Kinetic study of specific base catalyzed hydrolysis of ethyl acrylate in water-ethanol binary system

Article abstract of DOI:10.1134/S0036024413050348

Kinetic study of hydroxide anion catalyzed hydrolysis of ethyl acrylate has been carried in ethanol-water (10-50% v/v) binary systems at the temperature range 30 ± 0.1, 35 ± 0.1, 40 ± 0.1, and 45 ± 0.1 C. Calculated specific rate constant values decreases with increasing proportion of ethanol at all temperatures. The observed retardation of a base catalyzed hydrolysis reaction is explained on the basis of fact that the formation of polarized transition state is disfavored with increase in % of ethanol. The relation between the change in dielectric constant due to variation in binary mixtures and change in specific rate constant are explained on the basis of electrostatic and non electrostatic contributions of solvent mixtures. The variation of ΔG*, ΔH*, ΔS with solvent composition and the specific effect of water on the reaction rate kinetics are also discussed.

Full text of DOI:10.1134/S0036024413050348

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2013, Vol. 87, No. 5, pp. 730–736. © Pleiades Publishing, Ltd., 2013.
CHEMICAL KINETICS
AND CATALYSIS
Kinetic Study of Specific Base Catalyzed Hydrolysis
of Ethyl Acrylate in Water–Ethanol Binary System1
Sangita Sharma, Jayesh Ramani, Jasmin Bhalodia, and Bijal Vyas
Department of Chemistry, Hemchandracharya North Gujarat University, Patan 384265, Gujarat, India
Eꢀmail: sangitamem2000@gmail.com
Received April 5, 2012
Abstract—Kinetic study of hydroxide anion catalyzed hydrolysis of ethyl acrylate has been carried in ethaꢀ
nol–water (10–50% v/v) binary systems at the temperature range 30 0.1, 35 0.1, 40 0.1, and 45 0.1°C.
Calculated specific rate constant values decreases with increasing proportion of ethanol at all temperatures.
The observed retardation of a base catalyzed hydrolysis reaction is explained on the basis of fact that the forꢀ
mation of polarized transition state is disfavored with increase in % of ethanol. The relation between the
change in dielectric constant due to variation in binary mixtures and change in specific rate constant are
explained on the basis of electrostatic and non electrostatic contributions of solvent mixtures. The variation
of
Δ
G
*,
Δ
H
*, S* with solvent composition and the specific effect of water on the reaction rate kinetics are
Δ
also discussed.
Keywords: kinetics, alkaline hydrolysis, ethyl acrylate, binary system.
DOI: 10.1134/S0036024413050348
1
INTRODUCTION
Russian by Rekasheva [17]. So, it is planned to carry out
a detailed study on hydrolysis of ethyl acrylate in ethaꢀ
nol–water (10–50% v/v) binary systems at the temperꢀ
ature range 30 0.1, 35 0.1, 40 0.1, and 45 0.1°C.
In organic synthetic chemistry, hydrolysis is conꢀ
sidered as the reverse or opposite of condensation,
(a reaction in which two molecular fragments are
joined for each water molecule produced). As hydrolꢀ
ysis may be a reversible reaction, condensation and
hydrolysis can take place at the same time, with the
position of equilibrium determining the amount of
each product. The hydrolysis process is involved in
saponification in soap manufacturing, plant and aniꢀ
mals metabolism [1], enzymatic reactions, biosynꢀ
thetic reactions, inorganic ligand exchange reactions
[2]. Esters can be hydrolyzed to carboxylic acids by
nucleophilic acyl substitution mechanism either in
basic or acidic condition. Hydrolysis involves replacꢀ
ing the alkoxy group of the ester with a hydroxyl group.
It occurs when the nucleophiles (e.g., water or
hydroxyl ion) attacks the carbon of the carbonyl group
of the ester [3] has been extensively reported by numꢀ
ber of workers [4–14].
The variation of
Δ
G*,
Δ
H*, S* with solvent compoꢀ
Δ
sition and the specific effect of water on the reaction is
studied here.
EXPERIMENTAL
Material. All chemicals used were of A. R. grade
and conductivity water was used throughout the work.
The purity of ethyl acrylate was checked by distillation
at reduced pressure. Ethanol was purified by the
method reported earlier [18].
Kinetic measurements. Conductometric method
[19–21] was employed for the calculating hydrolytic
rate constant for ethyl acrylate at different temperaꢀ
tures using ꢀConductivity meterꢀ306 with cell conꢀ
μ
stant 1 0.01. Before the use, the new conductivity
cell was immersed in distilled water for at least 24 h.
The cell was kept in distilled water between the meaꢀ
surements. This distilled water was changed frequently
in order to avoid formation of algae, which tends to
cling to the pores of the platinumꢀblack layer of the
cell, reducing the active surface. It was ensured that
there are no air bubbles between the plates of the conꢀ
ductivity cell. Conductivity cells of different cell conꢀ
stants were chosen mainly to ensure that the actual
resistance between the plates, when cell is immersed in
the solution, is within practical measurable range.
Ethyl acrylate is extensively used in the production
of polymers including plastics, adhesives, resins, safety
glass, plastic bottles and dental restorations [15].
When consumed can cause a number of physical probꢀ
lems like eye, nose, throat irritation, eye redness, pain,
minor burns, skin redness, irritation of the skin and
rashes, loss of memory and headache [16]. Literature
survey reveals that there is only one review based on
kinetics and mechanism in vinyl esters reported in
1
The article is published in the original.
730

KINETIC STUDY OF SPECIFIC BASE CATALYZED HYDROLYSIS
731
Inaccuracy can occur in the results if the resistance to
be measured is too high or too low. A safe practical
range is 10 S/cm to 100 mS/cm.
(
C0 − Ct)/(Ct C )
−
∞
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
μ
In the present study High Precision Water Bath Ca.
No. MSWꢀ274 with the readability of 0.01°C is used
for maintaining the temperatures. Depending on the
experimental being done, two or four sets of apparatus
can be operated in a single bath of this size.
The reactions were initiated by mixing thermally
equilibrated solution of 0.02 N NaOH and 0.02N the
ester (in ethanol–water, % v/v) at the required temꢀ
perature. Beakers of the base and esters were mainꢀ
tained at the temperature of 30 0.1, 35 0.1, 40 0.1,
and 45 0.1°C respectively in water bath. After equilꢀ
ibration, 20 mL of ester solution was delivered in reacꢀ
tion vessel and similar amount of sodium hydroxide
solution was added to it. Measurement of time was
started only when one half of base was added to the
ester solution. The course of the reaction was followed
by measuring the conductance (Ct) of the reaction mixꢀ
ture at regular time intervals using a conductivity bridge
in the temperature range from 30 0.1 to 45 0.1°C.
0
5
10
15
20
, min
t
The reaction is found to be first order with respect
to [nucleophiles] and also first order with respect to
Fig. 1. Conductance measurement for the rate constant of
ethyl acrylate at 30 0.1°C.
[substrate]. The second order rate constant (
calculated from the linear plot of (C0 t)/(Ct
time, where C0 Ct are the conductance of the reacꢀ
tion mixture at zero,
kobs) are
–
C
– C ) vs
∞
hydrolysis of ethyl acrylate in water at 30 0.1°C is
given in Table 1 and graphical presentation is given in
Fig. 1.
,
, C
∞
t
and infinite time intervals [22].
The rate constants determined were found to be reproꢀ
ducible within 1% error. The possibility of esters
undergoing hydrolysis in ethanol–water (% v/v)
binary mixtures was checked by conducting an indeꢀ
pendent study under similar experimental conditions.
The rate constants for the hydrolysis of ethyl acrylate
in different solvents mixtures were determined at difꢀ
ferent selected temperatures. A personal computer was
used to carry out the multiple regression analysis.
RESULTS AND DISCUSSION
Table 2 gives the specific rate constant values at varꢀ
ious composition of aqueousꢀorganic solvent system at
different temperatures. The Arrhenius equation was
obeyed within the limits of experimental error in
selected systems. The plots of log
linear in all cases. Values of activation entropy
k
against 1/
T
S
were
* and
Calculation of rate constant (k). The second order
rate expression for a reaction in which the two reacꢀ
Δ
ΔG
* were calculated from the following well known
equations
tants are at equal concentration,
a, is,
1
x
K = ꢀꢀꢀꢀꢀeΔS*/Re–ΔH*/RT
,
kT
h
k = ꢀ × ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ.
a
a(a – x)
The amount of reaction that has occurred,
amount of reactants remaining, ( ), can be deduced
from the conductance ( ). The total amount of reaction
possible is proportional to total change of conductivity,
i.e., a C0 ). Therefore the amount of reaction
after time,
stituting the above expression,
(C0 – Ct)
x
, and the
ΔG* = ΔH* – TΔS*,
k = Kte–ΔG*/RT/h.
a – x
C
The calculated values of activation parameters Ea
*, *, * are presented in Table 2.
The results of present investigation can be summaꢀ
,
α(
– C
∞
ΔG
ΔH
ΔS
t
, namely, x is proportional to C0 – Ct. Subꢀ
rized as follows:
1
k = ꢀ × ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ.
(Ct – Ca)
(I) The rate of hydrolysis are much smaller in the
water–ethanol mixtures than in water–DMSO mixꢀ
tures [23]. The k2 values decrease with increase in the
organic cosolvent content in the reaction medium at
all temperatures. The graphical presentation in Fig. 2
shows that the fall is only gradual at first but when the
a
A graph (Fig. 1) of (C0 t)/(Ct
(time) should therefore give a straight line of slope
equal to [ . The units of
2 are dm3 mol–1 min–1 and
k2 are calculated using linear regression analysis with
–
C
– C ) against t
∞
a
]
k
k
r
2 = 0.918–0.995. A representative data for alkaline mole fraction of ethanol become greater than 0.062
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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732
SANGITA SHARMA et al.
Table 1. Conductance measurements for calculation of rate
constant of ethyl acrylate at 30 0.1 C in water
(III) The reaction rate decreases with decrease in
dielectric constant of the solvent mixture. The trend of
°
change in k2 values is thus just opposite to the results of
C0 – Ct
Ct
103, S
×
some workers [25, 26] but same as the results obtained
in alkaline hydrolysis of ester in aquoꢀDMSO mixꢀ
tures.
ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ
α
t, min
Ct – C
0
1
0.742
0.720
0.700
0.678
0.662
0.640
0.621
0.602
0.585
0.570
0.560
0.539
0.532
0.522
0.509
0.499
0.492
0.482
0.473
0.462
0.454
0
(IV) Plot of log
ing of specific interest, so plot of log
presented at 30 0.1°C. Figure 4 shows that the plot of
log vs. log[H2O] for ethanol is a straight line whose slope
k
against log[ethanol] reveals nothꢀ
0.030
0.060
0.095
0.122
0.161
0.198
0.236
0.273
0.307
0.330
0.383
0.402
0.429
0.466
0.496
0.518
0.550
0.580
0.619
0.648
k
against log[H2O] is
2
k
3
approaches the value 1.11 as the temperatures rises indiꢀ
cates that reaction is first order with respect to [H2O].
4
5
Reaction rate. Alkaline hydrolysis of several esters
has been studied in aqueousꢀorganic solvent system
[27–29]. Retardation of specific rate constant for
hydrolysis of 2 carbomethoxy propionate ion under
alkaline condition supports the results of the present
study [30, 31]. At very high concentration [32, 33] of
organic solvent, the rate of alkaline hydrolysis of esters
decreases with decreasing water concentration in aceꢀ
tone, dioxane and alcohol binary systems. The results
of investigations [31, 32] of alkaline hydrolysis of subꢀ
stituted ethyl benzoates in acetone–water and ethaꢀ
nol–water mixtures are similar to observations made
by Rawat and Rama [33]. The observed retardation of
a base catalyzed hydrolysis reaction with increase in
ethanol content in mixture may be explained on the
basis of fact that the formation of polarized transition
state is disfavored with increasing % of ethanol [34].
This is in accordance with the qualitative prediction of
Houghes–Ingold theory [35].
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
–3
3
According to Tommila et al. [36], the first and rate
determining step in alkaline ester hydrolysis is the
addition of hydroxide ion to the carbon of carbonyl
group of ester, after which a rapid reaction with water
follows. Thus if the activity of hydroxide ion is
decreased, the reaction rate must decrease. The lower
rates in ethanol–water mixtures suggest that the ethaꢀ
nol water interaction significantly increase the conꢀ
centration of “free” water available for solvation of
hydroxide ions. The curve representing the rate conꢀ
stant as a function of solvent composition (Fig. 2)
begins to reduce rapidly when the mole fraction of ethꢀ
Note:
C
= 0.742
×
10 S; concentration of reactants is equal
×
0
–2
–3
3
2 10 mol/dm ; C = 0.01 10 S, k = 1.60 dm
×
α
2
mol⎯ min.
1
(water in the solvent less than 800 mL/L) the fall
becomes very steep.
(II) Figure 3 shows the curves plotting the Arrhenius
parameter Ea as function of solvent composition have
the same form as the alkaline hydrolysis of ethyl acetate
in dioxane–water and acetone–water mixture [24].
Table 2. The alkaline hydrolysis of ethyl acrylate in ethanolꢀwater mixtures
Water in the solvent
k
2, dm3 mol–1 min–1
mL/L
wt %
x w
ε
30
°
C
35 40
°
C
°
C
45°C
1000
900
800
700
600
500
100
90
80
70
60
50
1.000
0.972
0.938
0.890
0.850
0.716
78.54
71.23
65.23
59.68
53.78
47.84
1.60
7.00
6.90
5.30
4.30
3.75
2.50
8.35
7.35
6.15
4.85
4.25
4.71
9.70
8.35
6.75
5.40
4.75
5.69
11.05
9.45
7.85
6.05
5.30
3
Note: Initial concentration of the ester and NaOH was 0.02 mol/dm ,
ε
is the dielectric constant at 30 0.1
°
C.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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KINETIC STUDY OF SPECIFIC BASE CATALYZED HYDROLYSIS
733
anol exceeds a value of greater than 0.062 i.e., when the
ratio of water molecule to ethanol molecules is 4 : 1 or
less, the reaction rate decreases vary rapidly with
increasing ethanol concentration. This is the point
where the amount of free water decreases rapidly with
increasing ethanol concentration. This implies that
the solvation of hydroxide ions being strong in this
range of ethanol ꢀ water binary mixtures and hydrolyꢀ
sis constant values decreases.
k
/kw
5
4
3
2
The values of
Δ
G*,
Δ
H*, S*. Variation of Ea with
Δ
solvent composition is plotted in Fig. 3, plots reveals
that with the increase in the proportion of ethanol in the
medium, activation energy of reaction has decreased
with decrease in water content (increase in methanol
content). Results observed here is in agreement with
those of earlier investigations [37]. Thus, it is anticiꢀ
pated that there is greater desolvation of the initial
state than the transition state.
900
700
500
[H2O], mL/L
Fig. 2. Variation of k/kw with solvent composition.
−
1
Ea, kmol
6.0
5.5
5.0
4.5
4.0
Values of G* depends on the rate constant and has
Δ
increased smoothly with increase in ethanol content in
binary mixtures reflecting the decrease in hydrolysis
constants (Fig. 5). Tommila and his coworkers [38, 39]
reported that the curves representing variation of
and * with mole fraction of solvent in binary mixꢀ
tures are much more complicated.
ΔG*
ΔH
In Fig. 6, plot of * against S* in aqua–ethanol
ΔH
Δ
media is straight line, which is in accordance with Barꢀ
clay–Butler rule [40]. These relations have been
observed [24, 41] in many cases and have often been
discussed. The behavior of curves in plots of
ΔH* and
ΔS
* as functions of solvent composition have been
attributed to changes in the solvation of the reactants
and the transition state as the composition of the solꢀ
vent is gradually varied [24].
0
0.1
0.2 0.3
[Ethanol], mole fraction
The orientation of the solvent molecules around
the solute particles affects its reaction with reacting
molecules, and thus solvation of reacting species lowers
the entropy of the system. Solvation is a spontaneous
Fig. 3. Variation of
E with solvent composition.
a
−
k [s ]
1
log
10
process, and so its
Δ
G* =
Δ
H* –
T
Δ
S* < 0, so S* < 0.
Δ
In aqueous solvent mixtures the water molecules have
a specific attraction for the hydrophilic part and the
organic molecules a specific attraction for the alkyl or
other groups of solute. Thus a variation in the compoꢀ
sition of the solvent leads in general to changes in the
solvation of the solute particles, i.e., in a reaction sysꢀ
tem in the solvation of the reactants and the transition
8
6
4
complex. The decrease in S* values again suggests the
Δ
formation of non mobile transition state indicating solꢀ
vation of transition state or desolvation of initial state.
1.4
1.6
1.8
log[H2O] [mL/L]
In Table 3 shows sharp increase in the values of
Δ
S*
and * from 10 to 20% increase in content of ethaꢀ
Δ
H
Fig. 4. Plot of logk against log[H O].
2
nol co solvent in selected binary mixtures. But reversal
trend is observed in their values up to 50% of the ethaꢀ
nol co solvent. This is indicative of strong structural
and solvation changes resulting in rearrangement and
reorientation of solvent particles around reactant molꢀ
plex behavior in thermodynamic activation parameꢀ
ters towards variations in solvent composition have
ecules sharply affecting the activation process. Comꢀ been reported by many workers [42].
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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734
SANGITA SHARMA et al.
−
1
−
1
−
1
Δ
G*, kcal mol
−ΔS*, mol K
21.9
21.6
21.3
21.0
20.7
59
57
55
53
51
4
5
500
700
900 1100
[H2O], mL/L
−
1
Δ
H*, kcal mol
Fig. 5. Variation of ΔG* with solvent composition at 30°C.
Fig. 6. Plot of
Δ
S
* against ΔH*.
According to Winstein and Franberg [43], in binary enthalpy of activation. This is in consistent with Frank
and Evans [45] concept.
solvent mixtures two types of solvent molecules interꢀ
act with reactants both in ground state and transition
state. Solvent–solvent interaction is also very compliꢀ
cated. Alcohol–water mixtures are probably the most
complex binary solvent systems. In water, a solute
molecule builds a structure of a considerable number
of water molecules around itself. Such a structure forꢀ
mation around ethyl acrylate is quite likely due to
The relation between the rate constant and dielectric
constant of the solvent. With the increase in the proporꢀ
tion of ethanol in the medium, the dielectric constant
values decrease; this has more pronounced effect than
the solvation factor. The plot of log
approximately linear over a wide range of mixtures
[46]. The plot of log against 1/ is a straight line for
k
again 1/ being
ε
k
ε
mixtures that contain less water than 850 mL/L and its
slope value is negative (Fig. 7).
electron releasing ꢀmethyl group in the vinyl ester
α
[44]. The structure formation around solute molecules
must compete for water molecules with the existing
waterꢀstructure. When a small amount of another solꢀ
vent like alcohol is added, the solute molecules also
compete with the structures built by the alcohol moleꢀ
cules. This leads to the decrease in entropy and
enthalpy of activation. When the water structure built
around solute molecules have essentially disappeared,
further addition of alcohol decreases entropy and
According to Laidler [47], the slope of the plot of
log
k
against 1/
for an ion(A–) dipole(B) reaction with
ε
net charge ZBE = 0 is given by the equation
ZA2 e2
dlnk
d(1/ε) kBT
1
1
1
3 μA2 μB2 μ*2
⎛
⎞
⎛
⎞
ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ = ꢀꢀꢀꢀꢀꢀꢀ ꢀꢀꢀꢀꢀꢀꢀꢀ ꢀꢀꢀ – ꢀꢀꢀ + ꢀ ꢀꢀꢀꢀꢀꢀꢀ + ꢀꢀꢀꢀꢀꢀꢀ – ꢀꢀꢀꢀꢀꢀ
,
3
r*3
rB3
⎝
⎠
⎝
⎠
2
rA r*
4
rA
where B is the Boltzmann constant and the
k
μ
’s and
r’s
are the dielectric constants and radii of the reactants
and the transitions complex, respectively. In all cases
r* < r
A, so that if the terms involving ZA2 e2 predomiꢀ
nate and the slope value would be negative, i.e., the
reaction rate would decrease with increasing dielectric
Table 3. Thermodynamic activation parameters for the
hydrolysis of ethyl acrylate in alkaline aqueous ethanol
medium
constant. The higher value of
tion of carbonyl group in transition state. The
μ
* arise from an ionizaꢀ
Ea
,
Δ
G
*,
Δ
H
*,
–
Δ
S*,
kcal mol–1
kcal mol–1
kcal mol–1 cal mol–1 K–1
rA value
is large in present study and hydroxide ion will be
strongly solvated. The ethanol molecules can fit easily
around the carbonyl oxygen, the ionization of carboꢀ
nyl group become more complete, thus transition state
will be more polarized. This factor leads to negative
5.796
4.896
4.715
4.408
4.334
21.710
20.997
21.120
21.521
21.666
5.160
5.415
4.401
3.741
3.644
52.76
51.46
54.40
57.02
57.09
value of slope of log
k
against 1/ . In present work, the
ε
slope value is not larger (–48.34), which in accorꢀ
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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KINETIC STUDY OF SPECIFIC BASE CATALYZED HYDROLYSIS
CONCLUSIONS
735
−
k [s ]
1
log
0.9
There is gradual decrease in the rate and also same
effect for the * and Ea values with successive
ΔS
increase in ethanol proportion in the aquaꢀorganic
solvent system. It may be attributed to the greater solꢀ
vation of polarized transition state relative to the reacꢀ
0.8
0.7
0.6
0.5
tant. Sharp increase in the values of
support to the placid behavior of
Δ
S
* and
Δ
H*, in
Δ
G*, indicates the
changes resulting in rearrangement and reorientation
of solvent molecules around reactant molecules,
sharply affecting the activation process. The plot of
ΔH
* against S*, gave straight lines confirming the
Δ
justification of isoꢀkinetic relationship. The decrease
in the rate with decreasing dielectric constant of the
medium can be attributed to the fact that the formaꢀ
tion of more polarized transition state. The plot of
0.012
0.016
0.020
1/
ε
logk against log[H2O] was found to be straight line
Fig. 7. Plot of log
k
against 1/ε at 30°C.
with slope close to 1.11, which is indicative of the fact
that the reaction going on is first order with respect to
[H2O] and overall bimolecular in nature.
dance with the fact that the solvation of hydroxide ion
increases gradually with increasing composition of
ethanol and hydrolysis constant values decrease.
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