The Oxidation of 2-( 2-Methoxyethoxy ) ethanol and 2-( 2-Ethoxyethoxy ) ethanol by Ditelluratocuprate ( III ) : A Kinetic and Mechanistic Study

Recently, more and more researchers have devoted themselves to the study of the higher oxidation state of transition metal. Cu(III), Ag(III), and Ni(IV) can be stabilized by chelation with polydentate ligands, such as ditelluratocuprate(III) [1, 2], diperiodatocuprate(III) [3, 4], diperiodatoargentate(III) [5, 6],ditelluratoargentate(III) [7], diperiodatonickelate(IV) [8] which are good oxidants in amedium with an appropriate pH. As early as 1844, the researchers have begun to study Cu(III) complex. Now, the use of Cu(III) not only plays a prominent role in many biological systems involving electron-transfer processes, but also makes contribution as an oxidation agent in organic mixture qualitative analysis [9]. Hence, the further research of Cu(III) is significant. In the present paper, the mechanism of oxidation of 2(2-methoxyethoxy)ethanol and 2-(2-ethoxyethoxy)ethanol by ditelluratocuprate(III) is reported. Both 2-(2-methoxyethoxy)-ethanol and 2-(2-ethoxyethoxy)-ethanol are colorless liquids and high boiling point solvents so that they have a wide application such as printing, dyeing, resin, cellulose, and coatings. In addition, 2-(2-methoxy ethoxy)-ethanol can be used for the extracting agent for hydrocarbon and chemical reagent in chemical analysis.


Introduction
Recently, more and more researchers have devoted themselves to the study of the higher oxidation state of transition metal.Cu(III), Ag(III), and Ni(IV) can be stabilized by chelation with polydentate ligands, such as ditelluratocuprate(III) [1,2], diperiodatocuprate(III) [3,4], diperiodatoargentate(III) [5,6],ditelluratoargentate(III) [7], diperiodatonickelate(IV) [8] which are good oxidants in a medium with an appropriate pH.As early as 1844, the researchers have begun to study Cu(III) complex.Now, the use of Cu(III) not only plays a prominent role in many biological systems involving electron-transfer processes, but also makes contribution as an oxidation agent in organic mixture qualitative analysis [9].Hence, the further research of Cu(III) is significant.In the present paper, the mechanism of oxidation of 2-(2-methoxyethoxy)ethanol and 2-(2-ethoxyethoxy)ethanol by ditelluratocuprate(III) is reported.
Both 2-(2-methoxyethoxy)-ethanol and 2-(2-ethoxyethoxy)-ethanol are colorless liquids and high boiling point solvents so that they have a wide application such as printing, dyeing, resin, cellulose, and coatings.In addition, 2-(2-methoxy ethoxy)-ethanol can be used for the extracting agent for hydrocarbon and chemical reagent in chemical analysis.

Experimental Section
2.1.Materials.All chemicals used were of AR grade, and double distilled water was used throughout the work.Ditelluratocuprate(III) (DTC) was prepared and standardized by the method reported by Chandra and Yadava [10,11].The purity of the complex was checked by comparing UV-Vis spectrum with the literature data, which showed a characteristic absorption peak at 405 nm.KNO 3 and KOH were used to maintain ionic strength and alkalinity of the reaction, respectively.Besides, solutions of DTC and reductants were always freshly prepared before using.

Kinetics Measurements and
Apparatus.The kinetics were followed under pseudo-first-order conditions.Solution (2 mL) containing required concentration of DTC, OH − , and TeO 4 2− and ionic strength and reductant solution (2 mL) of requisite concentration were transferred separately to the upper and lower branch tubes of a  type two-cell reactor.After thermostating the solutions at the desired temperature, the two solutions rapidly mixed and put into the galss absorption cell.The progress of the reaction was followed by measuring the decrease in absorbance of DTC at 405 nm.
The kinetic measurements were performed on a UV-vis spectrophotometer, which had a cell holder kept at constant temperature (±0.1 ∘ C) by circulating water from a thermostat.It was verified that the effect of other reagents at 405 nm was negligible.

Stoichiometric and Product Analysis.
Stoichiometric studies revealed that 1 mol of DTC consumed 1 mol of reductants by adding dropwise DTC of known concentrations to 0.1 mol⋅L −1 reductants until no further decolorization was observed [12] [Cu(OH) Under the kinetic conditions, the product of oxidation was identified as an aldehyde by its characteristic spot test [13].The  obs values were the average value of at least three independent experiments, and reproducibility was within ±5%.

Rate Dependence on the [Reductant]
. The [reductant] was varied in the range of 1.00 × 10 −2 to 5.00 × 10 −2 mol⋅L −1 at different temperature keeping all other [reactants] constant.The order  ap in reductant were calculated as fractional order form the slopes of the plots of ln  obs versus ln[reductant] (Tables 1 and 2).Besides, the  obs value increased with the increasing [reductant].The plots of  obs −1 versus [reductant] −1 were straight lines ( ≥ 0.998).

Rate Dependence on the [OH]
− .The effect of [OH − ] on the reaction had been studied in the range of 5.00 × 10 −3 mol⋅L −1 to 25.00 × 10 −3 mol⋅L −1 at constant [DTC], [reductant], [TeO 4 2− ], , and temperature (Table 3).It was found that  obs values increased with the increasing [OH − ] and the order with respect to [OH − ] was fractional.The plot of  obs −1 versus [OH − ] −1 was linear with a positive intercept ( ≥ 0.999).3).The  obs increased with the decreasing concentration of TeO 4 2− .The order with respect to TeO 4 2− was found to be a negative fraction, which revealed that TeO 4 2− was produced in equilibrium before the rate-controlling step.In addition, the plot of  obs −1 versus [TeO 4 2− ] was also straight line with a positive intercept ( ≥ 0.998).], and temperature (Table 3).The experimental results indicated that the rate constant decreased with the increasing  which showed that there was negative salt which was consistent with the common regulation of the kinetics [14].

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.The polymerization clearly appeared which indicated that free radical intermediates might be produced in the oxidation by DTC.And blank experiments in reaction system gave no polymeric suspensions.

Reaction Mechanism.
In the alkaline medium, the electric dissociation equilibrium of telluric acid was given earlier (here pK  = 14) Hence, the main tellurate species was H 4 TeO 6 2− in the concentration of OH − range used in this work.
The fractional order in OH − determined that OH − took part in a preequilibrium with Cu(III) before the ratedetermining step.The plot of  obs −1 versus [TeO 4 2− ] was straight line with a positive intercept indicating a dissociation equilibrium in which the Cu(III) lost a tellurate ligand H 4 TeO 6 2− from its coordination sphere forming an active species monotelluratocuprate(III) complex (MTC).The order with respect to reductant was fractional, which indicated complex formation between reductant and MTC.In addition, the plot of  obs −1 versus [reductant] −1 was straight line with a positive intercept proving the kinetic evidence for the formation of 2 : 1 complex.
In alkaline solution studied, (H 2 TeO 6 ) 4 − protonated easily and coordinated with central ion formed [Cu(H 4 TeO 6 ) 2 ] − .The plausible mechanism of oxidation was proposed as follows (R, resp.stand for OCH 3 and OCH 2 CH 3 ): [Cu ( The Cu * (III) stands for any kind of which Cu 3+ existed in equilibrium.The total concentration of Cu(III) at time  can be written as  Since reaction ( 6) was the rate-determining step, the rate of disappearance of [Cu(III)]  was represented as Rearranging ( 9) leads to Equation ( 9) suggested that the order with respect to Cu(III) was unity.From (10), the rate constants of the ratedetermining step at different temperature were determined by the intercept of the plots  obs −1 versus [reductant] −1 which were straight lines.Hence, the activation energy and the thermodynamic parameters were evaluated by the method given earlier at 298.2 K (Table 4).In addition, (11) indicated that the plots of  obs −1 versus [OH − ] −1 and  obs −1 versus [H 4 TeO 6 2− ] were also straight lines.All of the previous conclusions were consistent with experimental results.

Conclusion
Through the comparative study of oxidation of 2-(2methoxy ethoxy)-ethanol (MEE) and 2-(2-ethoxyethoxy)ethanol(EEE) by ditelluratocuprate(III), we found that both the MEE and the EEE formed the same intermediate compounds with Cu(III).In addition, the values of the activation parameters with respect to MEE were larger than those of EEE, which indicated that the reactivity of EEE was higher than that of MEE.The reason was that the electron-donating ability of EEE was larger than that of MEE, which indicated that the former was more stable than the latter.The previous conclusions were consistent with experimental results.
1 and  2 , respectively stand for the relative coefficient of the plots of ln  obs versus ln [EEE] and the plots of  obs −1 versus [EEE] −1 ;  ap stand for the order in EEE.

Table 4 :
Rate constants (k) and activation parameters of the rate-determining step.