Kinetics and mechanism of oxidation of some meta-dihydric alcohols by ditelluratocuprate(III) in alkaline medium

Shan Jinhuan, Wang Liping, Huo Shuying, Shen Shigang, Sun Hanwen
(College of Chemistry and Environmental Science. Hebei University, Baoding 071002 China)

Received  Apr. 4, 2002; Supported by the Natural Science Foundation of Hebei Province (295066).

Abstract The kinetics of oxidation of some meta-dihydric alcohols by ditelluratocuprater(III) (DTC) was studied spectrophotometrically between 25ºC and 40ºC in alkaline medium. The reaction rate showed first order dependence in oxidant and fractional order in reductant. It was found that the pseudo-first order rate constant k_obs increased with the increase of [OH^-] and the decrease of [TeO₄^2-]. There is a negative salt effect. A plausible mechanism involving a preequilibrium of adduct formation between the complex and reductant was proposed. The rate equations derived from mechanism can explain all experimental observations. The activation parameters along with rate constanas of the rate-determining step were calculated.
Keywords ditelluratocuprate(III), 1,3 –propylene glycol, 1,3-butylene glycol, redox reaction, kinetics and mechanism

1.EXPERIMENTAL
1.1 Material
All reagents used were analytical grade. All solutions were prepared with twice-distilled water. Solutions of DTC and reductant were always prepared freshly before use. The stock solution of DTC was prepared by method given by Jaiswal and Yadava^[5]. Its electronic spectrum was found to be consistent with that reported by Jaiswal and Yadava. The concentration of DTC was derived by its absorption at l =405nm. The ionic strength was maintained by addition of KNO₃ solution and the pH value was regulated by KOH solution.
1.2 Kinetic measurement and reaction product analysis     
All kinetics measurements were carried out under pseudo-first order conditions. Solution (2 mL) containing definite [Cu(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 l type two-cell reactor. After it was thermally equilibrated at desired temperature in thermobath (Shanghai), the two solutions were mixed well and immediately transferred to a 1cm thick glass cell in a constant temperature cell-holder (±0.1ºC). The reaction process was monitored automatically by recording the disappearance of Cu(III) with time (t) at 405 nm with a UV-8500 spectrophotometer (Shanghai). All other species did not absorb significantly at this wavelength.
    A solution with known concentrations of [Cu(III)], [OH^-], [TeO₄^2-] was mixed with an excess of reductant. With the complete fading of DTC color (brown red) marked the completion of the reaction, the aldehyde alcohols formed was estimated as its 2,4-dinitrophenyldrazine derivative by gravimetric analysis. It was found that one mole reductant consumed two mole Cu(III). The product of oxidation was the corresponding aldehyde which was confirmed by its characteristic spot test^[7].

2. RESULTS AND DISCUSSION
2.1 Evaluation of pseudo-first order rate constants
Under the conditions of [reductant]₀>>[Cu(III)]_0. The plots of ln(A_t-A_∞) versus time were lines, indicating the reaction is first order with respect to [Cu(III)], where A_t and A_∞ were 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≥0.9999). To calculate k_obs generally 8-10 A_t values within three times the half-lives were used. k_obs values were at least averaged values of three independent experiments and the reproducibility is within ±5%.
2.2 Rate dependence on [reductant]      
At fixed [Cu(III)],[OH^-],[TeO₄^2-], ionic strength m and temperature, k _obs values increased with the increase of [reductant] and the order in reductant was found to be fractional order.(Table 1). The plots of 1/k_obs versus1/ [reductant] were straight lines with a positive intercept. (r≥0.995) (Fig. 1).

Fig.1 Plots of 1/k_obsvs1/[PG] at different temperatures
[Cu(III)]=9.128×10^-5mol/L; [TeO₄^2-]=1.500×10^-3mol/L;
[OH^-]=3.007×10^-3mol/L; m=7.507×10^-3mol/L

2.3 Rate dependence on [OH^-]   
At fixed [Cu(III)], [TeO₄^2-] , [reductant], ionic strength m and temperature, k_obs increased with the increase of [OH^-] and the order with respect to OH^- were found to be fractional order (n_ap(PG)=0.736 and n_ap(BG)=0.719) (Table2A,B ) .The plots of 1/k_obs versus [OH^-] were lines (r ≥0.998).
2.4 Rate dependence on [TeO₄^2-] ionic strength m      
At fixed [Cu(III)], [OH^-], [reductant], ionic strength m and temperature, k_obs decreased with the increase of [TeO₄^2-] and the order with respect to TeO₄^2- were found to be fractional order (n_ap(PG)=-0.652 and n_ap(BG)=-0.678) (Table2A,B ) .The plots of 1/k_obs versus [TeO₄^2-] were lines (r ≥0.997).
    The rate was decreased by the addition of KNO₃ solution (Table2A,B), which indicate there was a negative salt effect which is consistent with the common regulation of the kinetics^[8].

Table 110³ k_obs /s^-1 varying with different [BG] at different temperatures [Cu(III)]=9.128×10^-5mol/L; [TeO₄^2-] =1.500×10^-3mol/L; [OH^-]=3.007×10^-3mol/L; m=7.507×10^-3mol/L

Table 2A10³ k_obs /s^-1varying with the different [TeO₄^2-],[OH^-], m at 30ºC [Cu(III)]=9.128×10^-5mol/L

Table 2B10³ k_obs /s^-1varying with the different [TeO₄^2-],[OH^-], m at 30ºC [Cu(III)]=9.128×10^-5mol/L

2.5 Free radicai detection
Acrylamide was added to the reaction system under the protection of nitrogen during the course of reaction. With the comparison to blank experiments without white polymeric suspensions, acrylamide had been polymerized under the initiation of free radical, which showed the production of free radical intermediates in the oxidation by Cu(III) complexes.
2.6 Discussion       
In alkaline medium, the dissociative equilibrium of [TeO₄^2-] was given earlier ^[9],(here K_w=14)
H₅TeO₆^- + OH^-H₄TeO₆^2- + H₂O           lg b₁=3.049          (1)
H₄TeO₆^2- + OH^- H₃TeO₆^3- + H₂O        lg b₂=-1                (2)
Hence the main tellurate species was H₄TeO₆^2-.
In view of the experiments, the mechanism was proposed as follows:
[Cu(OH)₂(H₄TeO₆)₂]^3- + OH^- [Cu(OH)₂(H₃TeO₆)]^2- + H₄TeO₆^2-+H₂O (3)
[Cu(OH)₂(H₃TeO₆)]^2- + RCHOHCH₂CH₂OH [Cu(OH)₂(H₃TeO₆)( RCHOHCH₂CH₂OH)]^2- (4)
[Cu(OH)₂(H₃TeO₆)( RCHOHCH₂CH₂OH)]^2- RCHOHCH₂^·CHOH + [Cu(OH)(H₃TeO₆)]^2-+H₂O (5)
Cu*(III) + OH^- + RCHOHCH₂^·CHOH Cu(II) + RCHOHCH₂CHO + H₂O (6)
Where Cu*(III) stand for any kind of form which Cu³⁺ existed in equilibrium. Reaction (3) and (4) belong to dissociation and coordination equilibrium, whose reaction rate are generally faster, reaction (5) belongs to electron-transfer reaction, whose reaction rate is generally slower, so reaction (5) is the rate –determining step.

                  (9)
Re-arranging equation (9) leads to equation (10-12)
(10)
(11)
(12)
From equation (10), the plots of 1/k_obs vs.1/[reductant] are straight lines and the rate constants of rate-determining step at different temperature was obtained from the intercept of the straight line. Equation (11) and (12) suggests that the plot of 1/k_obs vs 1/[OH^-] and 1/k_obs vs [H₄TeO₆^2-] are straight lines. Activation energy and the thermodynamic parameters were evaluated by the method given earlier^[10](Table 3). Dissociation equilibrium constant K₁ and coordination equilibrium constant K₂ of [PG] and [BG] are respectively 0.098,94.33 L/mol and 0.094,126.8L/mol. (t=30ºC).
    Based on the above results and discussion, It was concluded that the less the carbon chain , the larger the observed rate constants and the larger the rate constants of rate-determining step. The phenomena were consistent with the spatial hindrance of dihydric alcohols.

Table 3 Rate constants (k) and activation parameters of the rate-determining step

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