MEDIUM EFFECT ON ACTIVATION PARAMETERS FOR THE KINETICS OF REACTION BETWEEN β - BROMOPROPIONATE AND THIOSULFATE IONS

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1 MEDIUM EFFECT ON ACTIVATION PARAMETERS FOR THE KINETICS OF REACTION BETWEEN β - BROMOPROPIONATE AND THIOSULFATE IONS Zahida Khalid Rehana Saeed and Fahim Uddin * Department of Chemistry, University of Karachi, Karachi, 75270, Pakistan الخلاصة : سوف ندرس ف ي ه ذا البح ث ميكانيكي ة تفاع ل بروموبروبيوني ت B م ع أيون ات الثيوس لفيت عن د 333 K 283 في عدة مخاليط من الماء ن بروبانول. وقد قمنا بدراسة أثر الشدة الا يونية وثابت العزل على معدل التفاعل. فا ولنا التغيرات في معدل التفاعل وعوامل التنشيط على أساس الا ذابة وخواص العزل في الوسط. وتم حساب آل من طاقة التنشيط الحرة ) # E ) الانث البي ) # H ) و الانتروب ي ) # S ) ودراس ة تغي ر ه ذه الق يم م ع ال شدة الا يوني ة وثاب ت الع زل. وق د أظه رت النت اي ج أن ت عزى المعقدات المن شطة إلى توجيه المرآب إلى الحال ة الن شطة ول يس إل ى التفاع ل الكهرب اي ي ب ين الايون ات. وق د تحققنا من العلاقة الحرآية. ومن خلال تحليل طاقة التنشيط الانثالبي والانتروبي وطاقة الاستقطاب ظهر أن ذوبان الحالة الانتقالية يكون آبير ا عندما يكون ثابت العزل أآبر والذي ي حد من عملية تكون الروابط. *To whom correspondence should be addressed. fahim_uddin01@yahoo.com Paper Received 2 September 2002; Revised 12 April 2004; Accepted 5 January 2005 July 2006 The Arabian Journal for Science and Engineering, Volume 31, Number 2A 167

2 ABSTRACT The kinetics of the reaction between β-bromopropionate and thiosulfate ions has been studied at 283 to 333 K in different n-propanol water mixtures. The effect of ionic strength and dielectric constant on the rate of reaction was determined. Variations in the rate as well as activation parameters have been interpreted on the basis of solvation and the dielectric properties of the medium. Thermodynamic parameters such as free energy of activation ( E # ), enthalpy of activation ( H # ), and entropy of activation ( S # ) have been evaluated as a function of ionic strength and dielectric constant. The results show that the formation of the activated complex is due to the orientation in the configuration at the activated state rather then the electrostatic interaction among ions. The isokinetic relation ship has been verified. An analysis of the activation energy, enthalpy, entropy and polarisation energy reveals that solvation of transition state is larger in higher dielectric constant value which restrict the bond formation process. Key Words: Kinetics, iso kinetic temperature, Dielectric constant, molar polarisation energy change, activation parameters. 168 The Arabian Journal for Science and Engineering, Volume 31, Number 2 A July 2006

3 MEDIUM EFFECT ON ACTIVATION PARAMETERS FOR THE KINETICS OF REACTION BETWEEN β - BROMOPROPIONATE AND THIOSULFATE IONS INTRODUCTION An extensive kinetic study of the reaction of β-bromopropionate and thiosulfate ions was made by La Mer et al. [1]. The reaction was found to be kinetically second order. CH 2 BrCH 2 COO S 2 O 3 CH 2 (S 2 O 3 )CH 2 COO + Br (1) d[s 2 O -- 3 ] / dt = k [CH 2 BrCH 2 COO ] [S 2 O 3 ] (2) The rate law shows first order dependence on β-bromopropionate and thiosulfate ions. Whereas d[s 2 O 3 ]/dt is the rate of disappearance of thiosulfate ion and k is the bimolecular rate constant. The reaction is free from side reactions as reported earlier [2]. The kinetics of SN 2 reaction of β- bromopropionate and thiosulfate ions in presence of lithium and sodium chloride were studied by F. Uddin et al. [3]. The kinetics of the same reaction and primary salt effect produced by potassium and cesium chloride were also investigated as a function of ionic strength and temperature in non aqueous medium [4]. The activation parameters determined were found to be influenced by nature of added salt. The reaction has also been studied in n-propanol water [5] and ethanol water mixtures [6], in which the thermodynamic parameters were evaluated as a function of ionic strength in isocomposition of solvent. The results showed that the energy of activation decreases with an increase in the ionic strength in iso-dielectric media. The nature of the transition state regarding the shape of the activated complex has been investigated. The radii of singled sphere activated complex was calculated and compared with the literature [7,8]. The values of change in enthalpy ( H # ) and entropy ( S # ) of activation were used to calculate the isokinetic temperature, which shows that change in solvation do not causes any change in the usual mechanism of the reaction. Present work is the extension of reported work [4 6], in which the kinetics has now been studied as a function of dielectric constant and ionic strength at various temperatures. Isokinetic relationship has been verified, the effect of solvent composition on the activation parameters, the natures of transition state regarding their electrostatic and non-electrostatic contribution of free energy change ( G # ) and the polarizability has also been investigated. EXPERIMENTAL All the chemicals such as thiosulfate, iodine, sodium hydroxide, and n-propanol used were extra pure (reagent grade) from E. Merk and used without further purification, whereas β-bromopropionate from Riedel Haen was used after filtration. The experimental procedure was the same as described earlier [4 6]. Rate was measured in 5% to 25% (v/v) n-propanol water mixtures. The values of dielectric constants for different aqueous propanol mixtures were taken from literature [7]. All experiments were carried out in a thermostatic bath and the reaction temperature was held constant within ± 0.2 o C. Rates at five temperatures such as 10, 20, 30, 40, and 50 o C and at three ionic strengths in the range of 0.024, 0.03, and mol dm 3 were determined. The ionic strengths were varied by increasing the amount of standard solution of bromopropionate and thiosulfate.the reaction was followed by titrating aliquot portions of the reaction mixture with standard iodine solution and the rate constant was calculated by using an integrated form of second order rate equation. The variation in the rate constants as a function of various temperatures, ionic strength and solvent compositions are determined. The values of rate constants are found to be reproducible within ±2%. The activation energy ( E), the Arrhenius frequency factor (A), the entropy of activation ( S # ) and free energy of activation ( G # ) in different media have been calculated in the usual manner. RESULTS AND DISCUSSION The reaction between β - bromopropionate and thiosulfate was found to follow second order rate law with same initial concentration of reactants. The reaction was studied at three ionic strengths , , and mol dm 3. The concentration of reactants were kept constant at various dielectric constants and temperatures 10 o, 20 o, 30 o, 40 o, and 50 o C respectively. Tables 1, 2, and 3 provide the data for rate constant of β-bromopropionate and thiosulfate reaction at various compositions of n-propanol in aqueous solution. The values of rate constants are found to decrease with the increase in ionic strength and concentration of propanol, this deviation from the BrØnsted theory is in accordance to the literature [4, 7 11]. The reason for such behavior is due to the greater influence of ionic orientation for July 2006 The Arabian Journal for Science and Engineering, Volume 31, Number 2A 169

4 regulating the reaction velocity. The probability of ideal orientation is found to be maximum at extreme dilution and higher value of dielectric constant. The probability of formation of a more polar transition state is favorable in a medium of high dielectric constant [12,13]. The rate constant shows linear variation with temperature at all ionic strengths and composition of n-propanol in water. Table 1. Experimental values of Rate constant for the Reaction Between Bromopropionate and Thiosulfate Rate constant k 10 2 (mol -1 dm 3 s -1 ) Temperature at solvent composition (v/v) º C 5% 10% 15% 20% 25% 10% 0.91± ± ± ± ± % 1.43± ± ± ± ± % 2.02± ± ± ± ± % 2.58± ± ± ± ± % 3.29± ± ± ± ±0.01 Ionic strength = mol dm 3 Solvent = Aqueous n propanol Table 2. Experimental Values of Rate Constant for the Reaction Between Bromopropionate and Thiosulfate Rate constant k 10 2 (mol -1 dm 3 s -1 ) Temperature at solvent composition (v/v) º C 5% 10% 15% 20% 25% 10% 5.98± ± ± ± ± % 8.31± ± ± ± ± % 9.82± ± ± ± ± % 14.45± ± ± ± ± % 21.17± ± 11.14± ± ±0.01 Ionic strength = mol dm -3 Solvent = Aqueous n-propanol Table 3. Experimental Values of Rate Constant for the Reaction Between Bromopropionate and Thiosulfate Temperature º C Rate constant k 10 2 (mol -1 dm 3 s -1 ) at solvent composition 5% 10% 15% 20% 25% 10% 3.83± ± ± ± ± % 5.59± ± ± ± ± % 8.24± ± ± ± ± % 10.52± ± ± ± ± % 12.70± ± ± ± ±0.02 Ionic strength = mol dm -3 Solvent = Aqueous n-propanol 170 The Arabian Journal for Science and Engineering, Volume 31, Number 2 A July 2006

5 Activation Energies The Arrhenius plots, i.e log k vs 1/T for different ionic strengths and dielectric constant, are shown in Figures 1 and 2. The values of energy of activation ( E a ) were obtained from the Arrhenius slopes i.e. d (log k)/d (1/T). The values of energy of activation as a function of ionic strength and dielectric constant of medium are given in Table 4. The activation energies are found to be effected both by solvent composition and ionic strength of the medium. The activation energy increases by increasing the proportion of organic component in the solvent mixtures, i.e. with decrease in dielectric constant of the medium. This is quite expected in the view of the fact that the optimum proportion of the solvent composition, leads to stronger solvation of two reacting ions of similar charges and hence results the greater stabilization of the activated complex than in the mixture of other compositions. A similar response was found for the reaction in ethanol and water mixture [6, 11, 12], where a distinct minimum energy of activation was found in 10% ethanol water medium. The energy of activation increases with increasing the ionic strength of the medium. The trend is normal as the rate decreases the activation energy increases. Ionic Strength = mol dm log k % propanol % propanol 15% propanol /T (K -1 ) Figure 1. Plot of log k vs 1/T 1.4 Ionic Strength = mol dm log k % propanol % propanol /T (K -1 ) Figure 2. Plot of log k vs 1/T The Enthalpy of Activation ( H # ), Pre-exponential Factor (A), Entropy of Activation ( S # ): The values of H # (change in enthalpy of activation ), S # (change in entropy of activation) and G # (change in free energy of activation were also calculated by usual relations [12 14]. Results reported in Table 4, show that greater enthalpy change H # of the reaction in low dielectric constant of the media, containing greater percentage of propanol. This is because of preferential solvation of the activated complex as compared to that of the reactant. The change in entropy of activation in transition state is calculated as a function of ionic strength and dielectric constant of media. The negative values of entropy of activation are obtained at each ionic strength and solvent composition. These values are also found to decrease with increase in concentration of reactants and propanol in the media, i.e. at higher dielectric July 2006 The Arabian Journal for Science and Engineering, Volume 31, Number 2A 171

6 media. This behavior is in accordance to the idea of solvation of activated complex explained by Grundwald [15]. The values of standard enthalpy H o and entropy S o [16] were deduced by using the expressions; S # = S o + 1/D (3) H # = H o + 1/D (4) Table 4. Thermodynamics and Other Parameters for Reaction in Various n-propanol Water Media. Temp. = 30 o C Ionic strength = mol dm -3 Solvent Composition % E a H # S # (J K -1 ) G # Ionic strength = mol dm -3 Solvent Composition % E a H # S # (J K -1 ) G # Ionic strength = mol dm -3 Solvent Composition % E a H # S # (J K -1 ) G # Temperature = 30 ºC Where S o and H o are the change in standard entropy and enthalpy of activation at infinite dielectric constants respectively, while D is the dielectric constant of the media. The values of standard enthalpy change H o, and standard entropy change S o as a function of ionic strength, obtained from Figures 3 and 4 are tabulated in Table 5. Free Energy of Activation ( G # ): The free energy of activation G # is calculated by Eyring s relation [12, 13] as a function of ionic strength and dielectric constant of the media. The free energy change contribution due to electrostatic interactions ( G # e.s) [12] was calculated by using the equation: G # e.s = N A Z a. Z b e 2 / D R ab (5) 172 The Arabian Journal for Science and Engineering, Volume 31, Number 2 A July 2006

7 Table 5. Values of H o and S o at Different ionic Strengths Ionic strength 10 2 mol dm -3-1 H o k J mol S o J K where N A is the Avagadro s number, Z a and Z b are the charges on ions a and b, e is the electrostatic charge, D is the dielectric constant and R ab is the radius of the activated complex obtained by usual relation ship [6,8,10]. The free energy contribution due to the non-electrostatic attractions ( G # n.e.s) was evaluated from the following relationship [12]: G # total = G # e.s + G # n.e.s (6) A linear plot between log k and 1/D Figure 5 with negative slope in verified by literature [6, 8, 10], indicates that the electrostatic and non electrostatic interactions are contributing to changes in the total free energy of the activation of the β-bromopropionate reaction. Table 6 shows an increase in the values of the electrostatic and non electrostatic free energy change with a decrease in the dielectric constant of the media, which is possibly due to the approach of like charges in the formation of activated complex. The values reported in Table 6, shows that the major contribution in the total free energy change is due to the configurational orientation of the ions in the formation of activated complex i.e. G # e.s. Ionic Strength = mol dm S # J mol /D Figure 3. Plot of S # vs 1/D y = x Ionic Strength = mol dm H # k J mol y = x /D Figure 4. Plot of H # vs 1/D July 2006 The Arabian Journal for Science and Engineering, Volume 31, Number 2A 173

8 Isokinetic Temperature The variations in H # and S # obey the Barclay Butler rule [9, 17, 18] at each ionic strength. A linear relationship between these quantities is given as: H # = H # o + β S #. A linear plot of H # against S # at ionic strength mol.dm -3 is shown in Figure 6. The slope of the line, i.e. d H # / d S #, is equal to β, which is refered to as isokinetic the temperature. For β-bromopropionate and thiosulfate reaction this has been calculated in different propanol water mixtures. The average value of isokinetic temperature calculated as 452 K, which suggests that the change in solvation does not causes any changes in the actual mechanism [6], while the H # o is found to be 119 k J mol Solvent = Aqueous propanol Temperature = 30 o C log k Ionic Strength = 0.024mol dm mol dm mol dm /D Figure 5. Plot of k vs 1/D Table 6. Calculated Values of Non Electrostatic and Electrostatic (Free Energy Change) in Various n-propanol Water Mixtures. % Composition Dielectric constant G e. s G n. e. s Temperature = 30 o C Ionic strength = mol.dm -3 Molar Polarization Energy Change The variation in molar polarization energy (P m ) [18 20] is calculated as a function of ionic strength in 20% aqueous propanol and ethanol system by the relation given below; P m = RTA (1 1/D) 1.8 E D (8) Where P m and E D are the isodielectric activation energy and molar polarisation energy respectively. A is the slope of log k against 1/D, N is the Avogadro number and G is the number which expresses the distribution of charges on a spherical molecule. The plots of log k against 1/D as function of ionic strength are shown in Figure 5, gives straight lines with a negative slope. A comparative values of molar polarization energy (P m ) in 20% propanol and ethanol system are tabulated in Table 7. According to data negative values are obtained in both solvent systems. The extent of polarization of transition state is greater in case of ethanol, which suggests that the transition states are more polarized than in the initial states in case of later [20]. This means that the increase in carbon chain in the solvent system has a negative effect on polarization of the transition state, as far as comparison in the values at different ionic strengths are concerned, it is 174 The Arabian Journal for Science and Engineering, Volume 31, Number 2 A July 2006

9 suggested from our results that the bond formation process is much favored at lower ionic strength of the media in both solvent systems. Table 7. Molar Polarization Energy in Different Solvent System Ionic strength x10 2 mol dm -3 20% propanol 20% ethanol Temperature = 30 o C Ionic Strength = mol dm H # k J mol -1 y = x R 2 = S # k J mol -1 Figure 6. Plot of H # vs S # CONCLUSIONS The kinetic studies of β-bromopropionate and thiosulfate ions carried out in aquo-n-propanol solution showed that the rate of reaction is higher in solution containing higher percentage of water, i.e. at higher value of dielectric constant. The rate is also influenced by the nature of solvent, where as rate constant significantly increases at lower ionic strength. The trend of variation in the values of activation parameters as a function of ionic strength in non aqueous solution is similar to that reported in literature. The results also indicate that the activated complex is more solvated than the reactants. The total free energy change is the contribution of both the electrostatic and non electrostatic interactions. More over higher values of non electrostatic free energy change appear to be associated with the configurational orientation of the ions in the formation of the activated complex. REFERENCES [1] V.K. LaMer and M.E. Kamner, Chemical Kinetics II. The Influence of Relative Position of Electric Charge and Reacting Group on the Velocity of the Bromopropionate Reaction, J. Am. Chem. Soc., 53 (1931), pp [2] A. Slator, The Chemical Dynamics of the Reaction Between Sodium Thiosulfate and Organic Halogen Compound. Part II, J. Chem. Soc., 87 (1905), pp [3] F. Uddin and Z. Khalid, Cationic Salt Effects on the Rate of Reaction Between β- Bromopropionate and Thiosulfate Ions in Presence of Lithium and Sodium Chlorides, Acta Cientif. Venezol., 43 (1992), pp [4] F. Uddin and Z. Khalid, Effect of KCl and CsCl Electrolytes in the Kinetics of Reaction between β-bromopropionate and Thiosulfate Ions, Egypt. J. Chem., 38 (1) (1995), pp [5] F. Uddin and Z. Khalid, Study of the Rates of Reaction between β- Bromopropionate and Thiosulfate Ions in Isodielectric Media of Aqueous n- Propanol, Islamic Acad. of Sciences, 5 (1990), pp July 2006 The Arabian Journal for Science and Engineering, Volume 31, Number 2A 175

10 [6] F. Uddin, Z. Khalid, and T. Kausar, Solvent Effect on the Kinetics of Reaction between β-bonobromopropionate and Thiosulfate ions. Activation Parameters and Shape of the Activated Complex, J. Nat. Sci. and Maths., 36 (1) (1996), pp [7] F. Uddin, H. Tahir, and Noor-un Nisa, Study of the Rates of Bimolecular Ionic Reaction between Dibromosuccinate and Hydroxide ions, Pak. J. Sci. Ind. Res., 44 (2001), pp [8] F. Uddin and H. Kazmi, A Study of the Shape of Activated Complex in Reaction between Potassium Peroxodisulfate and Potassium Iodide, Arab. J. Sc. Engg, 28 (2A) (2003), pp [9] J. F. Loetzee and C. D. Ritchie, Solvent-Solute Interactions. New York: Marcel Dekker, vol. 11, 1976, p [10] K. J. Laidler, Chemical Kinetics, 2 nd edn. New York: McGraw Hill, 1965, p [11] M.Ghaziuddin Ahmed and M.N. Azam, Influence of the Dielectric Constant of the Medium on the Specific Rate Constant of Iodide Persulfate Reaction, Pak. J. Sci. Ind. Res., 14 (1971), pp [12] B.C. Bag and M.N. Das, Medium Effects on the Activation Parameters of Alkaline Hydrolysis of Monomethyl Succinate Ion, J. Ind. Chem. Soc., 60 (1983), pp [13] K. J. Laidler and P. A. Landskroener, The Influence of the Solvent on Reaction Rates, Trans Faraday Soc., 52 (1956), pp [14] I. M.Campbell, An Examples Course in Reaction Kinetics. Glasgow: Blackie, 1980, pp [15] E. Grundwald, G.Baughman, and G. Kohnstam, The Sovation of Electrolytes in Dioxane Water Mixtures, as Deduced from the Effect of Solvent Change on the Standard Partial Molar Free Energy, J. Am. Chem. Soc., 82 (1960), pp [16] S. A.H. Zaidi and T.A. Alvi, A Thermodynamic Study of the Reaction between Two Hypoiodite Ions in Ethanol Water Mixed Solvents, J. Chem. Soc. Pak., 12 (2) (1990), pp [17] D. Sitamanikyam and E. V. Sundaram, and E. V. Sundaram, Kinetics of Reaction of n-propyl Bromide with Thiosulfate Ion in Dioxane Water Media, Indian. J. Chem., 8 (1970), pp [18] J. F. Coetzee and C. D. Ritchie, Solvent Solute Interactions. New York: Marcel Dekker, 1976, p [19] E. Grundwald, G. Baughmanand, and G. Kohnstam, The Effect of Solvent Change on the Standard Potential of Electrolytes, from Precision Measurement of the Activities of the Solvent Components. The NaCl Dioxane Water, J. Am. Chem. Soc., 80 (1958), pp [20] V. Skivastava, R. I. Singh, S. B. Singh, P. Paasad, and L. Singh, Kinetic Study of the Solvent Effect on Alkali Catalysed Hydrolysis of Dimethyl Phthalate in Aquo Dioxane, Aquo Acetone and Aquo DMSO Media, J. Ind. Chem. Soc., 67 (1990), pp The Arabian Journal for Science and Engineering, Volume 31, Number 2 A July 2006