Mechanistic Study on the Oxidation of Sulfacetamide by Aqueous

Dec 5, 2008 - E-mail: [email protected]. ... and at a constant ionic strength of 0.50 mol/dm3, has been carried out spectrophotometrically at 36...
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Ind. Eng. Chem. Res. 2009, 48, 591–597

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APPLIED CHEMISTRY Mechanistic Study on the Oxidation of Sulfacetamide by Aqueous Alkaline Diperiodatoargentate(III) Suresh D. Kulkarni, Praveen N. Naik, and Sharanappa T. Nandibewoor* P. G. Department of Studies in Chemistry, Karnatak UniVersity, Dharwad 580 003, India

Oxidation of sulfacetamide (SUL), a sulfonamide drug by alkaline diperiodatoargentate(III) (DPA), a powerful oxidizing agent at 298 K and at a constant ionic strength of 0.50 mol/dm3, has been carried out spectrophotometrically at 360 nm. The results indicate that 3 mol of DPA consumed 1 mol of SUL (3:1). The oxidation product has been separated and characterized by IR and NMR spectral studies. The reaction is first order in [DPA] and has less than unit order in [SUL]. The rate constants increased with an increase in alkali concentration and decreased with increase in [IO4-]. Ionic strength and dielectric constant of the medium had negligible effect on the reaction rate. A mechanism has been proposed which explains the observed orders and experimental observations. Monoperiodatoargentate(III) (MPA) has been considered as the active species for the title reaction. The reaction proceeds through a SUL:MPA complex which decomposes in a slow step to give the p-hydroxylamine benzenesulfonamide and Ag(I) species. Further oxidation in the subsequent fast steps yeilds nitroso and nitro derivative of benzenesulfonamide, each transformation consuming 1 mol of MPA. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to the slow step of the mechanism are computed and discussed, and thermodynamic quantities are also determined. The probable active species of oxidation have been identified. Introduction Sulfonamides are important bacteriostatic agents still commonly used in human and veterinary medicine. They are excreted from the human body and animal organisms partially unmetabolized and also as biotransformation products. Expired and unused drugs containing sulfonamides are also introduced into wastewater from the households. Because of these facts, traces of sulfonamides are most frequently found in almost all kinds of surface water.1-4 Significant amounts of these compounds are introduced to surface water, through effluent from waste dumps.5 However, Halling-Sørensen et al.1 claim that sulfonamides are resistant to biodegradation. They have a long lifetime in the environment and can be accumulated in various organisms of the food chain.1,2 Although sulfonamides are present in the environment at low levels, they may cause pathogenic bacteria drug resistance to these compounds. This necessitates development of various advanced oxidation processes for the transformation of sulfonamides in water. Some of the advanced oxidation processes used have their own drawbacks. Chlorination may create and leave disinfection byproducts, and ozonation can form carcinogenic bromate ion by reacting with bromide present in water. The photocatalytic process in the presence of TiO2 is expensive and timeconsuming. Therefore, such processes are recommended only for the biodegradation resistant substances. One class of compounds repeatedly found at concentrations ranging from 0.13 to 1.9 cg/L1,4,6,7 is a group of antibiotics known as the sulfa drugs or sulfonamides, which are used in aquaculture,8 as agricultural herbicides,7,9 as a preventative measure for veterinary purposes,6,7 and in the treatment of respiratory and urinary tract infections in humans.10 * To whom correspondence should be addressed. Tel.: +91 0836 2215286. Fax: +91 0836 2747884. E-mail: [email protected].

Diperiodatoargentate(III) (DPA) is a powerful oxidizing agent in alkaline medium with a reduction potential11 of 1.74 V. It is widely used as a volumetric reagent for the determination of various organic and inorganic species.12 Jayaprakash Rao13 et al. have used DPA as an oxidizing agent for the kinetics of oxidation of various organic substrates. They normally found that order with respect to both oxidant and substrate concentrations was unity, and [OH-] was found to enhance the rate of reaction. It was also observed that they did not arrive at the possible active species of DPA in alkali, and on the other hand, they proposed mechanisms by generalizing the DPA as [Ag(HL)L](x+1)-. However, Anil Kumar14 et al. put an effort to give an evidence for the reactive form of DPA in the large scale of alkaline pH. DPA is a metal complex with Ag in +3 oxidation state like Cu3+ in diperiodatocuprate(III) (DPC) and Fe3+ in hemoglobin and Ni4+ in diperiodatonickelate(IV) (DPN). In the process Ag(III) is reduced to Ag(I) which can be easily separated in the form of AgCl and its concentration lowered to less than 0.1 mg/L (maximal admissible concentration in waste waters). The literature survey reveals that there are no reports on mechanistic studies of sulfacetamide (SUL) oxidation by DPA. Thus, sulfacetamide has been selected as a substrate in order to explore its oxidation by DPA in alkaline medium and to check the reactivity of sulfacetamide toward DPA. The present investigation in this chapter aimed to investigate the kinetics of redox chemistry of the Ag(III) in such media and to arrive at a plausible mechanism. Experimental Details Materials and Reagents. All chemicals used were of reagent grade and double distilled water was used throughout the work. A solution of sulfacetamide (Yarrow-chem products Mumbai) was prepared by dissolving a known amount of its sodium salt in double distilled water. The required concentration of SUL

10.1021/ie8000474 CCC: $40.75  2009 American Chemical Society Published on Web 12/05/2008

592 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009

was used from its aqueous stock solution. A stock solution of IO4- was prepared by dissolving a known weight of KIO4 (Riedel-de Haen) in hot water, the stock solution was used after 24 h. Its concentration was ascertained iodometrically15 at neutral pH maintained using phosphate buffer. KNO3 and KOH were used to maintain ionic strength and alkalinity of the reaction respectively. Aqueous solution of AgNO3 was used to study the product effect, Ag(I). Preparation of DPA. DPA was prepared by oxidizing Ag(I) as described elsewhere:11 The mixture of 28 g of KOH and 23 g of KIO4 in 100 cm3 of water along with 8.5 g AgNO3 was heated just to boiling and 20 g of K2S2O8 was added in several lots with stirring then allowed to cool. It was filtrated through a medium porosity fritted glass filter and 40 g of NaOH was added slowly to the filtrate, whereupon a voluminous orange precipitate agglomerates. The precipitate is filtered as above and washed three to four times with cold water. The pure crystals were dissolved in 50 cm3 water and warmed to 80 °C with constant stirring thereby some solid was dissolved to give a red solution. The resulting solution was filtered when it was hot and on cooling at room temperature, the orange crystals separated out and were recrystallized from water. The complex was characterized from its UV spectrum and exhibited three peaks at 216, 255, and 362 nm. These spectral features were identical to those reported earlier for DPA.16 The magnetic moment study revealed that the complex is diamagnetic. The compound prepared was analyzed17 for silver and periodate by acidifying a solution of the material with HCl, recovering and weighing the AgCl for Ag and titrating the iodine liberated when excess KI was added to the filtrate for IO4-. Prepared DPA is quite stable in water under our kinetic conditions. The aqueous solution of DPA was used for the required [DPA] in the reaction mixture. During the kinetics a constant concentration viz. 1.0 × 10-5 mol/dm3 of KIO4 was used throughout the study unless otherwise stated. Thus, the possibility of oxidation of SUL by periodate was also tested and found that there was no significant interference due to KIO4 under experimental condition. Kinetic Studies. The kinetics was followed under a pseudofirst order condition, [SUL] . [DPA] at 25 ( 0.1 °C unless specified. The reaction was initiated by mixing the required quantities of thermostatted solutions of sulfacetamide and DPA, which also contained definite quantities of IO4-, KOH, and KNO3 to maintain the required alkalinity and ionic strength. The total [OH-] was calculated considering the KOH in DPA as well as additionally added. Similarly, the total [IO4-] concentration was calculated by considering IO4- present in DPA solution and additionally added. The progress of reaction was followed by measuring the absorbance of unreacted DPA in the reaction mixture present in 1 cm cell in a thermostatted compartment of a Varian CARY 50 Bio UV-vis spectrophotometer at 360 nm. It was verified that there is a negligible interference from other species present in the reaction mixture at this wavelength. A Peltier accessory was used to control the temperature of cell holder and cell. The obedience of Beer’s law by DPA at 360 nm had been verified, giving a molar absorbance coefficient of 13900 ( 100 dm3/(mol cm). The effect of dissolved oxygen on the rate of reaction was studied by preparing the reaction mixture and following the reaction in an atmosphere of the nitrogen. No significant difference between the results was observed in presence and absence of nitrogen. In view of ubiquitous contamination of basic solutions by carbonate, the effect of carbonate on the reaction was also studied. Added carbonate

had no effect on the reaction rate. However, fresh solutions were used during the experiments. In view of the modest concentration of alkali used in the reaction medium, attention was also given to the effect of the surface of the reaction vessel on the kinetics. The use of polythene or acrylic ware and quartz or polyacrylate cells gave the same results, indicating that the surface had no effect on the rate. However, fresh solutions were used during the experiments. Regression analysis of experimental data to obtain the regression coefficient, r, and standard deviation, S, of points from the regression line was performed using the Microsoft Excel 2003 program. Results Stoichiometry and Product Analysis. Different sets of reaction mixtures containing varying ratios of DPA to SUL in the presence of constant amount of OH-, IO4-, and KNO3 were kept for 3 h at 298 K in a nitrogen atmosphere in a closed vessel. The remaining concentration of DPA was estimated spectrophotometrically at 360 nm. The results indicate that 2 mol of DPA were consumed by 1 mol of SUL (3:1) as in eq 1.

The acidified reaction mixture was concentrated and extracted with ether. This ether layer was subjected to column chromatography using a mixture of benzene (65%) and ethylacetate (35%) as mobile phase. 4-Nitrobenzenesulfonamide was identified as the main oxidation product by IR and NMR spectral studies. The IR spectrum, showed a (-NH2) stretch at 3348 cm-1 (asymmetric) and 3325 cm-1 (symmetric) and -NO2 stretching was observed at 1355 cm-1 (symmetric) and 1531 cm-1 (asymmetric). Further 4-nitrobenzenesulfonamide was characterized by its NMR spectrum (CDCl3); δ 8.24 ppm (d, Ar-2H (a)), δ 7.89 ppm (d, Ar-2H (b)), δ 7.66 ppm (s, NH2). It was observed that product does not undergo further oxidation under the present kinetic conditions. The formation of free Ag+ in solution was detected by adding KCl solution to the reaction mixture, which produced white turbidity due to the formation of AgCl. Reaction Orders. The reaction orders with respect to sulfacetamide, alkali, and periodate concentrations were determined from the slope of log kobs versus log(concentration) plots by varying the concentrations of one reactant at a time keeping all other concentrations and conditions constant. The [DPA] was varied as in Table 1. The linearity and parallelism of the plots of log(absorbance) versus time up to 80% completion of the reaction (Figure 1) indicates a reaction order of unity in DPA concentration. Constant values of rate constant also confirmed the order with respect to [DPA] as unity (Table 1). The [SUL] was varied in the range 5.0 × 10-4 to 5.0 × 10-3 mol/dm3 at 25 °C. The kobs values increased with the increase in [SUL] (Table 1) and found an apparent less than unit order dependence on [SUL] (r g 0.9842, S e 0.0009). The effect of alkali on the reaction rate was studied in the range of 0.05-0.50 mol/dm3. The rate constants increased with increasing [OH-] (Table 1), and the order was found to be less than unity (r g 0.9808, S e 0.00081). The [IO4-] effect was studied in the concentration range from 1.0 × 10-5 to 1.0 × 10-4 mol/dm3 at constant DPA

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 593 -

Table 1. Effect of DPA, SUL, Alkali, and IO4 Concentrations on DPA Oxidation of Sulfacetamide in Alkaline Medium at 25 °Ca [DPA] × 105 (mol/dm3)

[SUL] × 103 (mol/dm3)

[OH-] (mol/dm3)

[IO4-] × 105 (mol/dm3)

kobs × 103 (1/s)

1.0 2.0 4.0 8.0 10 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

5.0 5.0 5.0 5.0 5.0 0.5 1.0 2.0 4.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.05 0.1 0.2 0.4 0.5 0.4 0.4 0.4 0.4 0.4

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 4.0 8.0 10.0

5.03 5.05 5.10 5.09 5.06 1.05 1.89 3.00 4.41 5.05 2.97 3.55 4.22 5.05 5.17 5.03 4.43 3.72 2.93 2.73

a

Scheme 1

I ) 0.50 mol/dm3. Table 2. Thermodynamic Activation Parameters for DPA Oxidation of SUL in Aqueous Alkaline Medium with Respect to the Slow Step of Scheme 1 (a) Effect of Temperature

(b) Activation Parameters

T (K)

k × 103 (1/s)

parameters

values

298 303 308 313

8.40 8.67 8.99 9.42

Ea (kJ/mol) ∆Hq (kJ/mol) q ∆S (J/(K mol)) ∆Gq (kJ/mol)

5.9 ( 0.3 3.4 ( 0.4 -273 ( 22 84.8 ( 5.0

(c) Effect of Temperature on Equilibrium Constants K4, K5, and K6 for the Oxidation of SUL by Diperiodatoargentate(III) in Alkaline Medium T (K) K4 × 10 (dm3/mol) K5 × 104 (mol/dm3) K6 × 10-2 (dm3/mol) Figure 1. First order plots of oxidation of sulfacetamide by DPA in aqueous alkaline medium at 25 °C. [SUL] ) 5.0 × 10-3, [OH-] ) 0.40, and [IO4-] ) 1.0 × 10-5; I ) 0.50 mol/dm3. [DPA] × 10-5 mol/dm3: (1) 1.0, (2) 3.0, (3) 5.0, (4) 8.0, (5) 10.

and SUL concentrations and at constant ionic strength. The rate constants decreased by increasing [IO4-]. The order with respect to periodate concentration was negative and less than unity. The apparent less than unit order with respect to different reactants suggests a complex mechanism as explained in the Discussion section. Effect of Ionic Strength and Initially Added Products. The effect of ionic strength was studied by varying [KNO3] in the range of 0.50-1.0 mol/dm3 at constant concentrations of DPA, sulfacetamide, periodate, and alkali. Ionic strength had negligible effect on the rate constant. Initially added product, such as Ag(I) had no effect on the rate of reaction. Effect of Solvent Polarity. The effect of relative permittivity (D) was studied by varying the t-butanol-water (v/v) content in the reaction mixture with all other conditions being maintained constant. Decreasing the dielectric constant of the medium had no effect on the rate of the reaction. Test for Free Radicals (Polymerization Study). The intervention of free radicals was examined as follows: the reaction mixture, to which a known quantity of acrylonitrile scavenger had been added initially, was kept in an inert atmosphere for 1 h. Upon diluting the reaction mixture with methanol, no precipitate resulted, suggesting the absence of free radicals in the reaction.

298 303 308 313

1.58 ( 0.004 2.41 ( 0.06 2.96 ( 0.09 3.84 ( 0.12

5.60 ( 0.14 3.70 ( 0.11 2.35 ( 0.10 1.70 ( 0.06

3.47 ( 0.13 4.71 ( 0.16 7.57 ( 0.21 11.51 ( 0.33

(d) Thermodynamic Quantities Using K4, K5, and K6 thermodynamic quantities

values from K4

values from K5

values from K6

∆H (kJ/mol) ∆S (J/(K mol)) ∆G298 (kJ/mol)

44 ( 2 135 ( 19 3.44 ( 0.1

-62.0 ( 1.0 -272 ( 12 20.6 ( 0.7

63.0 ( 3.5 260 ( 11 -16.3 ( 0.7

Effect of Temperature. The influence of temperature on the kobs values were studied at 25, 30, 35, and 40 °C under varying concentrations of SUL, alkali, and periodate, keeping other conditions constant. Care was taken to allow reactant solutions to reach the required temperature before mixing. Other authors18,19 have also used similar temperature gap for their kinetic studies. The rate constant (k) of the slow step of Scheme 1 was obtained from the slopes and intercepts of 1/kobs versus 1/[SUL] plots at four different temperatures. The results were subjected to least-squares analysis. The activation energy for the rate determining step was obtained from the plot of log k(Y/calc) versus 1/T. The values are given in Table 2. Discussion Ag(III) species (two electron-oxidant) due to its strong versatile nature prompted studies on the kinetics of oxidation of various organic and inorganic substrates in the late 20th

594 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009

century. Among the various species of Ag(III), Ag(OH)4-, diperiodatoargentate(III), and ethylenebis (biguanide), (EBS), silver(III) are of maximum attention to the researchers due to their relative stability.20 The stability of Ag(OH)4- is very sensitive toward traces of dissolved oxygen and other impurities in the reaction medium whereupon it had not drawn much attention. However, the other two forms of Ag(III)12-14 are considerably stable; the DPA is used in highly alkaline medium and EBS is used in highly acidic medium. A literature survey16 reveals that the water soluble diperiodatoargentate(III) (DPA) has a formula [Ag(IO6)2]7- with dsp2 configuration of square planar structure, similar to diperiodatocopper(III) complex with two bidentate ligands periodate to form a planar molecule. When the same molecule is used in alkaline medium, it is unlikely to exist as [Ag(IO6)2]7- as periodate is known to be in various protonated forms21 depending on pH of the solution as given in following multiple equilibria (2)-(4). H5IO6 h H4IO6- + H+ -

H4IO6 h H3IO6

2-

+

+H

H3IO62- h H2IO63- + H+

K1 ) 5.1 × 10-4 -9

Scheme 2

(2)

K2 ) 4.9 × 10

(3)

K3 ) 2.5 × 10-12

(4)

Periodic acid exists as H5IO6 in acid medium and as H4IO6at pH 7. Thus, under the present alkaline conditions, the main species are expected to be H3IO62- and H2IO63-. Periodate also tends to dimerize at higher concentrations.11 However, the formation of this species is negligible under conditions employed for kinetic studies. On the contrary, the authors13 in their past studies have proposed DPA as [Ag(HL)2]x- in which “L” is a periodate with an uncertain number of protons and “HL” is a protonated periodate of an uncertain number of protons. This can be ruled out by considering the alternative form21 of IO4at pH > 7 which is in the form H3IO62- or H2IO63-. Hence, DPA could be as [Ag(H3IO6)2]- or [Ag(H2IO6)2]3- in alkaline medium. Therefore, under the present condition, diperiodatoargentate(III) may be depicted as [Ag(H3IO6)2]-. The similar speciation of periodate in alkali was proposed22 for diperiodatonickelate(IV). Since the rate of reaction was enhanced by [OH-], added periodate retarded the rate and first order dependency in [DPA] and less than unit order in [SUL], the proposed mechanism which explains all other experimental observations is shown in Scheme 1. In the prior equilibrium step 1, the OH- ion deprotonates the DPA to give a deprotonated diperiodatoargentate(III); in the second step, displacement of a ligand, periodate takes place to give free periodate which is evidenced by decrease in the rate with increase in periodate concentration. It may be expected that lower Ag(III) periodate species such as monoperiodatoargentate(III) (MPA) is more important in the reaction than the DPA. The inverse fractional order in [H3IO6]2- might also be due to this reason. Before the rate determining stage, this MPA combines with a molecule of SUL to give a complex, which decomposes in a slow step to give the 4-hydroxylamine benzenesulfonamide and Ag(I) species. The hydroxylamine derivative undergoes further oxidation in the subsequent fast steps to give nitroso- and nitro-derivatives of benzene sulfonamide where in each transformation consumes 1 mol of MPA. Thus, all these results indicate a mechanism of the type as in Scheme 1. The stepwise oxidation of aniline moiety may be described as given in Scheme 2. The analysis of the oxidative products indicates the presence of predominantly a nitro-group. It is

quite possible that the reaction proceeds through the hydroxylamine (after two electron oxidation of the -NH2 group). The initial attack on the NH2 group involves a single electron transfer mechanism as reported for oxidation of simple aniline.23 The resulting hydroxylamine may complex with oxidant leading to an intermediate, which further breaks to form the nitroso-group. Complexation between the hydroxylamine and oxidant has been proposed earlier in separate study.24 The nitroso-oxidation to the nitro-group may occur via electron pair loss to the Ag(III) after a water molecule’s attack on the nitroso-group. Such mechanism has also been proposed for oxidation of similar sulfonamides25 The second step in Scheme 2 is in accordance with the generally well-accepted principle of noncomplementary oxidations taking place in a sequence of one electron steps, the reaction between the substrate and oxidant generates a radical intermediate. The free radical scavenging experiment did not reveal their presence as given in the Experimental Details section. Sometimes the radicals are rapidly oxidized which might completely mask polymerization.26 Sometimes vinyl compounds themselves are oxidized under the experimental conditions used, and the test for free radicals fails.27 The oxidant-reductant complex formation was proved by the Michaelis-Menten plot, which explains less than unit order in SUL concentration. The structures of DPA, MPA, and Complex (C) could be as shown below.

The rate law for Scheme 1 could be derived as rate ) -

kK4K5K6[DPA][SUL][OH-] d[DPA] ) dt [H3IO62-] + K4[OH-][H3IO62-] + K4K5[OH-] + K4K5K6[OH-][SUL]

kobs )

kK4K5K6[SUL][OH-] [H3IO62-] + K4[OH-][H3IO62-] + K4K5[OH-] + K4K5K6[OH-][SUL]

(5)

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 595

different temperatures as given in Table 1. The plots of 1/kobs versus 1/[SUL] (r g 0.997, S e 0.0011), 1/kobs versus [H3IO6]2- (r g 0.9904, S e 0.008), and 1/kobs versus 1/[OH-] (r g 0.963, S e 0.0014) should be linear as shown in Figure 2. From the slopes and intercepts, the values of K4 are calculated at different temperatures. A van’t Hoff plot was made for the variation of K4 with temperature (i.e., log K4 versus 1/T (r g 0.9807, S e 0.101)), and the values of the enthalpy of reaction ∆H, entropy of reaction ∆S, and free energy of reaction ∆G were calculated. These values are also given in Table 2. A comparison of the latter values with those obtained for the slow step of the reaction shows that these values mainly refer to the rate limiting step, supporting the fact that the reaction before the rate determining step is fairly slow and involves high activation energy.29 In the same manner, K5 and K6 values were calculated at different temperatures, and the corresponding values of thermodynamic quantities are given in Table 2. The values of ∆Sq and ∆Hq were both favorable for electron transfer processes. The favorable enthalpy was due to release of energy on solution changes in the transition state. The low value of enthalpy of activation obtained might be due to the involvement of prior equilibrium steps30a as given in Scheme 1. The high negative value of ∆Sq (-273 J/(K mol)) suggests that the intermediate complex is more ordered than the reactants.30b The observed modest enthalpy of activation and a higher rate constant for the slow step indicates that the oxidation presumably occurs via an innersphere mechanism. This conclusion is supported by earlier observations.19,31 Conclusions

Figure 2. Verification of rate law (5) in the form of (6) for the oxidation of SUL by diperiodatoargentate(III). (a) 1/kobs versus [H3IO6]2-. (b)1/ kobs versus 1/[OH-]. (c) 1/kobs versus 1/[SUL].

The rate law (5) can be rearranged into the following form, which is suitable for verification [H3IO62-] [H3IO62-] 1 1 1 + + + ) kobs kK K K [OH-][SUL] kK2K3[SUL] kK3[SUL] k 1 2 3 (6) According to eq 6, other conditions being constant, the plots of 1/kobs versus [H3IO6]2-, 1/[OH-] and 1/[SUL] were linear as shown in Figure 2. From the intercepts and slopes of such plots, the reaction constants K4, K5, K6, and k were calculated as (1.58 ( 0.05) × 10-1 dm3/mol, (5.60 ( 0.19) × 10-4 mol/dm3, (3.46 ( 0.16) × 102 dm3/mol, and (8.40 ( 0.09) × 10-2 1/s, respectively. The values of K4 and K5 obtained are also in agreement with earlier literature.28 The negligible effect of ionic strength and dielectric constant of medium, on the rate of reaction explains qualitatively the involvement of neutral molecule, sulfacetamide, as seen in Scheme 1. The thermodynamic quantities for the different equilibrium steps, in Scheme 1, can be evaluated as follows. The SUL, periodate, and hydroxide ion concentrations were varied at

Among various species of DPA in alkaline medium, monoperiodatoargentate(III) (MPA) is considered as active species for the title reaction. The rate constant of the slow step and other equilibrium constants involved in the mechanism are evaluated, and activation parameters with respect to the slow step of the reaction were computed. Reaction proceeds through an intermediate complex, which is more ordered than the reactants. Oxidation presumably occurs via an innersphere mechanism. The overall mechanistic sequence described here is consistent with product, mechanistic, and kinetic studies. Appendix The rate law for the Scheme 1 could be derived as -d[DPA] ) k[Complex (C)] dt kK5K6[Ag(H2IO6)(H3IO6)]2) kK6[SUL][Ag(H2IO6)(H2O)2] ) [H3IO62-]

rate )

)

kK4K5K6[DPA][OH-][SUL]

[H3IO62-]

(I)

The total [DPA]T is given as (where the T and f stands for total and free, respectively) follows: [DPA]T ) [DPA]f + [Ag(H3IO6)(H2IO6)]2- + [Ag(H3IO6)(H2O)2] + Complex (C) )[DPA]f - K4[DPA]f[OH-] +

K5[Ag(H2IO6)(H3IO6)]2[H3IO62-]

+

596 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009

K4K5K6[SUL][Ag(H3IO6)2-][OH] 2-

[H3IO6 ] )[DPA]f + K4[DPA]f[OH-] +

K4K5[DPA] [H3IO62-]

+

K4K5K6[SUL][Ag(H3IO6)2-][OH-] [H3IO62-]

{

-

)[DPA]f 1 + K4[OH ] +

K4K5[OH-] [H3IO62-]

+ K4K5K6[OH-][SUL] [H3IO62-]

}

Therefore, [DPA]f )

[DPA]T[H3IO62-]

(II)

[H3IO62-] + K4[OH-][H3IO62-] + K4K5[OH-] + K4K5K6[OH-][SUL]

Similarly, [OH]T ) [OH]f + [Ag(H3IO6)(H2IO6)]2- + [Ag(H3IO6)(H2O)2] )[OH]f + K4[DPA][OH-] +

K4K5[DPA][OH-] [H3IO62-]

In view of the low concentrations of DPA and H3IO62used in the experiment, the second and third terms may be neglected. [OH]T ) [OH]f

(III)

Similarly, [SUL]T ) [SUL]f + Complex (C) ) [SUL]f + K4K5K6[DPA][OH-][SUL] [H3IO62-] In view of the low concentrations of DPA, OH-, and H3IO62used, [SUL]T ) [SUL]f

(IV)

Substituting eqs II-IV in eq I and omitting the subscripts T and f, we get rate ) -

kK4K5K6[DPA][SUL][OH-] d[DPA] ) dt [H3IO62-] + K4[OH-][H3IO62-] + K4K5[OH-] + K4K5K6[OH-][SUL]

kobs )

kK4K5K6[SUL][OH-] [H3IO62-] + K4[OH-][H3IO62-] +

(V)

K4K5[OH-] + K4K5K6[OH-][SUL] Literature Cited (1) Halling-Sørensen, B.; Nors Nielsen, S.; Lanzky, P. F.; Ingerslev, F.; Holten Luetzhoft, H. C.; Jørgensen, S. E. Occurrence, fate and effects of pharmaceutical substances in the environment-A review. Chemosphere 1998, 36, 357–393. (2) Kuemmerer, K. Drugs in the environment: emission of drugs, diagnostic aids and disinfectants into wastewater by hospitals in relation to other sources-a review. Chemosphere 2001, 45, 957–969. (3) (a) Heberer, T. Occurrence, fate, and removal of pharmaceutical residues in the aquatic environment: a review of recent research data.

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ReceiVed for reView January 12, 2008 ReVised manuscript receiVed November 4, 2008 Accepted November 4, 2008 IE8000474