Kinetics andMechanism of Oxidation of Triethylene Glycol and Tetraethylene Glycol by Ditelluratoargentate (III) in Alkaline Medium

e kinetics of oxidation of triethylene glycol and tetraethylene glycol by ditelluratoargentate (III) (DTA) in alkaline liquids has been studied spectrophotometrically in the temperature range of 293.2 K�313.2 K.e reaction rate showed �rst-order dependence in DTA and fractional order with respect to triethylene glycol or tetraethylene glycol. It was found that the pseudo-�rst-order rate constant (kkobs) increased with an increase in concentration of OH − and a decrease in concentration of H4TeO6 . ere was a negative salt effect and no free radicals were detected. A plausible mechanism involving a two-electron transfer was proposed, and the rate equations derived from the mechanism explained all the experimental results and observations. e activation parameters along with the rate constants of the rate-determining step were calculated.


Introduction
Recently, many researchers from many countries are interested in the study of the highest oxidation state of transition metals which in a higher oxidation state generally can be stabilized by chelation with suitable polydentate ligands.Metal chelates, such as diperiodatoargentate (III) [1], ditelluratoargentate (III) [2], ditelluratocuprate (III) [3], and diperiodatonickelate (IV) [4], are good oxidants in a medium with an appropriate pH.e oxidation of a number of organic compounds and metals in lower oxidation state by Ag(III) has also been performed [5,6].e research is focus on the kinetics of oxidation of small molecules by DTA.In this paper, the mechanism of the oxidation of triethylene glycol and tetraethylene glycol by DTA is reported.Both of triethylene glycol and tetraethylene glycol which serve as thinners, solvent, and dispersant, are used in coatings, inks, printing, dyeing, pesticide, cellulose and acrylic acid industry, and so forth.In addition, they also can be used as fuel antifreeze, cleaning agents, the extractant, nonferrous metal dressing agent and organic synthetic materials, and so forth.

Experimental
2.1.Materials.All of the reagents used were AR grade.All of solutions were prepared with doubly distilled water.Solution of DTA was prepared and standardized by the method reported earlier [7].Its UV spectrum was found to be consistent with that reported.e concentration of DTA was derived from its absorption at    nm.e solution of DTA was prepared with double-distilled water before using.e ionic strength  was maintained by adding the solution of KNO  , and the pH of the reaction mixture was regulated with the solution of KOH.e kinetic measurements were performed on a UV-Vis spectrophotometer (TU-1900, Beijing Puxi Inc., China), which had a cellholder kept at a constant temperature (±0.1 ∘ C) by circulating water from a thermostat (DC-2010, Baoding, China).None of the other species absorbed signi�cantly at this wavelength.

Kinetics Measurements and
known concentrations was mixed with an excess of reductants.e complete fading of DTA color (reddish brown) marked the completion of the reaction.e product of oxidation was identi�ed as ketone by its characteristic spot test [8].

Evaluation of Pseudo-First-Order Rate Constants. Under the conditions of [reductant]
≫ [Ag(III]  , the plots of ln(  −  ∞  versus time were straight lines, indicating the reaction is �rst order with respect to [Ag(III)], where   and  ∞ are the absorbance at time t and at in�nite time, respectively.e pseudo-�rst-order rate constants  obs were calculated by the method of least squares (  .e values of  obs were the average values of at least three independent experiments.e reproducibility was within ±5%.[Reductant].At �xed concentration of Ag(III), OH − , H 4 TeO 6 2− , and ionic strength , the values of  obs were determined at different temperatures.e plots of ln obs versus ln[reductant] were linear (  ), and from the slope of such plots, the order with respect to reductant was found to be fractional.e plots of [reductant]/ obs versus [reductant] were straight lines at different temperatures (Figures 1 and 2).

Rate Dependence on the [OH
, reductant, ionic strength , and temperature (298.2K), the values of  obs increased with increasing concentration of OH − .e order with respect to [OH − ] was fractional, and the plot of 1/ obs versus 1/[OH − ] was linear (Figure 3).

Rate Dependence on the
the experimental results indicated that  obs decreased while the [H 4 TeO 6 2− ] increased.e order with respect to H 4 TeO 6 2− was derived to be an inverse fraction, which revealed that H 4 TeO 6 2− was produced in equilibrium before the rate-determining step.A plot of 1/ obs versus [H 4 TeO 6 2− ] was a straight line (Figure 4).

Rate Dependence on the Ionic Strength.
With other conditions �xed, the reaction rate was decreased by the addition of KNO 3 solution (Table 1), which indicated there was negative salt effect which was consistent with the common regulation of the kinetics [9].

Reaction Mechanism.
In an alkaline medium, the electric dissociation equilibrium of telluric acid was given earlier (pK w = 14): e distribution of all species of tellurate in aqueous alkaline solution can be calculated from ( 1) and ( 2 According to the above experimental facts, the following reaction mechanism is proposed: Reactions ( 3) and ( 4) are dissociation and coordination equilibrium, the reaction rates of which are generally fast, reaction ( 5) is an electron-transfer reaction, the reaction rates of which are generally slow.Hence, reaction ( 5) is the ratedetermining step: [Ag(III)]  stands for any form of Ag(III) complex which exists in the equilibrium and R � stands for both of the reductants: Rearranging ( 8) leads to the following: From ( 9), the plots of

Conclusion
Based on the former discussion and results, we can know that the rate constants of the rate-determining step and the activation parameters for triethylene glycol and tetraethylene glycol are contiguous.Both of triethylene glycol and tetraethylene glycol form the same intermediate compounds with Ag(III), and the rate of tetraethylene glycol is a little quicker than that of triethylene glycol.e reason is that the electron-donating ability of tetraethylene glycol is larger than that of triethylene glycol.e transition complex formation between tetraethylene glycol and DTA is more stable than that of triethylene glycol.