Reaction Kinetics and Mechanism of Copper(II) Catalyzed Oxidative

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Reaction Kinetics and Mechanism of Copper(II) Catalyzed Oxidative Deamination and Decarboxylation of Ornithine by Peroxomonosulfate Paramasivam Suresh Kumar, Radhakrishnan Mohan Raj, Srinivasalu Kutti Rani, and Deivanayagam Easwaramoorthy* Department of Chemistry, B.S. Abdur Rahman University, Chennai 600048, India ABSTRACT: Copper(II) catalyzed and uncatalyzed oxidation of ornithine by peroxomonosulfate (PMS) was studied in acetic acid−sodium acetate buffered medium (pH 3.6−5.2). The catalyzed reaction was 2.6 times faster than the uncatalyzed reaction. The catalytic constant kc obtained in this study was 0.15 mol−1 dm3 s−1. A negative value of entropy of activation obtained in this reaction revealed that the transition state was more rigid than the reactants. ESR spectral data ruled out the participation of free radical intermediate. Cyclic voltammetric and absorption studies confirmed the formation of copper−ornithine−PMS complex. HPLC analysis revealed that the product formed in this reaction was 4-aminobutanal, which was confirmed by NMR spectra.

1. INTRODUCTION Peroxomonosulfate ion (HSO5−), the anion of Caro’s acid (H2SO5), is an inexpensive and environmentally friendly oxidant and has wide applications.1−3 Oxidation studies of peroxomonosulfate (PMS) with several compounds such as hypophosphorous acid,4 indole-3-acetic acid,5 oxovanadium(IV) and its aminopolycarboxylate,6 aromatic aldehydes,7 alkyl thiocyanates,8 tris(1,10-phenanthroline)iron(II),9 hydroxylamine,10 ascorbic acid,11 nickel(II) lactate,12 vanillin,13 and amino acids,14,15 have been reported. During the citric acid cycle, arginine, a protonic amino acid, is broken down to ornithine, which is a non-protein amino acid. Ornithine helps to build muscles and reduce body fat, especially in combination with arginine and carnitine. Ornithine helps to remove toxic ammonia from the liver and reduces the effects of cirrhosis of the liver and other disorders associated with malfunctioning of the liver. It is the major source of polyamines in mammalian physiological systems. Hence studies on the oxidation of ornithine are gaining importance. Few reports are available in the literature regarding the kinetics and oxidation of ornithine.16−18 Transition metals are known to catalyze many oxidation−reduction reactions, since they exhibit multiple oxidation states. Copper complexes have a major role in oxidation chemistry, due to their abundance and relevance in biological chemistry. Though copper(II) was used as the catalyst for several oxidation reactions,19−21 it has not been explored in the oxidation of α-amino acids by PMS. Hence the title study was carried out and the results obtained are discussed in this paper.

purification. PMS solution was freshly prepared every day, stored in a blackened vessel to prevent photodecomposition, and standardized iodometrically. Ornithine was obtained from Merck, India, and used as received. The chemicals such as sodium acetate and sodium perchlorate were of analar grade and used as received. Acetic acid was distilled to remove impurities and used to make the buffer solution. Analar grade solvents such as acetonitrile and 2-methyl-2-propanol were distilled and used for the reactions. 2.2. Kinetic Measurements. The kinetics of oxidation of ornithine by PMS, both in the presence and absence of copper(II) sulfate catalyst in acetic acid−sodium acetate buffered medium, was studied under pseudo-first-order conditions, i.e., [ornithine] ≫ [PMS] at various time intervals. A known volume of PMS solution, thermostatted at the desired temperature, was pipetted out into the reaction mixture of ornithine in buffer, and simultaneously a timer was started. Consumption of PMS in this reaction mixture was monitored by the iodometric method. The rate of the reaction followed first-order kinetics as shown in Figure 1, and the rate constant kobs was calculated from the linear plot of log [PMS]t vs time according to eq 1. log [PMS]t = log [PMS]0 − kt /2.303

The method of least squares was used to calculate the slope and intercept. The relative standard errors of the above mentioned rate constant for a single run and the relative standard errors of the mean were about 2%. The same methodology was adopted for the Cu(II) catalyzed oxidation. 2.3. Catalytic System. Copper sulfate pentahydrate was used as a homogeneous catalyst. The concentration of Cu(II) in the reaction mixture was kept at 2.5 × 10−3 mol dm−3.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. All the solutions used in this study were prepared with double-distilled water, and the solutions were freshly prepared every day from the reagent before starting the experiments. PMS was obtained from Aldrich, USA, and the purity of the sample was found to be 98% when tested by iodometric estimation22 and hence used without further © 2012 American Chemical Society

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Received: Revised: Accepted: Published: 6310

October 20, 2011 March 30, 2012 March 31, 2012 March 31, 2012 dx.doi.org/10.1021/ie202409p | Ind. Eng. Chem. Res. 2012, 51, 6310−6319

Industrial & Engineering Chemistry Research

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Figure 1. Plot of log [PMS]t vs time in the presence and absence of copper(II). (A) In the absence of copper(II): [ornithine] = 0.05 mol dm−3; [sodium acetate] = 0.085 mol dm−3; [Cu(II)] = 2.5 × 10−3 mol dm−3; pH 4.0 ± 0.1; [PMS] = 3.84 × 10−3 mol dm−3. (B) In the presence of copper(II): [ornithine] = 0.05 mol dm−3; [sodium acetate] = 0.085 mol dm−3; pH 4.0 ± 0.1; [PMS] = 3.84 × 10−3 mol dm−3.

Table 1. Effect of Varying Concentrations on the Reaction Rate at 308 K

a

103[PMS] (mol dm−3)

102[ornithine] (mol dm−3)

102[NaOAc] (mol dm−3)

pH ± 0.1

104kobsa (s−1)

103[Cu(II)] (mol dm−3)

104kobsb (s−1)

temp (K)

10kc

1.92 3.84 5.76 7.68 9.60 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84 3.84

5.00 5.00 5.00 5.00 5.00 2.50 5.00 6.25 7.50 10.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00

8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 2.13 4.25 6.38 8.50 10.63 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50 8.50

4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 3.6 4.0 4.4 4.6 4.8 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

2.49 2.42 2.15 2.26 2.34 1.46 2.42 3.07 3.68 4.38 2.03 2.11 2.26 2.42 2.53 2.07 2.42 2.88 3.15 3.26 2.42 2.42 2.42 2.42 2.42 1.34 2.42 3.64 5.14 8.33

2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 2.50 1.25 2.50 3.75 5.00 6.25 2.50 2.50 2.50 2.50 2.50

5.10 6.26 5.87 5.68 5.57 3.84 6.26 8.06 8.56 9.44 8.14 7.33 6.64 6.26 5.83 3.65 6.26 11.78 17.58 23.53 7.68 9.79 12.40 14.20 16.66 4.11 6.26 9.58 12.36 22.91

308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 308 303 308 313 318 323

1.04 1.54 1.49 1.37 1.29 0.95 1.54 2.00 2.00 2.02 2.44 2.09 1.75 1.54 1.32 1.58 1.54 3.56 5.77 8.11 4.21 2.95 2.67 2.36 2.28 1.11 1.54 2.38 2.89 5.83

Without Cu(II). bWith Cu(II).

2.4.1. ESR Spectral Analysis. The reaction mixture was scanned in the electron spin resonance spectrometer on a Varian E-112 (microwave power 20 μW; DPPH; g value = 2.002 32; magnetic field strength 3300 G) to decipher the types of intermediate (molecule/free radical) formed during the course of the reaction. The spectrum was taken immediately after preparation of the reaction mixture and monitored at different time intervals as well. ESR spectra were taken for the copper(II) catalyzed and uncatalyzed reactions at room temperature (298 K). Interestingly, an ESR signal was obtained for the Cu(II) catalyzed reaction only.

2.4. Spectral Analysis. The reaction mixture containing ornithine in buffer and PMS was scanned in the ultraviolet and visible regions on a Perkin-Elmer LS 25 UV spectrophotometer in order to unravel the intermediate formed during the course of the reaction. Spectra were taken for both copper(II) catalyzed and uncatalyzed reactions. The concentrations of ornithine and PMS in the spectral solution in both cases were kept in the ratio 10:1.5. The spectra were taken immediately after preparation of the reaction mixture in a 1 cm cell at room temperature (298 K). The time history of the absorption spectra was monitored at different time intervals. 6311

dx.doi.org/10.1021/ie202409p | Ind. Eng. Chem. Res. 2012, 51, 6310−6319


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Figure 2. Plot of kobs vs [ornithine] in the presence and absence of copper(II). (A) In the presence of copper(II): [sodium acetate] = 0.085 mol dm−3; [Cu(II)] = 2.5 × 10−3 mol dm−3; pH 4.0 ± 0.1; [PMS] = 3.84 × 10−3 mol dm−3. (B) In the absence of copper(II): [sodium acetate] = 0.085 mol dm−3 ; pH 4.0 ± 0.1; [PMS] = 3.84 × 10−3 mol dm−3.

Figure 3. Plot of kobs[H+] vs [H+] in the presence and absence of copper(II). (A) In the presence of copper(II): [ornithine] = 0.05 mol dm−3; [sodium acetate] = 0.085 mol dm−3; [Cu(II)] = 2.5 × 10−3 mol dm−3; [PMS] = 3.84 × 10−3 mol dm−3. (B) In the absence of copper(II): [ornithine] = 0.05 mol dm−3; [sodium acetate] = 0.085 mol dm−3; [PMS] = 3.84 × 10−3 mol dm−3.

2.4.2. Cyclic Voltammetric Studies. Electrochemical studies were carried out with a CHI 760C (CH Instruments, Inc., USA). Cyclic voltammetric measurements were performed at room temperature in an undivided cell (C-3 cell stand) with a glassy carbon electrode, platinum counter electrode, and a calomel reference electrode. All potentials were reported with respect to the saturated calomel electrode (SCE). Prior to the experiment, the solutions were deoxygenated by passing dry nitrogen for 30 min, and during the experiments the nitrogen flow was maintained.

when [PMS] > [ornithine] and [ornithine] > [PMS] was represented by eq 2.

Qualitative tests were carried out to identify the products of oxidation of ornithine by PMS. The reaction mixture containing a large excess of PMS over ornithine was allowed to stand for 48 h in a blackened vessel at room temperature. Excess PMS present in the reaction mixture was destroyed by adding sodium bisulfite, and then the mixture was extracted with dichloromethane. The organic layer was separated, dried, and given for IR analysis. From the IR data, absorption at 3427 cm−1 is due to amine, 2900 cm−1 is due to CH stretching, and 1711 cm−1 is due to CO of carbonyl group. The product was further confirmed by NMR studies. NMR studies were carried out with a JEOL AL 300 MHz spectrometer. The NMR data were given below δ 1.62 (CH2, 2H, m), δ 1.81 (CH2, 2H, m), δ 2.92 (CH2, 2H, t), and δ 9.47 (CHO, 1H, s), which confirmed the formation of 4-aminobutanal. The evolution of oxygen during the self-decomposition of PMS was confirmed by the color change with alkaline sodium dithionite activated by indigo carmine.23 3.2. Effect of [PMS] on kobs. The values of kobs were calculated for different concentrations of PMS by maintaining the other parameters at constant values. The results showed that the rate constant was unaffected with increase in [PMS] in

3. RESULTS AND DISCUSSION 3.1. Stoichiometry and Product Analysis. The stoichiometries of the reactions were determined for both copper(II) catalyzed and uncatalyzed reactions for the reaction mixtures containing a large excess of [PMS] over [ornithine]. Then the reaction mixture was allowed to stand for 48 h and the unconsumed PMS was estimated iodometrically. Corrections for the self-decomposition of PMS were made from the values obtained from the control experiments. The stoichiometry of the reaction was determined for the reaction mixture containing a large excess of [ornithine] over [PMS] in buffer as well. The excess [ornithine] was determined spectrophotometrically. The observed stoichiometry of the reaction in both copper(II) catalyzed and uncatalyzed reaction (ornithine:PMS = 1:1) 6312



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Figure 4. Plot of 1/kobs vs [NaOAc]. [ornithine] = 0.05 mol dm−3; [Cu(II)] = 2.5 × 10−3 mol dm−3; pH 4.0 ± 0.1; [PMS] = 3.84 × 10−3 mol dm−3.

Figure 5. Plot of kobs vs [copper(II)]. [ornithine] = 0.05 mol dm −3; [sodium acetate] = 0.085 mol dm−3; pH 4.0 ± 0.1; [PMS] = 3.84 × 10−3 mol dm −3.

Table 2. Kinetic and Thermodynamic Parameters for the Oxidation of Ornithine at 308 K ornithine

Ea (kJ mol−1)

ΔH‡ (kJ mol−1)

without Cu(II) catalyst with Cu(II) catalyst catalytic constant, kc

31.08 29.01 27.91

29.96 27.89 26.64

(J K

ΔS‡ mol−1)

−1

−151.28 −154.34 −138.39

ΔG‡ (kJ mol−1)

103k1K1 (m s−1)

104k2 (s−1)

10−4k3 (s−1)

102k2K2K3 (m−2 s−3)

76.56 75.43 69.27

0.87 −

1.85 −

2.37

− 5.75

in pH values in both cases. The plot of kobs[H+] vs [H+] gave a straight line with a high correlation coefficient (Figure 3). 3.5. Effect of [Sodium Acetate] on kobs. The effect of [sodium acetate] on kobs was studied by determining the values of kobs at different [sodium acetate], keeping other parameters constant. In this experiment, the ratio of acetic acid/sodium acetate was maintained at constant value to keep the pH constant. The values of kobs were unaffected in the absence of copper(II). However, in the presence of copper(II), the rate constant decreased with increase in [sodium acetate] as shown in Table 1. The plot of kobs−1 vs [sodium acetate] was linear with a positive intercept as shown in Figure 4, which suggested that the reaction was inhibited by acetate ion in the presence of copper(II) ions. 3.6. Effect of [Copper(II)] on kobs. The effect of [copper(II)] on the rate was studied by determining the values of kobs at different [copper(II)], by keeping other parameters at predetermined values. The kinetic results showed that the rate increased with an increase in [copper(II)] (Table 1), and the plot of kobs vs [copper(II)] was linear with a positive intercept (Figure 5).

both the presence and absence of copper(II) ion, presented in Table 1. This result ruled out the dimerization of PMS. 3.3. Effect of [Ornithine] on kobs. The values of kobs were calculated for the reactions conducted both in the presence and absence of copper(II) catalyst for different concentrations of ornithine, by keeping other parameters at constant values. Perusal of the kinetic results showed that the rate constant increased with increase in [ornithine] as presented in Table 1. Further, the plots of kobs vs [ornithine] were linear, in both cases shown in Figure 2. The positive intercept obtained in the above plots revealed that the reaction proceeded in two steps: one dependent on [ornithine] and the other independent of [ornithine]. The ornithine-independent step was due to the self-decomposition of PMS under the experimental conditions employed in this study. This was confirmed by conducting the reactions without ornithine in the reaction mixture at the same conditions. 3.4. Effect of pH on kobs. In order to ascertain the effect of pH on the rate of the reaction, reactions in the presence and absence of copper(II) ions at different pH values were carried out. The rate constant (kobs) values increased with an increase 6313



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Figure 6. (A) ESR spectrum of copper(II). (a) ESR spectrum of Cu(II) in water only. (b) ESR spectrum of Cu(II) in acetate buffer medium. (c) ESR spectrum of ornithine, Cu(II), and acetate buffer in the absence of peroxomonosulfate. (d) ESR spectrum of ornithine, Cu(II), acetate buffer, and peroxomonosulfate at 0 min after the addition of peroxomonosulfate. (B) ESR spectrum of ornithine in the presence of copper(II). (a) ESR spectrum of ornithine in the presence of copper(II) and acetate buffer in the absence of PMS. (b−d) ESR spectra of ornithine, Cu(II), acetate buffer, and peroxomonosulfate at (b) 0, (c) 11, and (d) 22 min, respectively, after the addition of peroxomonosulfate.

3.7. Effect of Temperature on kobs. The reaction was studied at five different temperatures, viz., 303, 308, 313, 318, and 323 K, by keeping all other parameters constant. The kobs increased with the increase in temperature, and the plot of log kobs vs 1/T was a straight line (Arrhenius plot). A plot of log kobs/T vs 1/T was drawn, which was also linear (Eyring’s plot). From the slope and intercept of the straight line, the thermodynamic parameters were calculated which are shown in Table 2. The positive values of free energy of activation (ΔG‡) and enthalpy of activation (ΔH‡) obtained in this study indicated that transition state was highly solvated, while the negative values of entropy of activation (ΔS‡) suggested the formation of a rigid transition state with reduction of degree of freedom of molecules. 3.8. Effect of Dielectric Constant on kobs. The effect of the dielectric constant (ε) of the reaction mixture on the reaction rate was studied by using two different solvents, such as 2-methylpropan-2-ol (tert-butyl alcohol) and acetonitrile. The kobs remained unaffected with the increase

in composition of the solvents, ruling out the formation of a polar intermediate. 3.9. Effect of Ionic Strength on kobs. The effect of ionic strength on the reaction rate was studied by varying the ionic strength of the medium from 0.05 to 0.2 mol dm−3, maintaining the other parameters at constant values. The increase in the ionic strength had no effect on the kobs value. This ruled out any interaction between the ions of the reactant. 3.10. Catalytic Activity. According to Moelwyn-Hughes24 the catalytic constant was calculated from eq 3. kc =

k T − kU [Cu(II)]x

(3)

where kT is the observed pseudo-first-order rate constant in the presence of Cu(II) catalyst, kU is the pseudo-first-order rate constant for the uncatalyzed reaction, kc is the catalytic constant, and x is the order of the reaction with respect to copper(II). In the present investigation, the x value was found to be unity. 6314



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Table 3. g and A Values for the ESR Spectrum

The values of kc were calculated from eq 3. The values of kc were evaluated for copper(II) catalyzed reaction at different temperatures. Further, kc vs 1/T and log kc/T vs 1/T were linear. From the slopes and intercepts of the above plots, ΔH‡ and ΔS‡ were also calculated and are presented in Table 2. kc was 1.54 × 10−1 at 308 K. 3.11. Test for Free Radical Intermediates. The reaction mixture failed to initiate the polymerization of acrylonitrile when added to the reaction mixture, which ruled out the formation of free radical intermediates. Moreover, with the addition of tertbutyl alcohol, the rate of the reaction was not affected, which clearly indicated the absence of sulfate free radicals, since tertbutyl alcohol is an effective scavenger of sulfate free radicals. The absence of free radicals in this reaction was further confirmed from ESR studies (Figure 6).

g

A (G)

descriptiona

site I

site II

site I

site II

1 2 3 4 5 6 7

2.19 2.17 2.18 2.14 2.14 2.14 2.14

− − − 2.11 − − −

− 40.28 42.00 58.42 58.61 59.18 58.03

− − − 55.94 − − −

4. DISCUSSION Ornithine exists as a dipolar ion in aqueous solutions. The dissociation of ornithine depends on the pH of the medium. The pKa values of ornithine are 1.71, 8.69, and 10.76.25 Hence, at pH 4.0, ornithine exists both in the protonated form and as zwitterions as shown in eq 4.

1, g value for the ESR spectrum of copper(II) (2.5 × 10−3 mol dm−3) only; 2, g and A values for the ESR spectrum of copper(II) in buffered (pH 4.0) medium; 3, g and A values for the ESR spectrum of copper(II) in buffered medium with PMS (3.86 × 10−6 mol dm−3); 4, g and A values for the ESR spectrum of ornithine (5 × 10−2 mol dm−3) with copper(II) (2.5 × 10−3 mol dm−3) in buffered (pH 4.0) medium; 5, g and A values for the ESR spectrum of ornithine with copper(II) in buffered medium at 0 min after the addition of PMS (3.86 × 10−3 mol dm−3); 6, g and A values for the ESR spectrum of ornithine with copper(II) in buffered medium at 11 min after the addition of PMS; 7, g and A values for the ESR spectrum of ornithine with copper(II) in buffered medium at 22 min after the addition of PMS.

Peroxomonosulfate ion (HSO5−) is a weak acid with pKa = 9.4. The standard potentials of HSO5−/SO42− and SO52−/ SO42− couples are 1.75 and 1.22 V, respectively. PMS exist as HSO5− in acidic condition, i.e., pH 3−6.26 ESR spectra were taken for the copper(II) catalyzed and uncatalyzed reactions at room temperature (298 K). Interestingly, no ESR signal was obtained for the reaction mixture in the absence of copper(II); however, an ESR signal was obtained for the reaction mixture in the presence of copper(II) ions. The ESR spectra are shown in Figure 6. The ESR spectrum of copper(II) in water showed only one signal; however, copper(II) in acetate buffer showed four signals revealing the formation of copper−acetate complex. ESR spectra were taken for the mixtures containing ornithine in buffered medium in both the the presence and absence of copper(II). There was no ESR signal for ornithine molecular species. However, the signal obtained in the presence of copper(II) corresponds to the formation of copper−ornithine complex. The ESR spectrum taken at various time intervals did not affect the position of the signal (g value) and the nature of the signal (Figure 6 and Table 3). This confirmed that the structure and oxidation state of Cu(II) remained unchanged throughout the reaction. The existence of site I and site II in the ESR spectrum was due to the equilibrium between the Cu(II)−ornithine monomer and the Cu(II)− ornithine dimer complex. When PMS was added to the reaction mixture, site II disappeared since the dynamic equilibrium of the above complexes was prevented. UV−visible spectral studies confirmed the formation of ornithine−copper(II) complex. The UV−visible spectrum of the mixture containing ornithine and PMS in acetic acid and sodium acetate buffered medium showed λmax at 236 nm. Scanning at different time intervals showed an increase in absorbance due to the formation of the intermediate imine, as shown in Figure 7. However, in the presence of copper(II), two absorption maxima were

obtained, one at 236 nm and another domain containing a wide band centered at 692 nm. λmax at 692 nm was due to the formation of ornithine−copper(II) complex; interestingly the peak was observed only at higher concentrations of [Cu] > 5 × 10−2 M and was attributed to the d−d transition (2Eg → 2T2g), specific for Cu(II) complexes with tetragonally distorted octahedral complex due to the Jahn−Teller effect. Further, copper(II) in water has λmax at 804 nm, whereas copper(II) in buffer has λmax at 761 nm, copper(II) with ornithine has λmax at 695 nm, and copper(II) with ornithine and peroxomonosulfate in acetate buffer has λmax at 682 nm, which indicates a hypsochromic shift revealing the formation of copper(II)−acetate complex initially, followed by formation of complexes with ornithine and PMS, viz., copper acetate− ornithine complex and copper acetate−ornithine−PMS complex. The cyclic voltammogram of Cu(II)−ornithine−PMS system is presented in Figure 8, which shows three peak potentials: −0.25, −0.45, and −0.77 V. The peak potential of Cu(II) in the Cu(II)−ornithine complex remained the same as that of free copper(II), whereas the peak potential of ornithine complex had shifted from −0.55 to −0.45 V for free ornithine. The peak potential of Cu(II) in Cu(II)−ornithine− PMS complex was −0.77 V compared to the peak potential of Cu(II) in Cu(II)−ornithine complex (−0.80 V) (Table 4). This establishes that Cu(II) forms complexes with ornithine which in turn forms complexes with PMS. The lowering of the peak potential of the Cu(II)−ornithine complex suggested that, although PMS donated electrons to form a complex with Cu(II), back-donation might be predominant, in order to decrease the reduction potential of Cu(II). Hence it is established that PMS interacts with free ornithine and also with Cu(II)−ornithine complex. ESR spectral data also confirmed distorted octahedron geometry. Further, the structure of copper−acetate complex has been arrived at by considering the effect of acetate on the rate of the reaction. The effect of acetate on the rate of the reaction revealed that there was no acetate effect on the rate in the absence of copper(II), whereas the rate decreased while the

a

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Figure 7. (a) Absorption spectra of the reaction mixture in the absence of copper(II) at various time intervals. [ornithine] = 0.05 mol dm−3; [sodium acetate] = 0.085 mol dm−3; pH 4.0 ± 0.1; [PMS] = 3.98 × 10−3 mol dm−3. (b) Absorption spectra of the reaction mixture in the presence of copper(II) at various time intervals. [ornithine] = 0.05 mol dm−3; [sodium acetate] = 0.085 mol dm−3; [Cu(II)] = 2.5 × 10−5 mol dm−3; pH 4.0 ± 0.1; [PMS] = 3.98 × 10−3 mol dm−3. (c) Absorption spectra of copper(II) in different reagents. [ornithine] = 0.05 mol dm−3; [sodium acetate] = 0.085 mol dm−3; [Cu(II)] = 2.5 × 10−3 mol dm−3; pH 4.0 ± 0.1; [PMS] = 3.98 × 10−3 mol dm−3.

The involvement of the ω-NH2 terminal group of ornithine in the complexation of copper(II) has already been established by means of potentiometry, calorimetry, and UV−visible and ESR techniques.27 It is already been established that a monomer complex between copper and ornithine was predominant at pH 4−6 and a dimer complex between ornithine and copper was predominant only above pH 7. Hence the structure of the complex (copper−ornithine−PMS) for the reaction conditions employed for our studies is given by

concentration of acetate increased. This observation suggested the existence of the equilibrium of eq 5.

Scheme 1. Mechanism of the Uncatalyzed Reaction Pathway

Based on the above discussion, the detailed mechanism of the uncatalyzed reaction pathway is given in Scheme 1. The 6316



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Figure 8. Cyclic voltammogram of ornithine with Cu(II) in acetate buffered medium in both the absence and presence of peroxomonosulfate. [ornithine] = 2.5 × 10−2 mol dm−3; [Cu2+] = 5 × 10−5 mol dm−3; pH 4.0 ± 0.1; [sodium acetate] = 8.5 × 10−2 mol dm−3; [PMS] = 8.0 × 10−4 mol dm−3.

Figure 9. NMR spectrum of the reaction mixture. [ornithine] = 2.5 × 10−2 mol dm−3; pH 4.0 ± 0.1; [sodium acetate] = 8.5 × 10−2 mol dm−3; [PMS] = 8.0 × 10−2 mol dm−3. 6317



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Table 4. Potential E (V) Values of the Cyclic Voltammogram

On linearizing eq 21

E (V) description 1 2 3 4 5

a

peak I

peak II

peak III

−0.71 −0.55 −0.80 −0.25 −0.25

− − − −0.45 −0.45

− − − −0.80 −0.77

kobs[H+] =

+ k 3[H+]

(22)

From eq 22 values of k2K2K3, k1K1, and k3 were calculated from different plots, and average values are given in Table 2.

1, E value for the cyclic voltammogram of copper(II) (5 × 10−5 mol dm−3) in buffered (pH 4.0) medium; 2, E value for the cyclic voltammogram of ornithine in buffered medium; 3, E value for the cyclic voltammogram of PMS (8 × 10−4 mol dm−3) in buffered medium; 4, E value for the cyclic voltammogram of ornithine (2.5 × 10−2 mol dm−3) with copper(II) (5 × 10−5 mol dm−3) in buffered medium; 5, E value for the cyclic voltammogram of ornithine (2.5 × 10−2 mol dm−3) with copper(II) (5 × 10−5 mol dm−3) and PMS (8 × 10−4 mol dm−3) in buffered medium. a

5. CONCLUSION The kinetics of the oxidation of ornithine by peroxomonosulfate in acetic acid−sodium acetate buffered medium (pH 3.6−5.2) in the presence and absence of copper(II) catalyst was studied at 308 K. The rate of the catalyzed reaction is 2.6 times faster than that of the uncatalyzed reaction. Variation of ionic strength did not show any effect on the rate of reaction. The formation of polar intermediate was ruled out since the solvent polarity did not affect the reaction rate. The reaction was carried out at five different temperatures, and the activation and thermodynamic parameters were calculated. A suitable reaction mechanism was proposed to explain the experimental observation. The time history of the ESR spectra confirmed that the reaction proceeded through a molecular intermediate. Cyclic voltammetric studies and absorption studies confirmed the formation of copper(II)−ornithine−PMS complex, the structure of which was also proposed.

detailed mechanism for the copper(II) catalyzed reaction pathway is given in Scheme 2. The product obtained in both copper(II) catalyzed and uncatalyzed reaction was 4-aminobutanal and 1H NMR spectrum is shown in Figure 9. kobs for Scheme 1 was derived as k1K1[ornithine] + k2 H+ On linearizing eq 11

k 2K 2K3[ornithine][Cu 2 +] + k1K1[ornithine] [OAc−]

kobs =

(11)

kobs[H+] = k1K1[ornithine] + k 2[H+]

(12)

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AUTHOR INFORMATION

Corresponding Author

Scheme 2. Mechanism for Copper(II) Catalyzed Reaction Pathway

*E-mail: [email protected]. Fax: +91 44 22750520. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the authorities of B.S. Abdur Rahman University for their constant encouragement and support.

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From eq 12 k1K1 and k2 were calculated from different plots, and the average values are given in Table 2. The mechanism for copper(II) catalyzed oxidation of ornithine by PMS is given in Scheme 2. kobs for Scheme 2 was derived as kobs =

k 2K 2K3[ornithine][Cu 2 +] k K [ornithine] + 1 1 + + k3 [OAc−][H+] [H ]

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