untitled European Journal of Chemistry 5 (2) (2014) 237‐240 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2014 Eurjchem Publishing ‐ Printed in the USA http://dx.doi.org/10.5155/eurjchem.5.2.237‐240.981 European Journal of Chemistry Journal homepage: www.eurjchem.com The oxidation of 2‐(2‐methoxyethoxy)‐ethanol and 2‐(2‐ethoxyethoxy)‐ethanol by dihydroxydiperiodato nickelate(IV): A kinetic and mechanistic study Jinhuan Shan * and Ziwei Zhang College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China *Corresponding author at: College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China. Tel.: +86.0312.5971129. Fax: +86.0312.5079386. E‐mail address: hbushanjh@163.com (J. Shan). ARTICLE INFORMATION ABSTRACT DOI: 10.5155/eurjchem.5.2.237‐240.981 Received: 22 November 2013 Received in revised form: 02 January 2014 Accepted: 04 January 2014 Online: 30 June 2014 KEYWORDS The kinetics of oxidation of 2‐(2‐methoxyethoxy)‐ethanol and 2‐(2‐ethoxyethoxy)‐ethanol by dihydroxydiperiodato nickelate(IV) (DPN) had been studied spectrophotometrically in alkaline medium in the temperature range of 293.2 to 313.2 K. The reaction rate showed first order dependence on DPN, 2‐(2‐methoxyethoxy)‐ethanol and 2‐(2‐ethoxyethoxy)‐ethanol. It was found that the pseudo‐first‐order rate constant kobs increased with an increase in concentration of OH‐ and a decrease in concentration of IO4‐. There was a positive salt effect and no free radicals were detected. A plausible mechanism is proposed and the rate equations derived from the mechanism can explain all the experimental observations. Kinetics Oxidation Mechanism 2‐(2‐Ethoxyethoxy)‐ethanol 2‐(2‐Methoxyethoxy)‐ethanol Dihydroxydiperiodatonickelate(IV) 1. Introduction As researchers have acknowledged the existance of oxidation state of transition metals, and successfully prepared and isolated in the purified sample, with the help of a variety of analytical methods to infer the true structure of these complexes and analysis mode. In order to provide more accurate theoretical basis for analytical method, the study of the oxidation state of transition metals quickly become a hot topic. A large number of studies have showed that: transition metals in a higher oxidation state generally can be stabilized by chelation with suitable polydentate ligands. Metal chelates, such as ditelluratocuprate (III) [1‐3] diperiodatocuprate (III) [4], diperiodatoargentate (III) [5,6], ditelluratoargentate (III) [7,8], diperiodatonickelate (IV) [9,10] are good oxidants in a medium with an appropriate pH. Ni(IV) complexes have been employed as oxidizing agents for the investigation of some organic compounds. Currently, using diperiodatonickelate (IV) to oxidation amino acid [11,12], drugs [13] and catalytic oxidation has become a research hotspot. In addition, a new chemiluminescence (CL) [14] reaction that occurs between luminol and di‐periodatonickelate in alkaline medium had been reported. In the present manuscript, the mechanism of oxidation of 2‐ (2‐methoxyethoxy)‐ethanol and 2‐(2‐ethoxyethoxy)‐ethanol by diperiodatonickelate(IV) is reported. Both 2‐(2‐methoxy ethoxy)‐ethanol (MEE) and 2‐(2‐ethoxyethoxy)‐ethanol (EEE) are colorless liquids and high boiling‐point solvents which means they will have a wide application, such as non‐polluting cleaning agents, extraction agents, diluent, medicine, additives and solvent, etc. 2. Experimental 2.1. Materials All chemicals used were of A.R. grade and double distilled water was used throughout this work. Solutions of [Ni(OH)2(H2IO6)2]4‐ DPN and reductants were always freshly prepared before use. The stock solution of DPN was prepared and standardized by the method report earlier [15]. The concentration of DPN was derived from its absorption at λ = 410 nm. KNO3 and KOH were used to maintain ionic strength and alkalinity of the reaction, respectively. The concentration of reductants must be more 20 times than the concentration of DPN. 2.2. Instrumentation The measurements of the kinetic were performed on a UV‐ Vis spectrophotometer (TU‐1900, Beijing Puxi Inc., China), which had a cell‐holder kept at a constant temperature 238 Shan and Zhang / European Journal of Chemistry 5 (2) (2014) 237‐240 (±0.1 °C) by circulating water from a thermostat (DC‐2010, Baoding, China). 2.3. Kinetics measurements All kinetic measurements were carried out under pseudo‐ first‐order conditions. Solution (2 mL) containing required concentration of Ni(IV), OH‐, IO4‐ and ionic strength and reductant solution (2 mL) of requisite concentration were mixed at the desired temperature, and immediately transferred into a 1 cm thick rectangular quartz cell in a constant temperature cell‐holder (±0.1 °C) When the DPN colour (wine red) was completed fading marked the completion of the reaction. 3. Results and discussion 3.1. Evaluation of pseudo‐first order rate constants Under the conditions of [reductant]0 >> [DPN]0, the plots of ln(At‐A∞) vs time were straight lines, showing that the reaction was first order with respect to Ni(IV), where At and A∞ were the absorbance at time t and at infinite time, respectively. The pseudo‐first‐order rate constants kobs were calculated by the method of least squares (r ≥ 0.996). Deviations in duplicate determinations were generally less than ±5%. 3.2. Rate dependence on the [reductant] At fixed concentration Ni(IV), OH‐, IO4‐ , ionic strength μ and temperature, The order nap were found to be first in [MEE] and [EEE], and the kobs value increased with the increasing [reductant]. Both the plot of kobs vs [MEE] and the plot of kobs vs [EEE] were straight lines, which through the origin at different temperature which the corresponding equation. (r ≥ 0.997) (Figure 1 and 2).  obs = a R'k  (1) From the Equation (1) we can indicate that the reaction order dependence on reductant was first order and R′ represents (2‐(2‐methoxyethoxy)‐ethanol (MEE) and 2‐(2‐ ethoxyethoxy)‐ethanol (EEE). Figure 1. Plots of kobs vs. [MEE] at different temperatures, [DPN] = 5.91×10‐6 mol/L, [IO4‐] = 1.00×10‐3 mol/L, [OH‐] = 1.00×10‐2 mol/L, µ = 3.10×10‐2 mol/L. 3.3. Rate dependence on the [IO4‐] Under the condition of [reductant]0 >>[DPN]0, at constant [reductant], [OH‐], ionic strength and temperature, kobs values decreased with the increase in concentration of IO4‐ and the order with respect to [IO4‐] was found to be fractional, which revealed that [IO4‐] was produced in equilibrium before the rate‐determining step. The plots of 1/kobs vs [IO4‐] were all straight lines without passing through the origin (Figure 3 and 4). 4 1 IO obs b c k       (2) The Equation (2) showing that there was a pre‐equilibrium involving the process of disassociation H2IO63‐ from Ni(IV) complex. Figure 2. Plots of kobs vs. [EEE] at different temperatures, [DPN] = 5.91×10‐6 mol/L, [IO4‐] = 1.00×10‐3 mol/L, [OH‐] = 1.00×10‐2 mol/L, µ = 3.10×10‐2 mol/L. Figure 3. Plot of 1/kobs vs 102[IO4‐] at 303.2 K, [DPN] = 5.91×10‐6 mol/L, [MEE] = 3.00×10‐2 mol/L, [OH‐] = 1.00×10‐2 mol/L, µ = 3.10×10‐2 mol/L (r ≥ 0.996). 3.4. Rate dependence on the [OH‐] At fixed concentrations of DPN, IO4‐, reductant, ionic strength µ and temperature (303.2 K), the value of kobs increased with increasing concentration of OH–. The order with respect to [OH‐] was fractional and the plots of 1/kobs vs  OH / OH         were observed which the corresponding linear equation at different temperatures; Shan and Zhang / European Journal of Chemistry 5 (2) (2014) 237‐240 239 Table 1. Effect of [OH‐], [IO4‐] and µ on the reaction at 303.2 K. 10‐6 [DPN] [reductant] µ×102 103 [IO4‐] 103 [OH‐] MEE EEE (mol/L) (mol/L) (mol/L) (mol/L) (mol/L) 103 kobs (s‐1) 103 kobs (s‐1) 5.91 0.03 3.10 1.00 5.00 13.10 10.49 5.91 0.03 3.10 1.00 10.00 19.97 14.09 5.91 0.03 3.10 1.00 15.00 26.23 16.07 5.91 0.03 3.10 1.00 20.00 31.05 17.76 5.91 0.03 3.10 1.00 25.00 35.23 18.75 5.91 0.03 3.10 0.50 10.00 20.77 14.53 5.91 0.03 3.10 1.00 10.00 18.52 13.03 5.91 0.03 3.10 1.50 10.00 16.13 12.08 5.91 0.03 3.10 2.00 10.00 14.49 11.14 5.91 0.03 3.10 2.50 10.00 13.51 10.42 5.91 0.03 1.10 1.00 10.00 13.23 11.61 5.91 0.03 2.10 1.00 10.00 14.12 12.53 5.91 0.03 3.10 1.00 10.00 15.34 13.52 5.91 0.03 4.10 1.00 10.00 17.12 14.71 5.91 0.03 5.10 1.00 10.00 18.41 15.24 Figure 4. Plot of 1/kobs vs 102 [IO4‐] at 303.2 K, [DPN] = 5.91×10‐6 mol/L, [EEE] = 3.00×10‐2 mol/L, [OH‐] = 1.00×10‐2 mol/L, µ = 3.10×10‐2 mol/L (r ≥ 0.998).  - -1 [OH ] / [OH ] obs d e k    (3) 3.5. Rate dependence on ionic strength µ The effect of ionic strength on the reaction was studied in the range of 1.10×10‐2 to 5.10×10‐2 mol/L at constant [DPN], [reductant], [OH‐], [IO4‐] and temperature. The experimental results indicated that the rate constant kobs increased with increased in ionic strength µ (Table 1), which showed that there was a positive salt effect that consistent with the common regulation of the kinetics [16]. 3.6. Free radical detection To study the possible presence of a free radical during the reaction, a known amount of acrylamide was added under the protection of nitrogen atmosphere. There was no polymeric suspensions appeared which indicated that no free radical intermediates produced in the oxidation by DPN. 3.7. Reaction mechanism In alkaline solution, Equilibrium (4‐6) was observed and the corresponding equilibrium constants at 298.2 K were determined by Aveston [17]. 2IO4‐ + 2OH‐ ⇋ H2I2O104‐ Log β1 = 15.05 (4) IO4‐ + OH‐ + H2O ⇋ H3IO62‐ Log β2 = 6.21 (5) IO4‐ + 2OH‐ ⇋ H2IO63‐ Log β3 = 8.67 (6) The distribution of all periodate species in alkaline solution can be calculated from the equilibriums (4‐6). The amount of dimer H2I2O104‐ and IO4‐ species can be neglected, the main species of periodate are H3IO62‐ and H2IO63‐, which was consistent with the result calculated from Crouthamel’s date by Murthy [18,19]. Based on such distribution, the formula of Ni(IV) periodate complex is represented by the less protonated ionic species [Ni(OH)2(H2IO6)2]4‐. We preferred to use [Ni(OH)2(H2IO6)2]4‐ to represent DPN because it is close to the formula suggested by Mukherjee [16] and will obtain support from kinetic studies. According to the above experimental facts, the plausible mechanism of oxidation was proposed as follows: [Ni(OH)2(H2IO6)2]4‐ + OH‐ K [Ni(OH)2(HIO6)]2‐ + H2IO63‐ + H2O DPN MPN (7) [Ni(OH)2(HIO6)]2‐ + R' k Adduct MPN (8) Adduct Fast Ni(IV) + Product (9) Reaction (8) is the rate‐determining step.        T obs3 T T 2 6 K OH R 'd Ni IV Ni IV Ni IV dt H IO K OH k k                         (10)  obs 3 2 6 K OH R ' H IO K OH k k               (11) Here:         3 2 6 e e eT H IO OH Ni IV DPN MPN MPN OH                      k k (12) Subscripts T and e stand for total concentration and concentration at equilibrium respectively. 240 Shan and Zhang / European Journal of Chemistry 5 (2) (2014) 237‐240 Table 2. Rate constants (k) and the activation parameters for the rate‐determining step *. T (K) 293.2 298.2 303.2 308.2 313.2 102 k (s‐1) MEE 33.37 42.72 62.66 81.30 112.84 EEE 34.22 44.66 64.60 92.34 143.06 Thermodynamic activation Parameters (298.2 K) MEE Ea = 32.42 kJ/mol, ΔH≠ = 29.94 kJ/mol, ΔS≠ = ‐234.09 J/K·mol EEE Ea = 56.80 kJ/mol, ΔH≠ = 54.32 kJ/mol, ΔS≠ = ‐228.96 J/K·mol * The plot of ln k vs T‐1 have following intercept (a) slope (b) and relative coefficient (r) : MEE: a = 10.05, b = ‐3898.85, r = 0.997; EEE: a = 18.59, b = ‐6831.39, r = ‐0.996. Ea represents the activation energy of the reaction; ΔH≠ represents enthalpy change of the reaction; ΔS≠ represents entropy change of the reaction. Neglecting the concentration of ligand dissociated from Ni(IV) and the species of periodate other than H2IO63‐ and H3IO62‐, equations (13) and (14) can be obtained from the Equations (5) and (6):  33 2 6 4 4ex ex 2 3 OH H IO IO OH IO OH                                 (13)  2 2 3 6 4 4ex ex 2 3 H IO IO OH IO OH                              (14) Here [IO4‐]ex represents the original overall entering periodate and equals approximately to the sum of [H2IO63‐] and [H3IO62‐]. Substituting equation (13) into (11), we can get the expression of pseudo‐first order rate constants as:      4 ex obs OHIO1 1 R ' K R ' OHk k k               (15)       4 ex obs OH1 1 IO R ' K R ' OHk k k                (16) The equation (11), (15), and (16) are consistent with equation (1), (2), and (3), respectively, which mentioned formerly. This is consistent with the experiments result. From the intercept of equation (16), we can obtain the rate constants of the rate‐determining step at different temperatures, and activation energy and the thermodynamic parameters are evaluated by the method given earlier [20]. If the formula of DPN was [Ni(OH)2(H3IO6) 2]2‐, equation (17) would be obtained instead of equation (14).    4 ex obs OHIO1 1 [R '] K R ' OHk k k               (17) The plot of 1/kobs vs.  OH / OH         should also be linear, but the linearity was not straight (Table 1), which substantially denies equation (13). Therefore, it seems advisable to represent DPN by [Ni(OH)2(H2IO6)2]4‐, which is consistent with the experimental result. Meanwhile, the pots of 1/kobs vs [IO4‐] were linear at different temperatures. From their slopes, the rate‐determining step constants k was evaluated. The activation parameters data of reductant obtained is presented in Table 2. 4. Conclusion In this study, we noted that the rate constants k of the rate‐ determining step and the activation parameters for 2‐(2‐ methoxyethoxy)‐ethanol and 2‐(2‐ethoxyethoxy)‐ethanol are contiguous. The reaction rate 2‐(2‐methoxyethoxy)‐ethanol is a little quicker than that of 2‐(2‐ethoxyethoxy)‐ethanol. The reason is that comparing 2‐(2‐methoxyethoxy)‐ethanol, 2‐(2‐ ethoxyethoxy)‐ethanol is larger and has larger spatial hindrance. The latter is more stable than the former. References [1]. Shan, J. H.; Li, Y.; Huo, S. H.; Yin, C. H. J. Chem. 2013, 2013, Article ID 627324. [2]. Shan, J. H.; Li, Y. Eur. J. Chem. 2013, 4(3), 203‐206. [3]. Shan, J. H.; Liu, Y. P.; Zhang, J. Y. Chinese J. Chem. 2011, 29(4), 639‐642. [4]. Shan, J. H.; Liu, Y. P.; Shen, H. X.; Zhang, J. Y.; Yang, Y. F. Int. J. Chem. 2011, 3(2), 111‐116. [5]. Naik, K. M.; Nandibewoor, S. T. Oxid. Commun. 2012, 35, 545‐559. [6]. Shan, J. H.; Wang, X. Q.; Zhao, N. Chinese J. Chem. 2010, 28(7), 1081‐ 1084. [7]. Ragunatharaddi, R. H.; Nagaraj, P. S.; Sharanappa, T. N. J. Phys. Org. Chem. 2009, 22(3), 234‐240. [8]. Jayant, I. G.; Sanjeevaraddi, R. S.; Sharanappa, T. N. Catal. Sci. Technol. 2012, 2, 2549‐2557. [9]. Shan, J. H.; Liu, H. M.; Huo, S. Y.; Fan, Y.; Shen, S. G. Trans. Met. Chem. 2006, 31, 999‐1002. [10]. Shan, J. H.; Shen, H. X.; Song, C. Y.; Wang, H. Y.; Wang, X. Q. Chem. J. Inter. 2009, 11(3),10 [11]. Shan, J. H.; Wei, H. Y.; Wang, L.; Liu, B. S.; Shen, S. G.; Sun, H. W. Indian J. Chem. 2001, 40(A), 865‐869. [12]. Shan, J. H.; Shen, H. X.; Wang, H. Y.; Wang, X. Q. Oxid. Commun. 2012, 35(3), 583‐390. [13]. Yang, C. Y.; Zhang, Z. J.; Wang, J. L. Microchim. Acta 2009, 167, 91‐96. [14]. Chandraiah, U.; Murthy, C. P.; Sushama, Indian J. Chem. 1989, 28(A), 162‐164. [15]. Murthy, C. P.; Sethuram, B.; Rao, T. N. Z. Phys. Chem. (Leipzig) 1986, 267, 1212‐1218. [16]. Mahesh, R. T.; Pol, P. D.; Nandibewoor, S. T. Monatsh. Chem. 2003, 134, 1341‐1352. [17]. Aveston, J. J. Chem. Soc. 1969, A, 273‐275. [18]. Crouthamel, C. E.; Meek, H. V.; Martin, D. S. J. Am. Chem. Soc. 1949, 71, 3031‐3035 [19]. Crouthamel, C. E.; Hayes, A. M.; Martin, D. S. J. Am. Chem. Soc. 1951, 73, 82‐87 [20]. Mukherjee, H. G.; Mandal, B.; De, S. Indian J. Chem. 1984, 23(A), 426‐ 428. [21]. Shan, J. H.; Liu, T. Y. Acta Chim. Sinica 1994, 52, 114. [22]. Keerti, M. N.; Sharanappa, T. N. J. Sulfur. Chem. 2011, 32(2), 123‐136. [23]. Feigl, F. Spot Tests in organic analysis, Elsevier Publishing Co., New York, NY, USA, 1956.