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 kobs, 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.
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. 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, ethylenediamine, glycolic acids, 2-aminoethanol 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.
- EXPERIMENTAL SECTION
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 and standardized 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¡Á105 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 (¡À0.1¡ãC, 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 (¡À0.1¡ãC). 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. The oxidation product was identified as the corresponding ketone alcohol by spot test.
- 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(At-A¡Þ) versus time were straight lines, details of the evaluation are described in our previous work.
2.2. The dependence of rate on the concentration of 2-amino-1-butanol
At constant temperature, kobs 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/kobs versus 1/[2-amino-1-butanol] were straight lines with a positive intercept (r ¡Ý 0.994) (Figure 1).
Figure 1 Plots of 1/kobs vs. 1/[2-amino-1-butanol] at different temperatures.
[Ni(IV)]= 7.87¡Á10-6 mol/L; [OH–]= 1.0¡Á10-2 mol/L; [IO4–]= 2.0¡Á10-3 mol/L; m= 0.11 mol/L
2.3. The dependence of rate on the concentration of OH–
At constant temperature, kobs 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/kobs versus f([OH–])/[OH–] was found to be straight linear (r = 0.999) (Figure 2).
Figure 2 The plot of 1/kobs vs. f([OH–])/[OH–] at 298.2 K
[Ni(IV)]= 7.87¡Á10-6 mol/L; [2-amino-1-butanol]= 6.0¡Á10-4 mol/L; [IO4–]= 2.0¡Á10-3 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 kobs decreases while increasing the concentration of IO4–. The order with respect to IO4– was negative fractional and the plot of 1/kobs versus [IO4–] was linear (r = 0.999) (Figure 3).
Figure 3 Plots of 1/kobs vs. [IO4–] at 298.2 K
[Ni(IV)]= 7.87¡Á10-6 mol/L; [2-amino-1-butanol]= 6.0¡Á10-4 mol/L; [OH–]= 1.0¡Á10-2 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¡Á10-6 mol/L; [2-amino-1-butanol]= 6.0¡Á10-4 mol/L; [OH–]= 1.0¡Á10-2 mol/L; [IO4–]= 2.0¡Á10-3 mol/L
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.
2IO4–+2OH– H2I2O104- log b 1 = 15.05 ( 1)
IO4– + OH– +H2O H3IO62- log b2 = 6.21 ( 2)
IO4– + 2OH– H2IO63- 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 H2I2O104- and IO4– species can be neglected, the main iodic acid species is H3IO62- and H2IO63-, 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. 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 [H2IO63-] and [H3IO62-]. In the [OH–] range used in this work, the main specie of periodate is H2IO63-. 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.
The fractional order dependence of kobs on [OH–] suggests that OH– takes part in pre-equilibrium with [Ni(IV)] before the rate-determining step. The plot of 1/kobs vs. [IO4–] is linear with a positive intercept indicating a dissociative equilibrium in which the Ni(IV) loses a periodate ligand H2IO63- from its coordination sphere, forming a reactive monoperiodatonickelate(IV) complex (MPN). The plots of 1/kobs 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–+ H2IO63- + H2O
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:
Neglecting the concentration of ligand dissociated from Ni( IV) , the main species of periodate are H2IO63- and H3IO62-, here:
[IO4–]t ¡Õ [H3IO62-]+[H2IO63-]
Equations ( 10) and ( 11) can be obtained from ( 2) , ( 3) and ( 9) :
Substituting eq (10) into ( 8) , we can get the following equations:
Eq(12) suggests that the plot of 1/kobs vs. 1/[R] are straight linear, and eq (14) shows that the plot of 1/kobs vs. [IO4–] is straight linear, and eq(13) shows that the plot of 1/kobs 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).
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¡Á104.
* based on our previous work
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, 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.