Solvent effect and Activation Parameters: A Kinetic Reaction of Ethyl Caprylate in Water-Acetone Media

A.K. Singh

Teerthanker Mahaveer University, Moradabad, India.

*Corresponding Author E-mail: dr.anilkumarsingh2009@gmail.com

ABSTRACT:

The kinetic result of hydrolysis of Ethyl Caprlyate has been investigated at different composition of aqueous-organic solvent with Acetone (30-70% v/v) over the temperature range of 20 to 40⁰c. The calculated result follows second order kinetics and is observed that the rate decreases with increasing proportion of Acetone. This behavior is attributed electrostatic nature that various solvent-solute interaction in reaction media. Linear plots of Logk against water concentration shows that equilibrium shifted from dense form to bulky form. Iso-kinetic temperature has been determined with the help of slopes of (ΔH^*) versus (ΔS*). Thermodynamic parameter has been calculated with the help of Wynne-Jones and Eyring equation.

KEYWORDS: Ethyl Caprylate, kinetics solvent effect, Acetone, water, activation parameter.

1. INTRODUCTION:

A large number of investigation are made on studies of solvent effect on hydrolysis of different ester in aqueous binary solvent system^1-5 but there is very little attention has put forward on alkali catalysed hydrolysis of. 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. Prediction of Ingold⁶ and Laidlar⁷ shows that the rate of reaction expected to increase with increase of dielectric constant of the reaction media. However Parker⁸ and Roberts⁹found decrease in rate with increase of dielectric constant under similar condition. Actually solvent- solute interaction and salvation of reactant and transition state are the dominating factors which influence the reaction rate in aqueous solvent system. In this studies an attempt has to be made to investigate the influence of dipolar aprotic solvent (Acetone) on reaction in aqueous solvent system at different temperature.

2. EXPERIMENTAL:

Kinetics of alkali catalyzed hydrolysis of Ethyl Capyrlate ester has been studied over different range of temperature (20 to 40⁰c). The entire chemical used was AnalaR or cp Merck. Water used during the experiment is double distilled. Organic solvents are purified by known procedure. Calculated amount of NaOH, water, organic solvent (Acetone) and ester are thermostated. From zero time onward 10 ml of of reaction mixture has been quickly poured into chilled flask containing 10ml standard hydrochloric acid which is quickly titrated against the barytha solution. The titer reading is noted with definite interval of time. The calculated rate, using second order reaction is inserted in Table-1.

3. RESULT AND DISCUSSION:

3.1 Effect of solvent on hydrolysis rate:

The second order rate constant of the reaction in water-Acetone have been calculated from linear plots of Logk against time. It is obvious from the result obtained in Table-1 that the values of rate decrease with gradual addition of solvent. The decrease in rate is due to precipitation of OH^- ion. The negative charge is believed to be dispersed on large area of the transition state. Hence this result is against the past theory of Haugh and Ingold⁶ but in some case it is found similar as result of this case^10,11. However it is observed that the rate is affected due to change in solvation and dielectric vales simultaneously.

Table–I Second orderrate constant values [k x10³(dm)3/mole/mint] at different solvent composition.

Temp in ^OC	% of Acetone
	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
40^0C	398.10	305.49	229.08	175.79	107.15

3.2 Mechanistic path ways and role of water molecule:

The slope of plots of Logk values with different water concentration, the number of water molecule involves in transition state has been determined as suggested by Tommila¹² and Lane¹³. The rate governing step of Ethyl Caprylate consists of attack of hydroxyl ion on carbonyl and water molecule on ester oxygen simultaneously as suggested by Laidler and Landskskroer⁷. The plots of Logk verses log [H₂O] (Fig-1, Table-3) have been found to be 1.16 to 2.34 with increasing temperature. This result shows that the nearly1.16 to 2.34 water molecule take part in formation of activated complex with increase in temperature.

Robertson [14] proposed a relation

logk = logk₀ + nlog[H₂O]

Where n is the salvation number (n) which is determined by plotting logk against log [H₂O] The salvation number helps us to decide the criterion for studies of mechanism of the reaction in aqueous solvent system. From the values of water molecules associated with transition state, it may be inferred that structure of water in water-acetone media changes from dense form to bulky form with increase in the temperature of reaction.

[H₂O] _d↔ [H₂O]_b

As suggested by Robertson R E, et al.¹⁵, it is observed that the mechanism of the reaction changes bimolecular to unimolecular Sharma Sangita et al.¹⁶ with increase in water concentration.

Table-2 Change in Log k values with log [H₂O] at different range of temperature.

% of EG	% of H2O	Log [H2O]	3 + Log k
			20^0C	25^0C	30^0C	35^0C	40^0C
30%	70%	1.569	1.430	1.745	2.030	2.320	2.600
40%	60%	1.522	1.375	1.669	1.950	2.215	2.485
50%	50%	1.4437	1.329	1.600	1.860	2.105	2.360
60%	40%	1.346	1.650	1.520	1.769	2.000	2.245
70%	30%	1.221	1.200	1.420	1.630	1.830	2.030

Fig. 1: Plot of log [H₂O] with Log K.

Table-3 Calculated values Slopes of log k verses log [H₂O] Water-acetone media

Temp^0C	Slope
20^0c	1.169
25^0c	1.372
30^0c	1.538
35^0c	2.191
40^0c	2.346

3.3 Activation Parameters of reaction (ΔH,^* ΔG^* ΔS*)

Activation parameter, like activation energy also plays an important role in ester hydrolysis. Wynne-jones and Eyring equation¹⁷, are used to calculate all these parameters and the results are inserted in Table-4 with increasing mole% of solvent. The change in free energy of activation (ΔG^*) with solvent composition appears to be very small and considered to be negligible. This behaviour of (ΔG^*) is also shown by^18,19. Again, decrease in entropy of activation with solvent composition attributed that the conversion of final state to transition state together with decrease in Entropy. It is also inferred that transition state is more solvated as compared to initial state with increase of solvent composition.

Large change in (ΔH^*) and (ΔS*) with negligible change in (ΔG^*) with varying composition of solvent usually attributed enthalpy entropy compensation effect. Metwally M S²⁰. Such compensation effect generally results in linear relationship between (ΔH^*) and (ΔS*). The plots of (ΔH^*) verses (ΔS*) for the investigated ester, are also found linear (fig-5) and follows Barclay and Butler rule^21,22. The slope of this line is called iso-kinetic temperature which comes to be 265. This suggested that very little interaction between the solvent and solute takes place.

Table-4 Thermodynamics Activation Parameters

% of EG	Mole %	∆H^* in KJ/Mole	20^0C		25^0C		30^0C		35^0C		40^0C
			∆G^*	∆S^*	∆G^*	∆S^*	∆G^*	∆S^*	∆G^*	∆S^*	∆G^*	∆S^*
30%	9.56	99.72	76.95	30.44	76.46	30.77	76.08	30.46	75.73	30.68	75.17	30.70
40%	14.11	94.86	77.26	12.79	76.90	12.85	76.55	12.87	76.24	12.88	75.85	12.87
50%	19.77	90.18	77.52	-4.061	77.29	-4.16	77.07	-4.290	76.90	-4.41	76.61	-1.182
60%	26.99	85.41	77.88	-21.56	77.75	-21.64	77.60	-21.78	77.52	-22.20	77.30	-21.85
70%	36.52	77.91	78.25	-48.43	78.32	-48.72	78.41	-48.08	78.52	-49.54	78.60	-49.93

(∆H^*and ∆G^* in KJ/Mole, ∆S^*in J/K/Mole)

4. CONCLUSION:

Result obtained shows that the values of rate decrease with successive addition of solvent which indicates that both salvation and dielectric change are responsible for decrease in rate.

The plots of Logk verseslog [H₂O] have been found to be 1.16 to 2.34 with increasing temperature. This result shows that the nearly1.16 to 2.34 water molecule take part in formation of activated complex with increase of temperature. Small change in free energy of activation with large change in (ΔH^*) and (ΔS*) appear to be enthalpy- entropy compensation effect. Linear plots of (ΔH^*) verses (ΔS*) justified the iso- kinetic relationship.

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Received on 15.09.2021                    Modified on 08.10.2021

Accepted on 30.10.2021                   ©AJRC All right reserved

Asian J. Research Chem. 2021; 14(6):441-444.

DOI: 10.52711/0974-4150.2021.00076