Oxidative Decarboxylation of N-Phenyl Glycine by Peroxomonosulfate

Jul 22, 2014 - Department of Chemistry, B. S. Abdur Rahman University, Vandalur, Chennai − 600048, Tamilnadu, India. •S Supporting Information...
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Oxidative Decarboxylation of N‑Phenyl Glycine by Peroxomonosulfate in a Perchloric Acid MediumCatalytic Effect of Cu2+ and VO2+ Ions L. Kanniappan, S. Kutti Rani, and D. Easwaramoorthy* Department of Chemistry, B. S. Abdur Rahman University, Vandalur, Chennai − 600048, Tamilnadu, India S Supporting Information *

ABSTRACT: The kinetics of oxidative decarboxylation of N-phenyl glycine (NPG) by peroxomonosulfate (PMS) in perchloric acid medium was investigated at 278 K. The mechanism of the catalytic effect of vanadium(IV) (VO2+) and copper(II) (Cu2+) in the oxidation was also discussed. The oxidation reaction was first-order, with respect to [NPG] in all of the cases. The rate of the reaction decreased with increase in [H+], i.e., the oxidation reaction was acid inhibited. The products were identified by gas chromatography and confirmed by comparing with the authentic sample. The reaction constants involved in the different steps of the reactions were calculated. The catalytic constant (Kc) was determined for the catalyzed reactions and observed that the catalytic efficiency was in the order of VO2+ > Cu2+.

1. INTRODUCTION

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. PMS was obtained from Lancaster, under the trade name “oxone”. The purity of the triple salt 2KHSO5·KHSO4·K2SO4 was estimated by iodometry and determined to be 98%. Other chemicals such as NPG, vanadyl sulfate, copper sulfate, perchloric acid, sodium perchlorate, and acetonitrile were from E. Merck, India. 2.2. Kinetic Measurements. Kinetic studies were carried out at 5 ± 0.1 °C in a perchloric acid medium under pseudofirst-order conditions with a large excess of NPG over PMS in both uncatalyzed and catalyzed reactions. The reaction rate was measured by monitoring the concentration of unreacted PMS at different time points iodometrically. The reaction mixture containing the required solution was thermostated in a 250-mL blackened iodine flask. A known volume of PMS was pipetted out into the reaction mixture and, simultaneously, a timer was started; the rate of oxidation of amino acids was followed by monitoring the concentration of unreacted PMS by iodometry. First-order kinetics was observed and the rate constant kobs were calculated from the plot of log [PMS]t versus time, which was linear up to 90% conversion of [PMS]. The standard deviation was ±2% (see Figure 1). 2.3. Stoichiometry. The stoichiometry of the reaction was determined both catalyzed and uncatalyzed reactions by allowing the reaction mixtures containing a large excess of PMS over NPG, i.e., [PMS]/[NPG] = 2.5 and allowed it to stand for 48 h at room temperature. After the completion of the reaction, the excess PMS present was then estimated iodometrically. The determination showed that one mole of PMS was consumed for one mole of NPG. Thus, the stoichiometric ratio for the reaction is as given in eq 1.

The kinetics and mechanism of oxidation of N-phenyl glycine (NPG) have gained much attention, probably with the objective of investigating model systems for the enzymatic oxidation of amino acids. It is used for the preparation of watersoluble conducting polymers.1 NPG and its derivatives are involved for oxidation in the presence of hydrogen peroxide as the oxidant via oxidative and nonoxidative decarboxylation.2,3 The major product aldehyde and aniline results from the oxidative decarboxylation process, but only the anilines are formed in the nonoxidative decarboxylation process. The high oxidation potential of peroxomonosulfate (PMS) and the propensity to react via oxygen transfer make the favorable one for the oxidation of various organic compounds in aqueous or solvent−water solution in the absence and also in the presence of transition-metal ions.4−12 In view of the superior ability of HSO4− to act as the leaving group, peroxomonosulfate oxidations would be expected to proceed very rapidly, compared to other peroxides, such as peroxodisulfate and peroxomonophosphoric acid.13−16 In recent years, the use of transition-metal ions such as vanadium, manganese, copper, iridium, etc., either alone or as binary mixtures, as catalysts in various redox processes have attracted considerable interest.17−21 Vanadium acts as a catalyst in the oxidation of many organic substrates.22 VO2+ ion catalysis in redox reactions involves different degrees of complexity, because of the formation of different intermediate complexes. Literature study reveals that few reports are available on the oxidation of NPG by various oxidants.23−27 The present study has been carried out to understand the kinetics and mechanistic aspects of the oxidation of NPG by PMS, as well as the catalytic effect of VO2+ and Cu2+ in perchloric acid medium, and also to study whether the oxidation occurs via oxidative decarboxylation or a nonoxidative decarboxylation process. © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13302

January 3, 2014 July 20, 2014 July 22, 2014 July 22, 2014 dx.doi.org/10.1021/ie500025r | Ind. Eng. Chem. Res. 2014, 53, 13302−13307

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complex. At higher concentrations of Cu2+ ion (0.2 mol dm−3), the spectrum consists of a broad weak band at 327 nm, which is attributed to 2Bg→2Ag; because of the formation of the NPG− Cu2+ complex, the absorption maximum was shifted to 723 nm by the addition of PMS. The shift in the λmax values of NPG− metal-ion complexes by PMS confirmed the oxidation of NPG. 2.6. Cyclic Voltammetric Studies. Cyclic voltammetric studies were used to investigate the interaction behavior of Cu2+ and VO2+ ions with NPG. The results revealed that both the cathodic and anodic peaks were shifted and developed new peaks upon the addition of NPG and also PMS. The development of the new peaks suggested the formation of the NPG−metal-ion complex. The voltammograms for both metal ions showed single peaks for both the cathodic and anodic modes, and every step was a single-electron process. The voltammogram of NPG in perchloric acid showed the anodic peak at 0.341 mV, which decreased upon the addition of PMS (shifted 0.091 mV in the positive direction), which suggested the complex formation between NPG and PMS. In the presence of the VO2+ ion catalyst, the anodic peak was observed at −0.054 mV. NPG was added to this solution and the peak potential shifted to 0.122 mV, because of the formation of the NPG−VO2+ complex, and peak potential shifted further, to 0.024 mV, by the addition of PMS (see Figure S3 in the Supporting Information). These observations suggested that the oxidation reaction was performed in a lower energy state. The voltammograms of Cu2+ ions showed the anodic peak at 0.089 mV. The addition of NPG to the copper solution shifted the potential to 0.005 mV, because of the formation of the NPG−Cu2+ complex, and the anodic peak potential further shifted to −0.151 mV after the addition of PMS, which confirmed the formation of a complex between the NPG−Cu2+ complex with PMS (see Figure S4 in the Supporting Information).

Figure 1. Plot of log [PMS]t vs time at 278 K. Conditions: [NPG] = 0.05 mol dm−3, [H+] = 0.10 mol dm−3, [PMS] = 3.5 × 10−3 mol dm−3.

2.4. Product Analysis. The product analysis was carried out under the following conditions. The reaction mixture containing 0.1 mol dm−3 NPG and 0.2 mol dm−3 PMS in perchloric acid was allowed to stand for 48 h for the completion of the reaction. The excess PMS was destroyed by adding NaHSO3 and then the product was extracted with dichloromethane. The same method was used for the metal-ion (VO2+, Cu2+) catalyzed reactions as well. The main products were identified as formaldehyde and aniline via gas chromatography−mass spectroscopy (GC-MS), and the purity of the products were compared with that of the authentic samples. It was observed that formaldehyde does not undergo further oxidation under the present kinetic conditions. 2.5. Ultraviolet−Visible (UV-Vis) Spectral Measurements. The absorption spectrum of the reaction mixture for the uncatalyzed reaction at low concentration showed an absorption maximum (λmax) at 308 nm for π → π* transitions and that at 282 nm corresponds to n → π* transitions is seen in Figure S1 in the Supporting Information. The addition of VO2+ ion catalyst to the reaction mixture shifted these bands to 304 and 280 nm, which corresponded to π → π* and n → π* transitions, respectively. These shifts might be due to interactions of π-orbitals of the carboxylate group on the Yaxis, which may have overlapped with the dxy orbital of the V4+ ion. From this observation, it was concluded that the VO2+ ion complexes with carboxylate group of the amino acid. The broad peak due to the NPG−VO2+ complex, which was asymmetric towards larger wavelengths (at λmax = 748 nm) was observed only at higher concentrations of the VO2+ ion (0.2 mol dm−3), which was due to 2B2→2E transition. This transition is allowed in xy polarization and this reflects the strong metal−ligand interaction in the VO2+ ion. The addition of PMS shifted the absorption maximum to 738 nm, as shown in the spectrum depicted in Figure S2 in the Supporting Information. Cu2+-ion-catalyzed oxidation of NPG also showed a shift of these band towards higher energies with λmax at 305 and 279 nm, which corresponds to π → π* and n → π* transitions, respectively. These shifts might be correlated with involvement of oxygen atom from the carboxylic groups of the ligand in the metal coordination, which caused the strengthening of the C− O bonds involved in the charge-transfer processes of the

3. RESULTS AND DISCUSSION 3.1. Effect of [NPG] on kobs. The kinetic runs were carried out with various initial concentrations of NPG by keeping other parameters constant, which yielded rate constants whose values were dependent on [NPG]. The pseudo-first-order rate constants thus obtained were found to increase with [NPG] for the catalyzed and uncatalyzed reactions in Table 1. Furthermore, the plots of kobs vs [NPG] gave a straight line with positive intercepts (see Figure 2). The positive intercept obtained in this study revealed that the reaction proceeded via two steps, i.e., one step that was independent of [NPG] and another step that was dependent on [NPG]. The independent step was due to the self-decomposition of PMS under these conditions. 3.2. Effect of Varying [H+] on kobs. The reaction rates were measured with various [H+] values (5.0 × 10−2−15.0 × 10−2 mol dm−3) by keeping other parameters constant. The reaction rate decreased as [H+] increased; greater increases in [H+] retarded the reaction rates significantly. The retardation of rate by [H+] may be mainly attributed to the conversion of the more-reactive neutral species of NPG to the less-reactive protonated form. The plot of kobs vs 1/[H+] was linear (r = 0.9970) (see Figure 3), indicating that this reaction was firstorder, with respect to [H+]. A similar observation was noticed for the oxidation reaction in the presence of metal ions as well. These results confirmed that the reaction was an acid-inhibited reaction. 13303

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Table 1. Effect of Varying Concentrations on the Reaction Rate for the Metal-Ion-Catalyzed and Uncatalyzed Oxidation of NPG by PMS in Perchloric Acid Medium at 278 K kobs (× 103 s−1) [NPG] (× 102 mol dm−3)

[VO2+] 4

2+

(× 10 mol dm−3)

[Cu ] (× 104 mol dm−3)

[H+] (mol dm−3)

[PMS] (× 103 mol dm−3)

2.5 3.8 5.0 7.5 10.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 2.5 3.8 5.0 6.3 7.5 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 2.5 3.8 5.0 6.3 7.5 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

10.0 10.0 10.0 10.0 10.0 5.0 7.5 10.0 12.5 15.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 1.8 3.5 5.3 7.1 8.9 3.5 3.5 3.5 3.5 3.5

without metal ion 1.04 1.24 1.37 1.69 1.98 1.58 1.18 0.99 0.81 0.72

1.27 1.30 1.32 1.45 1.50 1.42 1.42 1.41 1.41 1.42

with VO2+

with Cu2+

1.31 1.57 1.96 2.73 3.42 3.45 2.46 2.01 1.73 1.43 1.42 1.74 2.15 2.49 2.76 1.73 1.77 1.92 2.05 2.12 1.89 1.85 1.87 1.89 1.89

1.17 1.43 1.68 2.21 2.76 2.72 2.04 1.66 1.46 1.23 1.37 1.68 1.98 2.31 2.53 1.63 1.70 1.83 1.92 1.98 1.65 1.65 1.65 1.68 1.67

Figure 3. Plot of kobs vs 1/ [H+] at 278 K: (■) A = [NPG] = 0.05 mol dm−3, [PMS] = 3.5 × 10−3 mol dm−3; (●) B = [NPG] = 0.05 mol dm−3, [Cu2+] = 5.0 × 10−4 mol dm−3, [PMS] = 3.5 × 10−3 mol dm−3; and (▲) C = [NPG] = 0.05 mol dm−3, [VO2+] = 5.0 × 10−4 mol dm−3, [PMS] = 3.5 × 10−3 mol dm−3.

obtained in this plot confirmed that the reaction proceeded in the absence of metal ions as well.

Figure 4. Plot of kobs versus metal ion concentration at 278 K: (■) A = [NPG] = 0.05 mol dm−3, [H+] = 0.10 mol dm−3, [PMS] = 3.5 × 10−3 mol dm−3 (with VO2+); (●) B = [NPG] = 0.05 mol dm−3; [H+] = 0.10 mol dm−3, [PMS] = 3.5 × 10−3 mol dm−3 (with Cu2+).

3.4. Effect of Varying [PMS] on kobs. The oxidation of [NPG] was carried out under pseudo-first-order conditions for the catalyzed and uncatalyzed reaction, i.e., [NPG] ≫ [PMS] in perchloric acid medium. The concentration of PMS was varied from 1.8 × 10−3 to 8.9 × 10−3 mol dm−3 by keeping the other parameters constant. The reaction rate was not altered with increase in [PMS] in both catalyzed and uncatalyzed reaction (Table 1). The results revealed that the rate was independent of the concentration of PMS, which showed that the rate was first order in [PMS]. 3.5. Effect of Ionic Strength. The reaction rate were studied by the addition of NaClO4 (5.0 × 10−2−20.0 × 10−2 mol dm−3) and keeping the other parameters constant. It revealed that increasing the ionic strength (μ) had a negligible effect on the rate of reaction, which ruled out the interaction between the SO52− and NH2+ group of NPG. In an acid medium, NPG exists in protonated form only.

Figure 2. Plot of kobs vs [NPG] at 278 K: (■) A = [H+] = 0.10 mol dm−3, [PMS] = 3.5 × 10−3 mol dm−3; (●) B = [H+] = 0.10 mol dm−3, [Cu2+] = 5.0 × 10−4 mol dm−3, [PMS] = 3.5 × 10−3 mol dm−3; and (▲) C = [H+] = 0.10 mol dm−3, [VO2+] = 5.0 × 10−4 mol dm−3, [PMS] = 3.5 × 10−3 mol dm−3.

3.3. Effect of Varying Metal Ion Concentrations on kobs. The influence of metal ion concentration on kobs was studied by keeping the other parameters constant and varying [VO2+] (2.5 × 10−4−7.5 × 10−4 mol dm−3) and [Cu2+] (2.5 × 10−4−7.5 × 10−4 mol dm−3). The kobs values increased with increase in the concentration of VO2+ and Cu2+ (Table 1). Furthermore, the plots of kobs vs [metal ions] were straight lines with positive intercepts (see Figure 4). The positive intercept 13304

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3.6. Effect of Dielectric Constant. The relative permittivity (D) effect was studied by varying the composition of t-butyl alcohol−water (v/v) and acetonitrile content in the reaction mixture with all other parameters kept constant. It was determined that the dielectric constant of the medium had a negligible effect on the rate for the catalyzed and uncatalyzed reactions, which ruled out the formation of a more polar intermediate than the reactants. 3.7. Test for Free Radicals. The intervention of free radicals in the reaction was examined by adding a known volume of acrylonitrile monomer to the reaction mixture and maintaining it for 2 h under nitrogen atmosphere. Upon dilution with methanol, no precipitate was formed, ruling out the involvement of a free radical intermediate. Furthermore, the reaction had been studied in the presence of t-butyl alcohol, by keeping the other parameters constant. No significant effect of reaction rate was observed, confirming the noninvolvement of hydroxide-free radicals. 3.8. Effect of Temperature. The rate of catalyzed and uncatalyzed oxidation of NPG by PMS in a perchloric acid medium was measured at different temperatures (278−293 K). The rate of reaction increased as the temperature increased. The values of kobs at different temperatures were tabulated in Table 2. The energy of activation (Ea) was calculated from the

Here, kT is the observed pseudo-first-order rate constant in the presence of VO2+ or Cu2+ 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 [VO2+] or [Cu2+]. In the present investigations, X values for the standard run were determined to be unity for both VO2+ and Cu2+ catalysts. The value of Kc then can be calculated using the equation Kc =

temperature (K)

without metal ion

with Cu2+

Kc with VO2+

278 283 288 293

1.58 1.99 2.48 2.98

1.83 2.28 2.77 3.29

2.25 2.58 3.04 3.62

Ea (kJ mol−1) ΔH # (kJ mol−1) ΔS # (J mol−1 K−1) ΔG # (kJ mol−1) k1K1 (× 103 s−1) K2 (× 10−3 mol−1 dm3) k2 (× 103 s−1)

12.43 11.40 −194.97 65.67 1.26

11.16 10.48 −192.22 63.92

9.34 8.32 −185.11 59.78

3.51

4.57

0.66

0.58

0.54

with Cu2+

with VO2+

0.48 0.61 0.59 0.62

1.33 1.19 1.14 1.27

slope of the Arrhenius plot (log k obs vs 1/T) and thermodynamic parameters such as ΔH#, ΔS#, and ΔG# were calculated (Table 2) from the Eyring plot of log(kobs/T) vs 1/T. Fairly, the high positive value of free-energy activation (ΔG#) and enthalpy of activation (ΔH#) for the present study indicated that the transition state was highly solvated, while the negative value of entropy of activation (ΔS#) suggested the formation of a more-ordered transition state than the reactant with the reduction of degree of freedom of molecules. 3.9. Catalytic Activity. It has been pointed out by Moelwyn-Hughes28 that the reactions without a catalyst and in the presence of the catalyst proceed simultaneously, such that k T = kU + kc[catalyst]X

X

[catalyst]

=

kc [catalyst]X

(3)

where kT − kU = kc. The values of Kc were evaluated for both catalysts at different temperatures and determined to increase as the temperatures increased (see Table 2). The value of Kc for VO2+ is 1.33 at 278 K, whereas for Cu2+, the value is 0.48 at 278 K. It was observed from the value of Kc that the VO2+ ions represent a moreefficient catalyst, compared to Cu2+ ions in the oxidation of NPG by PMS in a perchloric acid medium. The pKa value of NPG is 5.42; hence, it exists in the protonated form in a perchloric acid medium. In the uncatalyzed reaction, NPG reacts with SO52−, which is the more-reactive species of PMS to form an imine intermediate. In the catalysed oxidation reaction the chelate complex formed by the interaction of non bonded electrons in the carboxylate oxygen of NPG with metal ions reacts with SO52− to give an imine intermediate. The formation of a moderately stable intermediate is supported by the observed thermodynamic parameters. The complex formation was favored by the enthalpy term, but a negative entropy indicated a rigid structure and the transition state became highly solvated. To ascertain the formation of the complex between VOSO4· 5H2O and NPG, the complex was prepared in an aqueous alcoholic solution at room temperature. This solution was stirred with a magnetic stirrer for 10−12 h, and the dark-greencolored complex was isolated, washed with ethanol, and dried under vacuum at room temperature to constant weight. The complex of NPG with copper was also prepared by the same method and the complexes were characterized by Fourier transform infrared (FT-IR) spectroscopy. Oxovanadium(IV) showed an intense strong band at 977 cm−1, which is characteristic of the VO group. The ν as (COO−) and ν s (COO−) stretching vibrations of −COOH group of free amino acid were observed at 1743 and 1405 cm−1, respectively. In the oxovanadium(IV) complex, a shift of these band was observed at 1737 and 1413 cm−1, respectively, which supported the monodentate coordination of the amino acid to carboxyl group. The complexation of VO2+ with oxygen donor atom was also confirmed by the appearance of ν(V−O) band at 746 cm−1. The band at 1577 cm−1, characteristic of νas(NH3+), and the ν(N−H) stretching vibration, which appears at 3146 cm−1, were not shifted after complexation with the VO2+ ion. Furthermore, the absence of a band at ∼630 cm−1 corresponds to ν(V−N) in the complex, supporting the evidence for the noninvolvement of coordination of −NH2 group of amino acid with VO2+ ion. From the kinetic and spectral data, it was striking that, at low pH, the complexation of NPG with metal ion occurred only through the carboxyl group. Based on the pKa value, Gillard et al.29 reported similar chelation for different amino acid complex at low pH range. The rate of metal-ion-catalyzed oxidation of NPG with PMS in a perchloric acid medium was high,

Table 2. Temperature Variation, Thermodynamic Parameters, and Catalytic Constant for the Oxidation of NPG by PMS in the Presence of VO2+ and Cu2+ Catalyzed and Uncatalyzed Reactions in Perchloric Acid Medium kobs (× 103 s−1)

k T − kU

(2) 13305

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aniline in these oxidation studies confirmed that the reaction proceeded via the oxidative decarboxylation of NPG.

compared to that of an uncatalyzed reaction. This is due to the high nucleophilicity of NPG, which would cause rapid electron transfer and the stabilization of transition state involving metal ions (VO2+/Cu2+) and NPG. Reactions and equations that describe this mechanism (numbered as eqs 4−16) are presented in Chart 1.

■

ASSOCIATED CONTENT

* Supporting Information S

This material is available free of charge via the Internet at http://pubs.acs.org.

■

Chart 1. Equations/Reactions 4−16

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors acknowledge the authorities of B. S. Adhur Rahman University for their constant encouragement. REFERENCES

(1) Nabid, M. R.; Taheri, S. S.; Sedghi, R.; Entezami, A. A. Chemical Synthesis and Characterization of watersoluble, Conducting Poly(Nphenylglycine). Iran Polym. J. 2008, 17, 1−7. (2) Totah, R. A.; Hanzlik, R. P. Oxidative and nonoxidative decarboxylation of N-Alkyl-N-phenylglycines by horseradish peroxidase. Mechanistic switching by varying hydrogen peroxide, oxygen, and solvent deuterium. Biochemistry 2004, 43, 7907−7914. (3) Totah, R. A.; Hanzlik, R. P. Non-oxidative decarboxylation of glycine derivatives by a peroxidase. J. Am. Chem. Soc. 2002, 124, 10000−10001. (4) Ramachandran, M. S.; Vivekanandam, T. S.; Malimmaniraj, R. P. Kinetics and mechanism of the oxidation of amino acids by peroxomonosulphate. Part 2. Effect of formaldehyde. J. Chem. Soc., Perkin Trans. 2 1984, 1345−1349. (5) Ramachandran, M. S.; Vivekanandam, T. S. Kinetics and mechanism of the oxidation of amino acids by peroxomonosulphate. Part 1. J. Chem. Soc., Perkin Trans. 2 1984, 1341−1344. (6) Ramachandran, M. S.; Vivekanandam, T. S. Kinetics and mechanism of the oxidation of amino acids by peroxomonosulphate oxidation of β-phenylalanine, isoleucine and threonine. Tetrahedron 1984, 40, 4929−4935. (7) Kannan, R. S.; Ramachandran, M. S. Studies on the autocatalyzed oxidation of amino acids by peroxomonosulfate. Int. J. Chem. Kinet. 2003, 35, 475−483. (8) Selvarani, S.; Medona, B.; Ramachandran, M. S. Kinetic evidence for the copper peroxide intermediate with two copper ions in proximity. Int. J. Chem. Kinet. 2006, 38, 439−443. (9) Thendral, P.; Shailaja, S.; Ramachandran, M. S. Nickel peroxide: A more probable intermediate in the Ni(II)-catalyzed decomposition of peroxomonosulfate. Int. J. Chem. Kinet. 2007, 39, 320−327. (10) Thendral, P.; Shailaja, S.; Ramachandran, M. S. The Role of Ni(II) in the oxidation of Glycylglycine Dipeptide by peroxomonosulfate. Int. J. Chem. Kinet. 2009, 41, 18−26. (11) Kim, J.; Edwards, J. O. A study of cobalt catalysis and copper modification in the coupled decompositions of hydrogen peroxide and peroxomonosulfate ion. Inorg. Chim. Acta 1995, 235, 9−13. (12) Kutti Rani, S.; Nirmal Kumar, S.; Wilson, C. Y.; Gopi, A.; Easwaramoorthy, D. Oxidation of vanillin by peroxomonosulphatethermodynamic and kinetic investigation. J. Ind. Eng. Chem. 2009, 15, 898−901. (13) Sundar, M.; Easwaramoorthy, D.; Kutti Rani, S.; Palanichamy, M. Mechanistic investigation of the oxidation of lysine by oxone. J. Sol. Chem. 2007, 36, 1129−1137. (14) Meenakshisundaram, S.; Sarathi, N. Kinetics and mechanism of oxidation of indole by HSO5−. Int. J. Chem. Kinet. 2007, 39, 46−51. (15) Shailaja, S.; Ramachandran, M. S. Studies on the oxygen atom transfer reactions of peroxomonosulfate: Oxidation of glycolic acid. Int. J. Chem. Kinet. 2009, 41, 160−167.

4. CONCLUSIONS The comparative study of VO2+- and Cu2+-catalyzed oxidation of NPG by PMS was made. The decrease in reaction rate with increase in perchloric acid concentration was attributed to the formation of protonated species of N-phenyl glycine (NPG), which is highly unreactive in the oxidation process. The oxidation products were identified as formaldehyde and aniline for both the catalyzed and uncatalyzed reactions. The activation parameters were evaluated for both catalyzed and uncatalyzed reactions. Ultraviolet−visible (UV-Vis) and Fourier transform infrared (FT-IR) spectra confirmed the complex formation between the −COO− group of NPG with metal ions. A catalytic constant with reference to the catalyst was also calculated. The catalytic efficiency has the following order: VO2+ > Cu2+. The formation of the products formaldehyde and 13306

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dx.doi.org/10.1021/ie500025r | Ind. Eng. Chem. Res. 2014, 53, 13302−13307