1. INTRODUCTION:

A change in solvent from a polar solvent to a non polar solvent has been suggested to increase or decrease reaction rates depending on the type of reactions¹ Solvent effect on reactivity in homogeneous media is explained in term of specific interaction of solvent and reactant molecules and also between solvent and transition state². In many physical and chemical process of solution, the solvent play very important role³. When the magnitude of solvent in elementary reaction is change, the rate of reaction will also change^4,5. For the understanding of solvent effect, a large effort has been made^6-9 which are sometime succeeded and also some time fails. In this contest here, hydrolysis of Ethyl caprylate is presented in aqueous solvent system at different composition in different range of temperature. Ethyl caprylate is fatty acid ethyl ester has prominent role as metabolite. It is suitable reagent used as standard for measurement of flavour-active compound by gas chromatography. It is also used in brown cocoa, dairy savory etc.

2. EXPERIMENTAL:

The kinetic of alkali catalised hydrolysis of Ethyl Pathylate in water-Acetone has been studied by keeping the concentration of alkali and ester M/10 and M/20 respectively. Water used was doubly distilled from KMnO₄. Chemical used in this experiment are either BDH (Analar) or Merck grade. Acetone was purified by known procedure. The conical flask containing the solution and small stopper bottle congaing Ethyl pathylate were thermostated half an hour. Then 0.4ml of ester was withdrawn with help of pipette and added quickly to alkaline solution of water-organic co-solvent mixture by constant shaking. Immediately 10ml of aliquot of the reaction mixture was withdrawn and allow to run into flask congaing 10ml of N/10 HCI solution. The excess of alkali of the solution was titrated by means of Standard solution using phenolphthalein as indicator. The moment at which half of the aliquot into the ice cold 0.1N HCI solution, the stop clock was started. This time is considered to be zero time or starting time. Taking into account of zero time kinetics of remaining ester was estimated after quenching the 10ml of aliquot in 10ml of ice cold 0.1N HCl at definite interval of time followed by titration as usual.

3. RESULT AND DISCUSSION:

3.1 Solvent Effect and calculated Specific Rate constant:

The specific rate constant of alkali catalyzed of Ethyl caprylate followed second order kinetics which has been calculated by slope of linear plots of Logk against1/T. The values of rate data (Table-1) show that with increasing proportion of organic solvent rate are decreases. The solvent effect on specific rate can also be observed when Logk is plotted against solvent composition (Table 2 and Fig. 1). Dipolar protic solvent like Acetone, DMSO, DMF are powerful bases and strong hydrogen bond acceptor so it interact with solute which are hydrogen bond donors¹⁰. The value of dielectric constant of the reaction media goes on decreasing with solvent composition so the result obtain in present study is against the Hugh and Ingold qualitative theory. However, some instant has been reported^11,12 in which the rate decreases in similar way as found in this case. The depletion of rate causes due to addition of Acetone in water-Acetone media may attribute due to combine effect of dielectric and salvation change taking place in the media. The dipolar aprotic solvent like DMSO, Acetone, DMF exerted greater effect on rate because such solvent produced inter molecular association of solvent in such aqueous solvent media. Solvent -solute interaction, salvation of reactant and transition state are also dominating factor which influencing also exerted greater effect on rate of reaction.

Table 1: Bimolecular rate constant k x10³(dm)³/mole/mint]

Temp in ^OC	% of Acetone
Temp in ^OC	30%	40%	50%	60%	70%
20^OC	26.91	23.71	21.33	18.48	15.84
25^OC	55.59	46.66	39.81	33.11	26.30
30^OC	107.15	89.12	72.44	58.74	42.65
35^OC	208.92	164.05	127.35	100.00	67.60
40⁰C	398.10	305.49	229.08	175.79	107.15

Table 2: Change in Log k Value with mole %,

Temp in ^OC		3 + Log k
Temp in ^OC	Mole%	20^OC	25^OC	30^OC	35^OC	40⁰C
30%	9.56	1.430	1.745	2.030	2.320	2.600
40%	14.11	1.375	1.669	1.950	2.215	2.485
50%	19.77	1.329	1.600	1.860	2.105	2.360
60%	26.99	1.265	1.520	1.769	2.000	2.245
70%	36.52	1.200	1.420	1.630	1.830	2.030

Fig. 1: plots of log k with mole %.

3.2 Thermodynamic Activation Energy (E_c) of bimolecular reaction:

With help of slopes (fig. 2) of linear plots of (Logk against 1/T) of Arrhenius, the value of activation energy (E_c) has been calculated and inserted in Table-4. The decreasing trend of iso composition activation energy with increasing composition of Acetone indicate that salvation in transition state and desolvation in initial state, because transition state being have more cation (ester+H⁺) available by the salvation of Acetone molecule than initial state.^13,14

Table 3: Logk Values with different Temperature, Water-acetone media

Temp in ^OC		3 + Logk
Temp in ^OC	10³/T	30%	40%	50%	60%	70%
20^OC	3.412	1.430	1.375	1.329	1.265	1.200
25^OC	3.355	1.745	1.669	1.600	1.520	1.420
30^OC	3.300	2.030	1.950	1.860	1.769	1.630
35^OC	3.247	2.320	2.215	2.105	2.000	1.830
40⁰C	3.195	2.600	2.485	2.360	2.245	2.030

Fig. 2: plots of Log K with 10³/T.

Table 4: Activation Energy at different composition of solvent

% of EG	30%	40%	50%	60%	70%
E_exp in KJ/mole	103.75	99.41	99.03	88.36	74.09

Table 5: Logk_D Values with different Temperature at constant D Water- acetone media.

Temp in ^OC	10³/T	D=40	D=45	D=50	D=55	D=60
20^OC	3.412	1.189	1.240	1.299	1.355	1.410
25^OC	3.355	1.420	1.500	1.580	1.660	1.740
30^OC	3.300	1.695	1.770	1.850	1.930	2.015
35^OC	3.247	1.895	2.015	2.140	2.260	2.385
40⁰C	3.195	2.152	2.280	2.410	2.539	2.669

Fig. 3: plots of Log K_D with 10³/T.

3.3 Effect of Dielectric Activation Energy:

An alternative aspect of solvent effect can be tested by considering the influence of dielectric constant D on reaction rate. Thus, an increase in D causes a consequent increase in rate. By the interpolation of Akerlof¹⁵ data, the dielectric constant values of reaction mixture have been obtained. When dielectric constant D of the medium is lowered, there is a considerably decrease in rate is found. Considering the reaction as dipole-dipole interaction, a linear relationship between logk and D-1/2D+1 should be obtained Lanndskroner¹⁶.

However, considering the reaction as ion-dipole interaction, linear relation is obtained by plotting Logk against1/D, other liner relationship of Logk and LogD¹⁷. Graphical application of different electrostatic theory for ion-dipole as well as dipole-dipole showed that the best linear plots are obtained when Logk is plotted as a function of D. Departure from linearity at low dielectric constant at low dielectric constant is however still preserved. In case of preferential salvation or salvation shorting of activated complex by water, with the higher component of solvent mixture there is deviation in linearity is observed. However, in this present study of alkaline hydrolysis of caprylate ester is based on dielectric constant effect can better be treated as an ion dipole interaction, in which ester represent the actual dipole rather than dipole interaction. Here Iso-dielectric activation energy was calculated by the slopes of interpolation of Logk against different temperature [Table-5, Fig-3] and the calculated values of dielectric activation energy are inserted in Table-6. The decreasing trend of (E_D) values with decreasing D is similar as previous views of^18,19

Table 6: Dielectric Activation Energy values at different D.

Dielectric constant(D)	D=40	D=45	D=50	D=55	D=60
E_D in KJ/mole	86.62	91.62	94.66	99.64	106.39

4 CONCLUSION:

The rate of hydrolysis of alkali catalyzed hydrolysis of ethyl caprylate decreases with increase component of solvent in the reaction mixture at different temperature range. Increase in value of Iso-composition energy with decreasing proportion of solvent indicate that there is salvation takes place in initial state and desolvation occurring in transition state. The result of this project to analyzed the solvent-solute interaction and solvation change in initial and transition state. Dielectric values also play important role in observing solvent effect in aqueous solvent media.

5. REFERENCES:

1. Parker A. 1969 Protic-dipolar aprotic solvent effects on rates of bimolecular reactions. J. Chem, Rev 69:1-32. doi.org/10.1021/ cr60257a001

2. Iglesias E 2005 Solvent effects versus concentration effects in determining rates of base-catalyzed ketoenoltautomerization. New J. Chem 29;, 625-632.

3. Yangjeh A.H, Nooshyar M 2005 Prediction of solvent effects on rate constant of [2+2] cycloaddition reaction of diethyl azodicarboxylate with ethyl vinyl ether using artificial neutral networks. Bull.Korean Chem. Soc. 26: 139-145.

4. Schmeer G, Six C, Steinkirchner J 1999 Investigations on substituent and solvent effects of solvolysisreactions. VIII. The influence of water and nonaqueous solvents on the imidazolysis of 4-nitrophenylacetate. J. Sol. Chem 28:211-222.

5. Kallol k Ghose 1999 Kinetic and solvent effect on hydrolysis of N-Benzylbenzo hydroximic acid in some binary aqueous solvent mixture J of Molecular Liquid. Vol-81: Issue-2 pp135-155

6. Seliverstova T S 2020 Kinetics and mechanism of hydrolysis of Benzyl Ether bond in aqueous-organic media. Russian Russian J of Physical Chemistry A. Vol-94: pp310-316.

7. Magda F Fathalla et al. 2019 The reaction of 2-chloroquinoxaline with piperidine in DMSO-H₂O and DMF- H₂O mixture. Kinetic and solvent effect. Journal of solution chemistry. Vol. 48:pp1287-1308

8. Seliverstova T S et al. 2020 The hydrolysis of Benzyl Ether bond in aqueous-organic media. Russian Journal of Physical Chemistry A, Vol-94: pp310-316.

9. Magdha F. Fathala. Kinetic reaction of 2-chloro quinoxaline with hydroxide ion in ACN-H₂O and DMSO- H₂O binary mixture. J of Solution Chemistry. Vol-40. Article no. 1258. 2011.

10. Parker A.J 1962 The effects of solvation on the properties of anions in dipolar aprotic solvents. Quart. Rev. London 16: 163.

11. Radha Krishnanmurty P S et al. 1970 Tetrahedron 26: p5503.

12. Varma DK et al. 2020 Study of solvent effect on kinetics of alkali hydrolysis of Ethyl Picolinate in water-Acetone media International J of Advance in Engineering and Management. Vol-2 Issue-9: pp5-6 DOI: 10.35629/5252-02090506

13. Metwally M S 1992 Kinetic study of resin catalised hydrolysis of Ethyl Propinoate in aqueous-organic system. Reaction kinetic catalysis and Letters vol-47:pp319-326.

14. Namami Shanker Sundhansu, Kinetic study of solvent effect of aqueous-DMSO solvent system on extensive thermodynamic properties of acid catalyzed solvolysis of higher Methanoate.Schlor Research Journal of Inter disciplinary Studies. Vol-8/61, 2020. Pp1448-14355

15. G. AKLOF 1932 Dielectric Constants of Some Organic Solvent-Water Mixtures at Various Temperatur J. Am. Chem. Soc 54 11: pp4125–4139

16. Laidler K.J and Landskroener P A 1956 Trans Faraday Soc. 52, 200

17. Fayez Y. Khalil and M. T. Hanna 1978 Kinetic Study of the Acid Hydrolysis of Ethyl Hydrogen Succinate in Binary Solvent Mixtures Z. Naturforsch. 33 b:1479-1483

18. Wolford R K, 1964 Kinetics of acid catalised hydrolysis of Acetal in dimethylsulfoxide-water solvent system at 15,25 and 35⁰c. Phys Chem, 68:p3392

19. Singh AK 2020 Effect of solvent on acid catalised solvolysis of Amyle Methanoate Formate in Water-ethylene Glycol (EG) mixture. Asian Journal of Research in Chemistry. Vol-13(6): pp469-472. 10.5958/0974-4150.2020.00083.8

Received on 06.08.2021 Modified on 29.08.2021

Asian J. Research Chem. 2021; 14(6):445-447.

DOI: 10.52711/0974-4150.2021.00077