Wang Liping, Ge Xusheng, Wang Yaru, Xu Mingyuan
(Department of Chemistry, Baoding University, Baoding 071000, China)
AbstractThe kinetics of oxidation of 1,3-butylene glycol by diperiodatocuprater(III)(DPC)in alkaline medium have been studied by spectrophotometry in the range of 298.2-313.2K. It is shown that the reaction was first order with respect to each reductant and Cu (III) , and reaction rates decrease with increase in [IO] and increase in [OH]. A plausible mechanism of reaction involving a pre-equilibrium between DPC and [Cu(OH) was proposed, which could be applied to explain all experimental phenomena, and the activation parameters of the rate-determining step have been also calculated.
Transition metals in a higher oxidation state can generally be stabilized by chelation with suitable polydentate ligands, while metal chelates such as diperiodatocuprate(III), ditelluratocuprater(III), diperiodatonickelate(IV) are good oxidants in a medium with an appropriate H value. The use of Cu(III) as an oxidation agent in analytical chemistry has been reported, but most of these are o-compounds, Therefore, it was thought worthwhile to study the kinetics of oxidation of some m-compounds such as 1,3-butylene glycol, and investigation will certainly provide us with more dynamical parameters theoretical foundation for the design of reaction route in the organic synthesis and quantitative analysis in analytical chemistry. In this paper, the mechanism of oxidation of 1,3-butylene glycol by diperiodatocuprater(III) is reported.
All reagents used were of AR grade. All solutions were prepared with doubly distilled water. Solutions of DPC and reductant were always freshly prepared before use. The stock solution of DPC in a strong alkaline medium was prepared by the method given by Jaiswal. The ionic strength was maintained by adding KNO solution and the pH value was regulated with KOH solution.
1.2 Kinetic measurement and reaction product analysis
Measurements of the kinetics were performed using a UV-1901spectrophotometer (Beijing) fitted with a CS-501 thermostat (±0.1ºC, Chongqing).The kinetics measurements were described previously. The product of oxidation was the corresponding aldehyde alcohols by their characteristic spot test
2. RESULTS AND DISCUSSION
2.1 Evaluation of pseudo-first order rate constants
The calculated measurements of the rate constant (kobs) values were described previously
2.2 Effect of varying [1,3-butylene glycol]
At fixed [Cu(III)], [OH], [IO], ionic strength and temperature, obs values increased with increase in [1,3-butylene glycol] and the plots of obs versus [1,3-butylene glycol] were line at passing through the origin (Fig.1) indicating that the reaction is first order in reductant.
Fig.1 Plots of obs vs [1,3-butylene glycol] at different temperatures
[Cu(III)]=8.00×10-5mol/L, [IO]=1.60×10-3mol/L, [OH]=0.10mol/L,=0.10mol/L
2.3 Effect of varying [OH
At fixed [Cu(III)],[1,3-butylene glycol], [IO], ionic strength and temperature, obs values increased with increase in [OH], the plots of lnobs versus ln[OH] were linear(r≥0.996) and from the slopes of such plots the observed order, nap, were found to be 1<nap<2. In addition, the plots of 1/obs versus f ([OH])/[OH were linear (Table1 ).
Table 1 10obs /s-1 varying with the different [OH] at different temperatures
[Cu(III)]=8.00×10-5mol/L, [IO]=1.60×10-3mol/L, [1,3-butylene glycol]=0.20 mol/L,=0.25 mol/L
b and r respectively stand for the slope and the relative coefficient of the plot of lnobs vs lnC
2.4 Effect of varying [IO] and ionic strength
At fixed [Cu(III)], [OH], [1,3-butylene glycol], ionic strength and temperature, obs decreased with increase in [IO]. The observed orders, nap, were found to be -1.36. The plots of 1/obs versus [IO was linear (Table 2). Table 2 reveals that ionic strength has negligible effect on the rate.
Table 2 Influence of variation of [IO], and ionic strength, =303.2K
2.5 Free radicai detection
Acrylamide was added under nitrogen blanket during the course of reaction. The appearance of white polyacrylamide was consistent with free radical intermediates in the oxidation by Cu(III) complexes. Blank experiments gave no polymeric suspensions.
In aqueous periodate solution equilibria (1)~(3) were detected and the corresponding equilibrium constants at 298.2K were determined by Aveston
2 IO + 2 OH H104- log=15.05 (1)
IO+ OH+HO HIO2- log=6.21 (2)
IO + 2 OH HIO3- logβ=8.67 (3)
The distribution of all species of periodate in aqueous alkaline solution can be calculated from equilibria (1)-(3). Under the conditions of [OH]=0.01~0.1mol·L-1, [HIO3-]: [HIO2-]: [H104-]: [IO≈(2.9~29): 1.0: (0.02~0.2): (6×10-5~6×10-2) , the dimer and IO species of periodate can be neglected, Neglecting the concentration of ligand dissociated from Cu (III) and the species of periodate other than HIO3- and HIO2-, Eqs. (4) and (5) can be obtained from(2) and(3):
Here [IOex represents the original overall entering periodate and equals approximately to the sum of [ HIO3- ] and [HIO2-].
Because the hexa-cyclic compound formed by m-compound is larger, will be more spatial hindrance compared to penta-cyclic compound formed by o-compound, and [Cu(OH), being small gives less sterical hindrance than [Cu(OH)(HIO)]2-. The plot of 1/obs versus f ([OH])/[OH is linear and the plot of 1/obs versus [IO is also linear with a positive intercept indicating that OH takes part in a pre-equilibrium with Cu(III) before the rate-determining step and a dissociation equilibrium in which the [Cu(III)] loses two periodate ligand HIO3- forming [Cu(OH) as reactive species.
In view of the above discussion, a plausible reaction mechanism was proposed:
[Cu(OH)(HIO5- + 2OH[Cu(OH) + 2HIO3- (6)
[Cu(OH)+ HCCHOHCHCHOHCu(II) + HCCHOHCH·CHOH (7)
Cu*(III) +OH + HCCHOHCH·CHOH Cu(II) + HCCHOHCHCHO (8)
Where Cu*(III) stand for any kind of form which Cu(III) existed in equilibrium. Subscripts T and e stand for total concentration and at equilibrium respectively. [Cu(III)] =[Cu(OH)(HIO5- +[Cu(OH)e. Reaction (8) is the rate-determining step.
As the rate of the disappearance of Cu (III) was monitored, the rate of the reaction can be derived as:
Substituting Eq (4) into (10), we get the following equation (11)
Rearranging Eq (11) leads to Eqs (12) and (13):
Equation (12) suggests that the plot of 1/obs versus [IOex should be linear, and Equation (13) shows that the plots of 1/obs versus ([OH])/[OH should also be linear at different temperatures. From their slopes and intercepts the rate-determining step constants () and equilibrium constants () were evaluated. The activation parameters data of 1,3-butylene glycol obtained by the method given earlier (Table 3).
Table 3Rate constants () and activation parameters of the rate-determining step, equilibrium constants (
|10 /L/mol· s||T /K||Activation parameters (298.2K)|
|1,3-propylene glycol see||1.161||0.976||0.934||0.921|
The plots of ln vs 1/T have following intercept (24.02) slope (-8379.95) and relative coefficient (0.999)
If the formula of DPC was [Cu(OH)(HIO3-, Equation (14) would be obtained instead of Equation (13).
Fig2 Plot of 1/obs vs([OH])/[OH at 298.2K
[Cu(III)]=8.00×10-5mol/L, [IO]=1.60×10-3mol/L,1,3-butylene glycol]=0.20mol/L,=0.25mol/L
The plot of 1/obs versus ([OH])/[OH is shown in Fig2 which rules out Equation (14). Based on the experimental observations, the formula of Cu(III) periodate complex may be represented by [Cu(OH)(HIO5-, which is support from kinetic studies.
Comparition 1,3-butylene glycol with 1,3-propyleneglycol (Table 3), we can say that smaller the carbon chain is, the larger the observed rate constants and the rate-determining step constants are, which is consistent with dihydric alcohol’s spatial hindrance.