Kinetics and mechanism of oxidation of sugars by alkaline potassium tetraoxomanganate (VII).
The kinetics of oxidation of sugars have been a subject of extensive research in recent years. This is attributed to the economic and biological importance of carbohydrates to the living organisms. The oxidations have been carried out in both acidic and alkaline media using such oxidants as transition metal ions, inorganic acids, organometallic complexes and enzymes [1-8]. The results of these experiments have revealed that in some cases the mechanisms were proposed based on the formation of intermediate complexes while in others the results were analyzed on the basis of formation of free radicals. Despite much work already done on the kinetics of oxidation of sugars, very little attention was given to the use of potassium tetraoxomanganate (VII) . The present study is therefore undertaken to clarify the mechanism of oxidation of D-glucose, galactose, fructose, maltose and sucrose by alkaline potassium tetraoxomanganate (VII) as a follow up of the previous studies in our laboratory on the oxidation of sugars .
The reagents were of analytical grade and were used without further purification. Stock solutions of both the oxidants and the substrates were freshly prepared using doubly distilled water.
The absorption spectra of solutions of different concentrations of the potassium tetraoxomanganate (VII) were measured in the visible regions between 300 and 800nm and kinetic data were collected at 520nm, the wavelength corresponding to maximum absorbance.
The kinetic studies were carried out under pseudo-first order conditions with the concentration of the sugars in large excess compared to that of the oxidant. All reactant solutions were placed in a thermostated water bath for at least one hour to attain a temperature of 30[degrees]C. Appropriate quantities of the reagent solutions were mixed in a 250[cm.sup.3] conical flask already placed in the thermostated bath. The reaction was initiated by introducing the oxidant solution into the mixture. The reaction rate was followed by measuring the decrease in absorbance at 520nm; the reaction was completed when KMn[O.sub.4]- sugar solution turned from purple to brown . The rate constants calculated were averages of at least two measurements.
Aqueous 20% acrylamide was added to various reaction mixtures; formation of gel was noticed in each case and on further addition of methanol polyacrylamide was precipitated. Blank experiments in which either the oxidant or substrate was excluded were also carried out and no gel formation was noticed. These indicate that free radicals were formed during the reactions [9, 10].
Stoichiometry of e Reactions
The procedure was adapted from that used by Sircar and Saika . An excess of the permanganate solution was added to the sugar solution and allowed to react for several hours. The excess of the permanganate was then determined. Blanks were prepared without addition of sugar solution for each set of the experiment. The volume of the primary standard ([Fe.sup.2+]) consumed by blanks was always found to be greater than the corresponding reacting species and consequently the consumption ratio, i.e. the number of moles of potassium tetraoxomanganate(VII) consumed per mole of the sugars were estimated by assuming that all the sugars were totally consumed under the reaction conditions.
Results and Discussion
Effect of reactants concentration
The pseudo-first order rate constants, [k.sub.obs], were determined at different initial concentrations of the sugars while maintaining constant the concentration of KMn[O.sub.4], ionic strength, pH and temperature at 30[degrees]C. The results as presented in table 1 show that the rate increases as the concentration of the sugars increase. This increase in rates is almost in direct proportion such that when divided by the corresponding reducing sugar concentration, fairly constant values were obtained. This suggests that the reactions are first-order with respect to the reducing sugars . The plots of [k.sub.obs] against the sugar concentrations were linear and passed through the origin (fig1), this confirms that the reactions are first-order with respect to the sugars . The average values of the second-order rate constants, [k.sub.2], at 30[degrees]C are 0.26, 0.25, 0.21, 0.044 and 0.01 [M.sup.-1][s.sup.-1] respectively for glucose, galactose, maltose, fructose and sucrose. Therefore the order of reactivities of these sugars is glucose ~ galactose > maltose > fructose > sucrose. This result agree in part with the one obtained by Upadhyay and Kambo. . In another set of experiments, the reactions were studied at various initial concentrations of the KMn[O.sub.4] but at constant sugar concentration, ionic strength, pH and temperature. The pseudo-first order rate constants recorded in table 2 indicates that the reactions are independent of the initial concentrations of KMn[O.sub.4]. This suggests a first-order dependence on KMn[O.sub.4].
[FIGURE 1 OMITTED]
Effect of ionic strength
The effect of ionic strength on the rate of the reaction was investigated by varying the concentration of KN[O.sub.3] in the range 0.05 to 0.25M. The initial rates of the reaction increased with increase in KN[O.sub.3] concentration. This suggests that the reactions occur between ions of similar charges .
Effect of pH
The effects of pH on the rate of oxidation of the sugars were studied at 30[degrees]C and 40[degrees]C in the pH range 10.1 to 11.6. The results show that at both temperatures, the rate of oxidation increased with increase in the pH of the reaction medium. The plots of log [k.sub.obs] against pH were linear (fig 2&3) for all the reactions. These results indicate that these reactions are base-catalysed .
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Effect of temperature
The oxidation of the sugars was carried out at different temperatures from 30[degrees]C to 80[degrees]C. The pseudo-first-order rate constants increased with increase in temperature, the second-order rate constants, [k.sub.2] were calculated from the relation [k.sub.2] = [k.sub.obs]/ [SUGAR]. The plots of log [k.sup.2] against 1/T were linear (fig 4) and the Arrhenius activation energy, [E.sub.a], and[DELTA] other thermodynamic activation parameters have been evaluated as described elsewhere . The results are presented in table 3. The negative values of entropies of activation ([DELTA][S.sup.#]) are indication that the reactions occur between ions of similar charges .
[FIGURE 4 OMITTED]
Mechanism of the reaction
It has been reported in the literature that in alkaline medium sugar will form enediol with the hydroxyl group and also that the rate of enolization is the same as the rate of oxidation  However, in the present study, the double reciprocal plots of the pseudo-first-order rate constants against the sugar concentrations were linear (fig. 5), this provides ample evidence for the formation of a 1:1 intermediate complex during the reactions . Therefore the rate of disproportionation of the activated complex is considered to be the slowest and hence the rate determining step. If we combine the kinetic data with the positive polymerization test, then the oxidation of the sugars takes place by the reaction between the [Mn.sup.VII] and the sugars resulting in election transfer between them to give enediol intermediate complex which then disproportionates in a slow step to give a free radical. The free radical then reacts with another molecule of [Mn.sup.VI] in a fast step to give the products.
[FIGURE 5 OMITTED]
The steps of the reaction are as shown below:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)
Where R is the free radical which reacts further with another molecule of Mn (VII) to give products.
The rate equation: The rate law derived from the above mechanism is:
d [[Mn.sup.VII]]/dt = [k.sub.s] [K.sub.1] [K.sub.s][S][[OH.sup.-]][[Mn.sup.VII]]T/1 + [K.sub.1] [K.sub.2] [S] [[OH.sup.-]] (4)
[k.sub.obs] = [k.sub.s][K.sub.1][K.sub.2][S][[OH.sup.-]]/1 + [K.sub.1][K.sub.2][S][[OH.sup.-]] (5)
Equation (5) thus confirmed the first-order dependence of reaction rates on the sugars at very low concentration of the sugars which is what we observed experimentally.
Rearrangement of equation (5) gives
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)
From the intercepts of the linear plots of 1/[k.sub.obs] against 1/[S] (figure 5) the rate constants for the disproportionation of the complex, [k.sub.s], were obtained and recorded in table 3 for each sugar.
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Odebunmi E.O. and Owalude S.O. *
Chemistry Department, University of Ilorin, P.M.B. 1515, Ilorin-Nigeria
* Corresponding Author E-mail: email@example.com, firstname.lastname@example.org
Table 1: Variation of rate constants with sugar concentrations at 30[degrees]C, pH=11, [KMn[O.sub.4]]=0.0004M,[KN[O.sub.3]]=0.2M Glucose [Sugar] [K.sub.obs] x [10.sup.3] [K.sub.2] [M.sup.-1] x [10.sup.2] M [s.sup.-1] [s.sup.-1] 1.0 2.02 0.20 1.2 3.08 0.26 1.4 3.25 0.23 1.6 4.26 0.27 1.6 4.26 0.27 1.8 -- -- 2.0 5.75 0.29 2.2 6.86 0.31 Galactose [Sugar] [K.sub.obs] x [10.sup.3] [K.sub.2] [M.sup.-1] x [10.sup.2] M [s.sup.-1] [s.sup.-1] 1.0 2.06 0.21 1.2 2.86 0.23 1.4 3.39 0.24 1.6 4.26 0.26 1.6 4.93 0.27 1.8 6.83 0.29 2.0 -- -- 2.2 -- -- Fructose [Sugar] [K.sub.obs] x [10.sup.3] [K.sub.2] [M.sup.-1] x [10.sup.2] M [s.sup.-1] [s.sup.-1] 1.0 4.82 0.048 1.2 5.56 0.047 1.4 5.67 0.041 1.6 6.85 0.043 1.6 7.47 0.042 1.8 8.66 0.043 2.0 9.63 0.044 2.2 -- -- Maltose [Sugar] [K.sub.obs] x [10.sup.3] [K.sub.2] [M.sup.-1] x [10.sup.2] M [s.sup.-1] [s.sup.-1] 1.0 1.85 0.19 1.2 2.35 0.20 1.4 2.58 0.19 1.6 3.75 0.23 1.6 4.07 0.23 1.8 4.26 0.23 2.0 5.22 0.21 2.2 -- 0.23 Sucrose [Sugar] [K.sub.obs] x [10.sup.5] [K.sub.2] [M.sup.-1] x [10.sup.2] M [s.sup.-1] [s.sup.-1] 1.0 7.41 0.01 1.2 7.69 0.01 1.4 8.45 0.01 1.6 8.86 0.01 1.6 11.27 0.01 1.8 12.57 0.01 2.0 17.73 0.01 2.2 -- 0.01 Table 2: Variation of rate constants with KMn[O.sub.4] concentration at 30[degrees]C, pH = 11, [sugar] = 0.02M, [KN[O.sub.3]] [KMn[O.sub.4]] x Glucose Galactose [10.sup.4] M [K.sub.obs] x [K.sub.obs] x [10.sup.3] [s.sup.-1] [10.sup.3] [s.sup.-1] 3 1.79 1.06 4 1.85 1.15 5 2.48 1.21 6 2.67 1.23 7 2.31 1.39 8 3.10 1.57 [KMn[O.sub.4]] x Fructose Maltose [10.sup.4] M [K.sub.obs] x [K.sub.obs] x [10.sup.3] [s.sup.-1] [10.sup.4] [s.sup.-1] 3 4.34 2.14 4 4.39 1.29 5 4.71 2.40 6 5.05 2.95 7 5.87 3.00 8 4.55 3.46 [KMn[O.sub.4]] x Sucrose [10.sup.4] M [K.sub.obs] x [10.sup.3] [s.sup.-1] 3 1.27 4 1.60 5 1.83 6 2.59 7 3.24 8 2.57 Table 3: Arrhenius and thermodynamic activation parameters for the oxidation of the sugars by alkaline KMn[O.sub.4] at 313K SUBSTRATE Ea A kJ [mol.sup.-1] [dm.sup.3] [mol.sup.-1] [s.sup.-1] GLUCOSE 53.32 1.58 x [10.sup.8] GALACTOSE 48.93 4.78 x [10.sup.8] FRUCTOSE 35.03 3.31 x [10.sup.7] MALTOSE 61.91 1.45 x [10.sup.8] SUCROSE 76.03 1.00 x [10.sup.7] SUBSTRATE [DELTA]H# [DELTA]S# [DELTA]G# kJ [mol.sup.-1] J [mol.sup.-1] kJ [mol.sup.-1] GLUCOSE 50.72 -88.27 78.35 GALACTOSE 46.33 -79.08 71.08 FRUCTOSE 32.44 -101.29 64.15 MALTOSE 59.31 -89.03 87.18 SUCROSE 73.42 -111.25 108.25 SUBSTRATE [k.sub.s] [s.sup.-1] GLUCOSE 8.70 x [10.sup.-3] GALACTOSE 1.00 x [10.sup.-2] FRUCTOSE 4.00 x [10.sup.-2] MALTOSE 9.09 x [10.sup.-3] SUCROSE 5.88 x [10.sup.-3]
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|Author:||Odebunmi E.O.; Owalude S.O.|
|Publication:||International Journal of Applied Chemistry|
|Date:||Sep 1, 2008|
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