Indian Journal of Chemical Technology
Vol. 12, January 2005, pp. 43-49
Mechanistic studies of saponification of some mono- and di-esters of
carboxylic acids through iso-kinetic relationships (ΔH≠ vs. ΔS≠)
in protic and aprotic solvent systems
B M Rao* & K Gajanan
Department of Chemistry, KITS Ramtek 441 106, Nagpur, India
Received 29 September 2003; revised received 16 August 2004; accepted 14 September 2004
The mechanism of saponification of structurally related and industrially important mono- and diesters in protic and
aprotic solvent systems 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. ΔE≠, −ΔH≠, ΔG≠, ΔS≠ and logA for the
steps which involve the competitive and consecutive saponification reactions. Further, the enthalpy-entropy correlations are
used to establish the plausible mechanism of saponification process for the esters. Comparative study of the slopes of linear
plots (ΔH≠ vs. ΔS≠) reveal that dilaurates undergo faster saponification process than distearates and oleostearates irrespective
of any solvent sytems. The experimental plots (ΔH≠ vs. ΔS≠) support a faster saponification process of mono esters than
diesters in all the protic and aprotic solvent systems studied.
Keywords: Saponification, monoesters, diesters, mechanism
IPC Code: C07C 27/02
Anchimerically assisted ester hydrolysis reactions are
important not only from a mechanistic point of view,
but also in a variety of biochemical transformations.
Due to the biological significance of saccharide
biomolecules, the rational design and development of
stereo-controlled glycosidation1,2 reactions are
becoming important not only in carbohydrate
chemistry but also in medicinal chemistry. An
aliphatic hydroxyl group situated in close proximity to
an ester bond has been found to facilitate the alkaline
hydrolysis of the ester without participating as a
nucleophile3-5. Henbest and Lovell6 postulated the
hydrolysis of cyclohexane-1:3 diol monoacetates a
hydrogen bonded structure involving ether oxygen as
important factor. Bruice and Fife7 examined the
significance of C=O and OH groups in the rates of
alkaline hydrolysis of a number of cyclopentane and
norbornane acetates and diol monoacetates. The
increased rates of hydrolysis of cyclopentyl and exo2-norbornyl acetates on substitution in the α or β
position were explained on the basis of inductive
effect. The results of the infrared studies showed that
the extent of facilitation of hydrolysis by a vicinal
hydroxyl group was not dependent on the type of
possible internal hydrogen bonding in the ground
___________
*For correspondence
([email protected]/[email protected])
state. Hence Bruice and Fife7 postulated an
intramolecular stabilization of the transition state,
which was later labelled as Bruice and Benkovic8
microscopic solvent effect.
In the present work a systematic approach is
adopted to explore the anchimeric assistance as well
as solvent effects in the saponification of the above
said esters. For a thorough insight into the
mechanistic patterns of saponification of some esters
viz. Ethylene glycol monostearate (EGMS), Glyceryl
monostearate (GMS), Glyceryl monooleate (GMO),
Methyl salicylate (MS), Propylene glycol distearate
(PGDS), Ethylene glycol distearate (EGDS), Glyceryl
distearate (GDS), Propylene glycol dilaurate (PGDL),
Ethylene glycol dilaurate (EGDL), Glyceryl dilaurate
(GDL) and Glyceryl oleostearate (GOS), a critical
comparative study of rate constants, thermodynamic
parameters, solvent effects and iso-kinetic studies has
been done.
Experimental Procedure
Theoretical treatment
In this investigation, the time ratio method9 is
adopted rather than Powell’s graphical method10 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
INDIAN J. CHEM. TECHNOL., JANUARY 2005
44
on a large scale (curve-fitting programme) and the
corresponding τ and κ values are noted from Swain’s
modified table9 for series first order reactions. From
the relation τ =β0k1t, the value of k1 and from the
other relation κ=k2/k1 the value of k2 are evaluated.
Thus, the rate constants k1 and k2 for the two
consecutive steps involved in saponification process
are calculated.
Materials and Methods
The esters employed in the present work were of
extra pure variety (BDH/E.Merck) and were further
purified by distillation or by crystallization from a
suitable solvent before use. The physical data, viz.
m.p./b.p., saponification value and IR spectra for
esters employed are in agreement with the data
available in the literature. Requisite amounts of the
reaction mixture (diesters and an excess of alkali
which is twenty times over and above the
stoichiometric equivalent concentration) were
pipetted 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 whole system was kept during the
reaction as well as titration under a stream of
nitrogen. The sodium hydroxide solution employed in
the saponification process as well as in the titration
was prepared carbonate-free by the reaction of
metallic sodium with conductivity water. The solvents
ethanol/acetone/dioxane, DMSO and DMF employed
in the saponification processes 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’. The rate constants for the consecutive, competitive
steps are evaluated. 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
ΔE≠, enthalpy of activation −ΔH≠, entropy of
activation ΔS≠, Gibb’s free energy ΔG≠ and frequency
factor (A) with respect to these individual steps are
calculated employing the necessary formulae. A
summary of comparative rate constants, and
thermodynamic parameters are presented in Tables 2
to 5. The iso-kinetic relationships describing entropyenthalpy correlationships are furnished in Figs 1 to 3.
Results and Discussion
Though a list of six different mechanisms for ester
hydrolysis are available in the literature, the
properties of four mechanisms with respect to
kinetics, expected configuration of an asymmetric
alkyl group R’ after reaction, the effect of electro
negativity of R and R’ on the rate, and the effect or
lack of effect of steric hindrance in R and R’ on the
rate of reaction are summarized in Table 1.
A critical study of comparative rate constants of
mono- and diesters as well as thermodynamic
parameters (Tables 2 to 5) exhibit that a change of
solvent i.e. from protic to aprotic (ethanol to DMF)
causes an increase in saponification process by about
79 times in EGMS while 82 times, 45 times and only
10 times in MS, GMO and GMS esters, respectively.
Such a type of behaviour could exclusively be
attributed to the solvent effect. However, MS,
undergoes a slower rate of saponification as compared
to aliphatic esters in the same solvent system and this
may be mainly due to the aromatic nature of MS. A
perusal of data furnished in Table 4 reveals that
distearates are less saponifiable than dilaurates and at
the same time, distearates and dilaurates of glycerol
are less saponifiable than the corresponding diesters
of ethylene and propylene glycols. Such a behaviour
could be attributed to the shorter alkyl group in
laureates.
A careful study of thermodynamic parameters
(Tables 3 & 5) reveal that in the values of energy of
Table 1—Mechanism for ester hydrolysis (Datta et al.11)
Mechanism
Position of
cleavage
Alkyl configuration
Kinetics
B’1
Acyl
Retention
[E]
B’2
Acyl
Retention
[OH−],[E]
B’’1
Alkyl
Racemization
[E]
B’’2
Alkyl
Inversion
[H2O][E]
+ sign means that an electron-donating substituent will favour hydrolysis.
− sign means that an electron-attracting group will be favourable for hydrolysis.
Electron requirement
R
R′
0
−
+
0
−
−
−
−
Steric
hindrance
No
Yes
No
Yes
RAO & GAJANAN: MECHANISTIC STUDIES OF SAPONIFICATION OF SOME MONO- AND DI-ESTERS
activation, enthalpy of activation, entropy of
activation and free energy of activation gradually
decrease. However, there is a progressive increase in
the values of frequency factor. These findings
necessarily indicate, that, the process of
saponification becomes faster from protic to aprotic
solvent systems i.e. ethanol to DMF. The energy of
activation of EGDS, PGDS and GDS decreases about
6-10 times from protic to aprotic solvent system. This
reflects that the saponification process is much faster
particularly in GDS irrespective of the solvent system,
this also supports the evidence of anchimeric
assistance.
Table 2—Rate constants of mono esters
[OH−] = 0.02 M; [Cl−] = 0.02 M; [Ester] = 0.001 M;
Temperature = 30± 0.05°C
Alcohol-Water (v/v: 72/28) = 0.445 mole fraction;
DMF-Water (v/v: 72/28) = 0.376 mole fraction
Name of
monoester
Alcohol-water
system (protic)
k × 102 sec−1
DMF-water
system (aprotic)
k × 102 sec−1
EGMS
GMS
GMO
MS
10.05
64.05
18.45
6.250
795.6
640.0
844.6
518.2
Table 3—Thermodynamic parameters of mono esters
[OH-] = 0.02 M; [Cl−] = 0.02 M; [Ester] = 0.001 M; Temperature
= 30± 0.05°C
Alcohol-Water (v/v: 72/28) = 0.445 mole fraction;
DMF-Water (v/v: 72/28) = 0.376 mole fraction
Name of
Monoester
ΔEa#
(kcal/
mole)
EGMS
GMS
GMO
MS
11.62
4.341
5.245
7.365
EGMS
GMS
GMO
MS
0.795
0.105
0.105
1.045
−ΔH#
(kcal/
mole)
ΔS#
(e.u.)
ΔG#
(kcal/
mole)
Alcohol-Water
10.34
12.47
−12.34
3.322
−4.025 −8.082
6.234
−10.42 −14.10
6.245
−11.05 −8.425
DMF-Water
0.675
−0.101 −25.67
0.076
−0.045 −19.85
0.017
−0.015 −26.67
0.757
−1.891 −20.05
45
Table 4—Rate constants of diesters
[OH-] = 0.02 M; [Cl-] = 0.02; [Ester] = 0.001 M;
Temperature = 30± 0.05°C
Alcohol-Water (v/v : 72/28) = 0.445 mole fraction;
DMF-Water (v/v : 72/28) = 0.376 mole fraction
logA
Name of
diester
14.24
13.56
15.02
7.222
Alcohol-water system
DMF-Water system
(protic)
(aprotic)
k1×102, sec-1 k2×102, sec-1 k1×102, sec-1 k2×102, sec-1
EGDS
PGDS
GDS
EGDL
PGDL
GDL
GOS
27.42
31.85
29.45
22.75
5.114
20.96
56.66
5.520
6.060
9.380
4.652
0.014
0.348
0.085
0.012
0.043
0.002
0.001
750.0
623.5
550.0
850.0
750.0
770.0
855.5
80.25
68.58
52.35
98.25
89.50
793.0
84.72
Table 5—Thermodynamic parameters of diesters
[OH-] = 0.02 M; [Cl-] = 0.02 M; [Ester] = 0.001 M; Temperature = 30± 0.05°C
Alcohol-Water (v/v : 72/28) = 0.445 mole fraction
Name of
ester
ΔEa# kcal/mol
ΔEa1#
ΔEa2#
−ΔH# kcal/mol
−ΔH1#
−ΔH2#
EGDS
16.51
17.28
16.23
PGDS
7.710
9.597
8.070
GDS
6.930
7.380
7.910
EGDL
20.42
28.96
18.09
PGDL
12.25
16.08
12.62
GDL
9.120
15.20
4.980
GOS
8.730
17.19
8.580
DMF-Water (v/v: 72/28) = 0.376 mole fraction
EGDS
2.401
4.128
1.345
PGDS
0.965
1.542
0.104
GDS
1.025
3.025
0.256
EGDL
2.351
4.049
4.750
PGDL
1.256
3.025
0.656
GDL
0.995
1.984
0.354
GOS
1.054
2.820
0.245
ΔS# e.u.
#
#
ΔG# kcal/mol
ΔG1#
ΔG2#
ΔS1
ΔS2
17.40
8.730
8.100
29.17
18.34
5.700
17.61
−16.16
−8.680
−6.470
−19.95
−12.21
−9.160
−13.67
−19.40
−10.53
−9.480
−20.30
−21.30
−9.630
−19.61
−10.84
−5.170
−5.808
−11.33
−8.530
−1.770
−4.050
3.521
0.862
0.802
6.775
1.224
0.981
0.834
−0.945
−0.912
−0.925
−1.849
−1.025
−1.061
−0.624
−1.502
−1.025
−1.450
−2.025
−2.145
−2.425
−0.981
−18.55
−16.65
−12.65
−25.42
−21.52
−13.85
−16.25
logA1
logA2
−10.94
−5.220
−4.990
−22.41
−12.72
−2.296
−10.08
8.762
7.121
7.242
9.593
7.896
7.227
8.216
9.459
7.514
7.284
9.657
9.876
7.317
9.506
−21.20
−21.75
−14.35
−29.10
−26.75
−16.45
−22.55
21.02
19.85
22.05
25.54
22.05
26.36
24.35
21.12
19.95
22.25
25.67
22.25
26.46
24.47
INDIAN J. CHEM. TECHNOL., JANUARY 2005
46
A critical examination of the Tables (6-9) pertinent
to thermodynamic parameters i.e. enthalpy of
activation and entropy of activation as well as isokinetic plots (Figs 1-3) enumerating the relationship
between the entropy-enthalpy correlations for the
saponification of mono- and diesters reveal the
following salient features:
(i)
As is evident from Figs 1 and 2, that, laurates
are easily saponifiable whatever may be the
solvent. Such evidence is mostly drawn from the
fact that the increase of the entropy values in the
range of –4.0 to –2.0 e.u. i.e. from alochol-water
to DMF-water system. Further, the values of
slopes of linear plots ‘a’ (EGDS) and ‘c’ (GDS)
are 1.016 and 0.77 showing a decrease of 1.32
times . On similar lines, the values of slopes of
linear plots ‘d’ (EGDL) and ’f’ (GDL) are 1.97
and 0.60 showing a downward trend of 3.28
times. Such a behaviour could be attributed to
the fact that as one proceeds from dilaurates of
ethylene and propylene glycols to glycerol, there
is a building up of -CH2 units which cause a
downward trend in the values of slopes
irrespective of the solvent system. The slope of a
linear plot ‘d’ (EGDL) records a very high value
i.e.1.97 than any of the diesters which reflects
that dilaurates are more saponifiable than the
corresponding stearates and particularly EGDL
is much more saponifiable than the
corresponding laurates i.e. PGDL and GDL.
(ii) Further, it is also evident from Figs 1 and 2, that,
in general PGDL and GDL are less saponifiable
than EGDL among the laurates. The –CH2 group
Table 6—Thermodynamic parameters of mono esters
[OH]=0.02 M [Ester]=0.001 M
[Cl−]=0.02 M Dioxane-Water = 0.352 mole fraction (v/v: 72/28)
Name of ester
Ethylene glycol monostearate
Glyceryl monostearate
Glyceryl mono oleate
Methyl salicylate
ΔEa#
kcal/mol
−ΔH#
kcal/mol
ΔS#
e.u.
ΔG#
kcal/mol
logA
2.205
1.025
1.955
5.325
3.184
0.975
2.655
3.155
−1.200
−1.067
−1.657
−8.055
−17.26
−12.25
−24.58
−13.65
18.65
23.47
21.25
14.75
Table 7—Thermodynamic parameters of mono esters
[Cl-]=0.02 M
Name of ester
Ethylene glycol monostearate
Glyceryl monostearate
Glyceryl mono oleate
Methyl salicylate
[OH]=0.02 M [Ester]=0.001 M
DMSO – Water = 0.398 mole fraction (v/v: 72/28)
ΔEa#
kcal/mol
−ΔH#
kcal/mol
ΔS#
e.u.
ΔG#
kcal/mol
logA
1.105
0.875
0.755
2.425
1.255
0.265
1.025
1.895
−0.654
−0.875
0.225
−22.65
−16.45
−24.75
−18.18
24.65
27.55
26.35
19.65
−4.345
Table 8—Thermodynamic parameters of di esters
[OH−]=0.02 M [Ester]=0.001 M
[Cl ]=0.02 M Dioxane-Water =0.352 mole fraction (v/v: 72/28)
−
Name of ester
ΔEa# kcal/mol
ΔEa1#
ΔEa2#
−ΔH# kcal/mol
−ΔH1#
−ΔH2#
Ethylene glycol distearate
Propylene glycol distearate
Glyceryl distearate
Ethylene glycol dilaurate
Propylene glycol dilaurate
Glyceryl dilaurate
Glyceryl oleostearate
3.720
3.150
2.540
5.804
4.804
2.974
3.889
4.872
1.525
1.396
12.44
2.985
1.596
5.569
9.380
4.254
5.230
10.29
7.550
5.582
5.720
8.452
5.226
2.250
14.23
5.456
2.306
10.84
ΔS# e.u.
ΔS1#
ΔS2#
−2.620
−4.212
−2.310
−7.230
−4.063
−3.959
−3.256
−5.424
−5.425
−5.120
−8.250
−8.235
.6.175
−4.625
ΔG# kcal/mol
ΔG1#
ΔG2#
−14.77
−8.550
−7.421
−20.44
−15.55
−8.958
−9.568
−16.64
−10.22
−10.32
−24.72
−18.23
−12.99
−18.84
logA1
logA2
14.45
13.58
18.34
16.21
15.22
21.59
18.56
14.56
13.68
18.45
16.31
15.32
21.89
18.75
RAO & GAJANAN: MECHANISTIC STUDIES OF SAPONIFICATION OF SOME MONO- AND DI-ESTERS
47
Table 9—Thermodynamic parameters of di esters
[OH-]=0.02 M [Ester]=0.001 M
[Cl ]=0.02 M DMSO-Water =0.398 mole fraction (v/v:72/28)
-
Name of ester
Ethylene glycol distearate
Propylene glycol distearate
Glyceryl distearate
Ethylene glycol dilaurate
Propylene glycol dilaurate
Glyceryl dilaurate
Glyceryl oleostearate
ΔEa# kcal/mol
ΔEa1#
ΔEa2#
−ΔH# kcal/mol
−ΔH1#
−ΔH2#
3.125
1.920
1.513
3.225
2.515
2.059
3.432
2.245
0.875
0.920
8.350
1.050
1.084
1.330
5.457
3.028
3.884
7.350
5.720
4.804
4.804
6.325
2.426
.1.023
10.22
3.209
1.377
1.841
ΔS# e.u.
ΔS1
ΔS2#
#
−1.754
−1.287
−1.083
−4.230
−2.060
−2.876
−1.563
−3.257
−2.473
−3.130
−5.315
−4.250
−3.631
−2.064
ΔG# kcal/mol
ΔG1#
ΔG2#
−16.22
−12.76
−10.22
−24.55
−18.31
−10.85
−13.31
−19.62
−16.28
−12.12
−28.57
−24.12
−13.73
−19.41
logA1
logA2
17.55
16.65
21.43
20.22
18.32
22.65
20.14
17.75
16.75
21.54
20.32
18.54
22.83
20.17
Fig. 1—Isokinetic relationship: Entropy-Enthalpy correlation for
the saponification of diesters
Fig. 3— Entropy-Enthalpy correlation for the saponification of
mono- and diesters
Fig. 2—Entropy-Enthalpy correlation for the saponification of
diesters
linkages gradually increase as one passes from
EGDL to PGDL and GDL. On the other hand,
EGDS is more saponifiable than PGDS and
GDS. Such a finding could also be attributed to
the structure-reactivity correlationships i.e. the CH2 group linkages gradually increase as one
proceeds from EGDS to PGDS and GDS. It is
also obvious from Figs 1 and 2 that GOS is less
saponifiable than GDS. Such a strange finding
could be rationalized in view of the unsaturation
present in the oleo (C17H33) alkyl unit of GOS.
The mechanistic pattern of saponification
process of these diesters as detailed in the
foregoing points is of the same type whatever
may be the solvent system.
(iii) As is evident from Fig. 3, that, the linear plots
‘b’ (EGMS), ‘d’ (GMS) and ‘e’ (GOS) are
sloping-away from the x-axis and recording
higher entropies of activation. At the same time,
48
INDIAN J. CHEM. TECHNOL., JANUARY 2005
the linear plot ‘g’ (MS), slopes towards the xaxis recording a lower value of entropy of
activation as compared to the mono stearates and
distearates of ethylene glycol and glycerol. This
observation reflects the fact, that, monoesters
e.g. mono stearates of ethylene glycol and
glycerol are more saponifiable than the
corresponding distearates whatever may be the
solvent system. The faster saponification process
of mono stearates of ethylene glycol and
glycerol may be due to the anchimeric assistance
(structures I and II) provided by the
neighbouring free hydroxyl group situated in
close proximity to an ester bond.
(iv) The slope value of straight line plot of ‘b’
(EGMS) is 2.38, while the slope value of
straight line plot ‘g’ (MS) is 0.56 (Fig. 3),
indicating a decrease of 4.25 times whatever
may be the solvent system. Such a behaviour
could be attributed to the aromatic nature of MS
when compared to aliphatic diesters, which
reflects structure-reactivity relationship showing
that monoesters (aliphatic type) are more
saponifiable than MS, an aromatic ester.
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 formation. This explanation of the
transition state was also supported by Haberfield
et al.12 by determination 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 saponification of oleostearates and dilaurates
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 structure-I.
As mentioned earlier one can surmise the rate of
saponification of diester is more than that of halfester/monoester and this could further be rationalised
on the basis of the transition state (structure II) which
is also formed from the intramolecular hydrogen bond
formation of the monoester is less solvated by aq.
solvent and therefore, responsible for the lowering of
the rate of saponification of the half-ester/monoester.
The vicinal hydroxyl groups formed in the
saponification process also play a prominent role in
the internal stabilization of the transition state
(structures I and II) causing lower saponification
rates. The thermodynamic parameters are also in
consonance with any type of ion-dipole reaction.
On the contrary, glycol monostearate or glyceryl
monostearate undergoes a faster saponification
process than the intermediate half-ester/mono-ester
formed. Perhaps this may be due to the unique
behaviour of the monoester undergoing a base
catalysed unimolecular acyl cleavage (Table 1) like
any other pure monoester. Further, during this rapid
saponification process of a fresh monoester, the
formation of intramolecular hydrogen bonding may
be a remote possibility.
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. The variable
susceptibility to polar effects suggests an increased
importance of transition state solvation in
acetone/dioxane/DMSO/DMF. 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, implies that
the attacking hydroxide ion and the carbonyl carbon
are separated by a greater distance in aq. DMF than
aq. DMSO/aq. dioxane/aq. acetone or aq. alcohol.
RAO & GAJANAN: MECHANISTIC STUDIES OF SAPONIFICATION OF SOME MONO- AND DI-ESTERS
While it is not permissible to make a quantitative
assessment of the contribution of transition state
solvation to the −ΔF term, the importance of the
contribution shows that substitution of aq. DMF/aq.
DMSO/aq. dioxane/aq. acetone than in aq. alcohol
leads to an enhanced rate of reaction. However, as per
the Ingold13 theory, both the reduced enthalpy/entropy
of activation support the involvement of a more
highly solvated transition state of diester in aq.DMF.
The state of anion solvation is frequently mentioned
by Benson14 and Tommila et al.15 as a contributing
cause of reactivity.
The present study show that the reactivity of
hydroxide ion depends the following equilibria:
where n equals the maximum number of water
molecules hydrogen-bonded to hydroxide ion. With
increasing acetone, 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 solvent system in going
from the initial state of a reaction to the activated
state. According to the simplest treatments of
electrostatic interactions particularly kinetics and
solution the charged ions are considered to be
conducting sphere and the solvent is regarded as a
continuous dielectric having a fixed dielectric
constant (ε). Initially the ions are at 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 a distance dAB apart. This
model is frequently referred to the double-sphere
model. A final conclusion from the solvent effect is
that the logk1 or logk2 versus 1/ε is almost linear and
the slope of the line is given by ZAZBe2/dABKT. From
this expression as well as from experimental slope it
is possible to calculate dAB i.e. inter ionic distance in
the double sphere model of an activated complex
(Table 10).
The effect of anion dissolution as the major
contributory cause for the increase of rate constant is
also, to some extent, unlikely for the following
reasons:
• The activity of the hydroxide ion with increasing
acetone/dioxane/DMSO and DMF concentrations
does not explain the dependency of medium effect
49
Table 10—dAB values: A comparative kinetic study of mono and
diesters
Name of
ester
Alcoholwater, Å
Dioxanewater, Å
DMSOwater, Å
DMFwater, Å
EGDS
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
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 hydrogen-bonded
water molecules as a single kinetic unit. If the
anion desolvation mechanism were operative this
would imply decreasing size of the nucleophile
with increasing acetone/dioxane/DMSO/DMF
content in the solvent medium. However, such
responses are not generally recorded in ‘δ’ at
higher mole fractions of acetone/dioxane/DMSO/
DMF.
Acknowledgements
The authors are grateful to the Management as well
as Dr. G. Thimma Reddy, Principal KITS, Ramtek for
necessary facilities and constant encouragement. One
of the authors (KG) is thankful to Prof. K. Vijaya
Mohan, Head, Department of Chemistry, KITS,
Ramtek for his encouragement.
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