Kinetics and Mechanism of EDTA-Catalyzed Oxidation of (S)-Phenylmercaptoacetic Acid by Chromium(VI)

Transcription

1 J. Ind. Eng. Chem., Vol., No. 5, (006) 7773 Kinetics and Mechanism of EDTACatalyzed Oxidation of (S)Phenylmercaptoacetic Acid by Chromium(VI) K. Sathiyanarayanan, C. Pavithra, and Chang Woo Lee* Department of Chemistry, Vellore Institute of Technology, Vellore 63 04, India *College of Environment and Applied Chemistry, Kyung Hee University, Yongin 44970, Korea Received March 5, 006; Accepted June 9, 006 Abstract: The conversion of (S)phenylmercaptoacetic acid to the corresponding sulfoxide was performed in 50 % (v/v) wateracetic acid mixture, in the presence of the disodium salt of ethylenediamminetetraacetic acid, the catalyst. The ionic strength had no appreciable effect on the reaction rate. The added Mn + retarded the rate considerably, suggesting a twoelectron transfer in the ratedetermining step. This notion is supported by the fact that added acrylonitrile had no effect on the rate of the reaction. The ratio k(d O) / k(h O) < clearly indicates a significant solvent isotope effect. Highly negative entropy (ΔS # ) values indicate a structured transition state. A mechanism is proposed involving the formation of a ternary complex, comprising ethylenediamminetetraacetic acid, Cr(VI), and (S)phenylmercaptoacetic acid, in a fast step. The complex hydrolyzes in a subsequent slow ratedetermining step, yielding the sulfoxide. Electronreleasing substituents in the phenyl ring accelerate the rate, while electronwithdrawing substituents retard the rate. Keywords: kinetics, mechanism, rate law, catalyst, oxidation Introduction ) Many oxidation reactions of (S)phenylmercaptoacetic acid have been studied extensively. The oxidations using bromamineb [], chloraminesb [], Ce(IV) [3], Mn (VII) [4], Cr(VI) [5], chloraminest [6], potassium peroxodisulphate [7], peroxydiphosphate [8], peroxomonophosphoric acid [9], pyridinium chlorochromate [], and Nchloronicotinamide [] as oxidants have been reported in the literature. The oxidation catalyzed by oxalic acid [5,] had been studied previously; now attempts have been made to study the ethylenediamminetetraacetic acid (EDTA)catalyzed reaction. Even though it is known that chromic acid can oxidize sulfides to their corresponding sulfoxides [5,,], the aim of the present study was to probe the role of EDTA in this type of oxidation. The metaledta catalyzed [3,4] and EDTAcatalyzed oxidations [5] of organic compounds have been reported previously. There are two possible roles played by EDTA: it may itself get oxidized, leading to a radical mechanism, or it may simply act as a catalyst To whom all correspondence should be addressed. ( sathiya_kuna@hotmail.com) without undergoing any change; we found that the latter situation occurs in the present study. Both roles have been reported in the case of oxalic acid [6,7]. Experimental Materials Potassium dichromate (AnalaR grade) and the disodium salt of EDTA were used without any further purification. Acetic acid was purified using a method similar to that reported by Weissberger [8]. All substituted (S)phenylmercaptoacetic acids and the parent compound were prepared using the method described by Gabriel [9]. Kinetic Analysis Kinetic measurements were performed in 50 % acetic acid/water (v/v) at temperature ranging from 93 to 308 K. To undertake the reaction at 93 K, a cryostat was used. All of the solutions were equilibrated at the required temperature. The reaction mixture was then prepared at that temperature and ml aliquots of the

2 78 K. Sathiyanarayanan, C. Pavithra, and Chang Woo Lee k Table. Effect of Varying [SPM], [Chromic acid] and [EDTA] on the Rate of the Reaction at 98 K Solvent: WaterAcetic Acid, 50 % (v/v) [SPM] [Chromic acid] 4 [EDTA] 4 k obs 4 mol dm 3 mol dm 3 mol dm 3 S dm 3 mol S Figure. Plot of log titre vs. time. [SPM] = M; acetic acidwater = 50 % (v/v); [EDTA] = 3 M; temperature = 98 K; [chromic acid] = 3 M; r = reaction mixture were pipetted out at convenient time intervals and quenched in ml of % KI solution. The liberated iodine was titrated against thiosulfate to a starch end point. The pseudofirstorder rate constants were evaluated from log titre vs. time plots. For all of the plots correlation coefficients (r) of at least were obtained. The pseudofirstorder rate constants (k obs ) obtained by the method of least squares afforded the secondorder rate constants (k ) when divided by [substrate]. Stoichiometry The stoichiometry of the reaction was determined by performing several sets of experiments with varying amounts of chromic acid in excess over [substrate]. The estimation of unreacted chromic acid showed that one mole of (S)phenylmercaptoacetic acid reacts with one mole of chromic acid. Figure. Plot of 4 + log k obs vs. + log [SPM]. [Chromic acid] = 3 M; acetic acidwater = 50 % (v/v); [EDTA] = 3 M; temperature = 98 K; S =.00; r = H CrO 4 + PhSCH COOH PhSCH COOH + H CrO 3 Product Analysis The reaction mixture was left to stand for a few hours under kinetic conditions. It was concentrated under reduced pressure. The residue was analyzed by IR spectroscopy. The following frequencies corresponding to the oxidized product were obtained: 30 (=S=O group), 70 (=C=O group), and 3430 cm (COOH group). The product was further confirmed by CoTLC. The yield of sulfoxide was 9 %, as determined by weight measurements of the reactant and product.

3 Kinetics and Mechanism of EDTACatalyzed Oxidation of (S)Phenylmercaptoacetic Acid by Chromium(VI) 79 Table. Effect of NaClO 4, Mn +, and Acrylonitrile on the Reaction Rate. [SPM] = 0.0 M; [Chromic Acid] = 0.00 M; [EDTA] = 0.00 M; Temperature = 98 K, Solvent: WaterAcetic Acid, 50 % (v/v). [NaClO 4] M [Mn + ] 3 M [Acrylonitrile] 3 M k obs 4 S Table 3. Effect of Solvent Composition on the Reaction Rate. [SPM] = 0.0 M; [Chromic Acid] = 0.00 M; [EDTA] = 0.00 M; Temperature = 98 K. Acetic acid : Water k obs 4 % 40 : : : : : 40 S Results and Discussion The reaction demonstrates a firstorder dependence on chromic acid, as is obvious from the linearity of the plot of log[chromic acid] vs. time (Figure ). The pseudofirstorder rate constants were determined from the graph. In Figure, the plot of log k obs vs. log[(s) phenylmercaptoacetic acid (SPM)] is linear, with a slope of unity, showing that the reaction is of first order in [SPM]. Table shows the effect of varying [SPM], [chromic acid], and [EDTA] on the rate of the reaction at 98 K. The reactivity decreases slightly as [chromic acid] increases. Such observations in chromic acid oxidations have been reported by several authors [0]. With the increase in the chromic acid concentration, a progressively small portion of the total amount exists in the form of the acid chromate ion, and the rate constant decreases upon increasing the Cr(VI) concentration. The second order rate constants at different [SPM] and [chromic acid] are listed in Table. The added EDTA increases the rate of the oxidation with a fractional order. This is obvious from the linearity of the plot of log k obs vs. log [EDTA], with a slope of (Figure 3). Table shows the effects of NaClO 4, Mn +, and acrylonitrile on the reaction rate. A free radical mechanism Figure 3. Plot of 4 + log k obs vs. 4 + log [EDTA]. [SPM] = M; Acetic acidwater = 50 % (v/v); [Chromic Acid] = 3 M; Temperature = 98 K; S = 0.599; r = is ruled out because the rate of the reaction is insensitive to the added acrylonitrile. The reaction does not exhibit any salt effect as it is evident from the rate constants at different ionic strengths, maintained by adding NaClO 4. The addition of Mn + retards the rate of the oxidation considerably, showing that the rate determining step involves a twoelectron transfer. Table 3 shows the effect of the solvent composition (mixtures of acetic acid and water) on the reaction rate. The reaction is favored by a low value of the relative permittivity. Figure 4 shows that a plot of log k obs vs /ε r is curved, with a tendency toward a positive slope. Table 4 indicates that heavy water (D O) affects the reactivity of the substrate. This isotope effect indicates the participation of water in the ratecontrolling step. The solvent isotope effect on the reaction rate is related to solvation effects in two solvents [3]. Table 5 shows the rate data and activation parameters for the EDTAcatalyzed oxidation of meta

4 730 K. Sathiyanarayanan, C. Pavithra, and Chang Woo Lee Table 4. Effect of D O on the Reaction Rate. [SPM] = 0.0 M; [Chromic Acid] = 0.00 M; [EDTA] = 0.00 M; Temperature = 98 K. Water : Acetic acid : D O k obs 4 % 50 : 50 : 0 48 : 50 : 46 : 50 : 4 S Figure 4. Plot of 4 + log k obs vs. /ε r. [SPM] = M; Acetic acidwater = 50 % (v/v); [Chromic Acid] = 3 M; Temperature = 98 K; [EDTA] = 3 M; S = 0.599; r = and parasubstituted (S)phenylmercaptoacetic acids by chromium(vi). Activation parameters, evaluated from Eyring s plot of log k obs /T vs. /T, are listed. The negative entropies of activation (ΔS # ) indicate that there is more order in the ratedetermining transition state relative to that in the reactant. The data in Table 5 reveals that electronreleasing substituents in the phenyl ring of (S)phenylmercaptoacetic acid enhance the rate, while electronwithdrawing substituents retard it. Figure 5 shows that the plot of log k vs. σ is linear, affording a ρ value of.503. This result suggests that the (S) phenylmercaptoacetic acid molecule becomes electron deficient in the ratedetermining step. This feature is in consonance with the proposed mechanism. The reaction series obeys the isokinetic relationship [4], as indicated by the linearity of the plot of log k (93 K) vs. log k (308 K) in Figure 6. Such a correlation indicates that all acids undergo oxidation through the same mechanism [4]. The substituents influence only the rate of the reaction and not the mechanism. From Table 5, the plot of ΔS # vs. ΔH # gives a straight line (not shown); the isokinetic temperature was found to be 307 K from the slope. Kinetic studies were undertaken by adding Co III, in the form of (Co(NH 3 ) 5 ) + Cl, a specific trapping agent for Cr II. The insensitivity of the reaction rules out the participation of Cr II in the reaction sequence. Howard and Levitt [5] reported that the sequence of the reaction taking place in sulfides is RSR RSOR. The conversion of a sulfide to a sulfoxide is faster than the conversion of a sulfoxide to a sulfone. This result supports the formation of sulfoxides in the present study. The absence of ionion interactions in the ratedetermining step is indicated by the insignificant rate variation with increasing ionic strength. The increase in the rate constant upon increasing the acetic acid content in the solvent suggests facile reactivity in a medium of low dielectric constant. In general, such rate enhancement in a less polar solvent can be attributed to the formation of Cr(VI) esters [6,7]. The formation of a ternary complex involving EDTA, Cr(VI), and (S)phenylmercaptoacetic acid is also found in a medium of low permittivity. Even though reports of both Cr(VI)EDTA mononuclear and binuclear complexes appear in the literature [8], in the present investigation the reaction proceeds through a Cr(VI)EDTA mononuclear complex. The structure of this complex has been characterized [9]. During Table 5. Rate Data and Activation Parameters for the EDTACatalyzed Oxidation of meta and parasubstituted (S) Phenylmercaptoacetic Acid by Chromium(VI) No Substituent H pch 3 pcl mch 3 mcl ΔS # k (l mol sec ) ΔH # 93 K 98 K 303 K 308 K kj mol J K mol

5 Kinetics and Mechanism of EDTACatalyzed Oxidation of (S)Phenylmercaptoacetic Acid by Chromium(VI) 73 Figure 5. Plot of + log k vs. σ. [SPM] = M; acetic acidwater = 50 % (v/v); [chromic acid] = 3 M; temperature = 98 K; [EDTA] = 3 M; ρ=.503; r = Figure 6. Plot of + log k (308 K) vs. 3 + log k (93 K). [SPM] = M; acetic acidwater = 50 % (v/v); [chromic acid] = 3 M; [EDTA] = 3 M; r = the oxidation, the complex may form another ternary complex of type C. Ternary complexes of a similar type have been suggested in the EDTAcatalyzed Cr(VI) oxidation of hydrazine [830] and EDTAcatalyzed chloramineb oxidative chlorination of αphenylpropionic acid [3]. The ternary complex formation is envisaged in the Cr(VI) oxidations of I [3], isopropyl alcohol [6], (S)phenylmercaptoacetic acid [5,], and cycloalkanes [33] in the presence of oxalic acid. Mechanism and Rate Law Blank experiments ruled out the oxidation of EDTA under the experimental conditions. Firstorder dependence on the concentration of (S)phenylmercaptoacetic acid (SPM), fractionalorder dependence on the concentration of EDTA, facile reactivity in the lowpermittivity medium, the absence of free radicals, and the presence of a solvent kinetic effect during the course of the reaction are all consistent with the following scheme. Thus, this rate law will characterize the reaction system. Conclusions The kinetics of the oxidation of (S)phenylmercaptoacetic acid catalyzed by EDTA have been studied systematically. A mechanism involving the hydrolysis of a ternary complex, comprising EDTA, Cr(VI), and (S)phenylmercaptoacetic acid, in the slow ratedetermining step is proposed. The product is the corresponding sulfoxide. A solvent isotope effect was observed in the reaction. The orders with respect to (S)phenylmercaptoacetic acid and Cr(VI) are both one, whereas the order with respect to EDTA is fractional. EDTA plays the role of a catalyst in this reaction. The

6 73 K. Sathiyanarayanan, C. Pavithra, and Chang Woo Lee negative sign of the entropy change suggests that the transition state is more orderly when compared with the reactants. Acknowledgment We express our gratitude to Mr. G. Viswanathan, the Chancellor, Vellore Institute of Technology, for providing the necessary facilities to undertake this research. References. R. Gurumurthy, K. Sathiyanarayanan, and M. Gopalakrishnan, Bull. Chem. Soc. Jpn., 65, 96 (99).. R. Gurumurthy, K. Sathiyanarayanan, and M. Gopalakrishnan, Int. J. Chem. Kinet., 4, 953 (99). 3. R. Gurumurthy, K. Sathiyanarayanan, and M. Gopalakrishnan, Tetrahedron, 50, 373 (994). 4. R. Gurumurthy and M. Gopalakrishnan, Indian J. Chem. Sect A, 5, 476 (986). 5. R. Gurumurthy, T. Anandabaskaran, and K. Sathiyanarayanan, Oxidation Commun.,, (998). 6. C. Srinivasan and K. Pitchumani, Bull. Chem. Soc. Jpn., 55, 89 (98). 7. C. Srinivasan and K. Pitchumani, Indian J. Chem. Sect A, 7, 6 (979). 8. C. Srinivasan and K. Pitchumani, Int. J. Chem. Kinet., 4, 789 (98). 9. G. P. Panigraghi and R. N. Nayak, Curr. Sci., 49, 740 (980).. G. Mangalam and S. P. Meenakshisundaram, Pol. J. Chem., 7, 58 (998).. K. Sathiyanarayanan, R. Suseela, and Chang Woo Lee, J. Ind. Eng. Chem.,, 80 (006).. C. Karunakaran and V. Chidambaranathan, Croatica Chemica Acta, 74, 5 (00). 3. M. M. Taqui Khan and R. S. Shukla, J. Mol. Catal., 7, 57 (99). 4. M. M. Taqui Khan and R. S. Shukla, J. Mol. Catal., 7, 6 (99). 5. S. Meenakshisundaram and R. Vinothini, Croatica Chemica Acta, 76, 75 (003). 6. J. Rocek and F. Hasan, J. Am. Chem. Soc., 94, 38 (97). 7. L. A. Radkowsley and J. Rocek, J. Am. Chem. Soc., 90, 968 (968). 8. A. Weissberger and E. S. Prabakaran, Organic Solvent Physical Properties and Methods of purification, nd Ed., p. 30, Interscience Publishers, London (955). 9. Gabriel, Ber.,, 939 (879). 0. G. V. Bakore and L. L. Jain, J. Inog. Nucl. Chem., 3, 805 (969).. G. Mangalam and S. P. Meenakshisundaram, J. Indian Chem. Soc., 68, 77 (99).. K. B. Wiberg, Ed., Oxidation in Organic Chemistry, p. 78, Academic Press, New York (965). 3. A. L. Nolan, R. C. Burns, and G. A. Lawrance, J. Chem. Soc., Dalton Trans., 304 (998). 4. O. Exner, Nature (London),, 488 (955). 5. E. Howard and L. S. Levitt, J. Am. Chem. Soc., 75, 67 (973). 6. F. H. Westheimer, Chem. Rev., 45, 49 (949). 7. S. P. Meenakshisundaram and R. M. Sockalingam, Collect. Czech. Chem. Commun., 66, 877 (00). 8. V. M. Sadagoparamanunjam, S. Sundaram, and N. Venkatasubramanian, Inorg. Chim. Acta 3, 33 (975). 9. P. Uma, K. Rao, and M. N. Sastry, React. Kinet. Catal. Lett., 39, 55 (989). 30. M. T. Beck and D. A. Durham, J. Inorg. Nucl. Chem., 33, 46 (97). 3. Satyendran, Ph. D. Thesis, Annamalai University (999). 3. G. Vandergrift and J. Rocek, J. Am. Chem. Soc., 98, 37 (976). 33. S. P. Meenakshisundaram and R. M. Sockalingam, Bull. Chem. Soc., Jpn., 74, 43 (00).