Kinetics and Mechanism of Oxidation of 3-phenoxy-1 , 2-propanediol by Ditelluratocuprate ( III ) in Alkaline Medium

The kinetics of oxidation of 3-phenoxy-1,2-propanediol by ditelluratocuprate(III) (DTC) in alkaline liquids has been studied spectrophotometrically in the temperature range of 293.2 K-313.2 K. The reaction rate showed first order dependence in DTC and fractional order with respect to 3-phenoxy-1,2-propanediol. 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 TeO4. The reaction rate changed with the position of hydroxyl group. There is a negative salt effect. A plausible mechanism involving a pre-equilibrium of a adduct formation between the complex and 3-phenoxy-1,2-propanediol was proposed. The rate equations derived from mechanism can explain all experimental observations. The activation parameters along with the rate constants of the rate-determining step were calculated.


Introduction
In recent years, the study of the higher oxidation state of transition metals has intrigued many researchers.This can provide new and valuable information in some fields.Transition metals in a higher oxidation state generally can be stabilized by chelation with suitable polydentate ligands.Metal chelates such as ditelluratocuprate(III) (Shan, J. H., Wei, H. Y., Wang, L., Liu, B. S., Shen, S. G., & Sun, H. W. 2001, Shan, J. H., Wang, L., Wei, H. Y., Shen, S. G., & Sun, H. W. 2002, Shan, J. H., Wang, L. P., & Sun, H. W. 2003), diperiodatocuprate(III) ( Shan, J. H., Shen, S. G., & Ma, Z. G. 1998), diperiodatoargentate(III) (Shan, J. H., Wang, L., & Shen, S.G. 2001), ditelluratoargentate(III) ( Gupta, K. K. S., Nandy, B. K., & Bera, A. K. 1997), diperiodatonickelate(IV) ( Shan, J. H., Wei, H. Y. & Wang, L. 2001) are good oxidants in a medium with an appropriate pH value.The use of Cu(III) as an oxidation agent is well known in analytical chemistry in the estimation of glucose, sugars and organic acids.The use of Cu(III) as an oxidation agent is also known in organic mixture qualitative analysis (Reddy, K. B., Sethuram, B., & Rao, T, N. 1981).Cu(III) is also shown to be an intermediate in the Cu(II)-catalyzed oxidation of amino acids by peroxodisulphate.Many biological systems involve electron-transfer processes wherein Cu(III) plays an important role (Feigl, F. 1956).Based on the studies of oxidation of some organic compounds by Cu(III) complex, Indian researchers proposed that in the alkaline medium the formula of DTC may be represented by [Cu(H 4 TeO 6 ) 2 ] -and the mechanism involving a pre-equilibrium of a adduct formation between the complex and reductant was also proposed (Zhao, X. Z., Luo, Y. R. 1993) .Because Cu(III) is in the higher oxidation state and the reaction is complicated in this kind of reaction system, it is of significance to have a further study on this kind of reaction system.Investigation on them will certainly provide us with more dynamical parameters, and will provide theoretical foundation for the design of reaction route in the organic synthesis and quantitative analysis in analytical chemistry.

Materials and apparatus
All the reagents used were of A.R. grade.All solutions were prepared with doubly distilled water.Solutions of [Cu(H 4 TeO 6 ) 2 ] -(DTC) and 3-phenoxy-1,2-propanediol were always freshly prepared before using.The stock solution of DTC was prepared and standardized by the method given by Jaiswal and Yadava (Chandra, S., Yadava, K. L. 1968, Jaiswal, P. K., Yadava, K. L. 1973).Its electronic spectrum was found to be consistent with that reported by Jaiswal and Yadava.

Kinetics measurements
All kinetics measurements were carried out under pseudo-first order conditions.Solution (2mL) containing definite concentration of Cu(III), OH -, TeO 4 2-and ionic strength and 3-phenoxy-1,2-propanediol solution (2mL) of appropriate concentration were transferred separately to the upper and lower branch tubes of a λ type two-cell reactor.The concentration of DTC was derived from its absorption at 405 nm.The ionic strength μ was maintained by adding KNO 3 solution and the pH of the reaction mixture was adjusted with a KOH solution.The kinetic measurements were performed on a UV-vis spectrophotometer (TU-1900, Beijing Puxi Inc., China), which had a cell holder kept at constant temperature (± 0.1 ºC) by circulating water from a thermostat (DC-2010, Baoding, China).

Product analysis
A solution with known concentrations of Cu(III), OH -, TeO 4 2-was mixed with an excess of 3-phenoxy-1,2-propanediol. With the complete fading of DTC color (reddish brown) marked the completion of the reaction.The product of oxidation was identified as aldehyde by its characteristic spot test (Feigl, F. 1966).It was found that one mole of 3-phenoxy-1,2-propanediol consumed two moles Cu(III) by weighting.

Evaluation of Pseudo-First Order Rate Constants
Under the conditions of [3-phenoxy-1,2-propanediol] 0 >>[DTC] 0 , the plots of ln(A t -A ∞ ) versus time were straight lines, details of the evaluation are described in our previous work.

Rate Dependence on the Ionic Strength
With other conditions fixed, the reaction rate does not change by the addition of KNO 3 solution (Table 1), which indicate there is negative salt effect (Jin, J. J. 1984).

Reaction Mechanism
Acrylamide was added under the protection of nitrogen atmosphere during the course of reaction.The appearance of white polyacrylamide was consistent with free radical intermediates in the oxidation by Cu(III) complexes.Blank experiments in reaction system gave no polymeric suspensions.
In the alkaline medium, the electric dissociation equilibrium of telluric acid was given earlier.(herepK w =14) The distribution of all species of tellurate in aqueous alkaline solution can be calculated from equilibriums (1)-( 2).In alkaline medium such as [OH -]=0.01mol] is line with a positive intercept which indicates a dissociation equilibrium in which the Cu(III) loses a tellurate ligand H 4 TeO 6 2-from its coordination sphere and forms a reactive monotelluratocuprate(III) complex (MTC).The fractional order in 3-phenoxy-1,2-propanediol indicated complex formation between 3-phenoxy-1,2-propanediol and MTC, and the plots of 1/k obs versus 1/[3-phenoxy-1,2-propanediol] was straight line with a positive intercept providing kinetic evidence for the formation of 2:1 complex.Hence the following inner-sphere electron transfer mechanism involving a pre-equilibrium of a adduct formation between the complex and 3-phenoxy-1,2-propanediol is proposed for the reaction.
In alkaline solution, [Cu(H 2 TeO 6 ) 2 ] 5-protonated easily and it coordinate with central ion.In view of the experiments, the mechanism was poposed as follows:(R stands for phenoxy-) As the rate of the disappearance of Cu (III) was monitored, the rate law of the reaction can be derived as: Re-arranging equation ( 10) leads to equation (11-13): From the equation ( 11), the plots 1/k obs vs 1/[ R' ] are straight lines and the rate constants of the rate-determining step at different temperature were obtained from the intercept of the straight line.Equation ( 12) and ( 13) suggest that the plots of 1/k obs vs 1/[OH -] and 1/k obs vs [H 4 TeO 6

2-
] are straight lines.Activation energy and the thermodynamic parameters were evaluated by the method given earlier(Table 2).

Conclusion
Among various species of Cu*(III) in alkaline liquids, monotelluratocuprate is considered as the active species for the title reaction.The rate constant of slow step and other equilibrium constants involved in the mechanism are evaluated and activation parameters with respect to the slow step of the reaction were computed.The overall mechanistic sequence described here is consistent with product studies, mechanistic studies and kinetic studies.

Table 1 .
Rate Dependence on onic Strength at 298.