Kinetics and mechanism of oxidation of 1,3-propylene glycol by dihydroxyditellutoargentate (III) in alkaline medium

Shan Jinhuan, Huo Shuying, Shen Shigang, Sun Hanwen
(Key Laboratory of Analytical Science of Hebei Province, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)

Received on Oct.3,2003; Supportted by the Nationaol Science Foundation of Hebei Province (NO.295066)

Abstract The kinetics of oxidation of 1,3-propylene glycol (PG) by dihydroxyditellutoargentate(III) (DDA) was studied spectrophotometrically between 303.2K and 318.2K in alkaline medium. The reaction rate showed first order dependence in DDA and1.18-1.26 order in PG. It was found that the pseudo-first order rate constant k_obs increased with an increase in concentration of [OH^-] and a decrease in concentration of [TeO₄^2-]. There was a negative salt effect and no free radical was detected. In view of this the dihydroxymonotelluratoargentate (III) species is assumed to be the active species. A plausible mechanism involving a two-electron transfer is proposed and the rate equations derived from mechanism can explain all experimental results. The activation parameters along with the rate constants of the rate-determining step were calculated.
Keywords dihydroxyditellutoargentate(III), 1,3-propylene glycol, redox reaction, kinetics and mechanism

Recently, the study of the higher oxidation state of transition metals has intrigued many researchers. Transition metals in a higher oxidation state generally can be stabilized by chelation with suitable polydentate ligands. Metal chelates such as diperiodatoargentate(III)^[1], ditellutsargentate(III)^[2], ditelluratocuprate(III)^[3], diperiodatonickelate(IV)^[4] are good oxidants in a medium with an appropriate pH value. The use of complexes as good oxidizing agents in analytical chemistry has been reported.^[5,6].The oxidation of a number of organic compounds and metals in lower oxidation state by Ag(III) have also been performed^[3]. But no further information on the kinetics is available. In this paper ,the mechanism of oxidation of PG by dihydroxyditellutoargentate(III) is reported.

1. EXPERIMENTAL SECTION
1.1 Materials
All the reagents used were of A.R. grade. All solutions were prepared with doubly distilled water. Solution of [Ag(OH)₂(H₄TeO₆)₂]^3- (DDA) was prepared and standardized by the method reported earlier^[7]. Its UV spectrum was found to be consistent with that reported. The concentration of DDA was derived by its absorption at l =351nm. Solution of DDA was always freshly prepared before use with solution and double-distilled water. The ionic strength m was maintained by adding KNO₃ solution and the pH value of the reaction mixture was regulated with KOH solution.
1.2 Apparatus and Kinetics Measurements
All kinetics measurements were carried out under pseudo-first order conditions. Solution (2 mL) containing definite [Ag (III)], [OH^-], [TeO₄^2-] and ionic strength m and reductant solution (2mL) of appropriate concentration were transferred separately to the upper and lower branch tubes of a type two-cell reactor. After it was thermally equilibrated at desired temperature in thermobath (made in Shanghai), the two solutions were mixed well and immediately transferred to a 1cm thick rectangular quartz cell in a constant temperature cell-holder (±0.1ºC). The reaction process was monitored automatically by recording the disappearance of Ag(III) with time (t) at 351 nm with a UV-8500 spectrophotometer (made in Shanghai). All other species did not absorb significantly at this wavelength. Details of the determinations are described elsewhere.^[8]
1.3 Product Analysis and Stoichiometry
Solution having known concentrations of [Ag (III)] and [OH^-] was mixed with an excess of PG. The completion of the reaction was marked by the complete discharge of Ag (III) color. After completion of the reaction, the oxidation product was identified^[9] as aldehyde alcohols, which was transformed into a precipitate of 2,4-dinitrophenyldrazone derivative by gravimetric analysis. It is found that one mole of PG consumed one mole Ag (III) by weighing.

2. RESULTS AND DISCUSSION
2.1 Evaluation of Pseudo-First Order Rate Constants
Under the conditions of [Reductant]₀>>[Ag(III)]_0, the plots of ln(A_t-A_∞) versus time are lines, indicating the reaction is first order with respect to [Ag(III)], where A_t and A_∞ are the absorbance at time t and at infinite time respectively. The pseudo-first-order rate constants k_obs were calculated by the method of least squares (r≥0.999). To calculate k_obs generally 8-10 A_t values within three times the half-life were used. k_obs values were at least averaged values of three independent experiments and reproducibility is within ±5%.
2.2 Rate Dependence on [PG]
At fixed [Ag(III)],[OH^-],[TeO₄^2-], ionic strength m and temperature, k _obs values increased with the increase of [PG] and the order in [PG]was found to be 1.18-1.26. The plots of [PG]/k_obs versus1/ [PG] are straight lines with a positive intercept. (r≥0.995) (Fig. 1).
2.3 Rate Dependence on [TeO₄^2-]
At fixed [Ag(III)],[OH^-],[PG], ionic strength m and temperature, k_obs decreased with the increase of [H₄TeO₆^2-] .The plots of 1/k_obs versus [H₄TeO₆^2-] are straight lines with a positive intercept .(r ≥0.999).(Fig 2.)
2.4 Rate Dependence on [OH^-] and Ionic Strength μ
At fixed [Ag(III)], [H₄TeO₆^2-], [PG], ionic strength m and temperature. k_obs increased with the increase of [OH^-].The plots of 1/k_obs versus1/ [OH^-] are lines (r ≥0.999).(Table1)The rate is decreased by the addition of KNO₃ solution(Table2), it indicates that there is a negative salt effect which is consistent with the common regulation of the kinetics^[10].

Fig.1. Plots of [PG]/k_obs vs 1/[PG] at different temperatures, [Ag(III)]=6.214×10^-4mol·L^-1, [OH^-]=1.721×10^-2mol·L^-1,[TeO₄^2-]=1.623×10^-3mol·L^-1, m=4.00×10^-2 mol·L^-1

Fig.2 Plots of 1/.k_obs vs [TeO₄^2-]. [Ag(III)]=6.214×10^-4mol·L^-1,[OH^-]=1.721×10^-2mol·L^-1,[PG]=0.10mol·L^-1,m=4.70×10^-2 mol·L^-1,T=308.2K.

Table 1 Rate dependence on [OH^-]

[Ag(III)]=6.214×10^-4mol·L^-1, [H₄TeO₆^2-]=1.623×10^-3mol·L^-1, [PG]=0.08mol·L^-1, m=8.50×10^-2mol·L^-1, T=308.2K.

Table 2 Rate dependence on ionic strength m

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