http://www.chemistrymag.org/cji/2001/03b055pe.htm

  Nov. 1, 2001  Vol.3 No.11 P.55 Copyright cij17logo.gif (917 bytes)

Kinetics and mechanism of oxidation of ethyleneglycol monobutylether by dihydroxydiperiodatonickelate(IV) complex in alkaline medium

Shan Jinhuan, Wei Haiying, Wang Li, Shen Shigang, Liu Baosheng, Sun Hanwen
(College of Chemistry & Environmental Science, Hebei University, Baoding 071002 China)

Abstract The kinetics of oxidation of ethyleneglycol monobutylether(EGB) by dihydroxydiperiodatonickelate(IV) complex(DPN) in aqueous alkaline medium at a temperature range of 25-45oC was studied by spectrophotometry. The reaction was found to be first order with respect to both Ni(IV) and EGB. The rate increased with the increase in [OH-] and decreased with the increase in [IO4-]. Added salts did not affect the rate and no free radical was detected. In view of these, a plausible mechanism of reaction involving a rapid preequilibrium is proposed. In addition, the rate equation which is derived from the mechanism can explain all experimental observations. Activation parameters of the rate-determining step were calculated.
Keywords dihydroxydiperiodatonickelate(IV), ethyleneglycol monobutylether, redox reaction, kinetics and mechanism

In recent years, study of the highest oxidation state of transition metals intrigued many researchers' interests, which can provide new and valuable information in some fields.
    Transition metals, in a higher oxidation state, can generally be stabilized by chelation with suitable polydentate ligands. Metal chelates such as diperiodatocuprate(III)[1], diperiodatoargentate(III)[2], diperiodatonickelate(IV)[3] and nickel(IV) oxime imine[4] complexes are good oxidants in a medium with an appropriate pH value. The use of Ni(IV) as an oxidizing agent is well known in the investigation of some organic compounds such as tetrahydrofurfuryl alcohol[5], 1,4-dioxane[6], ethylene diamine[7] etc. In the previous works, the existent form was [Ni(OH)2(H3IO6)2]2-. In this paper, we study the reaction kinetics and mechanism between DPN and EGB, and the existent form is less protonated complex [Ni(OH)2(H2IO6)2]4-, which is consistent with the experimental results.

1. EXPERIMENTAL
1.1 Materials
All reagents used were of A.R. grade. All solutions were prepared with twice-distilled water. Solutions of DPN and EGB were always freshly prepared before using with stock solution and twice-distilled water. The stock solution of DPN in a strong alkaline medium was prepared by the procedure given by Baker[8] and standardized by the method by Murthy[9]. Its electronic spectrum was found to be consistent with that reported by Murthy.
1.2 Kinetic measurement and reaction product analysis
The kinetic measurement was described elsewhere[10].The product of oxidation was the corresponding aldehyde by its characteristic spot test[11].

2. RESULTS AND DISCUSSION
2.1 Evaluation of pseudo-first order rate constants
Under the conditions of [EGB]o>>[Ni(IV)]o, the plots of ln(At-A03b05516.gif (65 bytes)) versus t were linear, indicating that the reaction is first order with respect to Ni(IV), where At and A03b05516.gif (65 bytes) were the absorbency at time t and at infinite time respectively. The pseudo-first order rate constants, kobs, were evaluated by the method of least squares (r>0.999). To calculate kobs, 8-10 At values within three times the half-life were used. kobs values of this paper were the averaged values of at least three independent experiments, and reproducibility is within ± 5%.
2.2 Rate dependence on [EGB]
At fixed [Ni(IV)], [OH-], [IO4-], ionic strength I and temperature, kobs increased with the increase of the [EGB]. Furthermore, the plots of kobs vs. [EGB] were linear through the origin at different temperatures (Fig 1), indicating the reaction order dependence on EGB was first order.

03b056001.gif (2282 bytes)
Fig.1 The plots of 103kobs vs. [EGB] at different temperatures
[Ni(IV)]=1.4854×10-4mol/L; [IO4-]=1.8378×10-3mol/L;[OH-]=1.3541×10-2mol/L; I=0.01651 mol/L

2.3 Rate dependence on [OH-]
At constant [Ni(IV)], [EGB], [IO4-], ionic strength I and temperature, the rate increased with the increase of [OH-]. The plot of lnkobs vs. ln[OH-] showed the order with respect to OH- was fractional (observation reaction order nap=0.38-0.52), indicating the reaction existed a preequilibrium involving OH- attended. The plots of 1／kobs vs. f([OH-])／[OH-] were straight linear with positive intercept (Fig. 2).
2.4 Rate dependence on [IO4-] and ionic strength I
At fixed [Ni(IV)], [EGB], [OH-], I and temperature, kobs decreased with the increase of [IO4-]. The order of IO4- was negative fractional (nap=-0.31) and the plot of 1／kobs vs. [IO4-] was linear (Table 1), showing that there was a preequilibrium involving the process of disassociation H2IO63-from Ni(IV)complex. At fixed conditions except ionic strength I, kobs hardly altered with the increase in I (Table 1).

03b056002.gif (2740 bytes)
Fig. 2 The plots of 1／kobs vs. f([OH-])/[OH-] at different temperatures
[Ni(IV)]=1.613×10-4mol/L; [IO4-]=1.79×10-3mol/L; [EGB]=2.50×10-2mol/L; I=1.636×10-2 mol/L

Table 1. 103kobs /s-1 varying with the different [IO4-], ionic strength I at 308.2K.

102I/mol/L

103 [IO4-]/mol/L

103kobs/s-1

1.636

1.79

7.446

2.940

1.79

7.893

4.256

1.79

8.253

5.721

1.79

8.212

6.636

1.79

8.284

1.636

2.79

6.507

1.636

3.79

5.93

1.636

4.79

5.576

1.636

5.79

5.098

[Ni(IV)]=1.613×10-4mol/L; [EGB]=2.50×10-2mol/L;[OH-]=1.313×10-2mol/L

2.5 Free radical detection
The addition of acrylonitrile or acrylamine to the reaction mixture under the protection of nitrogen neither changed the rate nor there was any polymerization, which showed the absence of free radical in the reaction.
2.6 Discussion
In aqueous periodate solution, equilibria (1)-(3) were detected and the corresponding equilibrium constants at 25°C were determined by Aveston[12].

2IO4- + 2OH- H2I2O104- logb 1=15.05                 (1)
IO4- +OH- + H2O H3IO62- logb 2=6.21                (2)
IO4- + 2OH- H2IO63- logb 3=8.67                          (3)

The distribution of all periodate species in aqueous solution was calculated from equilibria (1)-(3) . The dimer(H2I2O104-) and IO4- species of periodate can be neglected. The main species of periodate are H3IO62- and H2IO63-, consistent with the result calculated from Crouthamel's data[13] by Murthy. Based on such distribution, the formula of Ni(IV) periodate complex may be represented by either [Ni(OH)2(H3IO6)2]2- or the less protonated [Ni(OH)2(H2IO6)2]4-. We preferred to use the latter to represent DPN because it is close to that suggest by Mukherjee[14] and will obtain support from kinetic studies.
    In view of the above results and discussion, a plausible reaction mechanism was proposed:

[Ni(OH)2(H2IO6)2]4- +OH- [Ni(OH)2(H2IO6)]2- +H2IO63- +H2O                    (4)

         DPN                                                MPN

[Ni(OH)2(H2IO6)]2- + HOCH2CH2OC4H9 adduct                                        (5)

          MPN

adduct Ni(IV) + HCOCH2OC4H9                                                                    (6)

    Here, reaction (5) was the rate-determining step.
    As the rate of the disappearance of Ni(IV) was monitored, the rate of the reaction can be derived as:

                           (7)

                                                                           (8)

Here :   

    Subscripts T and e stand for total concentration and concentration at equilibrium respectively. Neglecting the concentration of ligand dissociated from Ni(IV) and the species of periodate other than H2IO63- and H3IO62-, equations (9) and (10) can be obtained from (2) and (3):

                          (9)

                           (10)

Here [IO4-]ex represents the original overall entering periodate and equals approximately to the sum of [H2IO63-] and [H3IO62-].

Substituting eq.(9) into (8), we can get the following equations:

                                          (11)

                                     (12)

    Eq.(8) suggests that the plot of kobs vs. [EGB] should be linear, and eq.(11) shows that the plot of 1／kobs vs. f([OH-])／[OH-] should also be linear, which are consistent with the experimental results.
    If the formula of DPN was [Ni(OH)2(H3IO6)2]2-, getting eq.  (10) into (8) obtained eq.(13):

                                                (13)

    The plot of 1／kobs vs.  j([OH-])／[OH-] should also be linear, but the linearity was not straight, which substantially denies eq.(13). Therefore, it seems advisable to represent DPN by [Ni(OH)2(H2IO6)2]4-, which is consisted with the experimental observation.

Table 2 Rate constants, equilibrium constant and activation parameters of rate-determining step

Constants

T／K

Activation parameters at 298.2K

298.2

303.2

308.2

313.2

318.2

k／mol/L/s-1

0.2096

0.2605

0.4034

0.6516

1.017

Ea*=63.88kJ/mol
DH¹ =61.40kJ/mol
DS¹ =-53.02J/K·mol

K

0.0725

0.0513

0.0423

0.0393

0.0388

* r=0.992, a=24.08, b=-7683.63 for the linear regression of lnk vs. 1／T.

    The plots of 1／kobs vs. f([OH-])／[OH-] were linear at different temperatures. From their intercepts, the rate-determining step constants k were evaluated. The activation parameters data were calculated[15] (Table 2).

REFERENCES       
[1] Niu W J, Zhu Y, Hu K C et al. Int. J. Chem. Kinetics,1996, 28 (12): 899.
[2] Shi T S. Science in China(series B) (Zhongguokexue B), 1990, 33: 471.
[3] Chandraiah U, Murthy C P, Sushama K. Indian J. of Chem. 1989, 28A: 162.
[4] Santanu B, Pradyot B. Bull. Chem Soc. Jpn., 1996, 69: 3475.
[5] Li Z T, Wang F L, Wang A Z. Int. J. Chem. Kinetics, 1992, 24: 933.
[6] Halligudi N N, Desai S M, Nandibewoor S T. Int. J. Chem. Kinetics, 1999, 31: 789.
[7] Li Z T, Chang Q, Li B W, et al. Chinese Research in Chinese Universities (Gaodengxuexiao Huaxuexuebao), 2000, 21 (5): 747.
[8] Baker L C W, Mukherjee H G, Sarkar S B et al.. Indian J. Chem., 1982, 21A: 618.
[9] Murthy C P, Sethuram B, Rao T. Z. Phys. Chem.(Leipzig), 1986, 287: 1212.
[10] Shan J H, Qie L J, Guo X S. Acta Chimical Sinica (Huaxue Xuebao), 1997, 55: 458.
[11] Feigl F. Spot Tests in Organic Analysis, New York: Elsevier Publishing Co., 1956.
[12] Aveston J. J. Chem. Soc.(A), 1969: 273.
[13] Crouthamel C E, Meek H V, Martin D S et al. J. Am. Chem. Soc., 1949, 71: 3031; 1951, 73: 82.
[14] Mukherjee H G, Mandal B, De S. Indian J. Chem., 1984, 23A: 426.
[15] Shan J H, Liu T Y. Acta Chimica Sinica (Huaxue Xuebao), 1994, 52: 1140.

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