Effect of Solvent on Kinetics of acid Catalyzed solvolysis of
Amyl Methanoate Formate in water-ethylene glycol (EG) mixture

 

A K Singh

Department of Chemistry, Teerthanker Mahaveer University, Moradabad, India.

 

ABSTRACT:

The hydrolysis of Amyl Methanoate was investigated volumetrically in water-EG media for kinetic studies at various composition having 30 to 80% of EG at five different temperature 20 to 40⁰c.The following conclusion were reached: first, the depletion of rate with gradual addition of organic co-solvent with increasing temperature of reaction media has been explained in term of salvation of initial and transition states to different extents. Secondly, the changes in value of iso-composition and iso-dielectric Activation energy of the reaction media has also been explained in light of salvation and desolvation of initial and transition states to different extant. The increase in numerical values of (DG*) with decrease in enthalpy of activation (DH*) and entropy of activation (DS*) of the reaction, reveals that ethylene glycol acts as entropy control solvent. Effect of ionic strength and [H⁺] ion on the reaction on the rate has also been studied and it is concluded that acid catalised hydrolysis of amyl methanoate is ion- dipolar reaction and it follows A_AC² mechanistic pathways of water-EG media. From the evaluated values of increasing number of water molecule associated in the formation of activated complex inferred that in presence of ethylene glycol, the bio molecular mechanistic pathways of the reaction has been change to unimolecular. Reaction is entropy controlled because numerical values of (DG^*) increase with simultaneous decrease in (DH^*) and (DS^*) values of the reaction media.

 

 

 

INTRODUCTION:

The effect of aquo-organic solvent on kinetic behavior of acid catalyzed can be of major importance in controlling reaction rate which is usually affected by specific interaction of the solvent with the reactant and with transition state. The dielectric constant of ethylene glycol is lesser than water, being miscible with water in all proportion and hence, varying composition of water solvent mixture will have different dielectric constant values lower than water^[1-3].

 

Properties of solvent alone, do not necessary predict the reaction and many workers^[4-10], in attempt to correlate the rate change with nature and properties of the solvent medium have conflicting results. Thus Parker^[11] and Robert^[12] found that rate constant values decreases in the system with increasing dielectric constant in the medium, in contradiction to the predictions of Hughes and Ingold^[13] and Laidler and Landskroener^[14], that rate of such reaction is expected to rise with increase of dielectric constant.

 

The kinetic studies of on acid canalized hydrolysis of amyl methanoate in binary aqueous mixture have revealed that the reorganization of the solvent surrounding the reactant in the activation processes greatly influences the reaction rate. Such reorganizations are affected by the addition of co-solvent its effect cannot, however, be treated as mere diluents of water modifying its dielectric properties only. In this project we have studied the solvent effect on kinetics of acid catalyzed amyl methanoate formate (which is used as flavoring agent in food) in varying composition of water-ethylene glycol mixture at different temperature.

 

2. EXPERIMENTAL:

Purified ethylene glycol of BDH grade and Amyl Methnoate of USSR made were used.

 

The kinetics of acid catalyzed hydrolysis of ester was studied volumetrically as usual by adding 0.60ml of ester in 50ml of 0.5M HCI solution. The specific rate constant was evaluated by making use of first order rate kinetics and is inserted in Table-1. Evaluated value of two activation energies (Iso-composition, E_C & Iso-dielectric activation, E_D) has been calculated by Arrhenius plots and tabulated in Table-4 and 5. For deciding the mechanistic pathways of reaction. The number of water molecule associated with activated complex (salvation number) and thermodynamic activation parameter have been calculated using Eyring equation ant tabulated in Table-7 and 8

 

3. RESULT AND DISCUSSION:

3.1 Solvent Effect on Specific Rate

The second order rate constant for acid catalyzed hydrolysis of Amyl methanoate formate in water-EG mixture were calculated at 30-40⁰ c by the slopes of linear plot of Logk against time (t). From these slope the specific rate constant at different temperature and composition are calculated in inserted in Table-1.For showing the solvent effect on specific rate constant of the reaction, the logarithmic value of k is also plotted against the mole % of added organic solvent (Ethylene glycol) of the reaction media tabulated in Table-2 and Fig-1, found decreasing trend with increasing composition of solvent. The depletion of rate constant with increasing temperature is responsible on any of the three factors. These are

I.      Decreasing polarity of the medium as changing from polar water molecule to less polar water-EG medium.

II.    Lowering of bulk dielectric constant values of the medium and

III. Depletion of H₂O⁺ ion of the solution by the organic co-solvent due to its basic character.

 

As Ethylne glycol is not basic, so it may not combine with H⁺ and H₂O⁺ ion of the acidic medium. Hence among the above mentioned three rate retarding factors, the first two factors are in and it is quite in agreement with the theory of Hughes and Ingold¹³ this explanation is also supported with the view of Laidler and Lanskroener¹⁴ and recent communication of Singh A. K.¹⁵

 

 

Table – 1 Specific rate constant ( k x10³) values of acid catalyzed

Solvolysis of Amyl Methanoate Formate in water-EG media in [(dm)³/mole/mint]

 

Table – 2 3+ Log k Values with mole %, in (Water-EG) media

 

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

 

3.2 Iso-composition Activation Energy (E_c) in water-EG media:

From the Table-4, it can be observed that Iso-composition activation energy decreases from 106.62 to 63.27 kJ/mole with increasing proportion of solvent in the reaction media. The enhancement in the value of Iso-composition activation energy is any of the following three cases;

a.     The greater solvation of transition state than initial state.

b.     The greater desolvation of the initial state than transition state, and

c.     Simultaneous salvation and desolvation of initial and transition state respectively.

 

In my case, out of the three factors the third one is applicable which is supported by depletion in entropy of activation with increase of organic–solvent in the reaction media as recorded in Table-8 This conclusion is also supported recently by Singh A K.¹⁶

 

 

Table – 3 Variation of Logk Values against 10³/T, Water- EG media.

 

 

Fig. 2: Variation of Log k with 10³/T.

 

Table – 4 Values of Iso-composition Activation Energy with increasing percentage of solvent (water-EG media)

 

3.3 Effect of Solvent on Iso- Dielectric Activation Energy:

The Iso-dielectric energy(E_D) of the reaction is obtained from interpolation of logk at different desired D values and from the slope of Arrhenius curve (logk vrs10³/T) the values of Iso-dielectric energy(E_D) is calculated which is tabulated in Table-5.On perpetual observation of table-5 it has been apparent that the value of E_D goes on increasing from 84.19 to 100.21 with increase of D in the reaction media. On basis of the above fact it is inferred that enhancement in Iso-composition activation energy (E_c) with depletion of Iso-dielectric Activation energy(E_D) are complimentary to each other. This finding is also supported earlier by past view of Wolford¹⁷ and it is recently supported by Singh A.K. et al.¹⁸

 

Table – 5 Iso-Dielectric Activation Energy values in water-EG Media

 

3.4 Effect of Solvent on Salvation number and mechanistic path of the reaction:

According to proposed Robertson¹⁹ relation

 

logk = logk₀ + n log[H₂O]

where n is the salvation number (n) which is determined by plotting logk against log [H₂O]

 

Number of water molecule associated with activated complex that is Salvation number (n) tells about the number of water molecule associated with transition state and also a criterion for studying about the mechanism of the reaction. Considering the values of slope from Table-6, it is inferred that the slope below log [H₂O] value of 1.3 which corresponds to 45.20% of water in reaction media, the value of slops increases from 0.264 to 0.784 with increase of temperature. This suggests that below 45.20% of water in reaction media, about 0.2 to 1.0 molecule of water are taking part in formation of activated complex as temperature increases from 20 to 40°C. Similarly, from the deep slopes, it is clear that in case of increase of water concentration (above 45.20%), the numerical value of slopes increases from 0.883 to 1.271.

 

Table 6 Variation 3+ Log k values with log [H₂O] at different temperature (water-EG media)

 

Fig. 3: Variation of log [H₂O] with Log k.

 

Table-7 Evaluated values of slopes of Water-EG media by Ploting o log k verses log [H₂O]

 

These values show that approximately from 0.8 to 1.3 molecule of water are taking part in formation of activated complex as the temperature rises from 20 to 40⁰c.From this, it may be concluded that when concentration of water fall in water-EG media, the number of water molecule taking part in formation of activated complex is about one but with increase in  water concentration of the reaction media, the number of water molecule associated with formation of activated complex become one and half.

 

Table-8 Rate constant and Thermodynamics Activation Parameters in Water- EG Media of the reaction (∆H^*and  ∆G^* in KJ/Mole, ∆S^*in J/K/Mole)

% of EG	Mole %	∆H* in KJ/Mole	20°C		25°C		30°C		35°C		40°C
			∆G*	-∆S*	∆G*	-∆S*	∆G*	-∆S*	∆G*	-∆S*	∆G*	-∆S*
30%	12.16	87.89	89.53	5.59	89.57	5.05	89.65	5.80	89.68	5.81	89.69	5.75
40%	17.73	83.76	86.58	22.59	90.20	21.61	90.32	21.65	90.33	21.33	90.41	21.24
50%	24.42	82.84	90.38	25.73	90.52	25.30	90.67	25.84	90.80	25.84	90.78	25.36
60%	32.42	82.09	90.96	30.27	90.86	27.75	91.04	29.53	91.19	31.88	91.44	29.87
70%	42.94	78.84	91.00	41.50	91.23	41.57	91.48	41.71	91.66	41.62	91.70	41.08
80%	56.34	77.41	91.33	47.50	91.70	47.95	92.09	48.44	92.40	48.66	92.64	48.65

 

Thus, it can be concluded that the acid catalysed hydrolysis of Amyl Methanoate in water-EG media follow unimolecular mechanism with respect to water.

 

It may also be inferred that with increase of solvent composition (30 to 70%) the number of water molecule associated with transition state increase with rise in temperature (20 to 40⁰c) which attributes to the fact that equilibrium of water molecule by addition of EG, is shifted from dense form to bulky form with rise in temperature.

 

[H₂O] _d ↔ [H₂O]_b

 

On guide line of Robertson R E, et al.²⁰, it is also inferred that the mechanistic path followed by acid catalyzed hydrolysis of Amyl methanoate is changed from bimolecular to unimolecular. It is also supported by Singh A K²¹

 

4. CONCLUSION:

The result of this work indicates that the rate of acid catalyzed hydrolysis of amyl mathanoate is influence by solvent composition. It decreases with increase of solvent composition. In this project solvent effect is also discussed considering dielectric constant of the medium and water concentration effect on reaction rate. The solvent effect is also discussed in term of activation energies (Iso-composition, E_C) which inferred the salvation and desolvation in initial and transition state. Salvation number represent the number water molecule involves in activated complex which decides the mechanistic pathways of reaction media. Activation parameter like entropy of activation, ∆S^* enthalpy of activation, ∆H^* & free energy, ∆G^* of activation show the solvent effect of salvation and desolvation on initial and transition state.

 

5. REFERENCE:

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2.      Akerlof. G. Dielectric constant of some organic solvent-water mixture at various temperature. J amm. Chem Soc. (1932), 54, 11, pp4125-4139.

3.      Akerlof. G. and Oliver., A Shorter The dielectric constant of dioxane-water mixture between 0 and 80°C. J Amm. Chem Soc (1936), 58,7, pp1241-1243.

4.      Singh A K. “Study of Solvent effect of Protic solvent on Solvolysis of Hexanoate ester and Activation parameters.” Asian journal of Research in Chemistry, 12(2) Nov 2020, pp 216-218

5.      Singh A K. Arjuman Bano,.“Kinetics and Solvent effect on Aquo-dipolar protic organic-solvent system of Hexanoate ester.” Asian journal of Research in Chemistry, 12(6) Nov 2019, pp 341-344

6.      Singh AK. Arjuman Bano., Solvent effect and kinetics on ethyl benzoate in aqueous solvent system. International Journal of Chemical Science, Volume 3; Issue 5; September 2019; Page No. 26-28

7.      Sharma Sangita et al. Kinetic study of specific base catalysed hydrolysis of Ethyl Acrylate in water-Ethanol binary system. Russian Journal of physical chemistry A. Vol. 87, No. 5, 2013. Pp730-736

8.      Magda F Fathalla., Kinetics of reaction of 2-chloro-quinosalin with Hydroxide ion in CAN- H₂O and DMSO- H₂O binary solvent mixture,” Journal of solution chemistry, 40, 1258-70,2011

9.      Singh A K., Kinetics and solvent effect on activation parameter of aquo-propanol solvent system for acid catalyzed solvolysis of propyl formate. International Journal of Chemical Science, vol-3, issue-4, July, 2019 pp85-88

10.   Singh A K., Solvent effect and kinetics on solvolysis of propyl formate in water-propanol solvent mixture. International Journal of Chemical Science, vol-3, issue-4, July, 2019 pp82-84.

11.   Parker A J., The effect of salvation on the properties of anion in dipolar Q Rev Issue-2(1962), p163.

12.   Roberts D. D., Solvent effect III. The influence of aqueous dimethyl solfoxide on alkyl benzoate ester saponification reaction. J Org. Chem. (1966,31,12,) pp4037-4041. aprotic solvents.                                              

13.   Hughes E.D. and Ingold C.K,” Mechanism of substitution at saturated carbon atom part IV, A discussion of constitution and solvent effect on mechanism, kinetics, velocity, and orientation of substitution^”. j chem. Soc 1935, 244- 255.

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

15.   Singh AK. Kinetics and solvent effect on activation parameter of aquo-propanol solvent system for acid catalyzed solvolysis of propyl formate. International Journal of Chemical Science vol-3, issue-4, July, 2019 pp85-88.

16.   Singh A.K. Solvent effect and kinetics on solvolysis of propyl formate in water-propanol solvent mixture. International Journal of Chemical Science vol-3, issue-4, July, 2019 pp82-84.

17.   Wolford, R K.: “Kinetics of the Acid-Catalyzed Hydrolysis of Acetal in Dimethyl Sulfoxide- Water Solvents at 15, 25, and 35°C” J. Phys.Chem., 1964, 68 (11), pp 3392–3398J. Phys.chem. 68, 3392, 1964.

18.   Singh AK. Arjuman Bano., Solvent effect and kinetics on ethyl benzoate in aqueous solvent system. International Journal of Chemical Science, Volume 3; Issue 5; September 2019; Page No. 26-28

19.   R.E. Robertson, “A survey of thermodynamic parameter for solvolysis in water”, Prog. Phy.Org. chem. 4, 1967 pp213

20.   Robertson. R E., Heppolitite. R L, Scott J.M.W: A survey of thermodynamic parameter for the solvolysis of water. Canad. Journal of chemistry. 37(4) pp803-824.

21.   Singh A K., “The influence of solvent on solvolysis of ethyl cinnamate in water-acetone mixed solvent system”.  Chemical science journal, vol-8 issue-1, March, 2017 pp 1-4

 

 

Received on 20.08.2020                    Modified on 11.09.2020

Accepted on 27.09.2020                   ©AJRC All right reserved