INTRODUCTION:

A number of authours^1,2,3,4,5 has been trying to study about hydrolysis of acid and amide. In this report, here I am trying to explain the result of base catalyzed hydroplysis of butyl salicylate which has greater importance as food additive as well as flavoring agents.

According to this theory⁶, the rate of reaction of aprotic solvent is greater than protic solvent in which the transition state passes through more polarization. This theory has been also supported by⁷. However, this statement is against the qualitative predication of Haugh and Ingold⁸ and also against the theoretical predication of⁹. According to both authors, the rate of such reaction Should decreases with increasing dielectric constant of the media. Therefore, its needs more investigation to reach at definite conclusion, particularly in the case of butyl salicylate. To study about activation parameter and mechanism of the rate process, here in this report it has been decided to study about the rate and mechanism of the base catalyzed hydrolysis of salicylate ester which have great importance in the chemical industry as well as also used as food additive.

MATERIAL AND METHOD:

The reactions kinetics has been studied under second-order conditions The chemicals used are Merck grade or BDH(Analar). Known procedure is followed for purifying the DMF. The salicylate ester containing in a conical flask has been thermo-stated for half an hour. Immediately after then 0.4mI of Salicylate ester was withdrawn quickly and added to organic co-solvent mixture with help of pipette. Again 10mI of aliquot has been withdrawn and allowed to pour into conical flask in which N/10 HCL solution was already present. Phenolphthalein indicator has been used to titrate the excess of alkali with standard solution. When half of aliquot into the cold 0.1N HCI solution the stop clock was started. Rest of ester has been estimated after quenching the 10ml of aliquot in 10ml of 0.1N HCl in a fixed interval of time. Evaluated value rate constant of the reaction has been recorted in Table 1.

RESULT AND DISCUSSION:

Reaction rate: The calculated rate of base-catalyzed hydrolysis butyle salicylate using second-order kinetics has been listed in Table 1. The rate of reaction decreases with increasing proportion of solvent composition with increasing temperature. The decrease in rate of reaction may either due to solvent-solute interaction or due to the dielectric effect although both factors are equally important for the depletion of rate but, according to¹⁰, solvent-solute interaction is the more probable factor for influence the rate, and the dielectric constant is secondary factor which influence the rate. Depletion of rate with an increasing proportion of rate has been also found recently by different workers.^11,12

Table 1: Calculated values of rate constant [k x10³(dm)3/mole/mint] at different solvent composition.

Temp in ^OC	% of DMF
Temp in ^OC	30%	40%	50%	60%	70%
20^OC	60.53	44.66	36.72	36.72	22.38
25^OC	118.85	95.27	66.07	66.07	53.70
30^OC	231.73	171.79	144.54	144.54	122.74
35^OC	428.54	350.75	307.60	307.60	269.15
40⁰C	829.85	707.94	656.14	656.14	602.55

Reaction rate and mechanism of reaction as a function of water concentration:

The rate can also been expressed as a function of water concentration by polting logk against- different concentrations of log [H₂O]. Table-2 The rate of reaction shows a marked increase with increaseing concentration of water and the linear plots has been obtaion in more water rich midia [Fig-1]. The number of water molecules associated with activated complex can be dermined with help of different slopes obtaioned by plots logk against different concentration of log [H₂O] as suggested by^13,14 are inserted in [Table 3]. From the Table 3, it has been observed that the number of water molecules associated with activated complex decreses with increasing temperature. From the values of number of water molecules associated with activated complex (Solvation numbers), it may inferred that structure of water molecules in water-DMF media changing from bulky form to dense form with increasing temperature of reaction media.

[H₂O] _b ↔ [H₂O]_d

The past investigation of different workers^15,16,¹⁷ also support this finding.

Table-2 Variation of logk values with log [H₂O] at different temperature.

% of DMF	% of H₂O	log [H₂O]	3 + log k
% of DMF	% of H₂O	log [H₂O]	20⁰C	25⁰c	30⁰c	35⁰c	40⁰c
30%	70%	1.569	1.782	2.075	2.365	2.632	2.919
40%	60%	1.522	1.650	1.979	2.285	2.580	2.890
50%	50%	1.4437	1.575	1.918	2.235	2.545	2.850
60%	40%	1.346	1.565	1.820	2.160	2.488	2.817
70%	30%	1.221	1.350	1.730	2.089	2.430	2.780

Fig. 1: Plot of log [H₂O] with log k

Table 3: Different values Slopes of log k against log [H₂O] Water-DMF media.

Temp⁰C	Slope
20⁰c	0.957
25⁰c	0.895
30⁰c	0.874
35⁰c	0.613
40⁰c	0.304

Activation parameters and it’s effect on rate and mechanism of the reaction:

Like iso composition activation energy and Iso-dielectric activation energy, some others thermodynamic parameters (enthalpy of activation, Gibb’s free energy of activation and entropy of activation) also have better indicator effects displayed by solvent on solvolysis reaction. Wynne-jones and Eyring equation¹⁸ has been used to calculate these parameters and obtained values are inserted in Table-4. By analyzing the different data arranged in Table 4, it has been found that, all the numerical values these three activation parameters are increase with increasing proportion of solvent composition in reaction media.

According to the fundamental relation of thermodynamic

ΔG^*= ΔH^*-TΔS*

It has been observed that the enhancement in numerical values of all these three thermodynamic parameters all together is possible only when the numerical increase in ΔH^*, is greater than that found in ΔS*. From the fact it has been concluded that, bas catalysed hydrolysis of butyle salicylate in water-DMF media is enthalpy stimulating and entropy control. In another words, the propanol present in reaction media activate the enthalpy but inhabits. The change in numerical value of these thermodynamic parameters also support solvation in initial state and desolvation in transition state. This finding is also supported recently by different authors.^19,20

Iso-kinetic Temperature (Barclay-Butlar rule):

In order to study the Iso- kinetic temperature, ΔH* values are plotted against ΔS* (fig 5). The observation show that the variation is well-linear in accordance with the Barclay-Butlar rule²¹. The numerical value of the slope has been found to be greater than 300. It is found in this case is 318.72. These higher values of slope indicate strong solute-solvent interaction in reaction media as alredy reported by²² and recently by different workers^16,23

Table 4: Consolidated values of thermodynamics Activation Parameters ∆H^*and ∆G^*in KJ/Mole, ∆S^*in J/K/Mole

% of Propanol-2	Mole %	*∆H^ in KJ/Mole**	20⁰C		25⁰C		30⁰C		35⁰C		40⁰C
% of Propanol-2	Mole %	*∆H^ in KJ/Mole**	∆G*	∆S*	∆G*	∆S*	∆G*	∆S*	∆G*	∆S*	∆G*	∆S*
30%	9.22	98.74	88.8.	33.85	88.71	33.65	88.54	33.66	88.42	33.50	88.19	33.70
40%	13.58	106.34	89.55	57.30	89.23	57.41	89.01	57.19	88.73	57.17	88.37	57.41
50%	18.96	111.85	89.95	74.74	89.58	74.73	89.30	74.42	88.94	74.38	88.61	74.24
60%	25.98	112.35	90.01	76.24	90.14	74.53	89.73	74.65	89.27	74.93	88.80	75.23
70%	35.27	124.26	99.22	85.46	90.66	112.75	90.15	112.57	89.62	112.46	89.03	112.55

Fig. 2: Variation of ∆G^* against mole % at 20⁰C

Fig. 3: Plot of ∆H^* against mole %

Fig. 4: Variation of ΔSˣ versus mole % at 20⁰C

Fig. 5: Variation of ∆H* with ∆S* at 20⁰C water-DMF media.

CONCLUSION:

On the basis of above discussion of this report, it has been found that in alkali catalysed hydrolysis of butyle salicylate, the rate of reaction decreases proportionally with increasing fraction of solvent. The number of water molecules associated in the activated complex has been decreases with increasing temperature. The increase in values of ∆G* together with ∆H* with ∆S* is possible only when extant of enhancement in entropy of activation is lesser than enthalpy of activation in the reaction media. The value of iso-kinetic temperature is approximate 318.72; indicating strong solvent- solute in reaction media.

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Received on 25.07.2024 Modified on 03.08.2024

Research J. Science and Tech. 2024; 16(3):176-180.

DOI: 10.52711/2349-2988.2024.00026