenylphosphine with acrylic acid in a
series of alcohols.
CH COOH
Ph3P + CH2
+
COO−
Ph3P CH2 CH2
,
ROH
R = Me, Et, Pr, i-Pr, Bu, s-Bu, i-Bu, t-Bu.
Note that such a study we have already initiated
earlier for itaconic acid [5], but the use of just acrylic
acid for the same purpose was the best because it
combines several important features: high reactivity,
second only to maleic acid (as found previously [4]),
the presence in the molecule of only one carboxy
group, which does not complicate the kinetic picture of
the reaction due to the possible emergence of parallel
channels of proton transfer, and the lack of restrictions
on the solubility in the selected solvents.
Use of alcohol medium to study the kinetics of
reaction of triphenylphosphine with acrylic acid led to
a complication of the kinetic pattern: In all cases the
first order of the reaction was revealed with respect to
triphenylphosphine and a fractional, greater than unity,
order with the respect to acrylic acid. Therewith, a
dependence was observed of the reaction order with
respect to the acid on its concentration in solution. A
trend was also traced in the change of order (n) toward
acrylic acid in the range of Cc from 0.03 to 0.5 M: it is
W = kIICpCc = kIIICsCpCc.
(1)
Here, Cc is the concentration of unsaturated carboxylic
acids, Cp is the concentration of phosphine, Cs is the
concentration of the solvent (acetic acid), kII and kIII are
respectively the second and third order rate constants.
We thought it interesting to see how the re-
placement of the solvent with strong acidic properties,
1738

KINETIC STUDY OF THE REACTION OF TRIPHENYLPHOSPHINE
1739
closest to unity in the primary alcohols, differs
significantly from unity in secondary alcohols, and
differs considerably in a tertiary alcohol.
0.6
0.5
1
R
Me
Et
Pr
i-Pr
Bu
s-Bu i-Bu t-Bu
0.4
0.3
0.2
0.1
0.0
7
n
1.07 1.13 1.22
1.33
1.25 1.38 1.27 1.68
2
3
5
Since a similar situation was observed formerly for
4
6
8
the reaction of triphenylphosphine with itaconic acid
[5], we assumed that in this case also there are two
parallel channels for the proton transfer, one of which
is of first order with respect to the acid, and the other is
of the second order. The kinetic equation of the
reaction is then given by Eq. (2).
0.0
0.1
0.2
0.3
Cc, М
0.4 0.5
0.6
Fig. 1. Separation of kII and kIII rate constants for the
reaction of triphenylphosphine with acrylic acid (30°C) in:
(1) MeOH: k'/Cc = 2.1×10–3 Cc + 4.3×10–3, R = 0.9984;
(2) EtOH: k'/Cc = 1.8×10–3Cc + 1.9×10–3, R = 0.9987;
(3) PrOH: k'/Cc = 2.3×10–3Cc + 1.4×10–3, R = 0.9997;
(4) i-PrOH: k'/Cc = 1.7×10–3Cc + 0.7×10–3, R = 0.9994;
(5) BuOH: k'/Cc = 2.5×10–3Cc + 1.2×10–3, R = 0.9989;
(6) s-BuOH: k'/Cc = 1.6×10–3Cc + 0.6×10–3, R = 0.9981;
(7) i-BuOH: k'/Cc = 3.6×10–3Cc + 1.7×10–3, R = 0.9992;
and (8) t-BuOH: k'/Cc = 1.2×10–3Cc + 0.1×10–3, R =
0.9986.
W = kIICpCckIIICpCc2 + = (kIICc + kIIICc2)Cp = k'Cp. (2)
Here k' = kIICc + kIIICc2 is pseudo-first order “effective”
rate constant at Cc >> Cp, observed in the kinetic
experiment.
The rate constants kII and kIII in this case can be
determined from Eq. (3), which is linear with the
concentration of acrylic acid.
k'/Cc = kII + kIIICc.
(3)
acrylic acid, whose proton-donor properties are
significantly higher, but its concentration is much lower.
High quality of the obtained linear dependences
(Fig. 1) is a criterion of validity of the approach used
to expand the effective rate constant over the
elementary components and the reliability of the deter-
mination on this basis of the rate constants kII and kIII.
The contribution of each channel of proton transfer
into the effective rate constant influences the observed
value of the order of the reaction with respect to the
acid, which generally ranges from one to two, and
leads to the above-noted trends in its change.
The nature of the constant kIII is clear: It
unambiguously describes the process of proton transfer
from a second molecule of acrylic acid. But with
respect to the constant kII there are two alternatives:
(1) kII reflects the intramolecular proton transfer from
attacked acid molecule through the transition state of
type A:
In favor of the latter option (intermolecular proton
transfer involving alcohol solvent) indicates the
variation of constants kII in parallel with the change in
acidity in the series of alcohols. Furthermore, there is a
good correlation between the corresponding values of
log kII and pKa of alcohols (Fig. 2, curve 1).
H δ+
log kII =–0.435pKa + 4.253,
δ−
+
−
N = 5, R = 0.990, s = 1.01×10–2.
+
Ph3P CH2 CH2 COO−
O
Ph3P CH2 CH
Clearly, the parameter pKa, being the most common
indicator of acid properties of the solvent, is by no
means universal and does not describe the totality of
the solvent effect on the reaction rate. Thus, the
attempt to include in this correlation acetic acid that
differs substantially in its properties (not only the
proton-donor ones) from alcohol reduces its quality
significantly:
C
O
A
(2) As in the case of acetic acid [4], kII reflects the
intermolecular proton transfer from solvent. The result
is a superposition of two channels of proton transfer to
the carbanion center of the attacked substrate: from
alcohol, a weak acidic reagent, but contained in the
solution in a high concentration as a solvent, and from
log kII =–0.197pKa + 0.247,
N = 6, R = 0.95, s + 1.26×10–1.
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1740
SALIN et al.
pKa
log k(20°C)
–3.5 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 0.0
15
16
17
18
19
20
0
0.0
–1
–2
–3
–4
–5
–6
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
AcOH
EtCO2H
EtOH
PrOH
PrOH
1
i-PrOH
t-BuOH
i-BuOH
BuOH
2
Fig. 2. Correlation between the values: (1) log kII, (2) log ks
and pKa of alcohol in the reaction of triphenylphosphine
with acrylic acid.
Fig. 3. Common isokinetic dependence log k(50°C)–
log k(20°C) for the reaction of triphenylphosphine with
acrylic acid in alcohols and carboxylic acids.
Using the parameter pKa for a series of related
compounds (alcohols) is reasonable, and it yields, of
course, much better linearity.
taken into account, in particular, in the calculation of
the activation entropy.
kII = ksCs.
(5)
For a more detailed study of the issue concerning
the role of alcohol in the quaternization of triphenyl-
phosphine by acrylic acid for three rather high-boiling
alcohols (PrOH, BuOH, and i-BuOH), we performed a
separation of the rate constants using Eq. (3) for
different temperatures. The high quality of the
obtained isokinetic dependence
Here, ks is the third-order rate constant describing the
proton transfer from the solvent, Cs is the con-
centration of solvent (alcohol). The third-order rate
constant describing the proton transfer from the second
molecule of acrylic acid we denote as kc, that is, kc = kIII.
Note that in the isokinetic relationship (4), along
with the constants kc can be used also the constants kII
without reducing them to the constants ks according to
Eq. (5) independent of alcohol concentration, as the
latter are directly proportional, and the change in Cs
within the operating temperature range can be
neglected, by considering it a constant value.
log k (50°C)–log k (20°C)
(4)
indicates that both the above channels of proton
transfer in alcohols are isokinetic, that is, are of
common nature, and therefore reflect the intermo-
lecular proton transfer from the medium.
log k(50°C) = 0.926 log k(20°C) + 0.539,
N = 6, R = 0.993, s =6.11×10–4.
The ks constants (see the table), “purified” from the
influence of solvent concentration, correlate with the
values of pKa of alcohol better than kII (Fig. 2, plot 2).
In this case, the determined second-order rate
constants are latent intermolecular in nature and
include the alcohol concentration [Eq. (5)] that must be
log ks = –0.355pKa + 1.752,
N = 5, R = 0.996, s = 2.55×10–3.
The effect of alcohol on the nature of the rate
constant kc is more complicated (see the table), and to
identify any correlation is difficult owing to the
relatively narrow range of their changes. However, it is
of some interest to consider the value kII/kIII = ksCs/kc =
C0, that is a constant for each alcohol in the
concentration units. Its physical meaning becomes
clear from the kinetic Eq. (2): C0 is a concentration of
acrylic acid providing equal rates of proton transfer
through both channels. Obviously, the C0 is smaller
when the alcohol acidity is lower, that is, lower its
tendency to release proton in the course of reaction
(see the table).
The rate constants ks and kc for the reaction of triphenyl-
phosphine with acrylic acid in alcohols ROH (30°C)
R
Cs, M C0, М ks×103, l2 mol–2 s–1 kc×103, l2 mol–2 s–1
Me
Et
24.4
17.0
13.3
13.0
10.8
10.8
10.7
10.5
2.05
1.06
0.61
0.41
0.48
0.38
0.47
0.08
0.18
2.1
1.8
2.3
1.7
2.5
1.6
3.6
1.2
0.11
Pr
0.11
i-Pr
Bu
0.054
0.11
s-Bu
i-Bu
t-Bu
0.056
0.16
0.0095
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 9 2010

KINETIC STUDY OF THE REACTION OF TRIPHENYLPHOSPHINE
1741
Activation parameters of the studied reactions
(subscripts correspond to the channels of the proton
transfer) in various alcohols are given below. The
closeness of the values of activation parameters of the
two reaction channels in each alcohol determines a
possibility of their parallel course. The large negative
activation entropy indicates a high ordering of the
activated reaction complex.
each reactant, and by analogy with acetic acid can be
assumed that the reaction kinetic equation (1) is also
true, and proton transfer occurs from the solvent.
Kinetic and activation parameters of this reaction in
comparison with acetic acid [4] are listed below. In
propionic acid the reaction rate is reduced, but the
general trend of the reaction acceleration in a
carboxylic acid medium persists.
R
ΔHs≠, kcal mol–1
–ΔSs≠, eu
ΔHc≠, kcal mol–1
–ΔSc≠, eu
Pr
10.7
41
Bu
10.0
44
i-Bu
10.2
42
R
Me
8.18
8.5
Et
3.32
9.9
37
kIII×103, l2 mol–2 s–1 (30°C)
ΔH≠, kcal mol–1
–ΔS≠, eu
10.8
35
9.9
38
9.9
37
40
If the above conclusions about the mechanism of
the reaction in propionic acid are correct, the kinetic
parameters of this reaction should obey the overall
isokinetic relationship (6), which in fact is observed
(Fig. 3).
If the above interpretation of the established
isokinetic relationship (4) is true, then should be a
general correlation with the previously used other
solvent, acetic acid, in which the proton transfer also
occurs from the environment and the protonating agent
is the acetic acid. This assumption is fully confirmed
by the high quality of the corresponding general
correlation.
log k(50°C) = 0.943 log k (20°C) + 0.593 ,
N = 8, R = 0.9994, s = 7.66×10–4.
So, we found a common isokinetic relationship for
the reaction of triphenylphosphine with acrylic acid in
alcohols and carboxylic acids. It is identical with the
isokinetic dependence found for a series of unsaturated
carboxylic acids [4], which confirms the general
character of the previously proposed stepwise
mechanism of their reaction with tertiary phosphines
regardless of the solvent nature.
log k(50°C) = 0.931 log k(20°C) + 0.554,
N = 7, R = 0.9996, s = 4.90×10–4.
(6)
In addition to acetic acid, we also studied the
kinetics of the reaction of triphenylphosphine with
acrylic acid in the medium of propionic acid. In this
case, as expected, the reaction is of first order toward
δ+
Ph3P
1
2
CR R CR3 COOH
+
Ph3P
δ−
R1R2C
CR3 COOH
В
δ+
Ph3P
δ+
+
+
−
CHR1R2−
CHR1R2−CH R3−
A−
δ−
3
Ph3P
[Ph3P
CH3R −COO
HA
COOH]
H A
+
3
δ−
R1R2
C
CR3
COOH
B
A– = RCOO–, CR1R2=CR3–COO–, RO–; R1, R2, R3 = H, Me, COOH.
According to this mechanism, the proton migration
to the carbanion center of the pre-reaction complex B
generated as a result of the nucleophilic attack of
phosphine, always occurs at the expense of an external
proton donor, the solvent or a second molecule of the
unsaturated acids, and it limits the whole process rate.
Thus, the study allowed us to establish that the
reaction of triphenylphosphine with acrylic acid in
alcohols is described by more complex, as compared to
carboxylic acids, kinetic equation of the form (2) with
two parallel channels of proton transfer, both of which
are intermolecular. Proton transfer in alcohols
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 9 2010

1742
SALIN et al.
proceeds with either direct participation of solvent
molecules, or involves a second molecule of acrylic
acid, while in the medium of strong proton donor
carboxylic acids it proceeds through the solvent only.
Both the possible channel of the proton migration
(through the solvent, or through second molecule of
unsaturated acid) obey the same general isokinetic
dependence, which reflects retaining of the main
features of the reaction mechanism and structure of the
activated complex, regardless of the nature of the
specific proton donor in the final act of the reaction.
The test solutions were prepared in the working cell
by mixing the reagent solutions of known concentra-
tion at a given temperature in required amounts,
stirring and placing the mixture in a temperature-
controlled spectrophotometer cell. At this moment the
time countdown and photometry began. Kinetic
measurements were repeated in all cases at least three
times. The error in determining the rate constants does
not exceed ± 5%.
The initial reagents (triphenylphosphine and acrylic
acid) and solvents were commercial products subjected
to additional purification by known methods [6–8].
The purity of solvents was checked by the absence of
changes in the spectra of prepared solutions of the
starting substances within 1 day.
EXPERIMENTAL
Investigations were carried out spectrophoto-
metrically on a Perkin Elmer Lambda 35 instrument
with a temperature-controlled cell (temperature control
accuracy ±0.1°C) in the media of the respective
solvents at a wavelength 300 nm (for alcohol) or
290 nm (for propionic acid) under conditions of
pseudo-first order with respect to triphenylphosphine
and at a large excessive concentration of acrylic acid,
which ranged from 0.03 to 0.55 M. The thickness of
the transmitting layer was 1 cm. Kinetic measurements
were carried out at temperatures from 20 to 50°C. The
pseudo-first order rate constants were determined from
the decrease in optical density of the absorption band
of triphenylphosphine in the reaction mixture, cal-
culating by the least squares method the slope of the
anamorphosis of the kinetic curve in the coordinates
log (Dx – D∞)–t, where Dx is current optical density,
D∞ is the final optical density upon completion of the
reaction, t is time. The true rate constants in alcohols
were determined as described in the main text. The
third order rate constant in propionic acid was
determined by dividing the rate constant of the pseudo-
first order by the concentration of excess reagent and
the concentration of EtCOOH. Concentrations of
solvents were taken constant and were calculated with
the formula Cs = d30/M. The values of d30, as well as
the pKa of solvents were taken from [6]. Activation
parameters were calculated from the temperature
dependence of the third order rate constant using the
known formulas [7].
ACKNOWLEDGMENTS
This work was financially supported by the Basic
Research and Higher Education Program (project
no. REC-007).
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