Alkaline KMnO4 oxidation of reducing sugars in microemulsions: Inhibition effect of surfactantsby R. Tripathi, S. K. Upadhyay

Kinetics and Catalysis

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Year
2015
DOI
10.1134/S0023158415030210
Subject
Chemistry (all) / Computer Science Applications / Modelling and Simulation / Catalysis

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ISSN 00231584, Kinetics and Catalysis, 2015, Vol. 56, No. 3, pp. 261–266. © Pleiades Publishing, Ltd., 2015. 261 1 In last two decades, the microemulsion was used as a novel reaction media for the organic reactions due to their large solubilisation capacities [1, 2]. The organic reactions in microemulsion media [3–12] were widely investigated and it was observed that the rate of chem ical reaction can be drastically modified by introduc tion of colloidal particles changing the local microen vironment. It was also found [13] that the enhance ment of the rate of reaction in microemulsion is usually larger than that in micellar media.

Recently it was reported [14–16] that an addition of a surfactant (anionic, cationic, nonionic) in the reaction mixture resulted in the retardation of the rate of oxidation of reducing sugars/amino acids by alka line KMnO4. The formation of an inactive complex between surfactant and KMnO4 was found to be responsible for the inhibition effect of the surfactant on the rate of oxidation in the reaction. Spectrophoto metric evidence also supported complexation/associ ation between the surfactant and KMnO4. Since the presence of the droplets can multiply enhance or retard chemical reaction rates, the application of microemulsions as molecular scale reactors in the above reactions may be interesting. In this context we chose to study the kinetics of the oxidation of some reducing sugars viz. glucose and fructose by alkaline

KMnO4 in microemulsion media. The goal of the study was motivated by the following conditions: (1) the kinetics behavior of the above reactions in aqueous and micellar media is well known [14–16]; 1 The article is published in the original. (2) a pH of the microemulsion was the same as that of the aqueous solutions indicating that the presence of microemulsion in reaction mixture does not affect its pH; (3) the reagents are the ionic species of same sign as well as of opposite sign.

In the present communication, the kinetics results of oxidation of reducing sugars vis. glucose and fruc tose by alkaline KMnO4 in cationic microemulsion (prepared from cetyl trimethyl ammonium bromide,

CTAB) and anionic microemulsion (prepared from aerosolOT, AOT) are reported and discussed.

EXPERIMENTAL

Materials

Permanganate solution was prepared by dissolving potassium permanganate (GR, Loba, India) in doubly distilled water. Freshly prepared solutions of Dglu cose and Dfructose (AR, Thomas Baker, India) were used in the experiments. The surfactants viz. cetyl tri methyl ammonium bromide and aerosolOT (Thomas

Baker) were used as received. However, their critical micelles concentrations (CMC) were determined by surface tension measurements and were in agreement with their reported values. All other reagents and sol vents used were of analytical grade. Doubly distilled water, prepared from alkaline KMnO4, was used throughout the experiments for the preparation of solutions.

Two types of microemulsion were used and were prepared as follows.

Alkaline KMnO4 Oxidation of Reducing Sugars in Microemulsions:

Inhibition Effect of Surfactants1

R. Tripathi and S. K. Upadhyay*

Department of Chemistry, H. B. Technological Institute, Kanpur, India *email: upadhyay_s_k@rediffmail.com

Received June 18, 2014

Abstract—The kinetics of oxidation of reducing sugars viz. Dglucose and Dfructose by alkaline KMnO4 in microemulsion media was investigated. The aqueous, cationic microemulsion was prepared from cetyl trim ethyl ammonium bromide, nbutanol, nhexane and water, whereas ndecane, aerosolOT and water were used to prepare the anionic microemulsion. The order of reaction in oxidant was always found to be unity, while that in substrate and alkali was decreased from unity to zero at higher concentrations substrate and alka lirespectively. On decreasing [H2O]/[Surfactant] ratio (increasing surfactant content) in microemulsion, the observed rates constants of oxidation (kobs) were decreased. The inhibition effect on the rate of oxidation was greater in cationic microemulsion. A mechanism consistent with kinetic data is proposed.

DOI: 10.1134/S0023158415030210 262

KINETICS AND CATALYSIS Vol. 56 No. 3 2015

TRIPATHI, UPADHYAY

The cationic microemulsion was prepared from combination (in weight percent) of cationic surfactant

CTAB (30%), cosurfactant nbutanol (4.5%), oil phase nhexane (1.5%) and water (64%). By mixing the components in the appropriate proportion the microemulsion was prepared and the solution was stirred vigorously until a clear transparent solution was obtained [17].

The anionic microemulsion was prepared [18] by mixing decane : aerosolOT : water in weight percent composition 80 : 10 : 10. The solution was stirred vig orously to obtain a perfectly microemulsion. The val ues of pH of microemulsions were determined by directly immersing a glass electrode into microemul sion solutions. The pH of microemulsion was same as that of the aqueous solutions.

Method

To a reaction mixture, containing appropriate quantities of KMnO4, NaOH, NaNO3, microemul sion was added so that the total volume of mixture was 50 ml after adding substrate. The reaction mixture was stirred well and then placed in a water bath maintained at a desired temperature (±0.1°C). The reaction mix ture was allowed to attain the bath temperature. The reaction then was initiated by adding requisite amount of substrate placed separately in the same bath.

Determination of Rate Constant

The rates were measured spectrophotometrically, by monitoring the absorbance due to KMnO4 as a function of time at 520 nm (λmax of KMnO4) on a spectropho tometer Toshniwal TVSP25 (India). The absorbance due to other reagents was negligible at 520 nm. The concentration of KMnO4 was kept within the limits of

Beer’s law.

The reactions were studied at different initial con centrations of the reactants. When the natural log of absorbance vs. time plots was plotted, each reaction showed a linear relationship with a negative slope up to 85–90%, suggesting a firstorder dependence of reac tion rate with respect to Therefore, pseudo firstorder rate constants in (kobs) were evalu ated from the slopes (slope = kobs/2.303) of the straight lines (r > 0.99) plotted as log of absorbance vs. time. The reported rate constant data, represent as an average of duplicate runs, were reproducible to within ±5%.