kinetic and mechanistic studies of saponification of...

Indian Journal of Chemical Technology Vol. 7, September 2001 pp.348-356

Kinetic and mechanistic studies of saponification of industrially important distearates, dilaurates of glycol, propylene glycol,glycerol and glyceryl

oleostearates

B Madhava Rao*, K Gajanan & K Vijaya Mohan Department of Chemistry, Visvesvaraya Regional College of Engineering, Nagpur440 OJ/ , India

Received 25 August 2000; revised 16 March 200/ ; accepted 26 March 200/

Kinetic and mechanistic studies of saponification of seven structurally related and industrially important diesters 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. 11E~,-11H~, !1G~,I1S~ and log A for both the steps which involve the competitive and consecutive saponification reactions which also indicate the first step of saponitication processes is quite faster than the second step and further more the laurates are more saponifiable than stearates and oleostearate in 72% alcohol-water moiety and in general these processes are much faster in acetone-water, dioxane-water, DMSO-water and DMF-water systems than in alcohol-water system.

The determination of rate constants from the experimental data in series first order reactions was fi rst explored in a great detail by Swain 1• A number of workers2-6 made detailed kinetic studies of alkaline hydrolysis of mono and diesters of different carboxylic acids in protic and aprotic solvents. Bruice and Fife7 forwarded logical explanation for a faster alkaline hydrolysis on the basis of internal solvation of the transition state followed by the attack of hydroxyl ion (OH-) at the ester carbonyl group. Several workers8-20 made detailed studies on the kinetics and kinetic laws, influence of temperature variance of thermodynamic parameters in respect of substituents, solvents and their dielectric constants, structure and reactivity, 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 literature21.36 with particular reference of alkali catalysed hydrolysis of mono and diesters. A critical analysis of the above literature37-54 reveals that most of the work is confined to the alkaline hydrolysis or saponification processes of either the normal esters or the synthesized esters with much emphasis on the study of correlationships pertinent to activation parameters or specific solvent effects or LFER relationships. However, very little

" For correspondence

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 the 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 't and K

values are noted from Swain's modified Table I for series first order reactions. From the relation T =f3ok1t the value of k1 and from relation K=k2/ k1 the values of k2 are evaluated. Thus the rate constants k1 and k2 for the two consecutive steps involved in saponification process at different temperatures are calculated.

Materials and methods

The liquid esters and semisolid 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., saponification value and IR spectra for esters employed are in agreement with the data collected from literature. The rate studies were carried out

RAO et al : KINETIC AND MECHANISTIC STUDIES 349

Table I -Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate

[Ester]= 0.001 M [OH-] = 0.02 M

rcrJ = o.o2 M Temp. 50° ± 0.05°C

Alcohol-Water= 0.445 mole fraction

Acetone-Water= 0.388 mole fraction

Dioxane-Water= 0.352 mole fraction

(v/v: 72/28) EtOH-Water, Acetone-Water and Dioxane-Water systems

Name of ester Structure Pseudo first order rate constant k1 x 102, s - I I st Step

Pseudo first order rate constant k2 x 102, s - I 2nd Step

Ethylene

glycol

distearate

Propylene

glycol

distearate

Glyceryl

distearate

Ethylene

glycol

dilaurate

Propylene

glycol

dilaurate

Glyceryl

dilaurate

Glyceryl

oleostearate

CH200CCnHJs

........-cH200CCn H35

c~ ~H200CCnH35

CH200CCnH35

CHOOCCnHJs

CH20H

CH200CCIIH23

I CH200CCIIH23

........-cH200CC11H23

CH,

~H200CC11H23 CH200CCIIH23

I CHOOCCIIH2)

I CH20H

CH200CCn H33

I CHOOCC11H35

I CH20H

(EtOHWater)

11.18

32.58

72.82

35.90

28.11

75.12

10.91

over 0.3 to 0.7 of the life period of the saponification reaction. Requisite amounts of the reaction mixture (diesters and excess of alkali which is twenty times over and above the stoichimetric 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 solvents and the whole system was kept during the reaction as well as titration in a stream of nitrogen. The sodium hydroxide solution employed in the saponification

(AcetoneWater)

98.43

39.65

104.0

112.0

29.91

120.6

57.20

(DioxaneWater)

156.0

45.75

163.0

176.0

126.0

187.0

150.0

(EtOH- (Acetone- (Dioxane-Water) Water) Water)

0.052 0.745 3.450

0.684 0.717 0.895

0.160 0.451 5.241

0.050 0.28 4.880

0.929 24.72 26.03

0.015 0.357 1.180

0.013 12.34 13.65

process as well as in the titration were prepared carbonate-free by the reaction of metallic sodium with conductivity water. The solvents ethanol/acetone/dioxane/DMSO/DMF used m the saponification process were purified by repeated distillation with CaO and also by azeotropic distillation methods.

Treatment of data Percentage 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

350 INDIAN 1. CHEM. TECHNOL., SEPTEMBER 2001

through these experimental points, the times for 15%, 35% and 70% of the reaction are recorded. From the time ratios, e.g. t3:/t15 or logt3:/t15 the corresponding K

values and r values are noted from the standard tables furnished by Swain 1 using the following relations:

r = {30k1t

K = k2 I k1

The rate constants for the consecutive, competitive steps are evaluated. The rate constants obtained in these investigations represent an average of at least three kinetic runs and are accurate within ±3%. Thermodynamic parameters, viz., energy of activation 11~, enthalpy of activation -M.f, entropy of activation !:J.s*, Gibb's free energy !:J.G-T- 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 4-13.

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 diesters of glycols and glycerol. In the saponification process of these esters, anchimeric assistance is provided by the neighboring 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 prompted for detailed study of 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 diesters (oils and fats-glycerides). Hence, anchimeric assistance from the neighbouring carboxyl group may not be possible. A meticulous critical analysis of the rate data furnished in Tables 1-8 show that the reactivity pattern in respect of saponification process of diesters into monoester/half-ester is more than that of monoester/half-ester hydrolytic reaction. This strange finding could be rationalized by

involving the concept of Kumuria et al 13• 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 -ve charge localized on the carbonyl oxygen atom making this a good proton acceptor through a hydrogen bond formation23• This explanation of the transition state was also supported by Haberfield et al 24 . by calorimetric determination of the relative enthalpies of reactant and transition states. In the alkaline hydrolysis of 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 oleostearates and dilaurates of glycerol the -ve charge on the carbonyl oxygen atom in the transition state decreases by diffusion through intramolecular hydrogen bond formation as shown in the following structure-!

H 0

H-J-0-~-R Where 'R' stands for

H-l-0-!~R Laury! = C11H23

I ~-H-C-0-!'I

I H

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)

H OH.

I t H-C-0-C-R

I ?· H-C - 0 - H

I

Where 'R' stands for

Laury! = C11 H23

Oleo =C17H33

H

which is also formed from the intramolecular hydrogen bond formation of the monoester and is less solvated by aq. ethanol or aq. acetone or aq. dioxane or aq. DMSO and DMF and therefore, responsible for the lowering of the rate of saponification of the monoester.

A perusal of thermodynamic parameters (Tables 9-13) indicates that in general the energies of activation are higher for laurates while the enthalpies of activa


Table 2- Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate

[OH"] = O.D2 M [Ester]= 0.001 M

[CI"] = O.D2 M Alcohol-Water= 0.445 mole fraction

Temp. 50° ± 0.05°C Acetone-Water= 0.388 mole fraction

Dioxane-Water= 0.352 mole fraction

(v/v: 72/28) EtOH-Water, Acetone-Water and Dioxane-Water systems

Name of ester Structure Pseudo first order rate constant Pseudo first order rate constant k1 x 102,s-1 lstStep k2 x 102, s - I 2nd Step

(EtOH- (Acetone- (Dioxane- (EtOH- (Acetone- (Dioxane-Water)

Ethylene CH200CC11H35 5.114

glycol I distearate CH200CC11H35

Propylene __....-CH200CC11H35 20.96

glycol c~ distearate H200CC11H35

Glyceryl CH200CC11H35 56.66

distearate

CHOOCC11H35

CH20H

Ethylene CH200CC I I H23 5.520

glycol I dilaurate CH200CC11H23

Propylene __....-cH200CC11H23 6.060

glycol CH2

Dilaurate ~H200CC11H23 Glyceryl CH200CC11H23 9.380

Dilaurate I CHOOCC1 1H23

I CH20H

Glyceryl CH200CC 11H33 4.657

Oleostearate I CHOOCC11H35

I CH20H

tion,entropies of activation and free energies of activation of dilaurates of ethylene glycol and propylene glycol are relatively lower than the values of the corresponding individual esters of distearates of glycols and glycerol, however, with a few exceptions of glyceryl dilaurate. Such a behaviour may be attributed due to the presence of a Jauryl unit (C11 H23) which is a shorter alkyl group than the oleic/stearic unit (cnHJfCnHJs) and, moreover, the vicinal hydroxyl groups formed in the saponification process also play a prominent role in

Water) Water) Water) Water) Water)

52.20 55.00 0.014 0.280 0.354

21.14 22.15 0.340 0.302 0.425

59.97 63.05 0.085 2.159 2.850

20.83 25.25 0.012 0.118 1.975

22.86 24.02 0.040 17.14 18.50

92.28 94.16 0.002 0.096 0.196

28.03 30.05 0.001 2.80 3.250

the internal stabilization of the transition state causing lower saponification rates. These thermodynamic parameters are also in consonance with any type of iondipole 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. The variable susceptibility to

352 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2001

Table 3- Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate

[OH-] = 0.02 M [Ester)= 0.001 M

[Cr] =0.02 M DMSO-Water = 0.398 mole fraction

Temp. 50° ± 0.05°C DMF-Water = 0.376 mole fraction

(v/v: 72/28) DMSO-Water, DMF-Water systems

Name of ester Structure

Ethylene CH200CC11H35 glycol I distearate CH200CC11H35

Propylene / CH200CC11H35 glycol CH2

distearate ........___ CH200CC11H35

Glyceryl CH200CC11H35 distearate

CHOOCC11H35

CH20H

Ethylene CH200CC11H23 glycol I dilaurate CH200CC II H23 Propylene /CH200CC11H23 glycol CH,

dilaurate "-. CH200CC11H23

Glyceryl CH200CC11H23

dilaurate I CHOOCC11H23

CHPH

Glyceryl CH200CC11H33

oleostearate I CHOOCC11H35

JH20H

polar effects suggests an increased importance of transitiOn state solvation in acetone/dioxane/DMSO/DMF. If pK value is equated with the degree of - ve 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 and aq. alcohol. While it is not permissible to make a quantitative assessment of the contribution of transition state solvation to the -t::.F term, the importance of the contribution shows that substitution of aq.DMF aq. DMSO/aq. dioxane/aq.

Pseudo first order rate Pseudo first order rate constant constant

k1 x 102, s - I k2 XJ02,S-I

1st Step 2nd Step

500.0 750.00 65.00 80.25

425.5 623.5 46.45 65.58

375.0 550.0 35.12 52.35

620.0 850.0 80.00 98.25

590.0 750.0 72.00 89.50

450.0 770.0 45.00 79.30

511.0 855.5 46.00 84.72

acetone than in aq. alcohol leads to .an enhanced rate of reaction.

However, more importantly, as per the HughesIngold 25 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 Benson26 and Tommila et at. 27 as a contribution cause of reactivity. Thus in aq.acetone, aq. dioxane and aq. DMSO and aq.DMF systems of the present study show 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 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 (Table 14), in going from the initial

Table 4- Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl

oleostearate : rate constants of I" step and 2"d step

[OH-] = 0.02 M

[Cr] = O.G2 M

Name of ester

Ethylene glycol distearate Propylene glycol distearate

Glyceryl distearate

Ethylene glycol dilaurate

Propylene glycol dilaurate

Glyceryl dilaurate

Glyceryl oleostearate

[Ester]= 0.001 M

Alcohol-Water= 0.445 mole fraction (v/v: 72/28)

5.114 10.97 20.96 26.44

56.66 70.88

5.520 21.96

6.060 19.44

9.380 14.35

4.657 7.780

K1 x 102 s-1

11.18

32.58

72.82

35.92

28.11

75.12

10.91

57.66 75.90

146.4

144.0

35.12

132.5

28.56

123.4 148.6

180.8

299.6

44.13

148.5

42.77

0.014 0.348

0.085

0.012

0.043

0.002

0.001

0.016 0.369

0.149

0.016

0.145

0.011

0.002

0.052 0.684

0.160

0.050

0.929

0.015

0.013

0.190 0.410 2.884 6.316

2.143 4.590

0.015 0.350

2.06 4.944

0.218 0.381

0.015 0.590

Table 5 - Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol,glycerol and glyceryl oleostearate : rate constants of I" step and 2"d step

Name of ester

Ethylene glycol distearate

Propylene glycol distearate

Glyceryl distearate



Glyceryl dilaurate


[OH-] = 0.02 M [Ester]= 0.001 M

[Cr] = 0.02 M Acetone-Water= 0.388 mole fraction (v/v: 72/28)

52.20 60.16 64.28 72.00 98.43 125.1 0.280 0.342 0.378 0.434 0.745 1.375

21.14 23.80 27.12 30.00

59.97 68.61 76.12 81.87

20.83 23.55 24.00 29.83 22.86 24.29 28.33 29.01

92.28 93.44 102.8 117.20

28.03 30.70 38.69 40.28

39.65 45.11 0.3021

104.05 145.6 2.159

112.0 0.118

29.91 34.0 17.14

120.6 125.2 0.096

57.20 63.16 2.800

0.388 0.406 0.420 0.717 0.776

0.241 2.644 3.681 4.510 0.572

0.140 0.160 0.256 0.280

18.21 21.25 21.76 24.72 2553

0.102 0.153 0.151 0.357 2.002

2.900 3.48 10.88 1234 15.79

Table 6- Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate : rate constants of I ' 1 step and 2"d step

[OH-] = 0.02 M [Ester]= 0.001 M

[Cr] = 0.02 M Dioxane-Water= 0352 mole fraction (v/v: 72/28)

Name of ester K1 x 102 Sec·1 K2 x 102 Sec·1

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

Ethylene glycol distearate 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

Propylene glycol 22.15 24.25 . 32.18 39.65 45.75 75.85 98.20 0.425 0.485 0.520 0.585 0.895 0.945 1.025 distearate

Glyceryl distearate 63.05 76.00 88.00 96.00 163.0 190.0 215.0 2.850 2.961 3.256 3.815 5.241 6.331 7.311 Ethylene glycol dilaurate 25.25 28.00 32.11 50.01 176.0 185.0 303.9 1.975 1.405 2.856 3.220 4.880 5.200 6.750

Propylene glycol dilaurate 24.02 28.00 35.00 46.00 126.0 135.0 142.0 18.35 20.65 23.33 24.45 26.03 28.95 30.02

Glyceryl dilaurate 94.16 99.25 125.0 141.0 187.0 224.0 314.0 0.196 0.260 0.920 1.060 1.180 2.430 3.100

Glyceryloleostearate 30.05 36.00 40.00 101.0 150.0 160.0 211.2 3.25 4.50 5.170 12.52 13.65 17.09 19.05

354 INDIAN J. CHEM. TECHNOL., SEfYfEMBER 2001

Table-7- Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate: rate constants of I" step and 2"d step

[OH"] = 0.02 M [Ester]= 0.001 M

[Cr] = O.Q2 M DMSO-Water = 0.398 mole fraction (v/v: 72/28)

Name of ester

Ethylene glycol distearate 90.00 108.0 148.0 360.0 500.0 0.720 8.000 14.30 39.00 65.00

Propylene glycol distearate 90.05 105.0 147.5 367.5 425.5 0.895 8.200 15.33 37.55 46.45

Glyceryl distearate 88.25 102.0 150.0 350.0 375.0 0.345 8.005 15.24 28.22 35.12

Ethylene glycol dilaurate 103.5 148.0 212.0 416.0 620.0 0.952 20.00 38.25 60.05 80.00

Propylene glycol dilaurate 92.42 133.0 195.0 395.0 590.0 0.859 12.00 20.00 41.05 72.00

Glyceryl dilaurate 94.65 128.0 200.0 338.0 450.0 0.425 12.65 21.05 31.25 45.00

Glyceryl oleostearate 107.7 137.0 273.0 470.0 511.0 0.865 17.50 26.25 35.00 46.00

Table 8- Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate : rate constants of I" step and 2"d step

Name of ester



Glyceryl distearate



Glyceryl dilaurate


98.00

115.6

118.0

165.0

136.0

145.6

155.2

[OH"] = 0.02 M

[CI"] =0.o2 M

126.0

132.0

152.0

280.0

195.7

183.3

198.5

220.0

185.0

192.5

327.5

210.0

305.5

325.1

542.0

495.0

450.0

712.5

533.3

458.8

520.2

[Ester]= 0.001 M

DMF-Water= 0.376 mole fraction (v/v : 72/28)

750.0

623.5

550.0

850.0

750.0

770.0

855.5

0.850

3.985

7.680

8.752

5.500

10.86

12.20

12.65

11.22

12.25

28.95

14.60

14.60

28.25

K2 x t02 s·'

18.10 46.07

21.05 47.05.

18.45 40.08

48.25 75.62

30.65 50.63

24.74 68.70

38.32 78.60

80.25

68.58

52.35

98.25

89.50

79.30

84.72

Table 9- Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate : thermodynamic parameters of I " step and 2"d step

[OH"] = 0.02 M

[Cl"] = 0.02 M

[Ester]= 0.001 M

Alcohol-Water= 0.445 mole fraction (v/v: 72/28)

Name of ester kcal/mol -I">.H kcal/mol e.u. kcal/mol log A1 logA2



Glyceryl distearate



Glyceryl dilaurate


16.51

7.710

6.930

20.42

12.25

9.120

8.730

17.28

9.597

7.380

28.96

16.08

15.20

17.19

16.23

8.070

7.910

18.09

12.62

4.980

8.580

17.40

8.730

8.100

29.17

18.34

5.700

17.61

-16.16 -19.40

-8.680 - 10.53

-6.470 - 9.480

- 19.95 - 20.30

-12.21 -2 1.30

-9. 160 -9.630

- 13.67 - 19.61

- 10.84

-5.170

- 5.808

- 11.33

-8.530

- 1.770

-4.050

/">.Gz

-10.94

- 5.220

-4.990

-·22.4 1

-12.72

- 2.496

- 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

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 (E).

Initially the ions are at infinite distance with each other. However, in activated state they are considered


Table 10- Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate : thermodynamic parameters of 1 .. step and 2nd step

[OH'] = 0.02 M

Name of ester



Glyceryl distearate



Glyceryl dilaurate


[Ester]= 0.001 M

11Ea

11Eal

6.520

5.420

4.470

10.14

6.020

4.824

4.670

kcallmol

t-.Ea2

12.05

8.020

6.360

16.47

9.093

7.907

7.180

[Cr] = 0.02 M; Acetone-Water= 0.388 mole fraction (v/v: 72128)

-till

-till,

5.079

7.220

6.598

17.86

3.056

3.247

7.500

kcal/mol

-till2

12.57

8.120

7.150

6.870

9.215

4.950

12.21

llS

llS,

-8.830

-6.120

- 5.270

-13.17

-7.607

-5.253

-11.73

e.u.

/lS2

-7.200

-7.840

-6.490

-13.74

-10.35

-8.048

-14.54

t-.G

t-.G,

-5.076

- 7.220

-6.600

-17.85

-6.662

-3.245

-7.490

kcaVmol

i1G2

-12.56

-8.120

-7.140

-6.870

-9.212

-4.947

-12.20

12.71

12.19

12.42

12.18

12.84

12.95

12.25

12.80

12.37

12.43

12.28

12.94

12.76

12.38

Table II -Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate : thermodynamic parameters of 1" step and 2nd step

[OW]=0.02M

Name of ester



Glyceryl distearate



Glyceryl dilaurate


to.£.

t1£al

3.720

3.150

2.540

5.804

4.804

2.974

3.889

[Cil =0.02 M

kcal/mol

t-.Ea2

9.380

4.254

5.230

10.29

7.550

5.582

5.720

-till

- !1H1

4.872

1.525

L396

12.44

2.985

1.596

5.569

[Ester]= 0.001 M; Dioxane-Water= 0.352 mole fraction (v/v :72128)

kcal/mol

- !1H2

8.452

5.226

2.250

14.23

5.456

2.306

10.84

llS

/lSI

- 2.620

-4.212

-2.310

-7.230

-4.063

-3.959

- 3.256

e.u.

/lS2

- 5.424

-5.425

-5.120

-8.250

-8.235

-6.175

-4.625

t-.G

i1G1

- 14.77

-8.550

-7.421

-20.44

- 15.55

- 8.958

-9.568

kcal/mol

i1G2

- 16.64

-10.22

-10.32

-24.72

-18.23

- 12.99

- 18.84

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

Table 12- Series first order reaction, saponification of distearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate : thermodynamic parameters of 1 .. step and 2nd step

[OH'] = O.Q2 M

Name of ester



Glyceryl distearate



Glyceryl dilaurate

Glyccryl oleostearate

[Cr] = 0.02 M [Ester]= 0.001 M; DMSO-Water = 0.398 mole fraction (v/v : 72/28)

t-.Ea kcal/mol -t-.H kcal/mol llS e.u. t-.G kcal/mol log A1 logA2

t1£al

3. 125

1.920

1.513

3.225

2.515

2.059

3.432

t-.Ea2

5.457

3.028

3.884

7.350

5.720

4.804

4.804

- till I

2.245

0.875

0.920

8.350

1.050

1.084

1.330

- till2

6.325

2.426

1.023

10.22

3.209

1.377

1.841

/lSI

- 1.754

-1.287

-1.083

-4.230

-2.060

- 2.876

- 1.563

/lS2

- 3.257

-2.473

-3.130

-5.315

-4.250

-3.63 1

- 2.064

i1G1

- 16.22

-12.76

- 10.22

-24.55

- 18.3 1

- 10.85

- 13.31

i1G2

-19.62

-16.28

-12.12

-28.57

-24.12

-13.73

- 19.41

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

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 log k2 versus 1/E is almost linear and the slope of the line is given by ZAZse2/dAB KT. From this expression as well as from experimental slope it is possible to calculate dAB· The investigations have revealed that the dAB values are normally from 1.6 to 2A in dioxane-water systems and

3.3 to 9A in alcohol-water system. 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) The activity of the hydroxide ion with increasing acetone/dioxane/DMSO and DMF concentrations does not explain the dependency of medium effect upon substrate steric substituent constant or the presence of medium effect discontinuites.

356 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2001

(ii) With a small ion such as hydroxide, it is more realistic to treat the ion plus the hydrogen-lxmded water molecules as a single kinetic unit. If the anion desolvation mechanism were operative this would imply decreasing size of the nucleophile with increasing acetoneldioxane/DMO/DMF content in the solvent medium. Such a charge in the steric bulk of the attacking reagent should be reflecting by the variation in the

reaction constant parameter 8 responses are not generally recorded in 8. However, such responses are not generally recorded in 8 at higher mole fractions of acetonedioxane/DMSO/DMF.

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 apJ1?tiC solvent influences on reaction rates as a criterion in the assessment of anchimeric assistance in reactions involving internally stabilized transition states. Acknowledgements

The authors are grateful to the authorities of V.R. College of Engineering, Nagpur for providing necessary laboratory facilities to carry-out this work The authors also wish to express their sincere gratitude to Dr V. Ramchandra Rao, the retired Prof and Head, Deptt of Chemistry, VRCE for his encouragement in the progress of this work. One of the authors (K. Gajanan) is grateful to the Management and Dr. G. Thimma Reddy, Principal, KITS, Ramtek for their kind permission and constant encouragement during this work and to Sqn.Ldr. K.R. Sharma and his family for their forbearance & constant moral support. References

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