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

Abstract The kinetics of oxidation of b-propylene-glycol (b-PG) by dihydroxydipriodatonickelate ( IV) ion ( DPN) was studied by spectrophotometry in alkaline medium. The reaction was found to be pseudo-first order with respect to Ni( IV) and to be fractional order with respect to b-PG. The rate increased with the increase of the concentration of OH^- and decreased with the increase of the concentration of  IO₄^-. Added salts did not affect the rate and no free radical was detected. A mechanism involving a pre-equilibrium of an adduct formation between the complex and b-PG was proposed. The rate equation derived from the mechanism can explain all the experimental results. The activation parameters of the rate-determining step were calculated.
Keywords dihydroxydipriodatonickelate (IV), b-propylene-glycol, 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-. Since Ni( IV) is in the highest oxidation state and the reaction is complicated in this system, it is of significance to have a further study on this kind of reaction system. In this paper, we study the reaction kinetics and mechanism between DPN and b-PG, have been studied and found that 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 b-PG 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 Kinetics measurement and reaction product analysis       
Measurements of the kinetics were performed using a UV-8500 spectrophotometer (Shanghai) fitted with a 501 thermostat ( ±0.1ºC) . Details of the determinations are described elsewhere ^[10]. The oxidation product was identified as the corresponding aldehyde by spot test^[11].

2 RESULTS AND DISCUSSION
2.1 Evaluation of pseudo-first order rate constants
Under the conditions of [b-PG]₀ >> [Ni( IV) ]₀, the plots of ln(A_t-A_∞) vs. time t for more than three times the half-life of the reaction were good straight lines ( r>0.9997) , indicating the order in Ni( IV) to be unity. The pseudo-first-order rate constants k_obs were calculated by using the least-squares method. The rate constants reported here are average of three independent runs. Deviations in duplicate determinations are generally less than ± 5% .
2.2 Effect of varying [b-PG]       
Under the condition of [b-PG]₀ > > [ DPN]₀, at fixed [Ni(IV) ], [OH^-], [IO₄^-], ionic strength I and temperature, k_obs values increased with the increase in concentration of b-PG and the order with respect to b-PG was found to be fractional ( in the temperature range of 25- 45ºC, n_ap= 0.44- 0.68) . The plots of 1/ k_obs vs. 1/ [b-PG] were straight lines with a positive intercept ( r>0.996) ( Fig.1) .

Fig.1 The plots of 1/ k_obs vs. 1/ [b-PG] at different temperatures
[Ni( IV) ]= 0.641×10^-4mol/ L; [IO₄^-]= 1.195×10^-3mol/ L;
[OH^-]= 1.498×10^-2mol/ L; I= 1.62×10^-2mol/ L

2.3 Effect of varying [OH^-]      
At constant [Ni( IV) ], [b-PG], [IO₄^-], ionic strength I and temperature, k_obs increased with the increase of [OH^-]. The plot of lnk_obs vs. ln[OH^-] showed the order with respect to OH^- was fractional order ( n_ap= 0.67) . The plot of 1/ k_obs vs. f([OH^-]) / [OH^-] was straight line with positive intercept ( Fig 2) .

Fig.2 The plot of 1/ k_obs vs  f( [ OH^-] / [ OH^-] ) at t= 35ºC
[ Ni( IV) ] = 0.641×10^-4mol/ L;[ IO₄^-] = 1.695×10^-3mol/ L; [b-PG] = 0.2mol/ L; I= 4.88× 10^-2mol/ L

2.4 Effect of varying [IO₄^-]
At fixed [Ni( IV) ], [b-PG], [OH^-], I and temperature, k_obs decreased with the increase of [IO₄^-].The order of IO₄^- was negative fractional ( n_ap =-0.75) and the plot of 1/ k_obs vs. [IO₄^-] was line ( r = 0.9992) ( Table 1) .
2.5 Effect of varying I
At fixed conditions except ionic strength I, k_obs hardly altered with the increase in I ( Table 1) .

Table 1 Influence of Variation of I, and [IO₄^-]

[b-PG]= 0.2mol/ L ; t= 35ºC

2.6 Discussion
In the alkaline medium, the dissociative equilibria ( 1) - ( 3) of the IO₄^- were detected and the corresponding equilibrium constants at 25ºC were determined by Aveston^[12].
2IO₄^{- +} 2OH^- H₂I₂O₁₀^4- logb_{1 =} 15.05               ( 1)
IO₄^- + OH^- + H₂O H₃IO₆^2- log b₂ = 6.21         ( 2)
IO₄^- + 2OH^- H₂IO₆^3- log b₃ = 8.67                    ( 3)
    The distribution of all periodate species in alkaline solution was calculated from equilibria ( 1) - ( 3) . The dimer ( H₂I₂O₁₀^4-) and IO₄^- species of periodate can be neglected. The main species of periodate are H₃IO₆^2- and H₂IO₆^3-, 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 the 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(HIO₆)]^2- + H₂IO₆^3- + H₂O ( 4)
                I                                                                  II
[Ni(OH)₂(HIO₆)]^2- + CH₂OHCH₂CH₂OH complex                           ( 5)
             II
complex [Ni(OH) ₂(HIO₆)]^4- + CH₂OHCH₂CHO + H₂O                    ( 6)
Here, reaction ( 6) was the rate-determining step.


Subscripts T and e stand for total concentration and concentration at equilibrium respectively.
As the rate of the disappearance of Ni( IV) was monitored, the rate of the reaction can be derived as:
    ( 7)

        ( 8)
Neglecting the concentration of ligand dissociated from Ni( IV) and the species of periodate other than H₂IO₆^3- and H₃IO₆^2-, here:
[IO₄^-]_ex @ [H₃IO₆^2-]+ [H₂IO₆^3-] ( 9)
Equations ( 10) and ( 11) can be obtained from ( 2) , ( 3) , ( 9) :
( 10)
( 11)
Substituting eq ( 10) into ( 8) , we can get the following equations:
( 12)

( 13)

( 14)
Eq ( 12) suggests that the plot of 1/ k_obs vs 1/ [b-PG] should be line, and eq ( 13) shows that the plot of 1/ k_obs vs  f ( [OH^-]) / [OH^-] should be line, eq ( 14) shows that the plot of 1/ k_obs vs [IO₄^-]should also be line, which are consistent with the experimental results.
    If the formula of DPN was [Ni(OH)₂ (H₃IO₆)₂]^2-, getting eq ( 11) into ( 8) obtained eq ( 15) :
     ( 15)
    The plot of 1/ k_obs vs f ( [OH^-]) / [OH^-] should also be line, but the linearity was not straight ( Fig.3) , which substantially denies eq( 15) . Therefore, it seems advisable to represent DPN by [Ni(OH)₂(H₂IO₆)₂]^4-, which is consisted with the experimental observation.

Fig.3 The plot of 1/ k_obs vs. f( [OH^-]) / [OH^-] at t=35ºC
[Ni( IV) ]= 0.641×10^-4mol/ L; [IO₄^-] = 1.695×10^-3 mol/ L; [b-PG] = 0.2 mol/ L; I = 4.88×10^-2 mol/ L

    Activation energy and the thermodynamic parameters ( t= 25ºC) were evaluated ( Table 2) by the method given earlier^[15].

REFERENCES         
[ 1] Niu W J, Zhu Y, Hu K C et al. Int. J. Chem. Kinet, 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 Kandlikar. Indian J. Chem. 1989, 28A: 248.
[ 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. Kinet, 1992, 24: 933.
[ 6] Halligudi N N, Desai S M, Nandibewoor S T. Int. J. Chem. Kinet, 1999, 31: 789.
[ 7] Li Z T, Chang Q, Li B W, et al. Chinese Research in Chinese Universities (GaodengxuexiaoHuaxuexuebao) , 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, Navaneeth Rao T. Z. Phys. Chemie, Leipzig, 1986, 287: 1212.
[ 10] Shan J H, Liu T Y. Acta Chimica Sinica (Huaxue xuebao) , 1994, 52: 1140.
[ 11] Feigl F. Spot Tests in Organic Analysis, New York: Elsevier Publishing Co., 1966, 7: 196.
[ 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.
[ 14] Mukherjee H G, Mandal B, et al. Indian J. Chem., 1984, 23A: 426.
[ 15] Shan J H, Wang L P et al. Chinese Journal of Inorganic Chemistry (Wujihuaxuexuebao), 2002, 18 ( 9) : 887.
[ 16] JIN J J. Kinetics Principle of Chemical Reaction in Liquid Phase., Shanghai: Science
Technique Press, 1984: 29.
[ 17] The Teaching and Research Section of Analytical Chemistry in Zhongnan Mining Institute, Handbook of Analytical Chemistry, Beijing: Science Press Co., 1984: 567.