Kinetics and
Solvent effect on Aquo-dipolar protic organic-solvent system of Hexanoate ester
A. K. Singh, Arjuman Bano
Department of
Chemistry, Teerthanker Mahaveer University,
Moradabad, India
*Corresponding Author E-mail: dr.anilkumarsingh@gmail.com
ABSTRACT:
Rate constant for the base
catalyzed hydrolysis propyl hexnoate was determined
volumetrically in water-n-propanol mixture for the composition varying from 30
to 70% (v/v) at different temperature ranging from 200C to 400C.
The initial slow but sharp decrease in rate with gradual addition organic co-
solvent in the reaction media and with increasing temperature of reaction has
been explained on the basis of salvation and desolvation
of initial and transition state to different extent. The Iso-composition
activation energy (EC) energy for same solution increases as
propanol content increases while Iso- Dielectric activation energy (ED)
with decrease as the dielectric constant of the solvent increases. The trends
of variation of numbers of water molecule associated with activated complex
increases with increasing temperature from 200C to 400C
tells about the fact that in presence of protic solvent(propanol) in reaction
media, the bimolecular mechanistic path of reaction changed to unimolecular.
The enhancement in DG* with simultaneous increase in (DH*) and
(DS*)
values of the reaction concluded that the reaction is enthalpy controlled.
KEYWORDS: Activated complex,
solvent effect, Propyl Hexanoate, Hydrolysis, Water-propanol mixture. Specific
rate constant, Iso-composition and Iso- dielectric Activation energy.
1. INTRODUCTION:
The kinetics of hydrolysis of base catalyzed reaction has received
considerable interest from Various researches [1-6] but the solvent effect on
the rate of reaction, mechanism of the reaction and thermodynamic activation
parameter of solvolysis of benzoate ester and solvent-solute interaction in the
reaction media consisting of propanol have not been reported so far. Hence, in
the present investigation the above untouched work will be considered, this
paper proposes to make a comprehensive study of kinetic solvent effect on the
alkali catalyzed hydrolysis of propyl Hexanoate in water-propanol solvent
system, especially Hexanoate ester was chosen for study in detail because Propyl Hexanoate is found in
alcoholic beverages as a flavoring ingredient. It is also constituent of
grapes, apples, papayas and other fruits and alcoholic beverage. It is also
used as solvent for polar organic compound.
2. EXPERIMENTAL:
Expert quality of Ethyl
Benzoate of fluke AG grade packed in Switzelarland
and propanol of Merck grade were used. The kinetics of the reaction was studied
volumetric by keeping the strength of alkali 0.1M and that of the ester 0.5 in
the reaction mixture. The reaction was found to obey second order kinetics
equation and the evaluated values of specific rate constant have recorded in Table-1.
The variation of Logk with mole % is mentioned in
Table-2. With help slope of Arrhenius plot of logk
verses 1/T (Table-3), the Iso-composition activation energy (EC)
Energy were calculated and inserted in Table-4. The values of Iso-Dielectric
activation energy (ED) calculated at different D were tabulated in
Table-5. The variation of Logk with log (H2O)
has mention in Table-6. The consolidated values of thermodynamic Activation
Parameter i.e. (DG*), (DH*) and
(DS*)
calculated with help of Wynne-Jons and Eyring [7]
equation and has been tabulated in Table-7.
3. RESULT AND
DISCUSSION:
3.1 Solvent Effect
on Specific Rate:
The second order rate constant for base catalyzed hydrolysis of propyl
hexanoate in water-propanol mixture were calculated at 30-400 c from
the slopes of linear plot of Logk against time (t).
The rate constant at different temperature and composition are collected in
Table-1. For showing the solvent effect on specific rate constant of the
reaction, the logarithmic value of k is also plotted against the mole % of
added organic solvent (propanol) of the reaction media tabulated in Table-2 and
Fig-1, found decreasing trend with increasing composition of solvent. The
depletion of rate constant with increasing temperature is responsible on any of
the three factors. These are
(I)
Decreasing polarity of the medium as changing from
polar water molecule to less polar water-propanol medium.
(II)
Lowering of bulk dielectric constant values of
the medium and
(III) Depletion of H2O+ ion of the
solution by the organic co-solvent due to its basic character.
As propanol is not basic, so it may not combine with H+ and H2O+
ion of the acidic medium. Hence among the above mentioned three rate retarding
factors, the first two factors are in and it is quite in agreement with the
theory of Hughes and Ingold [7] this explanation is also supported with the
view of Laidler and Lanskroener
[8] and recent communication of Singh AK [9]
Table–1 Specific rate constant
k x103(dm)3/mole/ mint] values of alkali catalyzed
Hydrolysis of propyl hexanoate in water- n-propanol
|
Temp in OC |
% of propanol |
||||
|
30% |
40% |
50% |
60% |
70% |
|
|
20OC |
61.65 |
53.70 |
46.23 |
38.90 |
34.27 |
|
25OC |
97.72 |
83.17 |
70.79 |
61.65 |
53.08 |
|
30OC |
142.88 |
123.02 |
107.15 |
92.25 |
80.35 |
|
35OC |
218.27 |
181.97 |
158.48 |
137.72 |
120.22 |
|
400C |
316.22 |
257.03 |
234.42 |
204.17 |
179.88 |
Table–2 Variation of 3+Log k
Value against mole %, (Water-n-propanol) System.
|
Temp in OC |
|
3 + Log k |
||||
|
Mole% |
20OC |
25OC |
30OC |
35OC |
400C |
|
|
30% |
9.33 |
1.790 |
1.990 |
2.155 |
2.339 |
2.500 |
|
40% |
13.79 |
1.730 |
1.920 |
2.090 |
2.260 |
2.410 |
|
50% |
19.35 |
1.665 |
1.850 |
2.030 |
2.200 |
2.370 |
|
60% |
26.47 |
1.590 |
1.790 |
1.965 |
2.139 |
2.310 |
|
70% |
35.90 |
1.535 |
1.725 |
1.905 |
2.080 |
2.255 |
Fig. 1: Variation of log k with
mole %.
3.2 Evaluated values of
Iso-composition Activation Energy (Ec) of
the reaction in water-propanol media.
From the Table-4, it can be
observed that the values of Iso-composition activation energy increases from 62.30 to 63.27 KJ/mole with increasing
proportion of solvent in the reaction media. The enhancement in the value of
Iso-composition activation energy is any of the following three cases;
a.
The
greater desolvation of transition state than initial
state.
b.
The
greater salvation of the initial state than transition state, and
c.
Simultaneous
salvation and desolvation of initial and transition
state respectively.
Out of the threes factor the
third one is applicable in this case and it is also supported by the increase
in entropy of activation with gradual addition of organic –solvent in the
reaction media as recorded in Table-8 This conclusion is also supported
recently by Singh A K. [10]
Table–3 Variation of Logk Values against 103/T, Water- propanol System.
|
Temp in OC |
|
3 + Logk |
||||
|
103/T |
30% |
40% |
50% |
60% |
70% |
|
|
20OC |
3.412 |
1.790 |
1.730 |
1.665 |
1.590 |
1.535 |
|
25OC |
3.355 |
1.990 |
1.920 |
1.850 |
1.790 |
1.725 |
|
30OC |
3.300 |
2.155 |
2.090 |
2.030 |
1.965 |
1.905 |
|
35OC |
3.247 |
2.339 |
2.260 |
2.200 |
2.139 |
2.080 |
|
400C |
3.195 |
2.500 |
2.410 |
2.370 |
2.310 |
2.255 |
Fig. 2: Variation of Log K with
103/T.
Table–4 Values of
Iso-composition Activation Energy with increasing percentage of solvent
(water-propanol media)
|
% of n-propanol |
30% |
40% |
50% |
60% |
70% |
|
Eexp in KJ/mole |
62.30 |
62.52 |
62.83 |
62.85 |
63.27 |
3.3 Effect of Solvent
on Iso- Dielectric Activation Energy.
The Iso-dielectric
energy (ED) of the reaction is obtained from interpolation of logk at different desired D values and from the slope of
Arrhenius curve (logk vrs103/T) the values
of Iso-dielectric energy(ED) is calculated which is tabulated in
Table-5.On perpetual observation of table-5 it has been apparent that the value
of ED goes on decreasing from 66.0 to 58.00 with increase of D in
the reaction media. On basis of the above fact it is inferred that enhancement
in Iso-composition activation energy (Ec)
with depletion of Iso-dielectric Activation energy (ED) are
complimentary to each other. This finding is also supported earlier past by
past view of Wolford [11] and it is recently supported by Singh R T. et al.
[12]
Table–5 Values of
Iso-Dielectric Activation Energy (water-propanol)
|
Dielectric constant(D) |
D=20 |
D=30 |
D=40 |
D=50 |
D=60 |
|
ED in KJ/mole |
66.00 |
64.00 |
63.83 |
60.06 |
58.00 |
3.4 Effect of
Solvent on Salvation number and mechanistic path of the reaction.
The salvation number (n) that
is number of water molecules associated with activated complex is determined by
plotting logk against log [H2O]. According
to proposed Robertson [13] relation
logk = logk0 +
nlog[H2O]
Salvation number
(n) that is the number of water molecule associated with activated complex
tells about the number of water molecule associated with transition state and
also a criterion for studying about the mechanism of the reaction. From noted
value of slope in Table-6, it is inferred that the value of slope increases
from 0.801 to 1.79 when the water concentration in the reaction increases and
the concentration of organic co-solvent decrease in reaction media. It may also
be inferred that with increase of solvent composition (30 to 70%) the number of
water molecule associated with transition state increase with rise in
temperature (20 to 400c) which attributes to the fact that
equilibrium of water molecule by addition of Propanol, is shifted from dense
form to bulky form with rise in temperature.
[H2O] d
↔
[H2O]b
On guide line of Parker and
Robertson [14], it is also inferred that the mechanistic path followed by
alkali catalyzed hydrolysis of propyl hexanote is
changed from bimolecular to unimolecular. It is also supported by Singh A K[15]
Table-6 Variation of 3+ Log k
with log [H2O] (water-propanol) at different temperature
|
% of Acetone |
% of H2O |
Log [H2O] |
3 + Log k |
||||
|
200C |
250c |
300c |
350c |
400c |
|||
|
30% |
70% |
1.5690 |
1.790 |
1.990 |
2.155 |
2.339 |
2.500 |
|
40% |
60% |
1.5229 |
1.730 |
1.920 |
2.090 |
2.260 |
2.410 |
|
50% |
50% |
1.4437 |
1.665 |
1.850 |
2.030 |
2.200 |
2.370 |
|
60% |
40% |
1.3468 |
1.590 |
1.790 |
1.965 |
2.139 |
2.310 |
|
70% |
30% |
1.2218 |
1.535 |
1.725 |
1.905 |
2.080 |
2.255 |
Fig. 3: Variation of
log [H2O] with Log K.
Table-7 Evaluated values of
slopes (Plot of log k verses log [H2O]) of Water-propanol media
|
Temp0C |
200c |
250c |
300c |
350c |
400c |
|
Slope |
0.801 |
0.826 |
0.843 |
0.875 |
1.279 |
Table-8 Rate constant and
Thermodynamics Activation Parameters (∆H*and ∆G* in
KJ/Mole, ∆S*in J/K/Mole) of the reaction in Water- propanol
Media.
|
% of Propanol |
Mole % |
∆H* in KJ/Mole |
200C |
250C |
300C |
350C |
400C |
|||||
|
∆G* |
-∆S* |
∆G* |
-∆S* |
∆G* |
-∆S* |
∆G* |
-∆S* |
∆G* |
-∆S* |
|||
|
30% |
9.33 |
51.00 |
88.74 |
127.09 |
89.17 |
128.08 |
89.76 |
127.92 |
90.98 |
129.74 |
90.719 |
126.36 |
|
40% |
13.79 |
54.81 |
89.68 |
118.89 |
89.57 |
116.64 |
90.08 |
116.40 |
90.66 |
116.39 |
91.13 |
116.03 |
|
50% |
19.35 |
55.59 |
89.45 |
115.56 |
89.92 |
115.20 |
90.43 |
114.98 |
91.01 |
115.00 |
91.37 |
114.31 |
|
60% |
26.47 |
57.49 |
89.87 |
110.51 |
90.32 |
110.16 |
90.81 |
109.96 |
91.37 |
110.00 |
91.61 |
109.00 |
|
70% |
35.90 |
57.96 |
89.87 |
108.90 |
90.61 |
109.83 |
81.16 |
76.56 |
91.72 |
109.61 |
92.06 |
108.94 |
5.
CONCLUSION:
In
this project of hydrolysis of propyl hexanoate, decreasing trend of specific
rate constant with increasing mole percent of co-solvent indicates that the
decreasing trend is either the decrease in bulk dielectric value or decrease in
polarity of reaction media by the addition of less polar propanol to it.
Increasing
trend of activation energy (Ec) with
increase co-solvent inferred simultaneous salvation and desolvation
o of initial and transition state respectively
The
increase in number of water molecules associated with activated complex with
increasing temperature shows that the mechanism of reaction media changes from
unimolecular to bio molecular with addition of the solvent.
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Received on
12.09.2019 Modified on
30.09.2019
Accepted on
14.10.2019 © AJRC All right
reserved
Asian J. Research Chem. 2019;
12(6):341-344.
DOI:
10.5958/0974-4150.2019.00064.6