Kinetics and mechanism of oxidation of 1,3-butylene glycol by diperiodatocuprate (III) complex in alkaline medium

1. EXPERIMENTAL
1.1 Materials                          
All reagents used were of AR grade. All solutions were prepared with doubly distilled water. Solutions of DPC and reductant were always freshly prepared before use. The stock solution of DPC in a strong alkaline medium was prepared by the method given by Jaiswal^[4]. The ionic strength was maintained by adding KNO₃ solution and the pH value was regulated with KOH solution.
1.2 Kinetic measurement and reaction product analysis                          
Measurements of the kinetics were performed using a UV-1901 spectrophotometer (Beijing) fitted with a CS-501 thermostat (±0.1ºC, Chongqing).The kinetics measurements were described previously^[5]. The product of oxidation was the corresponding aldehyde alcohols by their characteristic spot test^[6].

2. RESULTS AND DISCUSSION
2.1 Evaluation of pseudo-first order rate constants                          
The calculated measurements of the rate constant (k_obs) values were described previously^[5]
2.2 Effect of varying [1,3-butylene glycol]                          
At fixed [Cu(III)], [OH^-], [IO₄^-], ionic strength and temperature, k _obs values increased with increase in [1,3-butylene glycol] and the plots of k_obs versus [1,3-butylene glycol] were line at passing through the origin (Fig.1) indicating that the reaction is first order in reductant.

Fig.1 Plots of k_obs vs [1,3-butylene glycol] at different temperatures
[Cu(III)]=8.00×10^-5mol/L, [IO₄^-]=1.60×10^-3mol/L, [OH^-]=0.10mol/L, m=0.10mol/L

2.3 Effect of varying [OH^-]                          
At fixed [Cu(III)],[1,3-butylene glycol], [IO₄^-], ionic strength and temperature, k_obs values increased with increase in [OH^-], the plots of lnk_obs versus ln[OH^-] were linear(r≥0.996) and from the slopes of such plots the observed order, n_ap, were found to be 1<n_ap<2. In addition, the plots of 1/k_obs versus f² ([OH])/[OH^-]² were linear (Table1 ).

Table 1 10³ k_obs /s^-1 varying with the different [OH^-] at different temperatures
[Cu(III)]=8.00×10^-5mol/L, [IO₄^-]=1.60×10^-3mol/L, [1,3-butylene glycol]=0.20 mol/L, m=0.25 mol/L

b and r respectively stand for the slope and the relative coefficient of the plot of lnk_obs vs lnC

2.4 Effect of varying [IO₄^-] and ionic strength m                          
At fixed [Cu(III)], [OH^-], [1,3-butylene glycol], ionic strength and temperature, k_obs decreased with increase in [IO₄^-]. The observed orders, n_ap, were found to be -1.36. The plots of 1/k_obs versus [IO₄^-]² was linear (Table 2). Table 2 reveals that ionic strength has negligible effect on the rate.

Table 2 Influence of variation of [IO₄^-], and ionic strength m, t=303.2K

2.5 Free radicai detection                          
Acrylamide was added under nitrogen blanket 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 gave no polymeric suspensions.
2.6 Discussion                          
In aqueous periodate solution equilibria (1)~(3) were detected and the corresponding equilibrium constants at 298.2K were determined by Aveston^[7].
2 IO₄^- + 2 OH^- H₂I₂O₁₀^4- log b₁=15.05    (1)
IO₄^-+ OH^-+H₂O H₃IO₆^2- log b₂=6.21      (2)
IO₄^- + 2 OH^- H₂IO₆^3- logβ₃=8.67           (3)
    The distribution of all species of periodate in aqueous alkaline solution can be calculated from equilibria (1)-(3). Under the conditions of [OH^-]=0.01~0.1mol·L^-1, [H₂IO₆^3-]: [H₃IO₆^2-]: [H₂I₂O₁₀^4-]: [IO₄^-]≈(2.9~29): 1.0: (0.02~0.2): (6×10^-5~6×10^-2) , the dimer and IO₄^- species of periodate can be neglected, Neglecting the concentration of ligand dissociated from Cu (III) and the species of periodate other than H₂IO₆^3- and H₃IO₆^2-, Eqs. (4) and (5) can be obtained from(2) and(3):
     (4)
     (5)
    Here [IO₄^-]_ex represents the original overall entering periodate and equals approximately to the sum of [ H₂IO₆^3- ] and [H₃IO₆^2-].
    Because the hexa-cyclic compound formed by m-compound is larger, will be more spatial hindrance compared to penta-cyclic compound formed by o-compound, and [Cu(OH)₄]^-, being small gives less sterical hindrance than [Cu(OH)₂(H₂IO₆)]^2-. The plot of 1/k_obs versus f² ([OH])/[OH^-]² is linear and the plot of 1/k_obs versus [IO₄^-]² is also linear with a positive intercept indicating that OH^- takes part in a pre-equilibrium with Cu(III) before the rate-determining step and a dissociation equilibrium in which the [Cu(III)] loses two periodate ligand H₂IO₆^3- forming [Cu(OH)₄]^- as reactive species.
    In view of the above discussion, a plausible reaction mechanism was proposed:
[Cu(OH)₂(H₂IO₆)₂]^5- + 2OH^- [Cu(OH)₄]^- + 2H₂IO₆^3- (6)
[Cu(OH)₄]^-+ H₂CCHOHCH₂CH₂OH Cu(II) + H₂CCHOHCH₂^·CHOH (7)
Cu*(III) +OH^- + H₂CCHOHCH₂^·CHOH Cu(II) + H₂CCHOHCH₂CHO (8)
    Where Cu*(III) stand for any kind of form which Cu(III) existed in equilibrium. Subscripts T and e stand for total concentration and at equilibrium respectively. [Cu(III)]_T =[Cu(OH)₂(H₂IO₆)₂]_e^5- +[Cu(OH)₄]^-_e. Reaction (8) is the rate-determining step.
    As the rate of the disappearance of Cu (III) was monitored, the rate of the reaction can be derived as:
   (9)
   (10)
Substituting Eq (4) into (10), we get the following equation (11)
   (11)
Rearranging Eq (11) leads to Eqs (12) and (13):
        (12)
   (13)
    Equation (12) suggests that the plot of 1/k_obs versus [IO₄^-]²_ex should be linear, and Equation (13) shows that the plots of 1/k_obs versus f²([OH^-])/[OH^-]² should also be linear at different temperatures. From their slopes and intercepts the rate-determining step constants (k) and equilibrium constants (K) were evaluated. The activation parameters data of 1,3-butylene glycol obtained by the method given earlier ^[8](Table 3).

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

Fig2 Plot of 1/k_obs vs f²([OH^-])/[OH^-]² at 298.2K
[Cu(III)]=8.00×10^-5mol/L, [IO₄^-]=1.60×10^-3mol/L, [1,3-butylene glycol]=0.20mol/L, m=0.25mol/L

    The plot of 1/k_obs versus f² ([OH^-])/[OH^-]² is shown in Fig2 which rules out Equation (14). Based on the experimental observations, the formula of Cu(III) periodate complex may be represented by [Cu(OH)₂(H₂IO₆)₂]^5-, which is support from kinetic studies.
    Comparition 1,3-butylene glycol with 1,3-propyleneglycol (Table 3), we can say that smaller the carbon chain is, the larger the observed rate constants and the rate-determining step constants are, which is consistent with dihydric alcohol's spatial hindrance.

REFERENCES
[1] Wang L P. Shan J H. Xu M Y et al.   Journal of Hebei University, 2006, 26(6): 602.
[2] Shan J H. Wang L P. Huo S Y et al.  Chemical Journal on Internet, 2002, 4(10): 49.
[3] Shan J H. Qian Jing. Wang L P et al.  Chemical Journal on Internet, 2003, 5(3): 21.
[4] Jaiswal P K. Yadava K L.  Indian J Chem, 1973,11: 837.
[5] Shan J H. Wang L P. Shen S G et al.  Turkish Journal of Chemistry, 2003, 27: 265.
[6] Feigl F. Spot Tests in Organic Analysis. Elsevier Publishing Co.: New York, 1956 .
[7] Aveston J.  J Chem Soc(A), 1969,273.
[8] Shan J H. Liu T Y. Acta Chim Sinica, 1994, 52: 1140.

碱性介质中分光光度法研究二过碘酸合铜（Ⅲ）氧化1,3-丁二醇的反应动力学及机理
王立平 葛旭升 王亚茹 许明远
（保定学院化学系，河北 保定 071051）
摘要 在碱性介质中，用分光光度法研究了二过碘酸合铜（Ⅲ）配离子(DPC) 氧化1,3-丁二醇的反应动力学及机理。结果表明，反应对DPC和1,3-丁二醇均为一级，表观速率常数k_obs随着[IO₄^-] 增加而减小，随着[OH^-]增大而增大。据此提出了包括DPC和[Cu(OH)₄]^-形成的前期快速平衡的反应机理，由假设反应机理推出的速率方程能解释全部实验现象，进一步求得速控步的速率常数k和平衡常数K及活化参数。
关键词 二过碘酸合铜（Ⅲ）；1,3-丁二醇； 氧化还原反应； 动力学及机理