Indian Journal of Chemical Techno logy Vol. 9, July 2002, pp. 297-305

Articles

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

Department o f Chemi stry, Visvesvaraya Reg ional Co llege of Engineering, Nagpur440 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 workers2

-3 made detailed kinetic studies of alkaline

hydrolysis of mono and diesters of different carboxylic acids in protic and aprotic solvents. Bruice and Fife4 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 (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, 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 literature8

- '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 correspondence : (E-mail : bmrao @nagpur.dot.net. in Present address : Emeritus Professor, Department of Chemistry, KITS Ramtek 441 106, Dist., Nagpur, India.

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 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.

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

Indian J. Chern . Technol., July 2002

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

Indian J. Chem. Techno!., May 2002 Articles

Table I- First order reaction, saponification of mono and distearates, o leostearatcs of g lycol. glycerol and methy l salicy late: rate constants of I sl step and 2nd step

IOH'] = 0.02 M [Es ter] = 0.001 M [Cn =0.D2 M Alcohol-Water = 0.445 mole fraction (v/v : 72128)

Name of ester kl x 102 , S' I (lS I stel2) k2 x 102, S' I (2 nd stel2)

30°C 40°C 50°C 60°C 70°C 30°C 40°C 50°C 60°C 70°C

Ethylene glycol di stearate (diester) 5. 114 10.97 11.18 57.66 123.4 0.014 0.016 0.052 0. 190 0.410

Ethylene glycol monostearate (monoester) 10.05 15.45 30.25 42.25

Glycery l distearate (diester) 56.66 70.88 72.82 146.4 180.8 0.085 0.149 0.160 2.143 4.590

Glyceryl monostearate (monoester) 64.05 81.75 122.5 220.00

Glycery l oleostearate (diester) 4 .657 7.780 10.9 1 28.56 42.77 0.001 0.002 0.013 0.015 0.590

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

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 fEsterl = 0.001 M ICII =0.02 M Dioxane-Water = 0.352 mole fraction (v/v: 72/28)

Name of ester k, x 10' , S· I (I" step) k, x 10' . S· I (2nd 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

Ethylene glycol di stearate 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 (diester)

Ethylene glycol monostearate 62.05 76.45 102.4 182.2 236.3 301.25 (monoester)

Glycery l distearatt! 63.05 76.00 88.00 96.00 163.0 190.0 215.0 2.850 2.961 3.256 3.815 5.241 6.33 I 7.31 I (diester)

Glyceryl monostearate 72.45 96.05 126.0 174.0 212.0 248.0 (monoester)

Glyceryl oleostearate 30.05 36.00 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 (diester)

Glyceryl mono oleate 40.05 71.25 126.8 163.5 201.3 233.0 (monoester)

Methyl salicylate 29.45 74.35 135.4 187.0 211.0 265.0 (aromatic monoester)

Table 3-f'irst order reaction, saponification of mOllo and di stearates, o leostearates of glycol, glycerol and methyl salicy late : rate constants of 1 sl step and 2nd step

[OH'] = 0 .02 M [Ester] = 0.00 1 M [CI'] = 0.02 M DMSO- Water = 0.398 mole fraction (v/v: 72128)

Name of ester k l x 102,5' 1 (I SI stel2) k2 x 102, S'I (2nd stel2)

10°C 15°C 20°C 25°C 30°C 10°C 15°C 20°C 25°C 30°C

Ethylene glycol d istearate (diester) 90.00 108.0 148.0 360.0 500.0 0 .720 8.000 14.30 39.00 65.00

Ethylene glycol monostearate (monoester) 140.0 182.0 227.0 405.0 585 .0

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

Glycery l monostearate (monoester) 137.3 185.0 270.0 402.0 580.0

Glycery l oleostearate (diester) 107.7 137 .0 273 .0 470.0 5 11.0 0.865 17.50 26.25 35.00 46.00

Glyceryl monooleate (monoester) 160.0 217.0 325 .0 500.0 570.0

Methyl salicy late (aromatic monoester) 87.05 142.0 225.0 3 12.0 425.0

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Articles Indian J. Chern. Techno!. , July 2002

Table 4-First order reaction , saponification of mono and di stearates, oleostearates of glycol, glycerol and methyl salicy late: rate constants of I SI step and 2nd step

[OH'] = 0.02 M [Ester] = 0.001 M [Cr] = 0.02 M DMF-Water = 0.376 mole fraction (vlv: 72128)

Name of ester k , x 102, s" (1st s te~) k2 x 102

, S·, (2nd s te~) 10°C 15°C 20°C

Ethy lene glycol distearate(diester) 98.00 126.0 220.0

Ethylene glycol monostearate (monoester) 168.0 220.0 350.0

Glycery l distearate (diester) 11 8.0 152.0 192.5

Glyceryl monostearate (monoester) 168.0 202.0 3 14.5

Glyceryl oleostearate (d iester) 155.2 198.5 325. 1

Glyceryl monooleate (monoester) 190.0 270.0 398.0

Methyl salicy late (aromatic monoester) 120.8 260.0 380.0

formation 17. This explanation of the transition state was also supported by Haberfield et at. 18 by calorimetric 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 oleostearates 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

H 0

I II H-C-O-C-R Where ' R' stands for

I OH'

H-C-O-C-R

I ~ H- C-O-H

I H

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 lowering of the rate of saponification of the monoester.

On the contrary, a fresh monoester e.g. Glycol monostearate or glyceryl monostearate undergoes a

300

25°C 30°C 10°C 15°C 20°C 25°C 30°C

542.0 750.0 0.850 1~65 18.10 46.07 80.25

590.0 795.0

450.0 550.0 7.680 12.25 18.45 40.08 52.35

525.0 640.0

520.2 855.5 12.20 28.25 38.32 78.60 84.72

580.0 884.0

420.0 51 8.0

H OH' Where 'R' stands for I • •

H-C-O-C-R

I 6 Stearic = C17H35

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 unimolecular 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 the case of pure monoester e.g. methyl salicylate the following general mechanism which is a base catalysed unimolecular acyl cleavage could be envisaged vide Table 5.

o o

II II R - C - OR' "=7 R-{:+ + R'O'

A simple mechanism of basic hydrolysis of mono ester involves the following steps:

w~~ " IT OH·+R·- C'·-OR ..... R·-C-OR .... R' -C - OR + OH'

~ bH 1~

[ R - ~ -01.-,. HOR OH r 0"

Indian 1. Chern. Techno\., May 2002 Articles

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 mechanism

Mechanism Position of Alky l Kinetics Electron Steric cleavage configuration requirement hindrance

8'1 Acyl Retention E R ROO 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 [Ester) = 0.00 1 M [er) = 0.02 M Alcohol-Water = 0.445 mole fraction (v/v : 72128)

Name of ester 11£. kcallmol -I1H kcal/mol t1S e.u. I1G kcal/mol 10gA, logA2

11£. , l1£a2 -I1H, -I1H2 I1S, I1S2 I1G, I1G2

Ethylene glycol distearate 16.51 17.28 16.23 17.40 -16.16 -19.40 -10.84 -10.94 8.762 9.459 (diester) Ethylene glycol monostearate 11 .62 10.34 -12.47 -12.34 14.24 (monoester) Glyceryl distearate 6.930 7.380 7.9 10 8.100 -6.470 -9.480 - 5.808 -4.990 7.242 7.284 (diester) Glycery l monostearate 4.341 3.322 -4.025 -8.082 13.56 (monoester) Glyceryl oleostearate 8.730 17.19 8.580 17.61 -13.67 -19.61 -4.050 -10.08 8.2 16 9.506 (diester) Glyceryl monooleate 5.245 6.234 - 10.42 - 14.10 15.02 (monoester) Methyl salicylate 7.365 6.245 -1 1.05 -8.425 7.222 (aromatic monoester)

Table 7-First order reaction, saponification of mono and distearates, oleostearates of glycol, glycerol and methyl sal icylate : Thermodynamic parameters of I" step and 2nd step

[OH'l = 0.02 M [Ester) = O.DOI M [Cr] = 0.02 M Dioxane-Water = 0.352 mole fraction (v/v : 72128)

Name of ester I1Ea kcal/mol - I1H kcal/mol I1S e.u. I1G kcal/mol 10gA , logA2 11£., 11£.2 -I1H, -I1H2 I1S, I1S2 I1G , I1G2

Ethylene glycol distearate 3.720 9.380 4.872 8.452 -2.620 -5.424 -14.77 - 16.64 14.45 14.56 (diester) Ethylene glycol monostearate 2.205 3.184 -1 .200 -17.26 18.65 (monoester) Glyceryl distearate 2.540 5.230 1.396 2.250 -2.310 -5.120 -7.421 -10.32 18.34 18.45 (diester) Glycery l monostearate 1.025 0.975 -1.067 -12.25 23.47 (monoester) Glyceryl oleostearate 3.889 5.720 5.569 10.84 -3.256 -4.625 -9.568 -18.84 18.56 18.75 (diester) Glycery l mono oleate 1.955 2.655 -1.657 -24.58 21.25 (monoester) Methyl salicylate 5.325 3.155 -8.055 -13.65 14.75 (aromatic monoester)

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Articles Indian J. Chem. Techno!.. Ju ly 2002

Table 8-First order reaction, saponification of mono and distearates, oleostearates of glycol, glycerol and methyl salicylate : Thermodynamic parameters of 151 step and 2nd step

[OH-] = 0.02 M [Ester] = 0.00 1 M [Cr] = 0.02 M DMSO-Water = 0.398 mo le fraction (v/v: 72128)

Name of ester !'J.E" kcallmol -!'J.H kcallmol !1S e.lI. !'J.G kcallmol log Al logA2

!'J.E"I !'J.E,,2 -!'J.HI -!'J.H2 !'J.S I !'J.S2 !'J.GI !'J.Gz

Ethylene glycol di stearate 3.125 5.457 2.245 6.325 -1.754 -3.257 -16.22 - 19.62 17.55 17.75 (d iester) Ethylene glyco l Illonostearate 1.105 1.255 - 0.654 -22.65 24.65 (monoester) Glyceryl disteara te 1.513 3.884 0.920 1.023 -1.083 -3.130 -10.22 - 12.12 21.43 21.54 (diester) Glyceryl 1110nostearate 0.875 0.265 -0.875 -16.45 27.55 (monoester) Glyceryl oleostearate 3.432 4.804 1.330 1.841 -1.563 -2.064 -13.31 - 19.41 20.14 20.17 (diester) Glyceryl monooleate 0.755 1.025 -0.225 -24.75 26.35 (monoester) Methyl salicylate 2.425 1.895 -4.345 - 18.18 19.65 (aromatic monoester)

Table 9-First order reaction, saponification of mono and distearates, oleostearates of glycol, glycerol and methyl salicylate: Thermodynamic parameters of I sl step and 2nd step

[OH'] = 0.02 M [Ester) = 0.001 M [Cr) =0.02 M DMF-Water = 0.376 mole fraction (v/v: 72128)

Name of ester !'J.E" kcal/mol -!'J.H kcal/mol !'J.S e.lI. !'J.G kcal/mol logAI logA2

!'J.E"I !'J.E,,2 -!'J.HI

Ethylene glycol distearate 2.40 1 4.128 1.345 (diester) Ethylene glycol monostearate 0.795 0.675 (monoester) Glyceryl distearate 1.025 3.025 0.256 (diester) Glyceryl monostearate 0.105 0.076 (monoester) Glycery l oleostearate 1.054 2.820 0.245 (diester) Glycerylmonooleate 0.105 0.017 (monoester) Methyl salicylate 1.045 0.757 (aromatic monoester)

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 importance of transition state solvation in 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

302

-!'J.Hz !'J.S I !'J.S2 !'J.GI !'J.G2

3.521 -0.945 -1.502 -18.55 -21.20 21.02 21.12

-0.101 -25.67 27.42

0.802 -0.925 -1.450 -1 2.65 - 14.35 22.05 22 .25

-0.045 -19.85 31.85

0.834 -0.624 -0.98 1 - 16.25 -22.55 24.35 24.47

-0.015 -26.67 29.45

- 1.891 -20.05 22.25

hydroxide ion and 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 contribution 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

Indian J. Chem. Technol., May 2002

Benson20 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

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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

2IdABKT. 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) 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.

(ii) 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 [Ester] = 0.001 M [Cr] = 0.02 M Temp.= 50 ± 0.05°C

Alcohol-water Mole Pseudo first order Pseudo first order Dielectric constant composition (v/v) fraction rate constant rate constant (e)

k,x 102 , S-I k2 X 102 , S -I

1st Step 2nd Step

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 Mole Pseudo first order Pseudo first order Dielectric constant composition (v/v) fraction rate constant rate constant (e)

kl X 102 ,S-I k2 X 102 , S- I

1st Step 2nd Step

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

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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 [Ester] = 0.001 M [cr] =0.02 M Temp.= 30 ± 0.05°C

DMSO-water Mole Pseudo first order Pseudo first order Dielectric constant composition (vlv) fraction rate constant rate constant (€)

kl x 102, S -I k2 x I02,s -1

1st Step 2nd Step 72/28 0.398 500.0 65.00 14.33 60/40 0.276 245 .0 29.00 10.25 50/50 0.203 162.0 15.45 8.350 40/60 0.145 85.75 7.650 4.650 28/72 0.090 36.65 3.250 2.860

DMF-water Mole Pseudo first order Pseudo first order Dielectric constant composition (vlv) fraction rate constant rate constant (€)

kl x 102, S-I k2 x I02 ,s-1 1st Step 2nd Step

72/28 0.367 750.0 80.25 7.650 60/40 0.261 412.0 55.55 4.330 50/50 0.189 216.0 35.57 1.850 40/60 0.136 141.5 13.65 0.950 28172 0.084 74.25 6.050 0.025

Table 12-Effect of solvent composition, First order reaction, saponification of ethylene glycol monostearate (monoester) [OH·] = 0.02 M [Ester] = 0.001 M [Cn = 0.02 M Temp.= 50 ± 0.05°C

Alcohol-water Mole Pseudo first order Dielectric constant composition (vlv) fraction rate constant (€)

k x 102, S-I

72/28 0.4455 30.05 55.63 60/40 0.3141 11 .25 41.34 50/50 0.2339 6.850 37.91 40/60 0.1691 1.250 34.33 28172 0.1062 0.750 30.86

Dioxane-water Mole Pseudo first order Dielectric constant composition (vlv) fraction rate constant (€)

kx 102, S-I

72/28 0.3521 182.2 29.65 60/40 0.2408 112.8 20.97 50/50 0.1745 60.00 15.80 40/60 0.1235 32.05 11.87 28172 0.0759 10.25 8.119

Table 13-Effect of solvent composition, First order reaction, saponification of ethylene glycol monostearate (monoester)

[OH·] = 0.02 M [Ester] = 0.001 M [Cn =0.02 M Temp.= 30 ± 0.05°C

DMSO-water Mole Pseudo fi rst order Dielectric constant composition (vlv) fraction rate constant (€)

k x 102 , S- I

72/28 0.398 585.0 14.33 60/40 0.276 316.0 10.25 50/50 0.203 206.1 8.350 40/60 0.145 142.7 4.650 28172 0.090 68.05 2.860

DMF-water Mole Pseudo fi rst order Dielectric constant composition (vlv) fraction rate constant (€)

k x I02 ,s-1 72/28 0.367 795.0 7.650 60/40 0.261 525.0 4.330 50/50 0.189 463.0 1.850 40/60 0.136 275.0 0.950 28172 0.084 165.7 0.Q25

304

Indian J. Chem. Techno!., May 2002 Articles

Table 14-dAB values of EGOS & EGMS

ame of ester Alcohol-water. A 0 Dioxane-water, A 0 DMSO-water, AU DMF-water, AO

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

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

charge in the steric bulk of the attacking reagent

should be reflecting by the variation in the

reaction constant parameter 8. However, such

responses are not generally recorded in 8 at

higher mole fractions of dioxane/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 aprotic

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, retired Prof. and

Head, Dept. of Chemistry, VRCE for his

encouragement in the progress of this work. One of

the authors (K. Gajanan) is grateful to the

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.

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