1. INTRODUCTION:

The rate of chemical reaction is changed with the change of solvent composition of reaction mixture which has great importance in laboratory as well as in chemical industry. Some time it is found that rate is increased by a factor 10⁹ with change of solvent.¹ The solvent affect the reaction rate in two ways. One through static or equilibrium solvent effect and another through dynamic or fractional solvent effect.^2-5. The static solvent effect on reaction rate can be understood in term of Transition state theory. The second method are influence the rate of fast reaction. The solvent effect is also closely related to nature and extent of solvent-solute interaction. Solvent- solute interaction are very much complex in mixture of the solvent than pure solvent due to possibility of specific solvation of any of solvent present in the mixture.

In this investigation studies of solvent effect in hydrolysis of diethyl pathalate has been reported, which has greater use as solvent for the antimony reduction of aluminium chloride phosphorus, trichloride alkyl complex to form the corresponding alkyl phosphorus dichloride.

2. EXPERIMENTAL:

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 ethanol. The diethyl ester containing in a conical flask has been thermo- stated for half an hour. Immediately after then 0.4ml of pathlate ester was withdrawn quickly and added to organic co-solvent mixture with help of pipette. Again 10ml of aliquot has been withdrawn and allowed to pour into conical flask in which N/10 HCL solution was already present.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.

3. RESULT AND DISCUSSION:

3.1 Solvent Effect dependence and Rate constant:

The rate of hydrolysis of diethyl pathalate has been measured in water-ethanol medium by the slope of logk against time (Table-2 and Fig-1). The plot of logk against reciprocal of time has been found has been found to be linear. The retardation of rate with increase solvent composition may be explain on the basis of the fact that formation of polarized transition state is disfavored with increase proportion of ethanol. This fact is also supported by Hugh and Ingold⁶ qualitative theory. The rate determine step of hydrolysis of ester is addition of hydroxyl ion to the carbon of carbonyl group of and after which rapid reaction with water is followed. Hene if the activity of hydroxyl ion decreases, the reaction rate must be decreases Tommila et al.⁷ The lower the rate in water-ethanol mixture suggested that interaction between water and ethanol increases with increase concentration of free water available for salvation of hydroxyl ion. The curve logk against mole fraction of representing smooth decrease (fig-1), which implies that salvation of hydroxyl ion is stronger and hence the values of rate constant is decreases.

Table 1. Bimolecular rate constant k x10²(dm)³/mole/mint]

Temp in ^OC	% of Ethanol
Temp in ^OC	30%	40%	50%	60%	70%
20^OC	16.21	11.48	8.70	6.09	4.78
25^OC	24.54	18.40	14.45	10.47	8.31
30^OC	35.48	27.54	22.38	16.59	13.48
35^OC	50.11	42.65	33.88	28.31	22.64
40⁰C	74.47	67.45	56.23	48.97	38.01

Table 2: Log k values with different mole %

Temp in OC		2 + Log k
Temp in OC	Mole%	20^OC	25^OC	30^OC	35^OC	40^OC
30%	7.13	1.210	1.390	1.550	1.700	1.872
40%	11.63	1.060	1.265	1.440	1.630	1.829
50%	17.00	0.940	1.160	1.350	1.530	1.750
60%	31.54	0.785	1.020	1.220	1.452	1.690
70%	41.75	0.680	0.920	1.130	1.355	1.580

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

3.2 Activation Energy (Ec) and rate of reaction:

Arrhenius plots (Logk against 1/T) has been used to calculate value of activation energy (E_c). Table-4 collects the obtained values of activation energies under different solvent composition. The increase in (E_c) values with increase in solvent composition has been noticeable because it will only happened when there is salvation change take place either in initial level or transition level or both.

The increasing value of (E_c) as a function of solvent composition indicates that there is salvation taking place at initial level where as desolvation takes place at transition level.^10-12

Table 3: Different logk values as a function of temperature

Temp in ^OC		2 + Logk
Temp in ^OC	10³/T	30%	40%	50%	60%	70%
20^OC	3.412	1.210	1.060	0.940	0.785	0.680
25^OC	3.355	1.390	1.265	1.160	1.020	0.920
30^OC	3.300	1.550	1.440	1.350	1.220	1.130
35^OC	3.247	1.700	1.630	1.530	1.452	1.355
40⁰C	3.195	1.872	1.829	1.750	1.690	1.580

Fig. 2: Plots of log k with 10³/T

Table 4: Iso- composition activation energy at different solvent fraction

% of E_c	30%	40%	50%	60%	70%
E_exp in KJ/mole	61.64	68.56	74.25	82.00	82.36

Table 5. the values logk_D at different D

Temp in ^OC	10³/T	D=40	D=45	D=50	D=55	D=60
20^OC	3.412	0.750	0.850	0.940	1.040	1.140
25^OC	3.355	0.940	1.040	1.145	1.250	1.350
30^OC	3.300	1.160	1.275	1.390	1.501	1.620
35^OC	3.247	1.428	1.519	1.610	1.700	1.790
40⁰C	3.195	1.655	1.740	1.831	1.920	2.010

Fig 3: variation of log k_d with 10³/t

3.3 Dielectric Effect on reaction rate:

The another aspect of effect solvent may also be considered by observing the effect of dielectric constant D on reaction rate. There is an increase in rate is found to increase in D whereas the rate is decreses with decrease in D. The dielectric constant values are obtained by by interpolation of Akerlof data¹¹. Linear relationships are obtained between logk and D-1/2D+1 for dipole-dipole interaction Lanndskroner¹². However for ion-dipole linear relation are obtained by plotting Logk and LogD or Logk against1/D. With help of slope of plots of Logk_D against receprocal of absolute temperature, the iso-dielectric values has been calculated [ Table-5, Fig-3] with help of slope of different linear line. The depletion in (E_D) with decreasing D[Table-6], showed that depletion in (E_D) values followed similar trend as (Ec) vales with increasing proportion of solvent composition.^13-15.

Table 6: calculated values of (eD) at constsnt d

Dielectric constant (D)	D=40	D=45	D=50	D=55	D=60
E_D in KJ/mole	83.76	80.62	79.64	78.84	77.31

4. CONCLUSION:

The gradual decrease of rate is constantly found with increase proportion of solvent composition due to increase concentration of free water available for salvation of hydroxyl ion. The increase of activation energy with increasing composition of solvent indicates desolvation in transition state and salvation in initial state. Dielectric value follows similar trend as iso-composition activation energy.

5. REFERENCES:

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Received on 29.07.2022 Modified on 15.08.2022

Asian J. Research Chem. 2022; 15(5):337-339.

DOI: 10.52711/0974-4150.2022.00060