Kinetics of the Oxidation of Tetracycline Hydrate by Copper (II

Article pubs.acs.org/IECR

Kinetics of the Oxidation of Tetracycline Hydrate by Copper(II) Complexed with Bipyridyl in Alkaline Medium Using Chloro-Complex of Palladium(II) As Homogeneous Catalyst Ashok Kumar Singh,* Manjula Singh, Shahla Rahmani, Jaya Srivastava, and Jagdamba Singh Department of Chemistry, University of Allahabad, Allahabad 211002, India ABSTRACT: The present paper deals with the kinetics and mechanism of Pd(II)-catalyzed oxidation of tetracycline hydrate by Cu(Bip)22+ in alkaline medium. First-order kinetics with respect to [tetracycline hydrate] and fractional positive order kinetics in [Cu(II)] and [Pd(II)] was observed. Nil effect of [OH−] and [bipyridyl] on the rate of oxidation was observed. Almost no effect of ionic strength as well as dielectric constant of the medium on the rate of oxidation was observed. The reaction was studied at four different temperatures and observed values of rate constants were utilized to calculate various activation parameters specially the entropy of activation (ΔS#). With the help of the observed kinetic orders with respect to the reactants involved in the reaction, spectrophotometric evidence collected for the formation of reactive complexes and the positive entropy of activation, a most probable reaction mechanism for Pd(II)-catalyzed oxidation of tetracycline hydrate by Cu(Bip)22+ in alkaline medium has been proposed.

1. INTRODUCTION Tetracyclines1 are well-known antibiotics that have been extensively used in human and veterinary medicine to treat and prevent bacterial infections. It exhibits activity to a broad range of Gram-positive and Gram-negative bacteria including obligatory anaerobic ones. It acts as a bacteriostatic agent.2 It is also reported3 that it has antitumoral activity. Literature reveals that it has strong tendency to complex with metals. A report4 is also available where the extensive use of tetracycline antibiotics such as tetracycline, oxytetracycline, and chlorotetracycline, has been made to treat diseases for humans and in animal feed at subtherapeutic levels to prevent epidemics and increase the growth rate and weight gain in livestock and aquaculture. Mn(II) and Cu(II) significantly enhanced oxidative transformation of tetracyclines in presence of oxygen.5 It is also reported6 that copper−tetracycline complexes can bind DNA, whereas tetracycline itself can not. Several complexes of Cu(II) are frequently used as oxidant for various redox reactions in acidic as well as alkaline medium.7−12 Cu(II) is an essential element that is toxic at elevated concentration. Its reactivity and biological effects are strongly influenced by the extent of it complexation with ligands.13 Pd (II) acts as a efficient catalyst in many redox reactions.14−19 It is also reported20 that Palladiumbased catalysts are found to have high catalytic activity in clean deionized water. The literature survey shows that there is no report on the kinetics and mechanism of oxidation of tetracycline hydrate using Cu(Bip)22+ as oxidant and Pd(II) chloride as homogeneous catalyst. Hence, the present study has been undertaken with a view to ascertain whether 1. Cu(Bip)22+ in the oxidation of tetracycline hydrate behaves in the same way that other organic and inorganic oxidants behaved in the oxidation of various antibiotics like Ampicillin,21 chloramphenicol,22 sulfamethoxazole,23 sulfonamide,24 Norfloxacin,25 levofloxacin,26 and 4-hydroxycoumarin.27 2. tetracycline hydrate plays the role of a substrate in the reaction in the same way that other antibiotics played in their oxidation in alkaline medium.23−27 © 2012 American Chemical Society

3. there is any possibility of the formation of a complex between tetracycline hydrate and Cu(Bip)22+ and also between a complex thus formed, and reactive species of Pd(II)chloride in alkaline medium, that is [PdCl3(OH)]2−. 4. the role of Pd(II) in the oxidation of tetracycline hydrate by Cu(Bip)22+ in alkaline medium is similar to the role of Ru(III) in the oxidation of 4-hydroxycoumarin by alkaline diperiodatonickelate(IV).27

2. EXPERIMENTAL SECTION 2.1. Preparation of Solutions and Instruments Used. All the reagents used were of the highest purity available. The stock solution of copper sulfate, bipyridyl, sodium carbonate, and potassium chloride were prepared by dissolving the known amount of reagent in double distilled water. The solution of tetracycline hydrate was freshly prepared daily. Ionic strength was maintained by adding calculated amount of potassium chloride solution to the reaction mixture. A stock solution of Pd(II) chloride was prepared in known strength of HCl acid. For kinetic measurements, a Varian Carry Win UV−vis spectrophotometer connected with a Peltier accessory and computer was used. For pH measurement, EUTECH Instrument pH 510 was used. 2.2. Method of Study. The absorption spectra of the antibiotic (i.e. tetracycline hydrate) were recorded between 380 and 800 nm, and the maximum absorbance was found at 380 nm with molar absorptivity 0.11 × 105 dm3 mol−1cm−1 (Figure 1). Two sets of reaction mixture containing the reactants copper sulfate, sodium carbonate, catalyst, bipyridyl, and potassium chloride were taken in two separate conical flasks and were placed in a thermostatic water bath at constant temperature of 35 °C with an accuracy of ±0.1 °C. A solution of antibiotic was also kept in another conical flask at the same temperature. When the Received: Revised: Accepted: Published: 5728

February 4, 2012 March 26, 2012 March 27, 2012 March 27, 2012 dx.doi.org/10.1021/ie300306a | Ind. Eng. Chem. Res. 2012, 51, 5728−5736

Industrial & Engineering Chemistry Research

Article

slope of the tangent drawn between remaining concentration of tetracycline hydrate and time. First-order rate constant was calculated by using the formula k1 =

− dc dt

[Tc]

Kinetic study of oxidation of tetracycline was made at several initial concentrations of all reactants. The reaction follows firstorder kinetics with respect to [substrate] (i.e. tetracycline hydrate) throughout its 10-fold variation. When a plot was made between −dc/dt and [tetracycline], a straight line passing through the origin was obtained (Figure 2; correlation Figure 1. Spectra of tetracycline hydrate solutions recorded at room temperature. (1) [Tc] = 0.10 × 10−4 M, [Na2CO3] = 10.00 × 10−2 M; (2) [Tc] = 0.20 × 10−4 M, [Na2CO3] = 10.00 × 10−2 M; (3) [Tc] = 0.40 × 10−4 M; (4) [Tc] = 0.60 × 10−4 M, [Na2CO3] = 10.00 × 10−2 M; (5) [Tc] = 0.80 × 10−4 M, [Na2CO3] = 10.00 × 10−2 M; (6) [Tc] = 1.10 × 10−4 M, [Na2CO3] = 10.00 × 10−2 M.

reaction mixture had attained the required temperature, the calculated amount of antibiotic solution was added in one set of reaction mixture. Immediately, the reaction mixtures were transferred in a quartz cell of 1 cm width and placed in to the spectrophotometer. The progress of the reaction was monitored spectrophotometrically at 380 nm, where the antibiotic (i.e. tetracycline hydrate) absorbs maximum. Here, the mixture containing the solution of tetracycline hydrate acts as absorbing sample, and the mixture without tetracycline hydrate acts as reference sample. Absorbance versus time data over defined time ranges were obtained by spectrophotometric measurement. With the help of Beer’s law, the concentrations of absorbing species (i.e. tetracycline hydrate) at different time intervals were collected. The rate of reaction (i.e. −dc/dt) in each kinetic run was calculated with the help of the plots made between remaining concentrations of tetracycline hydrate and time. 2.3. Stoichiometry and Products Analysis. To find out the number of moles of oxidant consumed by one mole of substrate in the oxidation of tetracycline hydrate in presence of Pd(II) as catalyst, different sets of experiments were carried out, taking the concentration of oxidant as very large in comparison to the concentration of substrate. All the conical flasks having reaction mixtures were allowed to stand for several days at room temperature. On the basis of spectral evidence and kinetic orders, it has been concluded that one mole of tetracycline hydrate is oxidized by one mole of Cu(Bip)22+ in alkaline medium. As a result, the following stoichiometric equation for the aforesaid reaction can be proposed.

Figure 2. Plot between −dc/dt and [Tc] at 35 °C. [Cu(II)*] = 0.50 × 10−3 M; [Free Bip] = 40.00 × 10−4 M; [Pd(II)] = 5.64 × 10−6 M; [Na2CO3] = 10.00 × 10−2 M; μ = 20.00 × 10−1 M.

coefficient = 0.997). This shows that there is direct proportionality between −dc/dt and the concentration of tetracycline hydrate. The reaction shows fractional positive order kinetics in [Pd(II)]. The observation was supported by a plot made between log k1 and log [Pd(II)], where a straight line having positive intercept on log k1-axis was obtained (Figure 3;

Figure 3. Plot between log k1 and log [Pd(II)] at 35 °C. [Tc] = 2.00 × 10−5 M; [Cu(II)*] = 0.50 × 10−3 M; [Free Bip] = 40.00 × 10−4 M; [Na2CO3] = 10.00 ×10−2 M; μ = 20.00 × 10−1 M.

correlation coefficient = 0.996). It was also observed that the pseudo-first-order rate constant, k1, does not increase in the same proportion in which the concentration of Cu(II) is increased. This indicates fractional positive order kinetics in [Cu(II)], which is also supported by a plot made between log k1 and log [Cu(II)] (Figure 4; correlation coefficient = 0.991). There is no effect of [OH−] on the rate of oxidation of tetracycline hydrate by

3. KINETIC RESULTS Kinetic study of Pd(II)-catalyzed oxidation of tetracycline hydrate by Cu(Bip)22+ in alkaline medium was made at 35 °C. The rate of reaction in each kinetic run was determined by the 5729

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Figure 4. Plot between log k1 and log [Cu(II)*] at 35 °C. [Tc ] = 2.00 × 10−5 M; [Free Bip] = 40.00 × 10−4 M; [Pd(II)] = 5.64 × 10−6 M; [Na2CO3] = 10.00 × 10−2 M; μ = 20.00 × 10−1 M. Figure 5. Spectra of Cu(Bip)22+ for solutions [1−6] recorded at room temperature. (1) [Cu(II)*] = 3.00 × 10−3 M, [Free Bip] = 4.00 × 10−3 M, [Na2CO3] = 1.00 × 10−2 M; (2) [Cu(II)*] = 4.00 × 10−3 M, [Free Bip] = 4.00 × 10−3 M, [Na2CO3] = 1.00 × 10−2 M; (3) [Cu(II)*] = 5.00 × 10−3 M, [Free Bip] = 4.00 × 10−3 M; (4) [Cu(II)*] = 6.00 × 10−3 M, [Free Bip] = 4.00 × 10−3 M, [Na2CO3] = 1.00 × 10−2 M; (5) [Cu(II)*] = 7.00 × 10−3 M, [Free Bip] = 4.00 × 10−3 M, [Na2CO3] = 1.00 × 10−2 M; (6) [Cu(II)*] = 8.00 × 10−3 M, [Free Bip] = 4.00 × 10−3 M, [Na2CO3] = 1.00 × 10−2 M.

Cu(Bip)22+ in alkaline medium. The pseudo-first-order rate constant, k1, remains unaffected by change in bipyridyl concentration. Almost no effect of the ionic strength as well as dielectric constant of the medium on the rate of oxidation was observed in present investigation. The reaction was studied at four different temperatures (30, 35, 40, and 45 °C; Table 1). The rate constants calculated at different temperatures were utilized to calculate various activation parameters (Table 2).

librium constants for the equilibria shown below have been determined by several workers and all the results are in agreement with a value of log β4 between 11 and 12 at 25 °C.

Table 1. Observed Values of Rate and Rate Constant for Pd(II)-Catalyzed Oxidation of Tetracycline Hydrate by Cu(Bip)22+ in Alkaline Medium at 30, 35, 40, and 45 °Ca temp. (K)

−dc/dt × 108 (mol dm−3sec−1)

k1 × 104 (sec−1)

303 308 313 318

0.52 0.72 1.08 1.77

2.58 3.58 5.42 8.83

Pd2 + + Cl− ↔ PdCl+

K1

PdCl+ + Cl− ↔ PdCl 2

K2

PdCl 2 + Cl− ↔ PdCl3−

K3

PdCl3− + Cl− ↔ PdCl4 2 −

Solution conditions: [Tc] = 2.00 × 10−4 M; [Cu(II)] = 0.50 × 10−3 M; [Free Bip] = 4.00 × 10−3 M; [OH−] = 10.00 × 10−2 M; [Pd(II)] = 5.64 × 10−6 M; μ = 30.00 × 10−2 M. a

K4

The study for the stability constants and rates of reaction has been made by Elding.31 The values of log k1 to log k4 determined by him were 4.47, 3.29, 2.41, and 1.37, respectively, with log β4 equal to 11.5. A comparison can be made with values of 4.3, 3.54, 2.68, and 1.68 determined by Grinberg and co-workers.32 The probability of K4 being the most important stability constant for catalytic chemistry is also reported. In the present study of the oxidation of tetracycline, Pd(II)chloride has been used as homogeneous catalyst. Ayres33 has reported that when a reaction ratio of 2:1 for sodium chloride to Pd(II)chloride is maintained, it will result exclusively in the formation of tetrachloropalladate(II), [PdCl4]2−. Since throughout the study the ratio of chloride ion to Pd(II)chloride was maintained more than two, it will be reasonable to assume that Pd(II)chloride in HCl solution exist exclusively in the form of [PdCl4 ]2− . Further, because the experiments are performed in alkaline medium, there is every possibility of converting lone species, that is, [PdCl4]2−, of Pd(II)chloride into [PdCl3(OH)]2− according to the following equation:

4. DISCUSSION AND MECHANISM In the present study, Cu(II) has been used with bipyridyl as complexing agent in alkaline medium. On the basis of kinetic results obtained and spectroscopic evidence collected, it can be assumed that the reactive species of Cu(II) is Cu(Bip)22+ instead of free Cu(II). Formation of the complex, Cu(Bip)22+, is confirmed by its absorption at λmax= 670 nm with molar extinction coefficient 0.59 × 102 dm3 mol−1 cm−1 (Figure 5), which is also supported by literature.28 Going through the literature5,6,29 and making basis to the kinetic observations along with spectral information, it has been concluded that the tetracycline hydrate as such participates in its Pd (II)-catalyzed oxidation by Cu(Bip)22+ in alkaline medium. It is reported30 that Pd(II)chloride is insoluble in aqueous solution but does dissolve in the presence of chloride ion as PdCl3(H2O)− and PdCl42−. Literature also reveals that the equi-

[PdCl4]2 − + OH− → [PdCl3(OH)]2 − + Cl−

Table 2. Activation Parameters for Pd(II)-Catalyzed Oxidation of Tetracycline Hydrate by Cu(Bip)22+ in Alkaline Medium at 35 °Ca substrate

kr (mol−2 dm6 sec1)

Δ H# (kJ mol−1)

Δ G# (kJ mol−1)

Δ S# (eu)

A (mol−2 dm6 sec1)

tetracycline hydrate

1.27 × 10

66.59

45.57

68.21

5.18 × 1016

5

Solution conditions: [Tc] = 2.00 × 10−4 M; [Cu(II)] = 0.50 × 10−3 M; [Free Bip] = 4.00 × 10−3 M; [OH−] = 10.00 × 10−2 M; [Pd(II)] = 5.64 × 10−6 M; μ = 30.00 × 10−2 M.

a

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Kinetic data demonstrate that there is an increase in pseudofirst-order rate constant k1 with the increase in Pd(II)chloride concentration; hence, it can safely be assumed that [PdCl3(OH)]2− is the reactive species of Pd(II)chloride in alkaline medium. The existence of the species [PdCl4]2− in HCl and also the species [PdCl3(OH)]2− in alkaline medium is quite obvious from the spectra of the solution containing Pd(II)chloride and HCl, as well as of the solutions containing starting of Pd(II)chloride, that is, [PdCl4]2− and two different concentrations of OH− (furnished by Na2CO3 solution; Figure 6). Our assumption for the species

Figure 7. Spectra of solutions [1−5] recorded at room temperature. (1) [Tc] = 2.00 × 10−5 M; (2) [Tc] = 2.00 × 10−5 M, [Na2CO3] = 0.10 M; (3) [Tc] = 2.00 × 10−5 M, [Na2CO3] = 0.10 M, [Cu(II)*] = 0.25 × 10−3 M, [Free Bip] = 4.00 × 103 M; (4) [Tc] = 2.00 × 10−5 M, [Na2CO3] = 0.10 M, [Cu(II)*] = 0.50 × 10−3 M, [Free Bip] = 4.00 × 10−3 M; (5) [Tc] = 2.00 × 10−5 M, [Na2CO3] = 0.10 M, [Cu(II)*] = 1.00 × 10−3 M, [Free Bip] = 4.00 × 10−3 M.

Figure 6. Spectra of solutions [1−3] recorded at room temperature. (1) [Pd(II)] = 5.64 × 10−4 M; (2) [Pd(II)] = 5.64 × 10−4 M, [Na2CO3] = 0.05 M; (3) [Pd(II)] = 5.64 × 10−4 M, [Na2CO3] = 0.10 M.

[PdCl3(OH)]2− in alkaline medium also finds support from the literature where [PdCl3(OH)]2−, [PdCl2(OH)2]2−, [PdCl (OH)3]2−, and [Pd(OH)4]2− as the possible Pd(II) complex species in alkaline solution are reported.34 To verify the formation of a complex between Cu(Bip)22+ and tetracycline hydrate in alkaline medium, spectra for solutions containing tetracycline hydrate and OH− as well as tetracycline hydrate and OH− with three different concentrations of Cu(Bip)22+ have been taken with the help of a Varian Carry 300 Bio UV−vis spectrophotometer at room temperature (Figure 7). From Figure 7, it is quite evident that, with the increase in concentration of Cu(Bip)22+, there is increase in absorbance from 0.61 to 0.68, 0.86, and 1.07. This increase in absorbance can be considered as due to increasing

formation of the complex

Figure 8. Spectra of solutions [1−6] recorded at room temperature. (1) [Tc] = 2.00 × 10−5 M, [Na 2CO3] = 0.10 M; (2) [Tc] = 4.00 × 10−5 M; [Na2CO3] = 0.10 M; (3) [Tc] = 4.00 × 10−5 M, [Na2CO3] = 0.10 M, [Cu(II)*] = 0.40 × 10−3 M, [Free Bip] = 4.00 × 10−3 M; (4) [Tc] = 4.00 × 10−5 M, [Na2CO3] = 0.10 M, [Cu(II)*] = 0.80 × 10−3 M, [Free Bip] = 4.00 × 10−3 M; (5) [Tc] = 4.00 × 10−5 M, [Na2CO3] = 0.10 M, [Cu(II)*] = 0.80 × 10−3 M, [Free Bip] = 4.00 × 10−3 M, [Pd(II)] = 11.28 × 10−6 M; (6) [Tc] = 4.00 × 10−5 M, [Na2CO3] = 0.10 M, [Cu(II)*] = 0.80 × 10−3 M, [Free Bip] = 4.00 × 10−3 M, [Pd(II)] = 22.56 × 10−6 M.

, according

of Cu(Bip)22+ have been collected (Figure 8(2 and 3)). This information again confirms the formation of a complex to the following equilibrium: , between tetracycline hydrate

and Cu(Bip)22+ in alkaline medium. Further, when spectra for solutions containing tetracycline hydrate, OH−, and Cu(Bip)22+, and also tetracycline hydrate, OH−, and Cu(Bip)22+, with two different concentrations of Pd(II) were collected, it is found that there is an increase in absorbance from 0.91 to 1.06 and 1.22, with the increase in concentration of Pd(II) chloride.

Our observation finds support from the reported35 literature also. To verify the formation of the complex mentioned above in Pd(II)-catalyzed oxidation of tetracycline hydrate, spectra for solutions containing tetracycline hydrate and OH− as well as tetracycline hydrate and OH− with two different concentrations 5731



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This increase in absorbance can be regarded as due to more and

On putting the value of [C1] from eq 2 into eq 4, we have [S]T =

more formation of a complex

,

k1[S][Cu(II)*] + [S] k −1 + k 2[Pd(II)]

or according to the following equilibrium:

[S] =

[S]T {k −1 + k 2[Pd(II)]} k1[Cu(II)*] + k −1 + k 2[Pd(II)]

(5)

with the help of eqs 3 and 5, we can write eq 6: rate = −

On the basis of kinetic observations, it has been concluded

that the formation of the complex

k1k 2[S]T [Cu(II)*][Pd(II)] d[Tc] = dt k −1 + k1[Cu(II)*] + k 2[Pd(II)]

(6)

Equation 6 is the final rate law that is strictly in accordance with our experimental findings where unity order in [S] and fractional positive orders in [Cu(II)*] and [Pd(II)] were obtained. On reversing eq 6, we obtain

,

k −1 [S]T 1 = + k1k 2[Cu(II)*][Pd(II)] k 2[Pd(II)] rate 1 + k1[Cu(II)*]

in the proposed reaction path will not be shown in a reversible manner, but it will be shown in an irreversible manner.

(7)

Equation 7 can also be written as eqs 8 and 9: ⎧ [S]T k −1 1⎫ 1 1 =⎨ + ⎬ + rate k1 ⎭ [Cu(II)*] k 2[Pd(II)] ⎩ k1k 2[Pd(II)]

Further, since the solution remained homogeneous throughout the course of the reaction, there is no possibility of formation of Pd clusters, as reported36−38 earlier. With the help of the observed kinetic orders with respect to the reactants involved in the reaction, spectrophotometric evidence collected for the formation of reactive complexes and the positive entropy of activation, a most probable reaction mechanism in the form of Reaction Scheme 1 for Pd(II)-catalyzed oxidation of tetracycline hydrate by Cu(Bip)22+ in alkaline medium can be proposed. According to Scheme 1, the rate in terms of decrease in concentration of tetracycline hydrate can be expressed as rate = −

d[Tc] = k 2[C1][Pd(II)] dt

(8)

⎧ [S]T k −1 1⎫ 1 1 =⎨ + ⎬ + * *] rate k k [Cu(II) ] k [Pd(II)] k [Cu(II) ⎩ 1 2 2⎭ 1 (9)

Equations 8 and 9 show that if a plot is made between [S]T/rate versus 1/[Cu(II)*] or between [S]T/rate versus 1/[Pd(II)], a straight line having an intercept on y-axis will be obtained. When plots between [S]T/rate and 1/[Cu(II)*] and between [S]T/rate and 1/[Pd(II)] were made, the straight lines having intercepts on y-axis were obtained (Figure 9 (correlation coefficient = 0.991) and Figure 10 (correlation coefficient =0.979)). This proves the validity of the rate law in eq 6 and, hence, the proposed Reaction Scheme 1. From the intercepts and slopes, the values of constants k1, k2, and k−1 were calculated and found as 3.01 mol−1 dm3 sec−1, 0.27 × 103 mol−1 dm3 sec−1, and 5.78 × 10−3 s−1, respectively. Utilizing these values of the rate constants the rates for the variations of [Cu(II)*] and [Pd(II)] were calculated and are presented in Table 3. From a perusal of Table 3, it is quite evident that there is a close similarity between the observed rates and the calculated rates. This further proves the validity of the rate law in eq 6 and, hence, the proposed mechanism. Observed positive entropy of activation for Pd(II)-catalyzed oxidation of tetracycline hydrate supports the rate determining step (II) of the proposed reaction path, where the most reactive

(1)

where Tc represents tetracycline hydrate and Pd(II) represents [Pd(OH)Cl3]2−. On applying steady state approximation to the concentration of C1, we have eq 2: d[C1] = 0 = k1[S][Cu(II)*] − k −1[C1] − k 2[C1][Pd(II)] dt

or [C1] =

k1[S][Cu(II)*] k −1 + k 2[Pd(II)]

(2)

On substituting the value of [C1] from eq 2 into eq 1, we get rate = −

k k [S][Cu(II)*][Pd(II)] d[Tc] = 1 2 dt k −1 + k 2[Pd(II)]

(3)

activated complex

At any moment in the reaction, the total concentration of S (i.e. [S]T) can be shown in the following way: [S]T = [C1] + [S]

(4)

, with no net

charge, is being formed by the interaction of two oppositely 5732



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Scheme 1. Reaction Scheme for Pd(II)-Catalyzed Oxidation of Tetracycline Hydrate by Cu(Bip)22+ in Alkaline Medium

charged species, that is,

was compared with the studies reported for the oxidation of some other antibiotics, such as Ampicillin,21 chloramphenicol,22 sulfamethoxazole,23 sulfonamide,24 Norfloxacin,25 levofloxacin,26 and 4-hydroxycoumarin,27 by different organic and inorganic oxidants in acidic/alkaline medium. As far as the order with respect to oxidant (i.e. Cu(II)) is concerned, it is fractional positive throughout its variation, which is quite different from the observed first-order in all reported studies.21−27 In the present investigation, Cu(Bip)22+ is assumed as reactive species of Cu(II) in alkaline medium. In earlier reported21−27 studies [Mn(H2P2O7)33−], CBTH+, HFeO4−, and FeO42−, [MnO4·OH]2− and [Ni(OH)2(H3IO6)3(H2IO6)]3− had been considered as the reactive species of Mn(III),21 CBT,22 Fe(VI),23,24 Mn(VII),25,26

and

[Pd(OH)Cl3]2−. In this case the transition state will be less polar than the initial state.

5. COMPARATIVE STUDIES The present study of Pd(II)-catalyzed oxidation of an antibiotic (i.e. tetracycline hydrate by Cu(Bip)22+ in alkaline medium) 5733



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reported oxidation of chloramphenicol,22 Norfloxacin,25 levofloxacin,26 and 4-hydroxycoumarin.27 When efforts were made to find out the effect of the Pd(II)chloride on the rate of oxidation of tetracycline hydrate, it was found that the kinetic order is fractional positive, which is not similar to the reported unity order kinetics of Ru(III).27 Nil effect of [alkali] on the rate of oxidation of tetracycline hydrate by Cu(Bip)22+ was observed in the present investigation, which is quite different from other reported studies where rate of oxidation was either increased or decreased by enhancing the pH of medium. Observed positive entropy of activation in the present study provides support to the rate determining step where an interaction takes place between two oppositely charged species. This finding of positive entropy, on one hand, is similar to the reported oxidation of Ampicillin,21 sulfamethoxazole,23 sulfonamide24 and, on the other hand, it is entirely different from the reported oxidation of chloramphenicol,22 Norfloxacin,25 levofloxacin,26 and 4-hydroxycoumarin,27 where negative entropy was observed. In the light of the facts mentioned above, it can be concluded that the present study is almost different from the other studies reported earlier.

Figure 9. Plot between [S]T/rate and 1/ [Cu(II)*]. [Tc ] = 2.00 × 10−5 M; [Free Bip] = 2.00 × 10−3 M; [Na2CO3] = 10.00 × 10−2 M; [Pd(II)] = 5.64 × 10−6 M; μ = 20.00 × 10−1 M.

6. CONCLUSIONS The conclusions drawn from the observed kinetic data and also from the spectral information collected for the Pd(II)-catalyzed oxidation of tetracycline hydrate by Cu(Bip)22+ in alkaline medium are as follows: 1. Cu(Bip)22+, tetracycline hydrate, and [Pd(OH)Cl3]2− have been assumed as the reactive species of Cu(II), tetracycline hydrate, and Pd(II) chloride in alkaline medium, respectively. 2. The reaction shows first-order kinetics with respect to [tetracycline hydrate] and fractional positive order kinetics in [Cu(II)] and [Pd(II)].

Figure 10. Plot between [S]T/rate and 1/ [Pd(II)]. [Tc ] = 2.00 × 10−5 M; [Cu(II)*] = 0.50 × 10−3 M; [Free Bip] = 2.00 × 10−3 M; [Na2CO3] = 10.00 × 10−2 M; μ = 20.00 × 10−1 M.

Table 3. Calculated and Experimental Values of the Rate for the Variations of [Cu(II)*] and [Pd(II)] in the Pd(II)Catalyzed Oxidation of Tetracycline Hydrate by Cu(Bip)22+ Complex at 35 °Ca [Cu(II)*] × [Pd(II)] × 103 (M) 106 (M) 0.10 0.30 0.50 1.00 2.00 3.00 0.50 0.50 0.50 0.50 0.50 0.50

5.64 5.64 5.64 5.64 5.64 5.64 2.82 5.64 11.28 16.92 22.56 28.20

−dc/dt × 108 (mol dm−3 sec−1), calcd.

−dc/dt × 108 (mol dm−3 sec−1), exptl.

0.12 0.33 0.52 0.88 1.34 1.67 0.15 0.28 0.51 0.87 1.14 1.35

0.12 0.32 0.60 0.88 1.22 1.92 0.13 0.35 0.65 0.83 1.28 1.57

3. The complex,

, has been

proposed as the most reactive activated complex in the Pd(II)-catalyzed oxidation of tetracycline hydrate by Cu(Bip)22+ in alkaline medium. 4. Step (II) of Reaction Scheme1, where an interaction between two oppositely charged species, that is,

and [Pd(OH)Cl3]2−, is

shown, is well supported by the observed positive entropy of activation. 5. The rate of oxidation of tetracycline hydrate by Cu(Bip)22+ is unaffected by the change in concentration of hydroxyl ions and the change in dielectric constant of the medium. 6. The complex C1 formed between tetracycline hydrate and Cu(Bip)22+ in alkaline medium becomes more reactive when it interacts with the reactive species of the catalyst, that is, [Pd(OH)Cl3]2−, in the rate determining step of the propose reaction path.

a

Solution conditions for [Cu(II)*] variation and [Pd(II)] variation: [Free Bip] = 2.00 × 10−3 M; [OH−] = 1.41 × 10−4 M; μ = 2.00 M.

and DPN27 in acidic/alkaline medium respectively. First-order kinetics was reported in [substrate] (i.e. tetracycline hydrate), which is similar to the reported oxidation of Ampicillin,20 sulfamethoxazole,23 and sulfonamide24 but different from the 5734



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fructose by N-bromoacetamide in perchloric acid medium: A kinetic and mechanistic study. Carbohydr. Res. 2006, 341, 397−409. (18) Singh, A. K.; Yadav, S.; Srivastava, R.; Srivastava, J.; Rahmani, S. Kinetic and mechanistic investigation of Pd(II) catalysed and Hg(II)co-catalysed oxidation of d(+) melibiose by N-bromoacetamide in acidic medium. J. Organomet. Chem. 2010, 695, 2213−2219. (19) Hiremath, G. A.; Timmanagoudar, P. L.; Nandibewoor, S. T. Oxidation of allyl alcohol by alkaline periodate in the presence of micro amounts of palladium(II). J. Phys. Org. Chem. 1998, 11 (1), 31− 35. (20) Angeles-Wedler, D.; Mackenzie, K.; Kopinke, F. D. Palladiumcatalysed hydrodechlorination for water remediation: Catalyst deactivation and regeneration. Int. J. Civil Environ. Eng. 2010, 2 (1), 49−52. (21) Subrahmanyam, E. V. S.; Bhat, K. I.; Shrigara, B. S.; Satyanarayana, D. Kinetics and mechanism of oxidative transformations of (6R)-6-(α-phenyl-D-glycylamino)penicillanic acid (ampicillin) by manganese(III) in pyrophosphate medium. Pol. J. Chem. 2000, 74, 793−800. (22) Hiremath, R. C.; Jagdeesh, R. V.; Puttaswamy; Mayanna, S. M. Kinetics and mechanism of oxidation of chloramphenicol by 1-chlorobenzotriazole in acidic medium. J. Chem. Sci. 2005, 117 (4), 333−336. (23) Sharma, V. K.; Mishra, S. K.; Ray, A. K. Kinetic assessment of the potassium ferrate(VI) oxidation of antibacterial drug sulfamehtoxazole. Chemosphere 2006, 62, 128−134. (24) Sharma, V.; Mishra, S. K.; Nesnas, N. Oxidation of sulphonamide antimicrobials by ferrate(VI) [FeVIO42−]. Environ. Sci. Technol. 2006, 40, 7222−7227. (25) Naik, P. N.; Chimatadar, S. A.; Nandibewoor, S. T. Kinetics and oxidation of fluoroquinoline antibacterial agent, Norfloxacin, by alkaline permanganate: A mechanistic study. Ind. Eng. Chem. Res. 2009, 48, 2548−2555. (26) Khan, A. A. P.; Mohd, A.; Bano, S.; Husain, A.; Siddiqi, K. S. Trans. Met.Chem. 2010, 35, 117−123. (27) Shettar, R. S.; Nandibewoor, S. T. Kinetic, mechanistic and spectral investigations of ruthenium(III)-catalysed oxidation of 4hydroxycoumarin by alkaline diperiodatonickelate(IV) (stopped flow technique). J. Mol. Catal. A: Chem. 2005, 234, 137−143. (28) Prenesti, E.; Daniele, P. G.; Berto, S.; Toso, S. Spectrum− structure correlation for visible absorption spectra of copper(II) complexes showing axial co-ordination in aqueous solution. Polyhedron. 2006, 25, 2815−2823. (29) Chen, Y.; Hu, C.; Qu, J.; Yang, M. Photodegradation of tetracycline and formation of reactive oxygen species in aqueous tetracycline solution under simulated sunlight irradiation. J. Photochem. Photobiol. A: Chem. 2008, 197, 81−87. (30) Henry, P. M. Palladium Catalyzed Oxidation of Hydrocarbons; D. Reidel Publishing Company: Holland, 1928; pp 11−12. (31) Elding, L. I. Palladium(II) halide complexes. I. Stabilities and spectra of palladium(II) chloro and bromo aqua complexes. Inorg. Chim. Acta 1972, 6, 647−651. (32) Grinberg, A. A.; Kiseleva, N. V.; Gel’fman, M. I. O Konstantakh nestoikosti palladievikh kompleksov soedinenija tipa K2[PdX4]. Dokl. Acad. Nauk. SSSR 1963, 153, 1327−1329. (33) Ayres, G. H. Reactions involving colored complexes of the platinum metals. Anal. Chem. 1953, 24, 1622−1627. (34) Singh, A. K.; Negi, R.; Jain, B.; Katre, Y.; Singh, S. P.; Sharma, V. K. Pd(II) catalysed oxidative degradation of paracetamol by chloramines-T in acidic and alkaline media. Ind. Eng. Chem. Res. 2011, 50, 8407−8419. (35) Palm, G. J.; Lederer, T.; Orth, P.; Saenger, W.; Takahashi, M.; Hillen, W.; Hinrichs, W. Specific binding of divalent metal ions to tetracycline and to the tet repressor/ tetracycline complex. J. Bio. Inorg. Chem. 2008, 13, 1097−1110. (36) Mukhopadhyay, S.; Rothenberg, G.; Gitis, D.; Sasson, Y. Tandem one-pot palladium-catalysed reductive and oxidative coupling of benzene and cholrobenzene. J. Org. Chem. 2000, 65, 3107−3110.

AUTHOR INFORMATION

Corresponding Author

*Tel: +91-0532-2462266. Fax: +91-0532-2462266. E-mail: ashokeks@rediffmail.com, [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Kinetics of the Oxidation of Tetracycline Hydrate by Copper (II

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