Kinetics and Solvent Effect in Hydrolysis of Ethyl Cinnamate in Water- Methanol Mixture
Received Date: Jul 25, 2017 / Accepted Date: Aug 07, 2017 / Published Date: Aug 14, 2017
The solvent effect of ethanol on the alkali catalyzed solvolysis reaction was studied by carring out of the hydrolysis of ester namely ethyl cinnamate in water-methanol media of varying composition consisting of 30 to 70% of methanol (v/v) at different Temperature ranging from 20°C to 70°C. The Specific rate constant values of the reaction were found to depleted with increasing concentration of methanol in reaction media. Enhancement in ΔG* with simultaneous depletion in ΔH* and ΔS* of the reaction, it has been concluded that reaction is enthalpy stimulating and entropy inhibiting and specific salvation take place in water- methanol media. From the evaluated value of Iso kinetic temperature, which is less than 300 indicates that this reaction in water-methanol media obey Barclay-Butler rule and there is weak but considerable solvent –solute interaction taking place in reaction media.
Keywords: Solvent effect; Ethyl cinnamate; Hydrolysis watermethanol mixture; Specific rate constant; Iso-composition; Isodielectric activation energy; Activated complex
The effect of solvent on the rate and mechanism of alkali catalyzed hydrolysis of ester have received continued attention, but the explanation put forward is not satisfactory [1-3]. So, in order to explore the above idea, it was thought essential and useful to investigate about the fact that how ethyl cinnamate is useful as flavor and fragrance agent in cigarettes and cut tobacco. This compound is also used as food additive for human beings.
Kinetic of alkali catalyzed solvolysis of ethyl cinnamate was studied by adding different concentrations of the protic organic co solvent(Ethanol) from 30 to 70% (v/v) in reaction media at five different temperature i.e., 20°C, 25°C, 30°C, 35°C and 40°C using volumetric method. All chemicals used were either of BDH or Merck grades. The strength of the solution was kept 0.1M with respect to NaOH and 0.05 M with respect to ester. The reaction rates were determined by titrating the sample at different time interval using phenolphthalein indicator. The specific rate constant value was calculated using second order reaction and tabulated in Table 1. Variation of specific rate constant with different mole% and different temperature are inserted in Table 2. The value of slopes of Arrhenius plot of logk verses 1/T (Table 3) (Iso-composition activation energy (Ec) and plots of logk verse D(Iso-dielectric activation energy) were evaluated and inserted in Tables 4 and 5 respectively. From the plots of Logk verses Log [H2O] Table 6, the value of slopes were evaluated and tabulated in Table 7. Thermodynamic Activation Parameter were also calculated and inserted in Table 8.
|Temp in °C||% of Methanol|
Table 1: Specific rate constant k × 103 (dm)3/mole/mint values of alkali catalyzed hydrolysis of Ethyl-Cinamate in water-Methanol.
Table 2: Variation of 3+Logk Value against mole %, (Water-methanol) System.
|Temp in °C||3+Logk|
Table 3: 3+Logk Value against 103/T, Water- methanol System.
|% of MEOH||30%||40%||50%||60%||70%|
Table 4: Values of Iso-composition Activation Energy (water-MEOH media).
Table 5: Values of Iso-Dielectric Activation Energy (water-methanol).
|% of Acetone||% of H2O||Log [H2O]||3+Log k|
Table 6: Variation of 3+Log k with log [H2O] (water – methanol) at different temperature.
Table 7: Calculated values of slopes (Plot of log k verses log[H2O]) of water-methanol media.
|% ofMEOH||Mole %||∆H* in Kj/Mole||20°C||25°C||30°C||35°C||40°C|
Table 8: Consolidated values of Thermodynamics Activation Parameters (ΔH*and ΔG* in KJ/Mole, ΔS*in J/K/Mole) of the reaction in Water- methanol Media.
Results and Discussion
Solvent effect on specific rate
In order to highlight the effect of the solvent on specific rate constant values (Table 1) of the reaction, the logk values were plotted against the mole% of organic co-solvent (their values from Table 2 as shown in Figure 1) and were found to show decreasing trend. The plots show, that with increase in the temperature of the reaction, the degree of depletion in the rate constant of the reaction becomes slower. However, the possible rate depleting factors in the rate can be enlisted as follows:
1. Decrease in bulk dielectric constant value of medium,
2. Decrease in the polarity of the reaction media by adding less polar methanol to it.
The above noted two depleting factors are quite in operation and are with good agreement with theory of Hughes and Ingold  that rate ought to decrease with decreasing dielectric constant of the reaction media. Such decrease in rate constant with increasing proportion of organic co-solvent like methanol have also reported by Elsemongy , and recently by Singh .
Solvent effect on iso-composition activation energy of the reaction
From Table 4, (Figure 2) it is found that the values of Isocomposition Activation Energy goes on increasing [52.16 to 57.37 kj/mole] with increasing concentration of MEOH from 30% to 70% (v/v) in reaction media. The increasing trend in the Eexp values can be explained by any of the following three situations.
1. Transition state is desolvated less than Initial state
2. Transition state is solvated more than the initial state
3. Transition state is solvated and initial state desolveted.
The transition state being large cat ion (ester+H+) is available more for solvation by ethanol molecule than the initial state, so third factor seem to operative in my case and is also supported by decrease in entropy of activation (ΔS*). This conclusion is also supported recently by Singh et al. .
Solvent effect and iso- dielectric activation energy
From the slope of Arrhenius plot of logkD values against 1/T, the value of iso-dielectric activation energy has been evaluated and recorded in Table 5. From the Table 5 (Figures 3 and 4), it is inferred that ED value goes on increasing from 55.45 to 65.09 kj/mole with increasing D values. This trend of variation is similar as Ec values (62.31 to 64.88 KJ/mole) (Table 5) with gradual addition of more solvent in reaction media. This interpretation was supported by past view of Wolford  recently supported by Singh et al. .
Effect of concentration of water molecules [H2O], associated with the activated complex on mechanistic path of the reaction
The effect of water concentration [H2O] of the water-ethanol mixture, on rate and mechanism of alkali catalysed hydrolysis of ethyl cinnamate has been studies. For this, the number of water molecule associated with activated complex was determined by plotting Log k against Log[H2O], According to Robertson  relation
Here n is the solvation number and decide the criteria about the mechanism of reaction.
From noted value of slope mention in Table 7, it may be seen that with rise of temperature from 20 to 70°C, the value of slope (number of water molecule associated with activated complex) goes on increasing from (2.08 to 2.56). This observation may be attributed to the fact that with addition of ETOH in reaction media, the equilibrium of water component reaction media shifted from its dense form to its bulky form with rise of temperature.
Finally, it is inferred that with rise of temperature, the number of water molecule associated with activated complex increases as the proportion of bulky form of water molecule increase with increase in MEOH content in the water-ethanol mixture and it explain that when MEOH is added to water, the mechanistic path of reaction is changed from biomolecular to unimolecular in similar way as observed by parker and Tomilinson  and recently by Singh .
In hydrolysis of Ethyl cinnamate, the decreasing trend of specific rate constant at all temp with increasing mole % of co-solvent which show that the decrease is either due to bulk dielectric constant value of medium or decrease in the polarity of the reaction media by adding less polar methanol to it. The values of Iso-composition Activation Energy goes on increasing [52.16 to 57.37 kj/mole] with increasing concentration of MEOH from 30% to 70%(v/v) in reaction media indicate Transition state is solvated and initial state desolveted. Increase in the value of and ΔG* with simultaneous increase in ΔH* andΔS* for the hydrolysis ethyl cinanimate in water- Methanol is enthalpy dominating and Enthalpy control. The linear plots obtained by plotting logk as function of D represent the different electrostatic interaction for the ion-dipole as well as dipole - dipole reaction. Solvating power of MEOH change the mechanism of the reaction from bio-molecular to unimolecular due to increase in number of water molecule associated with activated complex (2.08 to 2.56).
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Citation: Singh AK (2017) Kinetics and Solvent Effect in Hydrolysis of Ethyl Cinnamate in Water-Methanol Mixture. J Phys Chem Biophys 7: 252. Doi: 10.4172/2161-0398.1000252
Copyright: © 2017 Singh AK, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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