Solvent play an important role in determining chemical reactivity. In particular, the rate of an elementary chemical reaction may change by orders of magnitude when solvent is changed. Kinetic solvent effect on chemical reaction in different media is correlated in terms of “solvent polarity” which is sums of all the specific and non-specific interaction of the media with initial and transition state”. A large effort has been directed towards understanding such solvent effect and a great deal of progress has been made recently [1-3] both theoretically and experimentally. However, the interpretation of kinetic result on the solvent polarity on medium some time fails and sometime succeeds. In this note, we have extended our studies on kinetic solvent on well-known base catalyzed hydrolysis of Ethyl cinnamete, which is important for use of adding to cigarettes and cut tobacco, that are used for flavor agent and fragrance agent.

The kinetics of base catalyzed hydrolysis of ethyl cinnamate has been carried out in water-acetone and having different concentration of solvent (acetone), varying from 30 to 70% (v/v) at five different temperature ranging from 20°C to 40°C at regular interval of 5°C. The specific rate constant calculated using second order reaction was found decrease with increase of acetone content, tabulated in Table 1 The iso dielectric Activation energy (E_D) is also calculated with help of slope of logkD against 103/T at constant and tabulated in Table 2. The iso kinetic temperature was calculated with help of Barclay and Butler relation. The evaluated thermodynamic activation parameter has been tabulated in Table 3.

Table 1: Specific rate constant k Ã� 10³ (dm)³/mole/mint values of alkali catalyzed Hydrolysis of Ethyl-Cinnamate in water-Acetone.

Table 2: 3+Logk_D Value against 10³/T, Water- Acetone System.

Table 3: Values of Iso-Dielectric Activation Energy (water-Acetone media).

Solvent effect on rate of reaction

In order to discuss the variation of specific rate constant values with gradual addition of dipolar aprotic organic solvent (acetone) in reaction media, the value logk tabulated in Table 1. The decrease observed in specific rate constant (Table 1), with increasing concentration of organic solvent is probably due to depletion of Dielectric constant of media and salvation change taking place in media [4].

Solvent effect and thermodynamic activation parameters

The thermodynamics Activation parameter such as Enthalpy of Activation (ΔH^*), Entropy of Activation (ΔS^*) and free energy of Activation (ΔG^*), are better indicator of the solvent effect exerted by the solvent on the solvolysis reaction. These thermodynamics Activation parameter has been calculated with help of Wyne-Jones Eyring [5] equation and absolute rate theory [6] and placed in Table 4.

Table 4: Thermodynamics Activation Parameters of the Reaction in Water- Acetone Media (?H^*and ?G^* in KJ/Mole, ?S^* in J/K/Mole).

On careful observation of the data Tabulated in Table 4 (wateracetone) ΔG^* value in case of water-acetone increases from 92.09 to 94.26 KJ/mole with change of proportion of solvents from 20% to 70% at 20°C. Though the enhancement is not large, however it is quite considerable (Figure 1). The report of Yagil et al. [7] indicate that OH? ion of NaOH is hydrated with three molecules of water. In mixed aquo-organic co-solvent mixture, the organic component molecules are expected to contribute to the process of desolvation as they may be regarded as poor anion solvater in comparison to water. A similar variation in ΔG^* values with increasing mole % of organic co solvent has been reported by Tommila [8] and Cleave [9] and recently Singh [10]. The variation of ΔG^* with mole % in the solvent are non-linear and increase smoothly with gradual addition of the solvent (Figure 2).

Figure 1: Variation of 3+Logk_D with 10³/T.

Figure 2: Variation of G^* with mole %.

This finding is indicative of salvation or desolvation of reactant as explained by Absolute Reaction rate theory and supported by Elsemongy [11] and Singh [12]. From data mention in the Table 3, the interesting feature comes in the light is that out of all the value of three thermodynamic parameters ΔH^*, ΔG^* and ΔS^*, the value of ΔH^* and ΔS^* goes on decreasing with simultaneous increase in G^* values with increasing proportion of acetone at all the temperature of the reaction, i.e., from 20°C to 40°C. From the fundamental thermodynamic equation ΔG^* =ΔH^* –TΔS^*

It may be inferred that the enhancement in the value of ΔG^* with simultaneous depletion in ΔH^* and ΔS^* value is only possible when the quantitative depletion in the value of ΔS^* is less than that found in the values of ΔH^* and from this fact, it is concluded that base catalyzed hydrolysis of Ethyl Cinnamate in water-acetone media is Enthalpy dominating and Entropy controlled. In another word, it may also be inferred that in presence of acetone in the reaction media activate the Enthalpy and control over the Entropy of the reaction.

The change found in the value of three thermodynamic activation parameters also support the fact that initial state of the reaction is desolvated and the transition state is solvated in the similar ways as recently reported [13-15]. The small but considerable increase in ΔG^* as shown in Table 3 and non-linear variation in ΔS^* curve with mole % of acetone shown in Figures 3 and 4 are indication of specific salvation taking place in the process of activation according to Seville and Hudson [13]. To explain the solvent effect on the thermodynamic parameters these three values namely ΔG^*, ΔH* and ΔS^* were plotted against mole % of acetone in reaction shown in Figures 2-4 respectively. The nonlinear variation in ΔH^* and ΔS^* give information specific salvation taking place in water-acetone as reported by Saviil et al. [13].

Figure 3: Variation of H^* with mole %.

Figure 4: Variation of ΔS^*+200 with mole %.

Solvent effect on the iso- dielectric activation energy of activation

From the slope of Arrhenius plot (obtain from interpolation of logkD value against 103/T at constant D of reaction media), the value of Iso dielectric activation energy of reaction have been evaluated and represented in Table 2. It is obvious that ED value goes on decreasing from 52.43 to 47.28 KJ/mole with increasing in D value of water – Acetone media, is similar trend to recent finding (Figure 1) [14].

Evaluation of Iso-kinetic temperature and solvent-solute interaction

In the light of Barclay and Butler [15] relationship between Enthalpy and Entropy of Activation, which is as follows

δm (ΔH^*) = βδm(ΔS^*)

It is straight-line equation representing the relationship between Enthalpy and Entropy of Activation. β is the Iso-kinetic temperature. From the data available in the Table 4 the plot of ΔH^* and ΔS^* in the solvent system in Figure 5 and from the slope of straight line, the value of Iso-kinetic temperature was calculated to be 244 (water-acetone) and which is less than 300. From the value of Iso-kinetic temperature which is much less than 300, it is concluded that there is slow change in the structure of the reactant or in the solvent or in due to weak interaction between solvent and solute present in the reaction media in similar way as reported by Lefler [16] Our this conclusion has been recently supported (Figures 2-5) [17].

Figure 5: Variation of ?H^* with ?S^*+200 at 30°C.

The result of this work indicate that the rate of hydrolysis of Ethyl cinnamate decreasing trend at all temp witch appear that transition state is more desolveted than initial state. The enhancement in the value of and ΔG^* with simultaneous depletion in ΔH^* ΔS^* for the hydrolysis ethyl inanimate in water- acetone 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. The much less value (>300) of iso kinetic Temps indicate weak interaction between solvent and solute.

My special gratitude to my supervisor Prof. Singh RT for his proper guidance and thanks to my friend Dr Ajit, Dr. Parag and Mr. Tiwari LK for his cooperation during preparation of this content. I am Thankful to Mrs Rolly Gupta & Miss Akansha for typing assistance.