Kinetics and mechanism of oxidation of 1,2-butyleneglycol by dihydroxydiperiodatonickelate ( IV) in alkaline medium

Received on Sep. 10, 2005.

Abstract The kinetics of oxidation of 1,2-butyleneglycol (a-BG) by dihydroxydiperiodatonickelate ( IV) were studied spectrophotometrically between 298.2K and 313.2K in alkaline medium. The reaction rate showed first order dependence on dihydroxydiperiodatonickelate ( IV) and 1<n_ap<2 order in a-BG. It was found that the pseudo-first order rate constant k_obs increased with increasing concentration of OH^- and decreasing concentration of IO₄^-. There is a negative salt effect and no free radical was detected. In view of this, the dihydroxydiperiodatonickelate ( IV) species is assumed to be the existent form. A plausible mechanism involving a two-electron transfer is proposed and the rate equations derived from the mechanism can explain all of the experimental results. The activation parameters, as well as the rate constants of the rate-determining step were calculated.
Keywords dihydroxydiperiodatonickelate( IV) , 1,2–butyleneglycol, redox reaction, kinetics and mechanism

In general, transition metal Ni only consistsin stabilizaed Ni(II), however, if there exist some ligands，the higher oxidation state of nickel, such as Ni(III), Ni(IV) can also exist stably, they can form complexes not only with iodic acid^[1], molybdic acid^[2] and telluric acid^[3] and so on, but also with organic substance, for example: [Ni^Ⅳ(L¹)]²⁺ and [Ni^IV(L¹) ₂]²⁺(where H₂L¹= 3,14-dimethyl -4,7,10,13-tetraazahexadeca-3,13 -diene- 2,15-dione dioxime and HL² = 6-amino-3-methyl-3-en-2-one oxime) ^[4].
    The use of Ni( IV) as an oxidizing agent is well known in the investigation of some organic compounds such as tetrahydrofurfuryl alcohol^[5], ethylene diamine ^[6], glycollic acids^[7] etc .Since Ni( IV) is in the higher 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, the kinetics and mechanism of oxidation of 1,2-butyleneglycol by dihydroxydiperiodatonickelate ( IV) is put forward, also, from the k and activation parameters got from the experiment, the microcosmic structure of transition state compound, has been studied for the sake of understanding the microcosmic information of the reaction.

1. EXPERIMENTAL
1.1 Materials
All the reagents used were of A.R. grade. All solutions were prepared with redistilled water. A solution of [Ni(OH)₂(H₂IO₆)₂]^4- (DPN) was prepared and standardized by the method reported earlier^[8]. Its UV spectrum was found to be consistent with that reported in the literature. The concentration of DPN was derived from its absorption at l = 410nm (e=1.06×10⁵ L·mol^-1·cm^-1).The solution of DPN was always freshly prepared before use with redistilled water. The ionic strength m was maintained by adding a solution of KNO₃ and the pH value of the reaction mixture was regulated with a KOH solution. Measurements of the kinetics were performed using a UV-8500 spectrophotometer (Shanghai) fitted with a 501 thermostat (±0.1^oC, Shanghai).
1.2 Kinetics measurements and product analysis   
All kinetics measurements were carried out under pseudo-first order conditions. A solution (2 mL) containing Ni(IV), OH^-, IO₄^- with ionic strength m and a a-BG solution (2mL) of an appropriate concentration were transferred separately to the upper and lower branch tubes of a type two-cell reactor. After thermal equilibration at the desired temperature in a thermobath, the two solutions were mixed well and immediately transferred in a 1cm rectangular cell quartz in a constant temperature cell-holder (±0.1^oC). The reaction process was monitored automatically by recording the disappearance of Ni(IV) with time (t) at 410 nm with a UV-8500 spectrophotometer. All other species did not absorb significantly at this wavelength.
    After completion of the reaction, the oxidation product was identified as an aldehyde alcohols which was precipitated as 2,4-dinitrophenyldrazone derivative. By gravimetric analysis, it is found that one mole of a-BG consumed one mole Ni(IV) .

2. RESULTS AND DISCUSSION
2.1 Evaluation of pseudo-first order rate constants                 
Under the conditions of [a-BG]₀>>[ Ni(IV)]₀, the plots of ln(A_t-A_∞) versus time were straight lines, indicating the reaction is first order with respect to [Ni(IV)], where A_t and A_∞ are the absorbance at time t and at infinite time ,respectively. The pseudo-first-order rate constants k_obs were calculated by the method of least squares (r≥0.999). Generally, 8-10 A_t values within three times the half-lives were used to calculate k_obs. The k_obs values were the average values of at least three independent experiments.
2.2 Rate dependence on [a-BG]
At fixed concentration of Ni(IV), OH^-, IO₄^-, ionic strength m, the values of k _obs were determined at different temperatures and a-BG concentrations. The k_obs values was found to increase with increasing concentration of a-BG at all temperatures with an apparent order of 1<n_ap<2. The plots of [a-BG]/k_obs versus 1/[a-BG] were straight lines with a positive intercept. (Table 1).

Table 1 10³ k_obs varying with different concentrations of a-BG at different temperatures

102[a-BG]    (mol/L)		0.50	0.60	0.80	1.30	2.50	nap	r	a	r1
T(K)	298.2	2.569	3.341	5.395	10.19	25.96	1.43	0.999	0.7548	0.995
	303.2	4.199	5.505	8.227	15.66	37.99	1.34	0.999	0.5642	0.992
	308.2	7.456	9.457	14.02	24.91	52.65	1.21	0.999	0.4230	0.998
	313.2	12.41	15.13	21.54	37.28	76.97	1.17	0.999	0.3002	0.996

[Ni(IV) ]＝8.04×10^－6mol/L; [OH^-]＝2.00×10^-2 mol/L; [IO₄^-]＝2.00×10^-3 mol/L; m＝0.12 mol/L; T=303.2K; a=l/k;  n_ap and r stand for the slope and relative coefficient, respectively, of the plot of lnk_obs versus ln[a-BG], a and r₁ stand for intercept and relative coefficient, respectively, of the plot of [a-BG]/k_obs versus 1/[a-BG].
2.3 Rate dependence on [IO₄^-]
At fixed concentration of Ni(IV), OH^-, a-BG、ionic strength m and temperature, the values of k_obs were determined.The value of k_obs decreased with the increasing concentration of IO₄^- .The plots of 1/k_obs versus [IO₄^-] were straight lines with a positive intercept.(Table 2 and Fig.1)

Fig.1 1/kobs versus [[IO4-],[Ni(IV) ]＝8.04×10－6mol/L; T= 303.2 K; [a-BG]＝2.00×10-2 mol/L; [OH-]＝2.00×10-2 mol/L; m＝0.12 mol/L

Table 2  Rate dependence on [IO₄^-]

103[IO4-] (mol/L)	1.00	2.00	3.00	4.00	5.00
102kobs  (s-1)	4.459	2.964	2.040	1.643	1.250

[Ni(IV) ]＝8.04×10^－6mol/L; T= 303.2 K; [α-BG]＝2.00×10^-2 mol/L; [OH^-]＝2.00×10^-2 mol/L; m＝0.12 mol/L

2.4 Rate dependence on [OH^-] and ionic strength m
At fixed concentration of Ni(IV), IO₄^-, a-BG, ionic strength m and temperature, the value of k_obs increased with the increase of concentration of OH^-. The order with respect to OH^- was found to be fractional. The plots of 1/k_obs versus f(OH)/[OH^-] were straight lines (r≥0.990) (Table3).The value of k_obs decreased with the addition of KNO₃ solution (Table4), which indicates that there was a negative salt effect, which was consistent with the common regulation of the kinetics^[9].

Table 3  Rate dependence on [OH^-]

102[OH-] (mol/L)	2.00	4.00	6.00	8.00	10.00
102kobs (s-1)	1.079	1.544	1.980	2.533	2.945

[Ni(IV) ]＝8.04×10^－6mol/L; T= 303.2 K; [a-BG]＝1.00×10^-2 mol/L; [IO₄^-]＝2.00×10^-3 mol/L; m＝0.12 mol/L

Table 4  Rate dependence on m

m mol/L	0.07	0.12	0.17	0.22	0.27
102kobs (s-1)	4.046	3.046	2.602	2.106	1.736

[Ni(IV) ]＝8.04×10^－6mol/L; T= 303.2 K; [a-BG]＝1.00×10^-2 mol/L; [IO₄^-]＝2.00×10^-3 mol/L; [OH^-]＝2.00×10^-2 mol/L

3 DISCUSSION OF THE REACTION MECHANISM
In an alkaline medium, the electric dissociation equilibrium of periodate was given earlier ^[10] (here pK_w=14)

2IO₄^- + 2OH^- H₂I₂O₁₀^4-          log b₁ = 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 aqueous 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 was H₃IO₆^2- and H₂IO₆^3-. According to the literature, the main existent form of oxidant was [Ni( OH) ₂₍ H₂IO_{6) 2}]^4-over the experimental concentration range of OH^-.
    The addition of acrylonitrile or acrylamide to the reaction mixture under a nitrogen atmosphere neither changed the reaction rate nor initiated any polymerization, showing no free radical were involved in the reaction^[11]. It is thought that under the employed conditions a type one-step, two-electron transfer mechanism is in operation.
    According to the above experimental fact, the following reaction mechanism is proposed
[Ni( OH) ₂(H₂IO₆) ₂]^4- + OH^- [Ni( OH) ₂(HIO₆) ]^2- + H₂IO₆^3-+H₂O                    (4)
[Ni( OH) ₂(HIO₆) ]^2-+[a-BG] [Ni( OH) ₂(HIO₆) ]^2-·[a-BG]                    (5)
[Ni( OH) ₂(HIO₆) ]^2-·[a-BG]+[ a-BG] [Ni( OH) ₂( HIO₆) ]^2-[a-BG]^2-[a-BG]        (6)
                                                                                      complex
complexcomplex Ni(II)+OCHCHOHCH₂CH₃+a-BG +H₂IO₆^3-+OH^-+H₂O         (7)
   Reaction (4) and (5) are in dissociation and coordination equilibria, the reaction rates of which are generally fast, reaction (6) belongs to an electron-transfer reaction, the reaction rate of which are generally slow, hence reaction (6) is the rate-determining step.
-d[Ni(IV)]_t/dt = k[Adduct] [a-BG]
    where [Ni(IV)]_t stands for any form of Ni(IV)complex which exists in the equilibrium.
-d[Ni(IV)]_t/dt = [ Ni(IV)]_t                               (8)

= k_obs[Ni(IV)]_t

                               (9)
                             (10)
                                          (11)
    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-]                                                          (12)
   Equations ( 13) and ( 14) can be obtained from ( 2) , ( 3) , ( 12) :
             (13)
             (14)

                (15)

                        (16)

    From equation (11), plots of [a-BG]/k_obs vs.1/[a-BG] should be straight lines which they were. Hence, the rate constants of rate-determining step at different temperatures were obtained from the intercept of the straight lines. Equation (15) suggests that a plot of 1/k_obs versus [IO₄^-] is straight line, also suggests that a plot of 1/k_obs versus f(OH)/[OH^-] is straight line which indicate the existent form of active intermediate is [Ni( OH) ₂( H₂IO₆) ₂]^4-. If Equation (16) be equal to this system which suggest that the existent form of active intermediate is [Ni( OH) ₂( H₃IO₆) ₂]^2-, it is not consistent with the experiment result.
    The activation energy and the thermodynamic parameters were evaluated by the method given previously.^[12](Table 5 and Fig. 2)

Fig.2 ln k versus 1/T [Ni(IV) ]＝8.04×10-6 mol/L; [OH-]＝2.00×10-2 mol/L; m＝0.12 mol/L; [IO4-]＝2.00×10-3 mol/L

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

k (L mol-1 s-1)	T (K)				Activation parameters
	298.2	303.2	308.2	313.2	Ea (KJ·mol-1)	DH# (KJ·mol-1)	DS#（J·mol-1K-1 ）
a-BG	1.325	1.772	2.364	3.331	47.60	45.12	-91.42
DEA*	11.90	15.49	21.16	29.74	46.28	43.81	-113.22

The plot of ln k vs -1/T has the following intercept (a),slope(b),and relative coefficient(r): a-BG a = 19.46 , b = -5.72×10³, r = 0.998 ; DEA a = 21.46 , b = -5.57×10³ , r = 0.998
^* based on our previous work

4. CONCLUTION                  
Contrast to the previous experiment, it can be concluded that the observed rate constants of the rate -determining step for diethanolamine are larger than those for 1,2-butyleneglycol. Because the -NH₂ embody the stronger alkalescence than -OH, according to coordination chemistry^[13] ,we know the stability of the coordination compound increased with the increase of the alkalescence. In addition, the 1,2-butyleneglycol embody a structure of ethyl, which has the special steric hindrance. So the formation of intermediate adduct between Ni(IV) complex and diethanolamine is more stable than that of 1,2-butyleneglycol, which leads up to the observed rate of the rate -determining step of diethanolamine faster than 1,2-butyleneglycol , this is consistent with the result from the experiment.

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碱性介质中二过(碘酸根)合镍(IV)酸根氧化b-丙二醇的反应动力学及机理
单金缓 刘红妹 霍树营 申世刚 孙汉文
( 河北省分析科学与技术重点实验室, 河北大学化学与环境科学学院，保定 071002)
摘要  用分光光度法在298.2-313.2K区间研究了碱性介质中二羟基二（高碘酸根）合镍(IV)酸根（DPN）氧化1,2-丁二醇（a-BG）的反应动力学及机理。结果表明：反应对DPN为准一级，对a-BG 为1<n_ap<2级；在保持准一级条件（[a-BG]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；存在负盐效应且没有自由基被检测出来。由此,作者认为[Ni( OH) ₂₍ H₂IO_{6) 2}]^4-是存在形式，并且提出了一种双电子传递机理,由假设反应机理推出的速率方程能很好地解释全部实验现象,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（高碘酸根）合镍（IV）酸根 1，2-丁二醇 氧化还原反应 动力学及机理