Kinetics and mechanism of oxidation of 2-amino-1-butanol by dihydroxydiperiodatonickelate ( IV) in alkaline liquid

Shan Jinhuan, Shen Haixia, Song Changying, Wang Heye, Wang Xiaoqian
(College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)

Abstract The kinetics of oxidation of 2-amino-1-butanol by dihydroxydiperiodatonickelate(IV) (DPN) in alkaline liquid was studied with spectrophotometry in the temperature range of 293.2 - 313.2 K. The reaction was found to be first order with respect to DPN and fractional order in 2-amino-1-butanol. The rate constant k_obs, increased with an increase in the concentration of OH^- , a decrease in increase in the concentration of IO₄^-. There is a positive salt effect and no free radical was detected. From the evaluation linear data, a reaction mechanism is proposed which involves a pre-equilibrium of an adduct formation between 2-amino-1-butanol and dihydroxydiperiodatonickelate(IV) (MPN), and the rate equations derived from the mechanism can explain all of the experimental results. the rate constants of the rate-determining step and the activation parameters were calculated.
Keywords Dihydroxydiperiodatonickelate( IV) , 2-amino-1-butanol, Kinetics and mechanism, Oxidation

The diperiodatonickelate(IV) as an oxidant in alkaline liquids is new and restricted to a few cases^[1-4] due to the fact of its limited solubility and stability in aqueous liquids. Reduction of Ni(IV) complexes has received considerable attention in order to understand the nature of intermediate oxidation states of nickel, such as Ni(III). Indeed, stable Ni(III) complexes are known^[5]. Moreover, when the nickel(IV) periodate complex is the oxidant, it needs to be known which of the species is the active oxidant, since multiple equilibria between the different nickel(IV) species are involved.
    Ni(IV) complexes have been employed as oxidizing agents for the investigation of some organic compounds such as tetrahydrofurfuryl alcohol^[6], ethylenediamine^[7], glycolic acids^[8], 2-aminoethanol^[9] etc. However, these reaction systems are so complicated that the Ni(IV) complexes-based oxidation mechanism has not been fully uncovered. So it is of great significance to have a further study on this kind of reaction system. In this paper, the kinetics and mechanism of oxidation of 2-amino-1-butanol by dihydroxydiperiodatonickelate ( IV) was studied in detail.

1. EXPERIMENTAL SECTION
1.1. Materials
All the reagents used were of A.R. grade. All solutions were prepared with doubly distilled water. The solution of [Ni(OH)₂(H₂IO₆)₂]^4-(DPN) was prepared^[10] and standardized^[11] by the method reported earlier. Its UV spectrum was found to be consistent with that reported in the literature. The concentration of DPN was derived from its absorption at 410 nm (e = 1.06×10⁵ L·mol^-1·cm^-1). The solution of DPN was always freshly prepared before use. The ionic strength m was maintained by adding KNO₃ solution and the pH of the reaction mixture was adjusted with a KOH solution. Measurements of the kinetics were performed using a TU-1900 spectrophotometer (Beijing) fitted with a 501 thermostat (±0.1°C, Shanghai).
1.2 Apparatus and Kinetics measurements
All kinetics measurements were carried out under pseudo-first order conditions. 2ml of the DPN solution containing a definite concentration of Ni(IV), OH^-, IO₄^- was transferred to upper branch of the l-type tube and 2ml of 2-amino-1-butanol solution with an appropriate concentration was transferred separately to the lower branch of this tube. After thermal equilibration at the desired temperature in a thermostat, the two solutions were mixed well and immediately transferred into a 1cm thick rectangular quartz cell in a constant temperature cell-holder (±0.1°C). The reaction process was monitored automatically by recording with a TU-1900 spectrophotometer. All other species did not absorb significantly at this wavelength. Details of the determinations are described elsewhere^[12]. The oxidation product was identified as the corresponding ketone alcohol by spot test^[13].

2. RESULTS AND DISCUSSION
2.1. Evaluation of Pseudo-First Order Rate Constants
Under the conditions of [2-amino-1-butanol]₀>>[ Ni(IV)]₀, the plots of ln(A_t-A_∞) versus time were straight lines, details of the evaluation are described in our previous work^[9].
2.2. The dependence of rate on the concentration of 2-amino-1-butanol
At constant temperature, k_obs values increase by increasing the concentration of 2-amino-1-butanol while keeping the concentration of Ni(IV), OH^-, IO₄^-, and m constant. The order with respect to 2-amino-1-butanol was fractional. The plots of 1/k_obs versus 1/[2-amino-1-butanol] were straight lines with a positive intercept (r ≥ 0.994) (Figure 1).

Figure 1 Plots of 1/k_obs vs. 1/[2-amino-1-butanol] at different temperatures.
[Ni(IV)]= 7.87×10^-6 mol/L; [OH^-]= 1.0×10^-2 mol/L; [IO₄^-]= 2.0×10^-3 mol/L; m= 0.11 mol/L

2.3. The dependence of rate on the concentration of OH^-
At constant temperature, k_obs values increase by raising the concentration of OH^- while keeping the concentration of Ni(IV), 2-amino-1-butanol, IO₄ ^- and m constant. The order with respect to OH^- was fractional. The plot of 1/k_obs versus f([OH^-])/[OH^-] was found to be straight linear (r = 0.999) (Figure 2).

Figure 2 The plot of 1/k_obs vs. f([OH^-])/[OH^-] at 298.2 K
[Ni(IV)]= 7.87×10^-6 mol/L; [2-amino-1-butanol]= 6.0×10^-4 mol/L; [IO₄^-]= 2.0×10^-3 mol/L; m=0.11 mol/L

2.4. The dependence of rate on the concentration of IO₄^-
At constant concentration of Ni(IV), 2-amino-1-butanol, OH^-, m and temperature, the experimental results indicate that k_obs decreases while increasing the concentration of IO₄^-. The order with respect to IO₄^- was negative fractional and the plot of 1/k_obs versus [IO₄^-] was linear (r = 0.999) (Figure 3).

Figure 3 Plots of 1/k_obs vs. [IO₄^-] at 298.2 K
[Ni(IV)]= 7.87×10^-6 mol/L; [2-amino-1-butanol]= 6.0×10^-4 mol/L; [OH^-]= 1.0×10^-2 mol/L; m= 0.11 mol/L

2.5. The dependence of rate on the ionic strength
With other conditions fixed, the reaction rate increased with increase in ionic strength when 2-amino-1-butanol was oxidized by Ni(IV), indicating that there was a positive salt effect to 2-amino-1-butanol (Table 1).

Table1 The dependence of rate on the concentration of m at 298.2 K
[Ni(IV)]= 7.87×10^-6 mol/L; [2-amino-1-butanol]= 6.0×10^-4 mol/L; [OH^-]= 1.0×10^-2 mol/L; [IO₄^-]= 2.0×10^-3 mol/L

3. DISCUSSION OF THE REACTION MECHANISM
In alkaline solution, equilibria (1-3) were observed and the corresponding equilibrium constants at 298.2 K were determined by Aveston^[14].
2IO₄^-+2OH^- H₂I₂O₁₀^4-                            log b 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 species of periodate in alkaline solution can be calculated from the equilibriums (1)-(3). The dimer H₂I₂O₁₀^4- and IO₄^- species can be neglected, the main iodic acid species is H₃IO₆^2- and H₂IO₆^3-, According to the literature, the main existent form of oxidant was [Ni(OH)₂(H₂IO₆) ₂]^4- over the experimental concentration range of OH^-.
The addition of acrylonitrile or acrylamide to the reaction mixture under the protection of nitrogen did not alter the rate and there was no polymerisation, showing the absence of free radicals in the reaction^[15]. It is thought that under the employed conditions a type one-step, two-electron transfer mechanism is in operation.
Here, [IO₄^-]_t represents the concentration of original over all periodate ions which is approximately equal to the sum of [H₂IO₆^3-] and [H₃IO₆^2-]. In the [OH^-] range used in this work, the main specie of periodate is H₂IO₆^3-. Based on the discussion, the formula of the Ni(IV) periodate complex may be

represented by either or the less protonated ionic species . We preferred to use the latter to represent DPN because it is close to that suggested by Mukherjee^[16].
    The fractional order dependence of k_obs on [OH^-] suggests that OH^- takes part in pre-equilibrium with [Ni(IV)] before the rate-determining step. The plot of 1/k_obs vs. [IO₄^-] is linear with a positive intercept indicating a dissociative equilibrium in which the Ni(IV) loses a periodate ligand H₂IO₆^3- from its coordination sphere, forming a reactive monoperiodatonickelate(IV) complex (MPN). The plots of 1/k_obs vs. 1/[2-amino-1-butanol] are linear, indicating a pre-equilibrium forming a 1:1 complex between MPN and 2-amino-1-butanol.
In view of the above results and discussion, a plausible reaction mechanism is proposed:
+ OH^-+ H₂IO₆^3- + H₂O               ( 4)
              I                                                         II

+R complex                                                (5)

complex[Ni(OH)₂(HIO₆)]^4-+product+NH₃                                   ( 6)
    Here, reaction ( 6) was the rate-determining step. The oxidation product of 2-amino-1-butanol is CH₃CH₂(CO)CH₂OH.



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

           ( 7)

                                                      (8)
Neglecting the concentration of ligand dissociated from Ni( IV) , the main species of periodate are H₂IO₆^3- and H₃IO₆^2-, here:
[IO₄^-]_t ≌ [H₃IO₆^2-]+[H₂IO₆^3-]                                                                      ( 9)
Equations ( 10) and ( 11) can be obtained from ( 2) , ( 3) and ( 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/[R] are straight linear, and eq (14) shows that the plot of 1/k_obs vs. [IO₄^-] is straight linear, and eq(13) shows that the plot of 1/k_obs vs. f([OH^-])/[OH^-] is straight linear which indicate the existent form of active intermediate is [Ni(OH)₂(H₂IO₆) ₂]^4-. The activation energy and the thermodynamic parameters were evaluated by the method given previously (Table 5)^[17].

Table 5 Rate constants (k) and the activation parameters for the rate-determining step at 298.2K

The plot of ln k vs. 1/T has the following intercept (a), slope (b), and relative coefficient (r):
r= -0.994, a= 29.98, b= -1.03×10⁴.
* based on our previous work^[9]

4. CONCLUTION
Among various species of Ni(IV) in alkaline liquids, monoperiodatonickelate is considered as the active species for the title reaction. The rate constant of slow step and other equilibrium constants involved in the mechanism are evaluated and activation parameters with respect to the slow step of the reaction were computed. The overall mechanistic sequence described here is consistent with product studies, mechanistic studies and kinetic studies.
Contrast to the previous work^[9], it can be concluded that the rate-determining step constants of 2-aminoethanol are larger than those for 2-amino-1-butanol. Because the 2-amino-1-butanol embody a structure of ethyl, which has the special steric hindrance. So the formation of intermediate adduct between Ni(IV) complex and 2-aminoethanol is more stable than that of 2-amino-1-butanol, which leads to the observed rate of the rate-determining step of 2-aminoethanol faster than 2-amino-1-butanol , this is consistent with the experimental observation.

REFERENCES
[1] Macartney D H, McAuley A, Inorg. Chem. 1983, 22: 2062.
[2] Acharya S, Neogi G, Panda R K, Ramaswamy D, Int. J. Chem. Kinet. 1982, 14: 1253.
[3] Lappin A G, Laranjeira M C, Youdi O L, J. Chem. Soc. Dalton Trans. 1981, 721.
[4] Siddiqui M A, Kumar C S and Chandraiah U, et al. Indian J.Chem. 1991, 30(A): 849.
[5] Haines RI, MacAulley A, Coord Chem. Rev. 1981, 39: 77.
[6] Li Z T, Wang F L and Wang A Z, Int. J. Chem. Kinetics. 1992, 24: 933.
[7] Li Z T, Chang Q and Li B W,et al. Chem. J. Chinese Universities. 2000, 21: 747.
[8] Chandraiah U, Murthy C P and Kandlikar S, Indian J. Chem. 1989, 28(A): 162.
[9] Qian J, Gao M Z and Shan J H, J. Guangxi Normal Univ. 2003,21(1): 248.
[10] Baker L C W, Mukerjee H G and Sarkar S B C, et al. Indian J. Chem. 1982, 21(A): 618.
[11] Murthy C P, Sethuram B and Rao T, Z Phys Chem. (Leizig). 1986, 287: 1212.
[12] Shan J H, Wang L P, Shen S G and Sun H W, Turk J. Chem. 2003, 27: 265.
[13] Feigl F, Spot Tests in Organic Analysis, Elsevier Publishing Co, New York, 1956.
[14] Aveston J J, Chem. Soc. 1969, (A): 273.
[15] Chandraiah U,Jaffar A K, Murthy C P, et al. Indian J. Chem. 1987, 26(A): 481.
[16] Mukherjee H G, Mandal B and De S, Indian J. Chem. 1984, 23(A): 426.
[17] Shan J H and Liu T Y, ACTA Chem Sin. 1994, 52: 1140.

分光光度法研究二羟基（过碘酸根）合镍（IV）氧化2-氨基-1-丁醇的反应动力学及机理
单金缓 申海霞 宋常英 王合叶 王晓倩
（河北大学与环境科学学院，保定 071002）
摘要 在碱性介质中，用分光光度法在293.2 K - 313.2 K区间研究了二羟基二（过碘酸根）合镍(IV)（DPN）氧化2-氨基-1-丁醇的反应动力学及机理。结果表明，反应对DPN为准一级，对2-氨基-1-丁醇为正分数级；在保持准一级条件（[2-氨基-1-丁醇]₀ >> [DPN]₀）下，表观速率常数随着[OH^-]的增加而增大，随着[IO₄^-]的增加而减小；存在正盐效应且没有自由基被检测出来。据此提出了包括2-氨基-1-丁醇和二羟基二（过碘酸根）合镍（IV）（MPN）形成加合物的前期平衡的反应机理，用假设反应机理推出的速率方程可解释全部实验现象，进一步求得速控步的速率常数和活化参数。
关键词 二羟基二（过碘酸根）合镍（IV）；2-氨基-1-丁醇；氧化还原反应；动力学及机理