Pd(II) Catalyzed Oxidative Degradation of Paracetamol by Chloramine

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Pd(II) Catalyzed Oxidative Degradation of Paracetamol by Chloramine-T in Acidic and Alkaline Media Ajaya Kumar Singh,*,† Reena Negi,† Bhawana Jain,† Yokraj Katre,‡ Surya Prakash Singh,§ and Virender K. Sharma^ †

Department of Chemistry, Govt. V. Y. T. PG. Autonomous College, Durg, 491001, Chhattisgarh, India Department of Chemistry, Kalyan Mahavidyalya, Sector-7, Bhilai, Durg, Chhattisgarh, India § National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305 0047, Japan ^ Chemistry Department, Florida Institute of Technology, Melbourne, Florida 32901, United States ‡

bS Supporting Information ABSTRACT: Pd(II) catalyzed oxidation of paracetamol (PA) by sodium N-chloro p-toluenesulfonamide (chloramine-T or CAT) was studied in HClO4 and NaOH solutions at 303 K. The stoichiometry and the oxidation product in both solutions were found to be the same. The determined stoichiometric ratio was 1:2 ([PA]:[CAT]) and quinone oxime was identified as the oxidized product of PA. However, the kinetics patterns in both media were different. In the acidic medium, the rate law was d[CAT]/dt = k[CAT][PA]0.9[Pd(II)]0.8[Hþ]0.4 and the rate law was d[CAT]/dt = k[CAT][PA]0.9[Pd(II)] [PTS]0.4[NaOH]1 in the alkaline medium. p-Toluenesulphonamide (PTS) is the reduced product of CAT. The kinetics of the reaction was studied as a function of temperature, ionic strength, concentration of the salt, concentration of the added reaction product, and dielectric constant of the medium to learn the mechanistic aspects of the reaction. A plausible mechanism is proposed, which is consistent with the kinetics, stoichiometry, and product of the reaction.

1. INTRODUCTION Platinum group metal ion-catalyzed reactions have evinced prodigious interest because of their involvement in many important industrial processes such as reduction, oxidation, halogenation, and alkylation of organic compounds.1 In recent years, platinum group metal ions including Ru(III), Os(VIII), Ir(III), Rh(III), and Pd(II) are widely used as catalysts due to their strong catalytic influences in various reactions. Palladium(II) chloride is the most important salt in the catalytic chemistry of palladium. Several authors have performed studies using Pd(II) because of the commercial importance of reactions catalyzed by Pd(II). The kinetics for the oxidation of ethylene by aqueous Pd(II) is an example.2,3 In this study, the effect of a chloride ion on the reaction rate was studied in order to establish the active species of the catalyst. Generally, the mechanism of catalysis depends on the nature of the substrate, the oxidant, and other experimental conditions.4,5 In most of the catalytic studies for organic transformations, the nature of the active form of Pd(II) remains obscure. The chemistry of aromatic sulfonyl haloamines (N-haloamines) is of interest due to its diverse behavior. N-Haloamines are a group of mild oxidizing agents, which have been used extensively for the oxidation of several organic compounds. The versatile nature of Nhaloamines is due to halonium cations and nitrogen anions in their structure, which can act as both a base and a nucleophile.610 Thus, these compounds can react with a wide range of functional groups to cause a variety of molecular changes. The prominent member of this class of compounds is sodium-N-chloro-4-methylbenzensulfonamide, generally known as chloramine-T (CAT). Several reviews have been published on the oxidizing behavior of CAT1113 and a r 2011 American Chemical Society

number of publications focus on the mechanistic aspects of the redox reactions in acidic media. In most of the studies one of the species, RNHCl (R = CH3C6H4SO2), HOCl, or H2OClþ, has been considered as the reactive species.14,15 Similar information on CAT in an alkaline medium is limited. In the present study, the kinetics and mechanism of the Pd(II) catalyzed oxidation of paracetamol (PA) by CAT were performed for the first time. Paracetamol (4-hydroxyacetanilide, acetaminophen, or 4-acetamidophenol) is a well-known drug that is involved in extensive applications in pharmaceutical industries. It is an antipyretic and analgesic compound of high therapeutic value.16,17 It is also used as an intermediate for pharmaceutical (as a precursor in penicillin) and azo dye.1821 Oxidation reactions are important in the synthesis of organic compounds and pharmaceuticals because these reactions create new functional groups or modify existing functional groups in a molecule.22,23 Various advanced oxidation processes such as electrochemical,2426 ozonation, and H2O2/UV oxidation2730 have been employed to remove aqueous paracetamol. The present study examines, in detail, the kinetic and mechanistic aspects of the Pd(II) catalyzed oxidation of PA by CAT in acidic and alkaline media with the following objectives: (i) to ascertain the reactive species of the catalyst and the oxidant; (ii) to deduce the rate law consistent with the kinetics results; (iii) to estimate activation parameters; (iv) to compare the catalyzed Received: August 3, 2010 Accepted: May 29, 2011 Revised: May 15, 2011 Published: May 29, 2011 8407

dx.doi.org/10.1021/ie101661m | Ind. Eng. Chem. Res. 2011, 50, 8407–8419

Industrial & Engineering Chemistry Research

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reactivity in acidic and alkaline media; (v) to establish the optimum condition to develop a simple procedure for the oxidative degradation of PA, and (vi) to elucidate the plausible reaction mechanisms based on the observed reaction rate law and stoichiometry.

2. EXPERIMENTAL SECTION 2.1. Materials and Method. Analytical reagent grade chemicals and double distilled water were used throughout the investigation. The paracetamol (PA) solution (SD. Fine Chem.) was prepared by dissolving an appropriate amount of the recrystallized sample in water. The required concentration of PA was obtained from its stock solution. The stock solution of CAT (Loba, AR) was prepared in water and standardized iodometrically. A solution of Pd(II) chloride (E. Merck) was prepared by dissolving a known weight of Pd(II) chloride in 0.1 M HCl and stored in a black-coated bottle to prevent photochemical decomposition. Sodium perchlorate, perchloric acid, and sodium hydroxide (E. Merck) were used without further purification. The permittivity of the reaction mixture was stabilized by the addition of acetic acid (040%). The kinetic measurements were carried out in a black-coated vessel at 303 K. The measurements were performed under pseudo-first-order conditions with [PA] . [CAT]. The specified volumes of CAT, HClO4/NaOH, Pd(II), and water were transferred into glass-stoppard Pyrex boiling tubes and placed in a thermostat-controlled water bath, which was pre-equilibrated at 303 K. An appropriate volume of PA solution, also equilibrated at the desired temperature in the same water bath, was rapidly poured into the reaction vessel to initiate the reaction. The reaction was monitored by iodometric determination of unconsumed [CAT] in known aliquots of the reaction mixture at different time intervals. Each kinetic run was studied for 85% completion of the reaction. The reaction was also carried out at various temperatures (303, 308, 313, and 318 K) to see the effect of temperature and to calculate the thermodynamic activation parameters. The effect of dissolved O2 on the rate of reaction was studied by preparing the reaction mixture and monitoring the reaction in a N2 atmosphere. A significant difference was not observed between the results. In view of the ubiquitous contamination of the basic solutions by carbonate, the effect of carbonate on the reaction was also studied. Additional carbonate did not have an effect on the reaction rate. Fresh solutions were used throughout the experiments. 2.2. Determination of Stoichiometry and Product Analysis. Different sets of the reaction mixture containing PA, Pd(II), HClO4, or NaOH with excess CAT were equilibrated for 72 h at 303 K. Estimation of unconsumed CAT in each set revealed that for the oxidation of 1 mol of PA, 2 mols of CAT were consumed. Accordingly, the following stoichiometry equation may be expressed as

The reaction products were extracted with ether after completion of the reaction (monitored by TLC). Evaporation of the

Figure 1. Sample individual time plots for log[CAT] for various concentrations at 303 K. [PA] = 1.00 102 mol dm3, [Pd(II)] = 2.60 105 mol dm3, [Hþ] = 1.00 103 mol dm3, [CAT] = 0.4 103, 0.5 103, 0.6 103, 0.8 103, 1.0 103 M.

ether layer was followed by column chromatography on silica gel (60200 mesh) using a gradient elution (from dichloromethane to chloroform). After the initial separation, the products were further purified by recrystallization. Acetic acid and quinone oxime were identified as oxidation products of PA, and p-toluenesulfonamide (PTS) was the reduction product of CAT. The reduction product, PTS (TsNH2), was detected by TLC.31 Further confirmation was determined by its melting point, 139 C, which was within close proximity to the reported temperature of 137140 C in previous works.16,17,19 PTS was quantitatively determined by its reaction with xanthydrol to yield the corresponding N-xanthyl-p-toluenesulfonamide.32 In a typical experiment, equal quantities of separated PTS and xanthydol (0.20 g) were dissolved in 10 mL of glacial acetic acid. The reaction mixture was stirred for 3 min at room temperature and allowed to stand for 90 min. The derivative was filtered, recrystallized with dioxane/water (3:1), and dried at room temperature. The mass of PTS was obtained with 8085% recovery in all of the cases. The quinone oxime was identified by its IR spectrum (1652 cm1, CdO stretching; 1615 cm1, CdN stretching of oxime; 3332 cm1, OH stretching). Identification was further confirmed by its melting point of 131 C (literature mp 132 C). Quinone oxime was also analyzed via GCMS (JEOL-JMS, Mate-MS system, Japan; Figure S-1, S-2 in the Supporting Information). GCMS results were analyzed by extraction of the reaction mixture with diethyl ether and by concentrating the ether layer by a slow evaporation procedure. The mass spectrum showed a molecular peak at 123 amu, confirming quinone oxime as the product. Acetic acid was identified by the spot test.33

3. RESULTS AND DISCUSSION At constant [PA], [Hþ]/[OH], and temperature, where [PA] . [CAT], a plot of log[CAT] versus time was linear (r > 0.989), indicating a first-order dependence of rate on [CAT] for both acidic and alkaline media. The kinetics for the oxidation of paracetamol by CAT, catalyzed by Pd(II), was investigated as a function of initial concentrations of the reactants in acidic and basic media at 303 K. The stoichiometry and oxidation product 8408

dx.doi.org/10.1021/ie101661m |Ind. Eng. Chem. Res. 2011, 50, 8407–8419


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are identical in both media, but their observed kinetics characteristics were different. This suggests that the mechanisms for the oxidation of PA by CAT in acid and alkaline media are different. The rate constant of the reaction in each kinetic run was determined by a plot of the log of the remaining [CAT] versus

time (Figures 1 and 2), which produced a straight line and indicates that the reaction under the chosen condition follows pseudo-first-order kinetics. 3.1. Kinetics of Oxidation of PA by CAT in HClO4 Medium. The pseudo-first-order rate constants (k1) obtained are listed in

Figure 2. Sample individual time plots for log[CAT] for various concentrations at 303 K. [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, [OH] = 1.00 103 mol dm3. [CAT] = 0.4 103, 0.5 103, 0.6 103, 0.8 103, 1.0 103 M.

Figure 3. Plot between rate of reaction (dc/dt) versus [CAT] on the reaction rate at 303K. (A) [PA] = 1.00 102 mol dm3, [Pd(II)] = 2.60 105 mol dm3, [Hþ] = 1.00 103 mol dm3. (B) [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, [OH] = 1.00 103 mol dm3, where A = acid medium and B = alkaline medium.

Table 1. Effect of [CAT], [PA], [Pd(II)], and [Hþ] on the Rate of Oxidation of Paracetamol at 303 Ka [CAT] 103 3

a

[PA] 102 3

[Pd(II)] x105 3

[Hþ] 103

dc/dt 107

3

3 1

(mol dm

s )

k1 104

k(cal) 104

k(exp) 104

1

1

(s )

(s1)

(mol dm )

(mol dm )

(mol dm )

(mol dm )

(s )

0.40

1.00

2.60

1.00

0.57

1.42

0.50

1.00

2.60

1.00

0.70

1.40

0.60 0.80

1.00 1.00

2.60 2.60

1.00 1.00

0.90 1.00

1.50 1.42

1.00

1.00

2.60

1.00

1.40

1.47

1.00

0.50

2.60

1.00

0.54

0.57

0.60

0.71

1.00

0.80

2.60

1.00

1.04

1.10

1.15

1.21

1.00

1.00

2.60

1.00

1.33

1.40

1.47

1.50

1.00

1.20

2.60

1.00

1.52

1.60

1.68

1.82

1.00

1.60

2.60

1.00

2.09

2.20

2.31

2.42

1.00 1.00

2.00 3.00

2.60 2.60

1.00 1.00

2.28 2.75

2.40 2.90

2.52 3.05

2.98 3.03

1.00

4.00

2.60

1.00

2.91

3.07

3.25

3.84

1.00

1.00

0.90

1.00

0.51

0.54

0.56

0.61

1.00

1.00

1.80

1.00

0.99

1.05

1.10

1.26

1.00

1.00

2.60

1.00

1.33

1.40

1.47

1.50

1.00

1.00

3.60

1.00

1.80

1.9

2.00

2.35

1.00

1.00

4.50

1.00

1.97

2.08

2.19

2.88

1.00 1.00

1.00 1.00

5.40 6.30

1.00 1.00

2.09 2.16

2.20 2.28

2.31 2.40

3.12 3.30

1.00

1.00

2.60

0.20

1.80

1.90

2.00

2.10

1.00

1.00

2.60

0.40

1.72

1.82

1.89

1.89

1.00

1.00

2.60

0.60

1.62

1.71

1.79

1.78

1.00

1.00

2.60

0.80

1.49

1.57

1.65

1.60

1.00

1.00

2.60

1.00

1.33

1.40

1.47

1.50

1.00

1.00

2.60

1.20

1.21

1.28

1.34

1.26

1.00 1.00

1.00 1.00

2.60 2.60

1.80 2.00

0.85 0.68

0.90 0.72

0.94 0.75

1.01 0.62

Values of k1 taken from experimental method, k(cal) from calculated method, k(exp) from expected method. 8409



Figure 4. Plot between rate of reaction (dc/dt) versus [PA] on the reaction rate at 303 K. (A) [CAT] = 1.00 103 mol dm3, [Pd(II)] = 2.60 105 mol dm3, [Hþ] = 1.00 103 mol dm3. (B) [CAT] = 1.00 103 mol dm3, [Pd(II)] = 6.70 105 mol dm3, [OH] = 1.00 103 mol dm3, where A = acid medium and B = alkaline medium.

Figure 5. Plot between rate of reaction (dc/dt) versus [Pd(II)] on the reaction rate at 303 K. (A) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3,[Hþ] = 1.00 103 mol dm3. (B) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3,[OH] = 1.00 103 mol dm3, where A = acid medium and B = alkaline medium.

Table 1. Furthermore, the values of k1 were unaltered with a variation of [CAT], confirming first-order dependence of [CAT]. This was also confirmed from the plot of (dc/dt) versus [CAT] (Figure 3). Fractional order with respect to [PA] and [Pd(II)] was observed for the oxidation of PA (Table 1, Figures 4 and 5). The values of the rate constant showed a negative effect for the concentration of Hþ on the rate of the reaction (Table 1 or Figure 6). The effect of ionic strength of the medium, chloride ion, and influence of the byproduct on the reaction rate were measured by varying the concentration of NaClO4, KCl, and PTS, respectively. The ionic strength, chloride ion, and byproduct had negligible effects on the rate of the reaction (Table 2). The rate of reaction decreased with decreasing dielectric constant of the medium (increasing the % of acetic acid by volume) (Table 3 or Figure 7). The reaction was studied at different temperatures (303318 K) (Table 4 or Figure 8) by holding other experimental conditions constant. From the linear Arrhenius plot of log k1 versus 1/T, the activation energy of Ea = 73.2 kJ mol1 was calculated. Values of the other activation

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Figure 6. Plot between rate of reaction (dc/dt) versus [Hþ] and [OH] on the reaction rate at 303 K. (A) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 2.60 105 mol dm3. (B) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, where A = acid medium and B = alkaline medium.

parameters, enthalpy of activation (ΔHq = 70.60 ( 0.04 kJ mol1), entropy of activation (ΔSq = 80.80 ( 0.08 J K1 mol1), Gibbs free energy of activation (ΔGq = 95.08 ( 0.91 kJ mol1), and Arrhenius factor (A = 8.78 ( 0.23) were also calculated (Table 7). 3.2. Kinetics of Oxidation of PA by CAT in NaOH Medium. The reaction concentrations were varied under pseudo-firstorder conditions: [PA] . [CAT] at constant [NaOH], [Pd(II)], and temperature. The plots of (dc/dt) versus [CAT] given in Table 5 and Figure 3 were linear; suggesting first-order kinetics with respect to [CAT]. The order of reaction with respect to [PA] was found to be fractional at 303 K (Table 5 or Figure 4). The plot of (dc/dt) versus [Pd(II)] was linear, indicating a first-order dependence for the rate of reaction on the [Pd(II)] (Table 5 or Figure5). With an increase in the concentration of OH, the value of the reaction rate decreased, which is also evident from the plot of (dc/dt) versus [OH] (Table 5 or Figure 6). This shows a negative effect of [OH] on the rate of reaction. Variations in the ionic strength of the medium and [Cl] did not show any significant change in the values of k1 under the constant experimental conditions (Table 2). The rate of reaction decreased with a decrease in the dielectric constant of the medium (increasing the % of acetic acid by volume) (Table 3 or Figure 7). The reaction was carried out at different temperatures (303318 K) (Table 4 or Figure 8). The addition of PTS into the reaction mixture showed that the rate of the reaction decreased with increasing [PTS] (Table 6 or Figure 9). From the linear Arrhenius plot of log k1 versus 1/T, the activation energy (Ea) was calculated as 63.8 kJ mol1. Other activation parameters determined were enthalpy of activation (ΔHq = 61.28 ( 0.06 kJ mol1), entropy of activation (ΔSq = 107.99 ( 0.94 J K1 mol1), Gibbs free energy of activation (ΔGq = 94 ( 0.48 kJ mol1), and Arrhenius factor (A = 7.38 ( 0.83) (Table 7). 3.3. Test of Free Radicals. The possible interference of free radicals was examined by adding acrylonitrile to the reaction mixture, which was kept in an inert atmosphere for 24 h. This reaction mixture was diluted with methanol and no precipitate was observed, which suggests that free radicals were not involved in the reaction. 8410



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Table 2. Effect of Ionic Strength and Chloride on the Rate of Oxidation of Paracetamol at 303 Ka dc/dt 107 (mol dm3 s1) [KCl] 105 (mol dm3)

NaClO4 (I)

k1 10 4 (s1)

acid medium

alkaline medium

acid medium

alkaline medium

blank

1.40

2.2

1.47

2.3

1.0

1.50

2.5

1.57

2.6

2.0

2.00

2.0

2.10

2.1

3.0

1.62

2.3

1.70

2.4

4.0

1.85

2.4

1.94

2.5

5.0

1.45

2.6

1.52

2.7

6.0

2.10

2.0

2.21

2.1

8.0 10.0

1.65 1.85

2.6 2.2

1.73 1.94

2.7 2.3

blank

1.40

2.2

1.47

2.3

2.0

1.60

1.6

1.70

1.7

4.0

2.42

2.4

2.50

2.5

6.0

2.21

2.2

2.31

2.3

10.0

1.50

2.5

1.57

2.6

12.0

1.48

2.4

1.55

2.5

16.0 25.0

2.20 1.40

2.2 2.4

2.31 1.47

2.3 2.5

a Solution conditions: (A) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 2.60 105 mol dm3, [Hþ] = 1.00 103 mol dm3. (B) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, [OH] = 1.00 103 mol dm3, where A = acid medium and B = alkaline medium.

Table 3. Effect of Solvent on the Rate of Oxidation of Paracetamol at 303 Ka dc/dt 107 (mol dm3 s1)

k1 104 (s1)

%CH3COOH (v/v)

acid medium

alkaline medium

acid medium

alkaline medium

0

1.40

2.2

1.47

2.3

5

1.25

1.9

1.31

2.0

10

0.84

1.5

0.88

1.57

20

0.62

1.15

0.65

1.2

30

0.42

1.04

0.44

1.1

40

0.18

0.9

0.19

0.94

Solution conditions: (A) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 2.60 105 mol dm3, [Hþ] = 1.00 103 mol dm3. (B) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, [OH] = 1.00 103 mol dm3, where A = acid medium and B = alkaline medium. a

3.4. Reactive Species of CAT. CAT acts as a mild oxidant in both acidic and alkaline media. In general, CAT undergoes a twoelectron change in its reactions to form the reduction products, PTS (p-CH3C6H4SO2NH2 or TsNH2) and sodium chloride. The oxidation potential of the CAT-TsNH2 redox couple decreases with an increase in pH of the medium (Eo is 1.14 V at pH 0.65, 0.778 V at pH 7.0, 0.614 V at pH 9.7, and 0.50 V at pH 12). The existence of similar equilibria in acidic and alkaline solution of CAT has been reported by Morris et al.,34 Ruff and Kucsman,35 Bishop and Jennings,36 Hardy and Johnston37 and Higuchi et al.38 The aqueous solution of chloramine-T (TsNClNa) behaves as a strong electrolyte,35 and, depending on the pH, it results in different types of reactive species. The possible oxidizing species in an acidified CAT solution are the

conjugate free acid (TsNHCl), dichloramine-T (TsNCl2), hypochlorous acid (HOCl), and H2OClþ. In alkaline solutions, TsNHCl, HOCl, TsNCl, and OCl species are considered the oxidizing agents. 3.5. Reactive Species of Pd(II) Chloride in Acidic Medium. The complexes of platinum or palladium group metals are wellknown. Palladous chloride is very soluble in HCl and exists as [PdCl4]2. The different possible mononuclear complexes of Pd(II) are [PdLCl3], [PdL2Cl2], [PdL3Cl]þ, and [PdL4]2þ (where L represent a ligand such as amine, phosphine, sulphide, and thioether). The existence of different species of palladium chloride, specifically Pd2þ, PdClþ, PdCl2, PdCl3, and PdCl42, has been observed in HClO4 medium depending upon the [Cl]/[Pd] ratio. The observed results39 showed that the species Pd2þ, PdOHþ, and PdClþ were present when [Cl]/[Pd] was up to 0.8; PdClþ and PdCl2 were present when [Cl]/[Pd] was 2.22.8; and only PdCl2, PdCl3, and PdCl42 were present when [Cl]/[Pd] was 4.0 to 4.8. Therefore, it may be considered that Pd2þ, PdClþ and PdCl2 are reactive species of the catalyst when [Cl]/[Pd] is low. Moreover, when [Cl]/[Pd] is high, the reactive species of palladium(II) are PdCl2, PdCl3 and PdCl42. The hydrolysis of palladium(II) chloride species in aqueous medium has been applied to many investigations.4043 The equilibrium constants correspond to the following equilibrium: Ka

Pd2þ þ Cl s F PdClþ s R Kb

PdClþ þ Cl s F PdCl2 s R

ðBÞ

Kc

ðCÞ

Kd

ðDÞ

PdCl2 þ Cl s F PdCl3 s R PdCl3 þ Cl s F PdCl4 2 s R 8411

ðAÞ



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Figure 7. Plot between log k versus 1/D at 303 K. (A) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 2.60 105 mol dm3, [Hþ] = 1.00 103 mol dm3. (B) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, [OH] = 1.00 103 mol dm3, where A = acid medium and B = alkaline medium.

Table 4. Temperature Dependence and Activation Parameters for the Oxidation of PA by CAT in HClO4 Medium and NaOH Medium with Pd(II) Catalysta k1 104 (s1) temperature K

acid medium

alkaline medium

303

1.47

2.2

308

2.5

3.6

313

3.42

4.9

318

6.3

7.71

Solution conditions: (A) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 2.60 105 mol dm3, [Hþ] = 1.00 103 mol dm3. (B) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, [OH] = 1.00 103 mol dm3, where A = acid medium and B = alkaline medium. a

It has also been reported4042 that Kd is probably the most important stability constant for catalytic chemistry. The reported values of log K are 4.47, 3.29, 2.41, and 1.37 for reactions A, B, C, and D, respectively. Studies have shown that in the presence of chloride ion, Pd(II) chloride exists as [PdCl4]2, and in aqueous solution it may be further hydrolyzed to [PdCl3(H2O)].44 Other equilibria in solutions are expressed by reactions E . PdCL4

2

Ke

þ H2 O s F ½PdCl3 ðH2 OÞ þ Cl s R

ðEÞ

The reported value of log β4 is 11.54 (β4 is the equilibrium constant) and the value of the hydrolytic constant (Ke) is 2.5 103. The existence of Pd(II) chloride exclusively in the form of the complex [PdCl4]2- has been suggested previously.45 However, in the present study, no effect of the chloride ion on the reaction rate was observed. This ruled out the possibility of [PdCl4]2 as a reactive species of Pd(II) in the present study. Tait et al. reported41 the existence of [PdCl(H2O)3]þ (written as PdClþ aq in the mechanism for brevity) which is the hydrolyzing species of PdCl2 and may represent the catalyzing species of Pd(II) in the present system.46 3.6. Spectral Evidence for the Reactions Shown in the Proposed Reaction Scheme. Spectroscopic evidence for the complex formation between the oxidant and substrate was obtained from UVvis spectra of PA, CAT, and a mixture of both. The absorption maxima for PA, CAT, and PA-CAT were 240, 220, and 225 nm, respectively. A hypsochromic shift of

Figure 8. Plot between log k versus temprature. (A) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 2.60 105 mol dm3, [Hþ] = 1.00 103 mol dm3. (B) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, [OH] = 1.00 103 mol dm3, where A = acid medium and B = alkaline medium.

15 nm from 240 to 225 nm of PA suggests an occurrence of complexation between CAT and PA. Therefore, reactions ii and vii in Schemes 1 and 2 are likely participating in the oxidation of PA by CAT. The formation of the complex between Pd(II) and the CATPA mixture was also verified from the spectra of both CATPA and CATPAPd(II) in which shifts of CATPA were observed from 225 to 230 nm for the acidic medium (Figure 10) and 225 to 235 nm for the alkaline medium (Figure 11). 3.7. Reaction Mechanism and Rate Law in Acidic Medium. The suggested oxidizing species32,3437 in acidified CAT solutions are TsNHCl, TsNCl2, HOCl and H2OClþ. If TsNCl2 and HOCl were to behave as the reactive species, then the rate law would have followed a second-order dependence on [CAT] and first-order retardation by the added [TsNH2]. However, experimental observations are opposite of these expectations. Similarly, if H2OClþ were to be the reactive species, there would have been a positive effect of [Hþ] at the rate, which did not occur. Therefore, it can be assumed that TsNHCl is the reactive oxidizing species for the oxidation of PA by CAT. The protonation constant (Kp) for the reaction Kp

TsNHCl þ Hþ s F TsNH2 Clþ s R

ðFÞ

is determined to be 1.02 102 at 298 K.16,17 In the present study, an inverse fractional-order in [Hþ] suggests a regeneration of TsNHCl, which appears to be the active oxidant involved in the mechanism of PA oxidation. On the basis of the results above, Scheme 1 has been proposed for the oxidation of PA by CAT catalyzed by Pd(II) in an acidic medium. In Scheme 1, the reaction between CAT and PA in the acidic medium in the presence of Pd(II) has a 1:2 stochiometry of oxidant to reductant with first-order dependence on [CAT]. Scheme I also suggests an apparent order, less than a unit order, in [PA] and [Pd(II)] and an inverse fractional order in [Hþ] in the rate of the reaction. The order of less than a unit order in [PA] may result from the formation of a complex between CAT and PA prior to the formation of the products. Spectroscopic evidence for the complex formation (C2) between the oxidant and substrate was obtained from UVvis spectra of PA, CAT, and a mixture of both. A hyper-chromicity shift of about 5 nm from 219.9 to 225 nm in the CAT spectra was observed (Figure 10). The complex formation between 8412



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Table 5. Effect of [CAT], [PA], [Pd(II)], and [OH] on the Rate of Oxidation of Paracetamol at 303 Ka

a

[CAT] 103

[PA] 102

[Pd(II)] 105

[OH] 103

dc/dt 107

k1 104

k(cal) 104

k(exp) 104

(mol dm3)

(mol dm3)

(mol dm3)

(mol dm3)

(mol dm3 s1)

(s1)

(s1)

(s-1)

0.40

1.00

6.70

1.00

0.82

2.10

0.50

1.00

6.70

1.00

1.12

2.20

0.60

1.00

6.70

1.00

1.30

2.10

0.80

1.00

6.70

1.00

1.70

2.10

1.00

1.00

6.70

1.00

2.20

2.31

1.00

0.20

6.70

1.00

0.52

0.55

0.45

0.55

1.00 1.00

0.40 0.80

6.70 6.70

1.00 1.00

1.10 1.40

0.86 1.78

0.87 1.68

1.01 1.80

1.00

1.00

6.70

1.00

1.60

2.31

2.05

2.22

1.00

1.20

6.70

1.00

2.20

2.50

2.42

2.59

1.00

2.00

6.70

1.00

2.94

3.10

3.70

4.02

1.00

1.00

1.30

1.00

0.42

0.44

0.41

0.43

1.00

1.00

2.60

1.00

0.85

0.90

0.81

0.86

1.00

1.00

5.30

1.00

1.79

1.89

1.61

1.73

1.00 1.00

1.00 1.00

6.70 9.30

1.00 1.00

2.19 3.13

2.31 3.30

2.05 2.80

2.22 3.01

1.00

1.00

10.7

1.00

3.51

3.70

3.21

3.45

1.00

1.00

13.4

1.00

4.08

4.30

4.10

4.31

1.00

1.00

6.70

0.40

4.75

5.00

4.50

4.96

1.00

1.00

6.70

0.80

2.70

2.85

2.50

2.67

1.00

1.00

6.70

1.00

2.19

2.31

2.05

2.22

1.00

1.00

6.70

1.20

1.72

1.81

1.81

1.85

1.00 1.00

1.00 1.00

6.70 6.70

1.60 2.00

1.35 1.16

1.42 1.23

1.33 1.08

1.43 1.17

Values of k1 from experimental method, k(cal) from calculated method, k(exp) from expected method.

Table 6. Effect of [PTS] on the Rate of Oxidation of Paracetamol at 303 Ka. [PTS] 103 (mol dm3)

dc/dt 107 (mol dm3 s1)

k1 104 (s1)

k(cal) 104 (s1)

k(exp) 104 (s1)

1.00

2.18

2.30

2.05

2.22

2.00

1.72

1.81

1.87

1.74

5.00

1.43

1.51

1.47

1.28

7.00

1.24

1.31

1.28

1.15

10.0

1.01

1.07

1.08

1.02

15.0

0.77

0.81

0.85

0.89

20.0

0.68

0.72

0.71

0.81

Values of k1 from experimental method, k(cal) from calculated method, k(exp) from expected method. Solution conditions: [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, and [OH] = 1.00 103 mol dm3. a

CAT and PA results in fractional order dependence on [PA]. Significantly, a plot of [CAT]/rate versus 1/[PA] shows a straight line with a nonzero intercept (Figure 12). K2 is the composite equilibrium constant which involves the equilibrium required to bind CAT to PA. This intermediate species (C2) further reacts with the Pd(II) catalyst and forms the complex, C3. This complex undergoes oxidative decomposition in the next step (rate determining), leading to the formation of the intermediate species of PA with regeneration of the Pd(II) catalyst. This intermediate species further reacts with another molecule of CAT species in a fast step to yield the product. The given results support the mechanism given in scheme 1. According to Scheme 1 and in consideration that 1 mol of PA is oxidized by 2 mol of CAT, the rate in terms of decrease in the

concentration of CAT can be expressed as rate ¼ ðRÞ ¼

d½CAT ¼ 2k½C3 dt

ð1Þ

On the basis of Scheme 1 equilibrium steps iiv, eqs 27 can be obtained in the following forms, respectively: R ¼

2kK1 K2 K3 ½PdðIIÞ½PA½CAT ½Hþ

ð2Þ

At any time in the reaction, the total concentration of CAT, that is, [CAT]T, can be expressed as ½CATT ¼ ½CAT þ ½C1 þ ½C2 þ ½C3 8413

ð3Þ



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Substitution of the variables of [C1], [C2], and [C3] in eq 3, eq 4 is obtained. ½CAT ¼

½Hþ þ K

½CATT 1 þ K1 K2 ½PA þ K1 K2 K3 ½PdðIIÞ½PA ð4Þ

Substitute the expression for [CAT] (eq 4) into eq 2 to obtain the expression for eq 5. R ¼

2kK1 K2 K3 ½PdðIIÞ½PA½CATT 1 þ K1 K2 ½PA þ K1 K2 K3 ½PdðIIÞ½PA

½Hþ þ K

ð5Þ

Equation 5 is the rate law based on the observed kinetic orders with respect to each reactant involved in the reaction. The rearrangement of eq 5 gives eq 6. þ

3.8. Reactive Species of Pd(II) in Alkaline Medium. Palladium(II) chloride catalysis has been observed during various redox reactions.44,47 The palladium complexes are somewhat less stable, kinetically and thermodynamically, than platinum analogues. Palladium complexes are known to exist as different complexes in alkaline solutions44,4750 and the possible Pd(II) complex species are [Pd(OH)Cl3]2, [Pd(OH)2Cl2]2, and [Pd(OH)4]2. The species [Pd(OH)3Cl]2 and [Pd(OH)4]2 are not commonly found due to their insoluble characteristic. Furthermore, the rate decreases with an increase in [OH] and there was no effect of [Cl] on the rate of reaction, which clearly rules out [Pd(OH)Cl3]2 as the reactive species. Hence, the [Pd(OH)2Cl2]2 complex ion (written as Pd(II) in the mechan-

Scheme 1

½CATT ½H 1 ¼ þ 2kK1 K2 K3 ½PdðIIÞ½PA 2kK2 K3 ½PdðIIÞ½PA rate 1 1 ð6Þ þ þ 2kK3 ½PdðIIÞ 2k Equation 6 suggests that if a plot is made between [CAT]T/rate and [Hþ], 1/[PA], or 1/[Pd(II)], straight lines with positive intercepts can be obtained (Figure 12). Experimental data resulted in straight lines with positive intercepts in the plots of [CAT]T/rate versus [Hþ], 1/[Pd(II)], and 1/[PA] (Figure 12). This supports the proposed reaction scheme on the basis of which the rate law (eq 6) was derived. From the values of the intercept and slope of the plots, the values of k, K1, K2, and K3 were calculated as 6.9 104 s1, 1.7 103 mol dm3, 18.19 mol1 dm3, and 4.5 104 mol1 dm3, respectively.

Figure 9. Plot between rate of reaction (dc/dt) vs [PTS] on the reaction rate at 303 K. (A) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 2.60 105 mol dm3, [Hþ] = 1.00 103 mol dm3. (B) [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3, [OH] = 1.00 103 mol dm3, where A = acid medium and B = alkaline medium.

Table 7. Activation Parameters for Paracetamol with Different Catalyst and Medium (for Isokinetic Temperature) PA þ catalyst þ medium PA þ Ir(III) þ H

þ

k1 104 (s1)

k2 104 (s1)

Ea (kJ mol1)

ΔHq (kJ mol 1)

ΔSq (J K1 mol 1)

ΔGq (kJ K1 mol 1)

ref

5.78

8.70

54.9

52.03 ( 1.32

129.40 ( 3.69

92.15 ( 1.93

14

PA þ Ru(III) þ Hþ PA þ Pd(II) þ Hþ

4.00 2.20

6.80 3.60

61.5 73.2

58.98 ( 0.27 70.60 ( 0.04

- 111.05 ( 3.69 80.80 ( 0.08

92.62 ( 0.07 95.08 ( 0.91

15 present work

PA þ Pd(II) þ OH

1.47

2.50

63.8

61.28 ( 0.06

107.99 ( 0.94

94.00 ( 0.48

present work

8414



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Scheme 2

Figure 10. UVvisible spectra: (1) [CAT] = 1 104 mol dm3; (2) [PA] = 1 104 mol dm3; (3) [Pd(II)] = 1 106 mol dm3, [Hþ] = 1 103 mol dm3; (4) [CAT] = 1 104 mol dm3, [PA] = 1 104 mol dm3.

ism for brevity) has been assumed to be the reactive species in the present study. 3.9. Reaction Mechanism and Rate Law in Alkaline Medium. In alkaline solutions of CAT, the species TsNCl2 and H2OClþ do not exist and the possible species are TsNHCl, HOCl, and TsNCl. The following equilibria in alkaline solutions of CAT have been reported.36 Kx

TsNCl þ H2 O s F TsNHCl þ OH s R Ky

TsNHCl þ H2 O s F TsNH2 þ OHCl s R Ky ¼ 4:21 103

at

ðGÞ ðHÞ

298 K

In the present investigation, an addition of TsNH2 in the reaction mixture decreased the rate of reaction in an alkaline medium, which suggests that the pre-equilibrium step involves TsNH2 as one of the products. When HOCl was considered as the reactive species of CAT, the obtained rate law showed a negative effect on TsNH2 and led to a rate law that explains all of the kinetics observations. A general mechanism (Scheme 2) was proposed to substantiate the observed kinetics during the oxidation of PA by CAT in an alkaline medium. Scheme 2 involves a chlorine transfer through the intermediacy of HOCl. The protonated oxidant species HOCl (X1), formed from TsNHCl, reacts with the substrate in a fast equilibrium step to form the substrate-CAT complex (X2) with the elimination of the hydroxide ion. The inverse negative order in [OH] may also be due to this analysis. In the prerate determining step, the X2 complex combines with a molecule of Pd(II) to give an intermediate complex, X3, which then

Figure 11. UVvisible spectra: (1) [CAT] = 1 104 mol dm3; (2) [PA] = 1 104 mol dm3; (3) [Pd(II)] = 1 106 mol dm3, [OH] = 1 103 mol dm3; (4) [CAT] = 1 104 mol dm3, [PA] = 1 104 mol dm3; (5) [CAT] = 1 104 mol dm3, [PA] = 1 104 mol dm3, [Pd(II)] = 1 106 mol dm3, [OH] = 1 103 mol dm3.

decomposes slowly to produce the intermediate species by the regeneration of the Pd(II) catalyst and without the interference of a free radical. One more molecule of CAT further reacts with an intermediate in a fast step to yield products. Spectroscopic evidence for the complex formation between the oxidant and substrate was also evident from the UVvis spectra of PA, CAT, and the mixture of both. A hypsochromic shift of about 5 nm was observed from 220 to 225 nm in the CAT spectra (Figure 11). According to the reaction scheme and in consideration that 1 mol of PA was oxidized by 2 mol of CAT, the rate is expressed as d½CAT ¼ 2k½X03 dt

ð7Þ

K 01 K 02 K 03 ½PA½PdðIIÞ½CAT ½OH ½TsNH2

ð8Þ

rate ¼ ðRÞ ¼ ½X03 ¼

Substituting the expression for [X03] (eq 8) into eq 7, eq 9 is obtained. R ¼ 8415

2k0K 01 K 02 K 03 ½PA½PdðIIÞ½CAT ½OH ½TsNH2

ð9Þ



ARTICLE

Figure 12. Verification of rate law for 1/[PA] and 1/[Pd(II)] oxidation of paracetamol by chloramine-T in HClO4 medium at 303 K. [CAT] = 1.00 103 mol dm3, [Hþ] = 1.00 102 mol dm3.

Figure 13. Verification of rate law for [OH] and [PTS] oxidation of paracetamol by chloramine-T in NaOH medium at 303 K. [CAT] = 1.00 103 mol dm3, [PA] = 1.00 102 mol dm3, [Pd(II)] = 6.70 105 mol dm3.

At any time during the reaction, the total concentration of CAT (i.e., [CAT]T) can be displayed as ½CATT ¼ ½CAT þ ½X1 þ ½X2 þ ½X3

ð10Þ

Further substitution of the values of [X1], [X2] and [X3] yields eq 11. ½CAT ¼ ð½CATT ½OH ½TsNH2 Þ=ð½OH ½TsNH2 þ K 01 ½OH þ K 01 K 02 ½PA þ K 01 K 02 K 03 ½PA½PdðIIÞÞ

ð11Þ

Substituting the expression for [CAT] (eq 11) into eq 9 produces eq 12. R ¼ ð2k0K 01 K 02 K 03 ½PA½PdðIIÞ½CATT Þ=ð½OH ½TsNH2 þ K 01 ½OH þ K 01 K 02 ½PA þ K 01 K 02 K 03 ½PA½PdðIIÞÞ

ð12Þ

Since [Pd(II)] is on the order of 105, the inequality K01K02K03[PA][Pd(II)] , 1, can be taken as a valid one. With this inequality, eq 12 is converted to eq 13. R ¼ ð2k0K 01 K 02 K 03 ½PA½PdðIIÞ½CATT Þ=ð½OH ½TsNH2 þ K 01 ½OH þ K 01 K 02 ½PAÞ

ð13Þ

Equation 13 is the rate law based on the observed kinetic orders with respect to each reactant of the reaction. By reversing eq 7, eq 14 can be expressed as ½CATT ½OH ½TsNH2 ½OH ¼ þ 0 0 0 0 2k0K 1 K 2 K 3 ½PA½PdðIIÞ 2k0K 2 K 03 ½PA½PdðIIÞ rate 1 þ ð14Þ 2k0K 03 ½PdðIIÞ Equation 14 indicates that if a plot is made between [CAT]T/rate and [OH], [TsNH2]1/[PA], or 1/[Pd(II)] (Figure 13 and 14), straight lines with positive intercepts on the y-axis will be obtained (Figure 13 and 14). The proposed reaction scheme supports the rate law represented in eq 14. From the values of the intercepts and slopes of the plots, the values of k0 K03, K01, and K02 were calculated as 15.98 s1, 8.05 103 mol dm3, and 11.96 103 mol1 dm3, respectively. The proposed mechanism is also supported by the moderate values of energy of activation and other activation

Figure 14. Verification of rate law for 1/[Pd(II)] and 1/[PA] oxidation of paracetamol by chloramine- T in NaOH medium at 303 K. [CAT] = 1.00 103 mol dm3, [OH] = 1.00 103 mol dm3.

parameters. The fairly high positive values of free energy of activation and enthalpy of activation indicate that the transition state is highly solvated, while the large negative entropy of activation suggests the formation of the compact activated complex with fewer degrees of freedom. The reaction product (PTS) does not influence the rate in acidic media since it has been observed that it is not involved in the pre-equilibrium process. Change in the ionic strength of the medium does not alter the rate, indicating that nonionic species are involved in the rate-limiting step. Addition of halide ions had no effect on the rate, which suggests that no interhalogen or free chlorine forms in the reaction. All of the observations also confirm the proposed mechanism. 3.10. Effect of Dielectric Constant and Calculation of the Size of the Activated Complex. To find out the effect of the dielectric constant of the medium on the rate of the reaction, the reaction was studied at different dielectric constants (D) of the medium with constant concentrations of all other reactants and at a constant temperature. The dependence of the rate constant on the dielectric constant of the medium is given by the 8416



ARTICLE

following equation: log k1 ¼ log k0

the validity of the rate laws expressed in eqs 5 and 13, and hence, the proposed reaction mechanisms.

ZA ZB e2 N 1 2:303ð4πεÞdAB RT D

where k0 is the rate constant in a medium of infinite dielectric constant, ZA and ZB are the charges of the reacting ion, dAB represents the size of the activated complex, T is the absolute temperature, and D is the dielectric constant of the medium. This equation suggests that if a plot is made between log k vs 1/D, a straight line can be attained with a slope of 108.2 in an acidic medium and 47.3 in an alkaline medium (Table 3 and Figure 7). A negative dielectric effect supports the proposed mechanism. The effect of changing the solvent composition on the rate of reaction has been discussed in detail.33 For the limiting case of the zero angle approach between two dipoles or an iondipole system, it has been shown that a negative slope of a linear line in the plot of log k versus 1/D is a general result for a reaction between a negative ion and a dipole or between two dipoles. A positive slope relates to a positive iondipole interaction.34 The former concept agrees with the observations in the present study. The values of dAB were evaluated using slopes as 3.2A and 7.4A in acid and alkaline media, respectively. The activation parameters for the oxidation of PA with different catalysts by CAT are summarized in Table 7. The entropy of activation for the designated reaction changed within the observed range. The isokinetic temperature (β) was evaluated from the Exner equation.51 β ¼ T 1 T 2 ðb 1Þ=T 2 b T 1 A plot of log k2 at 308 K versus log k1 at 303 K gave an isokinetic temperature of 407.29 K. Higher values of β (407.29 K) than the experimental temperature (303 K) indicate that the rate is governed by the enthalpy of activation. 3.11. Multiple Regression Analysis. To find out the relationship between dependent, that is, pseudo-first-order variable rate constant k1 and three independent variables, [PA], [PD(II)], and [Hþ] in acidic medium and [PA], [Pd(II)], [OH], and [PTS] in alkaline medium to arrive at a conclusion whether the proposed mechanism is well in accordance with our experimental kinetic data or not, we have taken the help of multivariate regression analysis using the computer package “STATGRAPHICS”. With the aid of a multivariate regression analysis, a relationship between the observed pseudo-first-order rate constant (k1) and concentrations of all the reactants of the reaction, except CAT determined, k1 ¼ k½PA0:8 ½PdðIIÞ0:8 ½Hþ 0:4

ð15Þ

k1 ¼ k½PA0:9 ½PdðIIÞ½OH 0:9 ½PTS0:3

ð16Þ

where k = 1.28 and 3.55 102 for acidic and basic medium, respectively. Since for both the cases p-value in the ANOVA is than 0.01, there is a statistically significant relationship between the variables at the 99% confidence level. This supports the validity of the rate laws given in eqs 5 and 13 for the acidic and alkaline media, respectively. The proposed reactions in Schemes 1 and 2 are also valid because these equations were used to calculate the rates based on the multiple regression analysis (eqs 15 and 16). The similarity among the three rates, that is, the observed (experimentally), calculated (from rate law), and predicted (from regression analysis) results, clearly support

4. COMPARATIVE STUDIES An attempt was made to compare the experimental results in the present study with the results reported earlier for the Ir(III) catalyzed16 and Ru(III) catalyzed17 oxidation of PA by CAT. The reactive species of CAT is HOCl in Ir(III) and Ru(III) catalysts, but TsNHCl was the reactive species of CAT in the Pd(II) catalyzed oxidation of PA in an acidic medium. All catalysts have first- to zero-order kinetics with respect to the concentration of PA. The order with respect to the catalyst varied from first- to zero-order while other catalysts were only first-order. The chloride ion showed a positive effect on the reaction rate in the Ir(III) catalyzed oxidation of PA, while the present study as well as the Ru(III) catalyzed oxidation of PA by CAT had no effect with respect to the chloride ion. Furthermore, efforts were made to compare the finding of this paper with the results reported for the Ir(III) catalyzed16 and Ru(III) catalyzed17 oxidation of PA by CAT in an acidic medium. It can be seen from Table 7 that the activation energy is the highest for the slowest reaction in the Pd(II) catalyzed oxidation of PA in an acidic medium. From the given rate constants and energies of activation (Table 7), the relative reactivity of the catalysts for the oxidation of PA by CAT is in the order: Ir(III) > Ru(III) > Pd(II) alkaline medium > Pd(II) acidic medium. Pd(II) has a d8 electronic configuration and is expected to have the least catalytic efficiency among the catalysts used. 5. CONCLUSIONS The oxidation of PA by CAT experienced a slow reaction rate in both acidic and alkaline media, but increased in rate in the presence of the Pd(II) catalyst. The reactive species involved was only the Pd(II) ion for the Pd(II) chloride catalyzed oxidation of PA by CAT. The reactive species for the oxidation of CAT in an acidic medium was TsNHCl and in an alkaline medium was HOCl . The rate constant of a slow step and other equilibrium constants involved in the mechanism were evaluated and activation parameters with respect to the slow step of the reaction were estimated. Under comparable experimental conditions, the reactive ability of acidic and alkaline media toward the oxidation of PA by CAT was in the order: (OH) > (Hþ). The observed results were explained by plausible mechanisms and the related rate laws were deduced. It can be stated that Pd(II) acts as an efficient catalyst for the oxidation of paracetamol by chloramineT in acidic as well as alkaline media. ’ ASSOCIATED CONTENT

bS

Supporting Information. GCMS results for Quinone oxime. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Address correspondence to [email protected].

’ ACKNOWLEDGMENT A. K. Singh is thankful to UGC, Regional office, Bhopal, M. P., India for Research Project grants. We are thankful to DST-FIST 8417


Industrial & Engineering Chemistry Research for providing us instrumental facilities for research work in our chemistry department. We wish to thank reviewers for the critical and useful comments that refined the manuscript.

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Pd(II) Catalyzed Oxidative Degradation of Paracetamol by Chloramine

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