Articles
Indian Jo urn al of Chemi cal Tec hno logy
Vol. 9, July 2002, pp. 297-305
A comparative kinetic and mechanistic study of saponification of industrially
important esters viz., mono and distearates, oleostearates of glycol,
glycerol and methyl salicylate in alcohol-water, dioxane-water,
DMSO-water and DMF-water moieties
B Madhava Rao* & K Gajanan
De partment o f Che mi stry, Visvesvaraya Reg ional Co llege of En gineering, Nag pur440 Oil , India
Received 12 October 2001; revised received 29 April 2002; accepted 2 May 2002
Kinetic and mechanistic studies of saponification of the above structurally related and industrially important
mono and diesters and their comparative behaviour of saponification process have been investigated. Time ratio
method and Swain's standard data for series first order reactions have been utilized for the evaluation of rate data
and thermodynamic parameters viz. fll?', -Mr, !la", D,S# and log A for mono and diesters which involve the
competitive and consecutive saponification reactions. These investigations also indicate that mono esters undergo
saponification process much faster than diesters and furthermore the oleostearates are more saponifiable than
stearates.ln general, these processes are much faster in dioxane-water, DMSO-water and DMF-water systems than
in alcohol-water system. Further, the dAB (inter-ionic distance in the double sphere model of an activated complex) of
mono esters are much smaller than diesters confirming that saponification processes are relatively faster in mono
esters than diesters.
The determination of rate constants from the
experimental data in series first order reactions was
first explored in a great detail by Swain I. A number of
workers 2-3 made detailed kinetic studies of alkaline
hydrolysis of mono and diesters of different
carboxylic acids in protic and aprotic solvents. Bruice
4
and Fife forw arded logical explanation for a faster
alkaline hydrolysis on the basis of internal solvation
of the transition state followed by the attack of
hydroxyl ion (OR) at the ester carbonyl group.
Several workers5-7 made detailed studies on the
kinetics and kinetic laws, influence of temperature,
variance of thermodynamic parameters in respect of
substituents, sol vents and their dielectric constants,
structure and reacti vity, influence of dipolar, aproticprotic solvents in the saponification processes of
saturated esters and also aliphatic dicarboxylic esters.
Instances of the meaningful application of activation
parameters along with LFER relationships are also
encountered in the literature 8- 'o with particular
reference to alkali catalysed hydrolysis of mono and
diesters. A critical analysis of the available
literature" -15 reveals that most of the work is
*For correspo ndence :
(E-mail : bmrao @nagpur.dot.net. in
Present address : Emeritus Professo r, Department of Chemistry,
KITS Ramtek 441 106, Dist., Nagpur, Indi a.
confined to the alkaline hydrolysis or saponification
processes of either the normal esters or the
synthesized esters with much emphasis on the study
of correlation ships pertinent to activation parameters
or specific solvent effects or LFER relationships.
However, very little work is encountered in literature
as regards the detailed study of kinetics and
mechanism of saponification of industrially important
esters in protic-aprotic solvent systems and
simultaneously
evaluating
rate
data
and
thermodynamic parameters.
Experimental Procedure
Theoretical treatment
In present investigations, the time ratio method is
adopted rather than Powell's graphical method since
time ratio method yields comparatively more precise
rate constants. In this method, times for 15%, 35 %
and 70% of the reaction are recorded from a graph
drawn on a large-scale and the corresponding l' and K
values are noted from Swain's modified Table ' for
series first order reactions . From the relation T =f30k,t
the value of kl and from the other relation K=k7/k l the
value of k2 are evaluated. Thus, the rate constants kl
and k2 for the two consecutive steps involved in
saponification process of diesters at different
temperatures are calculated.
Indian J. Chern . Technol., July 2002
Materials and methods
The liquid esters and semisolid esters employed in
the present work were of extra pure variety
(BOHlE.Merck) and were further purified by
distillation or by crystallization from a suitable
solvent before use. The physical data, viz. m.p.,
saponification value and IR spectra for esters
employed are in agreement with the data collected
from literature. The rate studies were carried out over
0.3 to 0.7 of the life period of the saponification
reaction. Requisite amounts of the reaction mixture
(mono and diesters and excess of alkali which is
twenty times over and above the stoichimetric
equivalent concentration) were pi petted out at noted
time intervals into a solution containing a known
excess of potassium hydrogen phthalate which served
to arrest the reaction . Carbon dioxide was carefully
removed from the original reactants; the solvents and
the whole system was kept during the reaction as well
as titration in a stream of nitrogen . The sodium
hydroxide solutions employed in the saponification
process as well as in the titration were prepared
carbonate-free by the reaction of metallic sodium with
conductivity
water.
The
solvents
ethanolldioxane/OMSO/OMF
used
In
the
saponification process were purified by repeated
distillation with CaO and also by azeotropic
distillation methods.
Treatment of data
Percentages of a saponification reaction for a
particular kinetic run are plotted as a function of time
t on a large scale. From a smooth curve drawn
through these experimental points, the times for 15%,
35 % and 70% of the reaction are recorded. From the
time ratios, e.g. t3s1tlS or logt3s1tls the corresponding K
values and r values are noted from the standard tables
furnished by Swain 1 using the following relations:
T = ~oklt
K=k 2 /k l
The rate constants for the consecutive, competitive
steps are evaluated. However, the rate constants of
mono esters were calculated using first-order rate
expression, as the [OR] is isolated in these
saponification processes. The rate constants obtained
in these investigations represent an average of atleast
three kinetic runs and are accurate within ±3 %.
Thermodynamic parameters, viz. energy of activation
f..~, enthalpy of activation -f..H*, entropy of
298
activation Mt, Gibb's free energy f..C* and frequency
factor (A) with respect to these individual steps are
calculated employing the necessary formulae . A
summary of the rate constants and also the
thermodynamic parameters are presented in Tables.
Further, the effect of the above solvent systems as
well as evaluation of dAB (inter ionic distance in the
double sphere model of an activated complex) were
also explored.
Results and Discussion
A number of instances concerning anchimeric
assistance are encountered in the literature. However,
very little work is on record in respect of the
saponification of mono and diesters of glycol,
glycerol and their comparative behaviour. In the
saponification process of diesters, anchimeric
assistance is provided by the neighbouring hydroxyl
group located in close proximity to an ester bond.
Further, the aliphatic hydroxyl group is found to
facilitate the alkaline hydrolysis of the ester without
participating itself as a nucleophile. Such anchimeric
assistances are also encountered in a variety of
biochemical transformations and this has promoted us
to study in detail the saponification of the above said
industrially important diesters.
Anchimeric assistance from the neighbouring
carboxyl group in the second stage of hydrolysis
could be surmised provided the neighbouring
carboxyl group remains intact with further
stabilisation or neutralisation or linking to the
glyceride molecule. Such a situation is difficult to
visualise in the saponification process of industrially
important mono and diesters (oils and fatsglycerides). Hence, anchimeric assistance from the
neighbouring carboxyl group may not be possible.
Such anchimeric assistance from neighbouring group
cannot be contemplated with respect to monoesters as
they have exclusively a single ester group. A
meticulous critical analysis of the rate data furnished
in Tables 1-4, shows that the reactivity pattern in
respect of saponification process of diesters into
mono-ester/half-ester is more than that of mono-ester/
half-ester hydrolytic reaction . This strange finding
could be rationalized by involving the concept of
Kumuira et ai. 16 that the longer carbon chains present
in the alkyl groups of the fatty acid units of the diester
provide such a difference in rates . A survey of
literature also reveals that the transition state formed
from the saponification of an ester has a negative
charge localized on the carbonyl oxygen atom making
this a good proton acceptor through a hydrogen bond
Articles
Indian J. Chem. Techno!., May 2002
Table I- First order reaction, saponification of mono and distearates, o leostearatcs of g lycol. g lycero l and methy l salicy late:
rate constants of I sl step and 2 nd step
IOH '] = 0.02 M
[Cn =0.D2 M
[Es ter] = 0.001 M
Alcoho l-Water = 0.445 mo le fraction (v/v : 72128)
30°C
40°C
50°C
60°C
70°C
30°C
k2 x 10 2 , S' I (2 nd stel2)
50°C
60°C
40°C
5. 114
10.97
11.18
57.66
123.4
0.014
0.016
0.052
0. 190
0.410
180.8
0.085
0.149
0.160
2.143
4.590
42.77
0.001
0.002
0.013
0.015
0.590
Name of ester
x 102 ,
kl
Ethylene glycol di stearate (diester)
stel2)
S' I (lS I
Ethylene glycol monostearate (monoester)
10.05
15.45
30.25
42.25
G lycery l distearate (diester)
56.66
70.88
72.82
146.4
G lyceryl monostearate (monoester)
64.05
81.75
122.5
220.00
Glycery l oleostearate (diester)
4 .657
7.780
10.9 1
28.56
Glycery l mono oleate (monoester)
18.45
38.25
102 .5
180.0
Methyl salicy late (aromatic monoester)
6.75
13.75
86.05
182.4
70°C
Table 2-First order reaction, saponification of mono and distearates, oleostearatcs of glycol, glycerol and methyl salicylate:
rate constants of I" step and 2nd step
IOH'] = 0.02 M
ICII =0.02 M
Name of ester
fEsterl = 0.001 M
Dioxane-Water = 0.352 mole fraction (v/v: 72/28)
k, x 10' . S· I (2 nd step)
k, x 10' , S· I (I" step)
30°C
35°C
40°C
45°C
50°C
55°C
60°C
30°C
35°C
40°C
45°C
50°C
55°C
60°C
Eth ylene glycol di stearate
(diester)
55.00
62.00
68.90
76.00
156.0
162.0
272.0
0.354
0.530
1.700
2.700
3.450
4.100
6.200
Ethylene glycol monostearate
(monoester)
62.05
76.45
102.4
182.2
236.3
301.25
Glycery l distearatt!
(diester)
63.05
88.00
96.00
163.0
190.0
215.0
2.850
2.961
3.256
3.815
5.241
6.33 I
7.31 I
Glyceryl monostearate
(monoester)
72.45
96.05
126.0
174.0
212.0
248.0
Glyceryl oleostearate
(diester)
30.05
40.00
10 1.0
150.0
160.0
211.2
3.25
4.50
5.170
12.52
13.65
17.09
19.05
Glycery l mono oleate
(monoester)
40.05
71.25
126.8
163.5
201.3
233.0
Methyl salicy late
(aromatic monoester)
29.45
74.35
135.4
187.0
211.0
265.0
76.00
36.00
Table 3-f'irst order reaction, saponification of mOllo and di stearates, o leostearates of glyco l, glycerol and methyl salicy late :
rate constants of 1sl step and 2 nd step
[OH'] = 0 .02 M
[CI'] = 0.02 M
[Ester] = 0.00 1 M
DMSO- Water =0.398 mole fraction (v/v: 72128)
10°C
x 102,5' 1 (I SI stel2)
15°C
20°C
25 °C
30°C
Et hylene g lycol d istearate (diester)
90.00
108.0
148.0
360.0
500.0
Et hylene glyco l monostearate (monoester)
140.0
182.0
227.0
405.0
585 .0
Name of ester
10°C
k2 x 102 , S'I (2 nd stel2)
15°C
20°C
25°C
30°C
0 .720
8.000
14.30
39.00
65.00
0.345
8.005
15.24
28.22
35.12
0.865
17.50
26.25
35.00
46.00
kl
Glyceryl distearate (diester)
88.25
102.0
150.0
350.0
375.0
Gl yce ry l monostearate (mo noester)
137.3
185.0
270.0
402.0
580.0
Glycery l o leostearate (diester)
107.7
137 .0
273 .0
470.0
5 11.0
Glyceryl mo noo leate (monoester)
160.0
217.0
325 .0
500.0
570.0
Methy l salicy late (aromatic mo noester)
87.05
142.0
225.0
3 12.0
425.0
299
Articles
Indi an J. Chern . Techno!. , July 2002
Table 4-First order reaction , saponification of mono and di stearates, oleostearates of glycol, glycerol and meth yl salicy late:
rate constants of ISI step and 2nd step
[OH'] = 0.02 M
[Cr] = 0.02 M
[Ester] = 0.001 M
DMF-Water = 0.376 mole fraction (vlv: 7212 8)
10°C
k , x 102 , s" ( 1st ste~)
15°C
20°C
25°C
Ethy lene glycol distearate(diester)
98 .00
126.0
220.0
Name of ester
168.0
220.0
350.0
590.0
795.0
11 8.0
152.0
192.5
450.0
550.0
Glyceryl monostearate (monoester)
168.0
202.0
3 14.5
525.0
640.0
Glyceryl oleostearate (d iester)
155.2
198.5
325. 1
520.2
855.5
Glyceryl monooleate (monoester)
190.0
270.0
398.0
580.0
884.0
Methyl salicy late (aromatic monoester)
120.8
260.0
380.0
420.0
51 8.0
0
I
I
H-C-O-C-R
I
Where ' R' stands for
OH'
H-C-O-C-R
I
H-
~
1~65
18.10
46.07
80.25
7.680
12.25
18.45
40.08
52.35
12.20
28.25
38.32
78.60
84.72
10°C
750.0
Ethylene glycol monostearate (monoester)
H
30°C
0.850
30°C
542.0
Glycery l distearate (diester)
formation 17. This exp lanation of the transition state
was also supported by Haberfield et at. 18 by
calorimetri c determ'ination of the relative enthalpies
of reactant and transition states. In the alkaline
hydrolysis of an ester, the transition state resembles a
species such as an alkoxide ion much more than a
delocalized anion having a weak hydrogen-bonding
interaction with the solvent. In the present study,
particularly in the saponification of o leostearates of
glycerol the negative charge on the carbonyl oxygen
atom in the transition state decreases by diffusion
through intramolecular hydrogen bond formation as
shown in the following Structure I
k2 x 102 , S·, (2 nd s te~)
15°C
20°C
25°C
OH'
H
Where 'R' stands for
•
I
•
H-C-O-C-R
Stearic
=
C 17 H35
6
I
H-C--O--H
I
H
Structure II
faster saponification process than the intermediate
half-ester/mono-ester formed vide Tables 1-4.
Perhaps thi s may be due to the unique behaviour of
the fresh monoester undergoing a base catalysed
unimolecul ar acyl cleavage like any other pure
monoester. Further, during this rapid saponification
process of a fresh monoester, the formation of
intramolecular H-bonding may be a remote
possibility . However, in th e case of pure monoester
e.g. methyl sali cy late the following general
mechanism which is a base catalysed unimolecular
acyl cleavage could be envisaged vide Table 5.
o
II
C -O-H
I
H
R-
C-
o
II
OR'
"=7
R-{:+ + R'O'
Structure I
As mentioned earlier the rate of saponification of
diester is more than that of monoester could further be
explained on the basis the transition state (Structure
II) which is also formed from the intramolecular
hydrogen bond formation of the monoester is less
solvated by aq.ethanol or aq.dioxane or aq.DMSO and
aq.DMF and therefore, responsible for the loweri ng of
the rate of saponification of the monoester.
On the contrary, a fresh monoester e.g. Glycol
monostearate or glyceryl monostearate undergoes a
300
A simple mechanism of basic hydrolysis of mono
ester involves the following steps:
OH·+R· -
w~~
"
IT C'· -OR ..... R·-C-OR .... R' - C
~
OR + OH'
bH
1~
HOR
[ R -~ - 01.-,.
r
OH
0"
Articles
Indian 1. Chern. Techno\., May 2002
In a saponification process the sequence of these
reactions proceeds to completion because equilibrium
is displaced by a proton exchange between the acid
and alkoxide ion. A perusal of thermodynamic
parameters (Tables 6-9) indicates that in general the
energies of activation, the enthalpies of activation,
entropies of activation and free energies of activation
of mono esters of ethylene glycol and glycerol are
relatively lower than the values of the corresponding
individual diesters, half-esters/mono esters of glycols
and glycerol. Further, in diesters as well as halfester/mono-ester the vicinal hydroxyl groups formed
in the saponification process also play a prominent
role in the internal stabilization of the transition state
causing
lower
saponification
rates .
These
thermodynamic parameters are also in consonance
Table 5- Saponification mecha nism
Mechanism
Position of
cleavage
Acyl
8'1
Alky l
configuration
Retention
Kinetics
E
Electron
requirement
R ROO
Steric
hindrance
No
-00
Table 6-First order reaction, saponification of mono and distearates, oleostearates of glycol, glycerol and methyl salicylate:
Thermodynamic parameters of I SI step and 2nd step - A comparative kinetic study
[OH') =0.02 M
[er) = 0.02 M
Name of ester
Ethylene glyco l distearate
(diester)
Ethylene glycol monostearate
(monoester)
Glyceryl distearate
(diester)
Glycery l monostearate
(monoester)
Glyceryl oleostearate
(diester)
Glyceryl monoo leate
(monoester)
Methyl salicylate
(aromatic monoester)
[Ester) =0.00 1 M
Alcohol- Water =0.445 mole fraction (v/v : 72128)
11£.
11£. ,
kcallmo l
l1£a2
-I1H
-I1H,
kcal/mol
t1S
e.u.
I1G
kcal/mol
-I1H2
I1S,
I1S2
I1G,
I1G 2
16.51
17.28
16.23
17.40
-16.16
-19.40
-10.84
-10.94
11 .62
6.930
10.34
7.380
4.341
8.730
7.9 10
8.100
3.322
17.19
8.580
-6.470
-9.480
-4.025
17.61
-13.67
- 5.808
-4.990
-4.050
8.762
9.459
7.242
7.284
13.56
-8.082
-19.61
logA 2
14.24
-12.34
-12.47
10gA,
-10.08
8.2 16
5.245
6.234
- 10.42
- 14.10
15.02
7.365
6.245
-1 1.05
-8.425
7.222
9.506
Table 7-First order reaction, saponification of mono and distearates, oleostearates of glycol, glycerol and methyl sal icylate :
Thermodynamic parameters of I" step and 2 nd step
[OH'l =0.02 M
[Cr] =0.02 M
Name of ester
Ethylene glycol distearate
(diester)
Ethylene glycol monostearate
(monoester)
Glyceryl distearate
(diester)
Glycery l monostearate
(monoester)
Glyceryl oleostearate
(diester)
Glycery l mono oleate
(monoester)
Methyl salicylate
(aromatic monoester)
[Ester) =O.DOI M
Dioxane-Water =0.352 mole fraction (v/v : 72128)
I1Ea
11£.,
kcal/mol
- I1H
-I1H,
kcal/mol
I1S
e.u.
I1G
kcal/mol
11£.2
-I1H2
I1S,
I1S 2
I1G ,
I1G2
3.720
9.380
4.872
8.452
-2.620
-5.424
-14.77
- 16.64
2.205
2.540
3.184
5.230
1.025
3.889
1.396
-1 .200
2.250
0.975
5.720
5.569
-2.310
-1.067
10.84
-3.256
-7.421
-10.32
-9.568
14.45
14.56
18.34
18.45
23.47
-12.25
-4.625
logA2
18.65
-17 .26
-5.120
10gA ,
-18.84
18.56
1.955
2.655
-1.657
-24.58
21.25
5.325
3.155
-8.055
-13.65
14.75
18.75
301
Articles
Indian J. Chem. Techno!.. Ju ly 2002
Table 8-First order reaction, saponification of mono and distearates, oleostearates of g lycol, glycerol and methyl salicylate :
Thermodynamic parameters of 151 step and 2 nd step
[OH -] = 0.02 M
[Cr] = 0.02 M
[Ester] = 0.00 1 M
DMSO-Water = 0.398 mo le fraction (v/v: 72128)
Name of ester
!'J.E"
!'J.E"I
kcallmol
!'J.E,,2
-!'J.H
-!'J.HI
kcallmol
-!'J.H2
!1S
!'J.S I
e.lI.
!'J.S2
!'J.G
!'J.G I
kcallmo l
!'J.G z
log Al
logA 2
Ethylene glycol di stearate
(d iester)
Ethylene glyco l Illonostearate
(monoester)
Glyceryl disteara te
(diester)
Glyceryl 1110nostea rate
(monoester)
Glyceryl oleostearate
(diester)
Glyceryl monooleate
(monoester)
Methyl salicylate
(aromatic monoester)
3.125
5.457
2.245
6.325
-1.754
-3.257
-16.22
- 19.62
17.55
17.75
1.105
1.513
1.255
3.884
1.023
0.265
0.875
3.432
0.920
4.804
-1.083
-3.130
-0.875
1.330
1.841
-1.563
24.65
-22.65
- 0.654
-10.22
- 12.12
-13.31
21.54
27.55
-16.45
-2.064
21.43
- 19.41
20.14
0.755
1.025
-0.225
-24.75
26.35
2.425
1.895
-4.345
- 18.18
19.65
20.17
Table 9-First order reaction, saponification of mono and distearates, oleostearates of glycol, glycerol and methyl salicylate:
Thermodynamic parameters of I sl step and 2 nd step
[OH'] = 0.02 M
[Cr) =0.02 M
[Ester) = 0.001 M
DMF-Water = 0.376 mole fraction (v/v: 72128)
Name of ester
!'J.E"
!'J.E"I
kcal/mo l
!'J.E,,2
-!'J.H
-!'J.HI
kcal/mol
-!'J.Hz
!'J.S
!'J.S I
e.lI.
!'J.S2
!'J.G
!'J.G I
kcal/mol
!'J.G 2
logAI
logA2
Ethylene glycol distearate
(diester)
Ethylene glycol monostearate
(monoester)
G lyceryl distearate
(diester)
Glyceryl monostearate
(monoester)
Glycery l oleostearate
(diester)
Glycerylmonooleate
(monoester)
Methyl salicylate
(aromatic monoester)
2.40 1
4.128
1.345
3.521
-0.945
-1.502
-18.55
-21.20
21.02
21.12
0.795
1.025
0.675
3.025
0. 105
1.054
0.256
0.802
0.076
2.820
0.245
-0.925
-1.450
-0.624
-1 2.65
- 14.35
-0.98 1
- 16.25
22.05
22 .25
31.85
-19.85
-0.045
0.834
27.42
-25.67
-22 .55
24.35
0.105
0.017
-0.015
-26.67
29.45
1.045
0.757
- 1.891
-20.05
22.25
with any type of ion-dipole reaction. The results of
the present work also show that systems which
involve a high degree of internal stabilization of the
transition state are susceptible to the solvent influence
and there is much variance in the thermodynamic
parameters and also in the value of the rate constants
for both the steps of diesters. The variable
susceptibility to polar effects suggests an increased
state solvation
in
importance of transition
dioxane/DMSO/D MF. If pK value is equated with the
degree of negative charge developed in the transition
state or alternatively the degree of 'tightness' of the
transition state complex, imp li es that the attacking
302
-0.101
24.47
hydroxide ion an d the carbonyl carbon are separated
by a greater distance in aq. DMF than aq. DMSO,
aq. Dioxane, and aq.alcohol. Whi le it is not
permissible to make a quantitative assessment of the
contri bution of transition state solvation to the -D..F
term, the importance of the contribution shows that
substitution of aq .DMFI aq. DMSO/aq.dioxane than in
aq.alcohol leads to an enhanced rate of reaction.
However, more importantly, as per the HughesIngold 19 theory, both the reduced enthalpy, entropy of
activation support the involvement of a more highly
solvated transition state of diester in aq.DMF. T he
state of anion solvation is frequently mentioned by
Articles
Indian J. Chem. Technol., May 2002
Benson 20 and Tommila et al. 21 as a contribution cause
of reactivity. Thus in aq .Dioxane, aq.DMSO and
aq.DMF systems, the present study shows that the
reactivity of hydroxide ion is dependent upon the
following equilibrium:
where n equals the maximum number of water
molecules hydrogen-bonded to hydroxide ion. With
increasing dioxane, DMSO and DMF content, the
equilibrium would be shifted to right, resulting in a
less solvating hydroxide ion. Estimates may be made
of the free energy change caused due to the variation
of sol vent system (Tables 10-13), in going from the
initial state of a reaction to the activated state.
According to the simplest treatments of electrostatic
interactions particularly kinetics of reactions in
solution the charged ions are considered to be
conducting spheres and the solvent is regarded as a
continuous dielectric having a fixed dielectric
constant (£). Initially the ions are at a infinite distance
with each other. However, in activated state they are
considered to be intact (i .e. there is no smearing of
charge) and they are at distance dAB apart. This model
is frequently referred to the double-sphere model. A
final conclusion from the solvent is that the log k
versus 1/£ is almost linear and the slope of the line is
given by Z AZBe 2Id AB KT. From this expression as well
as from experimental slope it is possible to calculate
dAB (inter-ionic distance in the double sphere model of
an activated complex). These investigations have
revealed that the dAB values (Table 14) for diesters,
half-ester/monoesters are much more than the pure
mono esters and the cause for such behaviour may be:
1) the size of diester, 2) solvation of the attacking OR
and 3) the anchimeric assistance provided by the
neighbouring group in the half-ester.
The effect of anion dissolution as the major
contribution cause for the increase of rate constant is
also to some extent unlikely for the following reasons.
(i)
(ii)
The activity of the hydroxide ion with increasing
dioxanelDMSO and DMF concentrations does
not explain the dependency of medium effect
upon substrate steric substituent constant or the
presence of medium effect discontinuities.
With a small ion such as hydroxide, it is more
realistic to treat the ion plus the hydrogenbonded water molecules as a single kinetic unit.
Table IO--Effect of solvent composition, series first order reaction, saponification of ethylene glycol distearate (diester)
[OH-] = 0.02 M
Alcohol-water
composition (v/v)
[Ester] = 0.001 M
Mole
fraction
Temp.= 50 ± 0.05°C
[Cr] = 0.02 M
Pseudo first order
rate constant
k,x 102 , S-I
Pseudo first order
rate constant
k2 X 102 , S -I
1st Step
2nd Step
Dielectric constant
(e)
72/28
0.4455
11.18
0.052
55.63
60/40
0.3141
7.250
0.021
41.34
50/50
0.2339
2.450
0.001
37.91
40/60
0.1691
0.925
0.007
34.33
28172
0.1062
0.015
0.001
30.86
Dioxane-water
composition (v/v)
Mole
fraction
Pseudo first order
rate constant
kl X 102 ,S-I
1st Step
Pseudo first order
rate constant
k2 X 102 , S- I
2nd Step
Dielectric constant
(e)
72/28
0.3521
156.0
3.450
29.65
60/40
0.2408
95 .65
2.015
20.97
50/50
0.1745
41.25
1.257
15.80
40/60
0.1235
17.35
0.875
11.87
28172
0.0759
6.450
0 .121
8. 119
303
Articles
Indian J. Chern. Techno!., July 2002
Table II-Effect of solvent composition, series first order reaction, saponification of ethylene glycol distearate (diester)
[OH·] = 0.02 M
[cr] =0.02 M
Temp.= 30 ± 0.05°C
[Ester] = 0.001 M
DMSO-water
composition (vlv)
Mole
fraction
72/28
60/40
50/50
40/60
28/72
0.398
0.276
0.203
0.145
0.090
DMF-water
composition (vlv)
Mole
fraction
72/28
60/40
50/50
40/60
28172
0.367
0.261
0.189
0.136
0.084
Pseudo first order
rate constant
kl x 102 , S -I
Pseudo first order
rate constant
k2 x I02,s - 1
1st Step
500.0
245 .0
162.0
85.75
36.65
2nd Step
65 .00
29.00
15.45
7.650
3.250
Pseudo first order
rate constant
kl x 102, S-I
1st Step
750.0
412.0
216.0
141.5
74.25
Pseudo first order
rate constant
k2 x I0 2 ,s-1
2nd Step
80.25
55.55
35.57
13.65
6.050
Dielectric constant
(€)
14.33
10.25
8.350
4.650
2.860
Dielectric constant
(€)
7.650
4.330
1.850
0.950
0.025
Table 12-Effect of solvent composition, First order reaction, saponification of ethylene glycol monostearate (monoester)
[Cn = 0.02 M
Temp.= 50 ± 0.05°C
[OH·] = 0.02 M
[Ester] = 0.001 M
Alcohol-water
composition (vlv)
Mole
fraction
Pseudo first order
rate constant
k x 102, S-I
Dielectric constant
72/28
60/40
50/50
40/60
28172
0.4455
0.3141
0.2339
0.1691
0.1062
30.05
11 .25
6.850
1.250
0.750
55.63
41.34
37.91
34.33
30.86
Dioxane-water
composition (vlv)
Mole
fraction
Dielectric constant
72/28
60/40
50/50
40/60
28172
0.3521
0.2408
0.1745
0.1235
0.0759
Pseudo first order
rate constant
kx 102, S-I
182.2
112.8
60.00
32.05
10.25
(€)
(€)
29.65
20.97
15.80
11.87
8.119
Table 13-Effect of solvent composition, First order reaction, saponification of ethylene glycol monostearate (monoester)
[OH·] = 0.02 M
304
[Ester] = 0.001 M
[Cn =0.02 M
Temp.= 30 ± 0.05°C
DMSO-water
composition (vlv)
Mole
fraction
Pseudo fi rst order
rate constant
k x 102 , S- I
Dielectric constant
72/28
60/40
50/50
40/60
28172
0.398
0.276
0.203
0.145
0.090
585.0
316.0
206.1
142.7
68 .05
14.33
10.25
8.350
4.650
2.860
DMF-water
composition (vlv)
Mole
fraction
Dielectric constant
72/28
60/40
50/50
40/60
28172
0.367
0.261
0.189
0.136
0.084
Pseudo fi rst order
rate constant
k x I0 2 ,s-1
795.0
525.0
463.0
275.0
165.7
(€)
(€)
7.650
4.330
1.850
0.950
0.Q25
Articles
Indian J. Chem. Techno!., May 2002
Table 14-dAB values of EGOS & EGMS
Alcohol-water. A0
Dioxane-water, A0
U
DMSO-water, A
DMF-water, A
EGOS
4.2-9.6
2.1-3.0
0.8-1.0
0.1-0.3
EGMS
2.5
1.5
0.2
0.03
ame of ester
O
EGOS: Ethylene glycol distearate, EGMS: Ethylene glycol monostearate
If
the
anion
desolvation
mechanism
were
operative this would imply decreasing size of the
nucleophile
with
increasing
dioxane/DMSOI
DMF content in the solvent medium. Such a
References
I
2
reaction constant parameter 8. However, such
responses are not generally recorded in 8 at
higher mole fractions of dioxane/DMSO/DMF.
Anantkrishnan S V & Venkatratnam R V, Illdiall J Chelll. 4
(1966)379.
3
charge in the steric bulk of the attacking reagent
should be reflecting by the variation in the
Swain C G, J Alii Chelll Soc. 66 (1944)1696.
Anantkrishnan S V & Venkatratnam R V, Proc Illdiall Acad
Sci, Sec A 65 (1967)188.
4
Bruice Thomas C & Thomas Fife, J Alii Chelll Soc, 84 (1962)
1973.
5
Rao G V & Venkatasubramanian N, Illdiall J Chelll, 10
(1972)178.
6
Balakrishnan M, Rao G V &Venkatasubramanian
,J Chelll
Soc Perkill Trails, 2 (1974)6.
It is also obvious that systems which involve a high
degree of internal stabilization of the transition state
are susceptible to pronounced dipolar aprotic solvent
influences. Further, on the basis of this observation, it
would be seen that one can use di-polar aprotic
solvent influences on reaction rates as a criterion in
the assessment of anchimeric assistance in reactions
7
8
9
10
II
Nagpur
for
providing
necessary laboratory facilities to carry-out this work.
wish
to
express
their
sincere
gratitude to Dr V. Ramchandra Rao, retired Prof. and
Head,
Dept.
of
Ghosh Kallol K & Kishore K. Illdiall J Chelll, Sec B. 38
B(3)(1999) 337.
Engineering,
also
Ghosh Kallol K & Thakur Santhosh S, Illdiall J Chelll Soc.
76(1)(1999)15.
The authors are grateful to the authorities of V.R.
authors
Wheeler, Graham & Cock-Croft W E, Edllc Chelll, 34(2)
(1997)47.
12
The
Niyaz M & Arifin, J Colloid IlIfellace Soc, 180(1) (1996)
132.
Acknowledgements
of
Singh 0 P, Prasad & Sunil Kumar. J Illdiall Chelll Soc,
67(2),
involving internally stabilized transition states.
College
Holba V, Benko & Komadel P Z. Phys Chem (Leipzig). 262
(1981)445.
Chemistry,
VRCE
for
his
13
Yu He. Zhang & Juxian, fllIaxe Yallji Yll Yillg Yall!?, 12(2)
(2000)234.
14
Zhan Chang, Guo Landry, i30nald W & Rick L, J Phys
15
Shi Ji-Liang, Xu Jia-yi. Yi Hu-Non & Xi-kui, Chill J Chelll,
Chelll. 104(32)(2000)7672.
18(4)(2000)617.
16
Kumuira C K, Murai Koichi & HidelOshi Y, Kogva Kegllka
Zasshi, 71 (9)(1968)1569.
encouragement in the progress of this work. One of
17
the
Parker A J, Chelll Revs, 69 (1981) I.
18
Haberfield P, Friedman J & Pinkston M F, J Alii Chelll Soc.
19
Ingold C K, Structllre alld MechallislII ill Orgallic Chelllistr"
authors
(K.
Gajanan)
is
grateful
to
the
94 (1972)7I.
Management, Dr. G. Thimma Reddy, Principal and
Dr. K. Vijaya Mohan,
Head Dept.
of Chemistry,
KITS, Ramtek for their kind permission and constant
encouragement during this work and also to Sqn.Ldr.
K. R. Sharma and other family members for their
forbearance and continued moral support.
(Cornell University Press, Ithaca, New York), 1953,345.
20
Benson
S
W,
Tile
FOlllldarioll
of
Chelllical
Killetics
(McGraw-Hill Book Co Inc. New York), 1960,535.
21
Tommila E & Murto M L, Acta Chem Scalld, 17 (1963)
1947.
305