KINETICS OF OXIDATION OF SOME REDUCING SUGARS BY POTASSIUM PERMANGANATE IN ACIDIC MEDIUM BY VISIBLE SPECTROPHOTOMETRY

Abstract

Sugars containing either aldehyde (aldose), ketone (ketose ) or hemiacetal groups can be oxidized and are classified as reducing sugars. As oxidation of carbohydrates is widely studied under the field of organic chemistry, the present research has been conducted to study the oxidation of reducing sugars (galactose, fructose, maltose and lactose) with potassium permanganate as an oxidizing agent in sulphuric acid medium. The rate of oxidation of sugars was monitored by recording the change in optical density of MnO4- ion at λmax 545nm. The reactions exhibit first order with respect to [H+], [Sugar] & [MnO4-]. Plots of kobs vs [substrate] were found to be linear for the oxidation of galactose, fructose, maltose and lactose. A plot of log[sugar] vs logk gave straight line with slope of the order of unity (0.81, 0.84, 0.48 and 1.20 in galactose, fructose, maltose and lactose respectively). The oxidation showed that configuration of sugars has some bearing on rate of oxidation. At lower concentration of oxidants, the linear dependence of reaction rate tends towards new order at their higher concentration. Poor dependence on ionic strength suggests the presence of molecular species in the rate determining step. The rate of reaction was affected at elevated temperature where thermodynamic activation parameters like activation energy (Ea), enthalpy change of activation (∆H#), free energy change of activation (∆G#) and entropy change of activation (∆S#) were determined by Arrhenius and Erying equations. The negative value of entropy of activation suggests the existence of highly solvated transition intermediate state and the value of energy of activation suggests the slow kinetics. Hg catalyst was used to increase the rate of reaction in case of maltose and lactose where reactions proceed very slowly with respect to time as compared to other sugars used. +2 OH [Hg(H2O)6]+2 + MnO4-+ H3O+ O Mn O Hg(H2O)5 + 2 H2O The positive value for the free energy of activation indicated high electrostatic interaction between solute and solvent which was supported by the negative value of ∆S# indicating the solvated intermediate state. By considering the first order kinetics with respect to sugars concentration a mechanism consistent with above findings has been proposed in the relevant section of this thesis. TLC and conventional (spot test) methods were used for the verification of oxidation products of sugars. The main products were formic acid and arabinonic acids which were detected in the oxidation of all sugars (Galactose, Fructose, Maltose and Lactose). The other respective acids of each sugar were identified as galacturonic, 1 fructuronic, malturic and lacturic acid by Fab mass, H-NMR and 13 C-NMR spectroscopy. C 6 H12O6 + 2 MnO4 + − ⎯H ⎯→ C5 H10O6 + HCOOH + 2 MnO3 Galactose / Fructose Arabinonic acid + − − Formic acid +2 C 12 H 22O11 + 4 MnO4 + H 2O ⎯H /⎯ → 2C5 H10O6 + 2 HCOOH + 4 MnO3 ⎯ Hg ⎯ Maltose / Lactose C 6 H12O6 + 2 MnO4 Arabinonic acid + − Maltose / Lactose Formic acid ⎯H ⎯→ C6 H10O7 + H 2O + 2 MnO3 − galacturonic / fructuronic acid Galactose / Fructose C12 H 22O11 + 4 MnO4 − − + +2 ⎯H /⎯ → 2C12 H18O13 + 2 H 2O + 4 MnO3 ⎯ Hg ⎯ − malturic/ lacturic acid The reactions pathways leading to the formation of above acids have been proposed by presenting four schemes in relevant section.