Asian Journal of Chemistry; Vol. 25, No. 2 (2013), 595-611 http://dx.doi.org/10.14233/ajchem.2013.13111 REVIEW Kinetics and Mechanism of Oxidation of Reducing Sugars: A Review 1,* 1 2 2 3 J.V. SINGH , ANUPAM AWASTHI , DIPTI , ASHISH TOMAR and DEVENDRA SINGH 1Department of Chemistry, Nehru College, Chhibramau-209 721, India 2Department of Chemistry, Meerut College, Meerut-250 001, India 3Faculty of Engineering and Technology, Chandra Shekhar Azad University of Agriculture and Technology Campus, Etawah-206 001, India *Corresponding author: E-mail: [email protected] (Received: 25 November 2011; Accepted: 8 August 2012) AJC-11938 This review summaries the kinetic and mechanistic aspects of the oxidation reaction of reducing sugars with various oxidants, with emphasis to the Cr(VI) oxidation of reducing sugars. An attempt has been made to cover the recent literature available on oxidation kinetics of reducing sugars upto June 2011. Key Words: Oxidation, Kinetics, Mechanism, Reducing sugars. INTRODUCTION Reducing sugar in solution: A reducing sugar, may exist in solution as acyclic aldehydes, the corresponding hydrates, Carbohydrates including sugars are multifunctional naturally α- and β-pyranoses and as α- and β-furanoses (Scheme-I)7. occurring material, the oxidation products of which play a key role as intermediates for the synthesis of more complex O O molecules and also display, in many cases, varied biological activities. Although a number of oxidation procedure for carbo- Acyclic aldehyde OH OH CHO hydrates have been developed but clear information or an overall α-pyranose β-pyranose view on the kinetics and mechanism of reducing sugars with various oxidants is lacking which promoted us to document OH HO O HO O OH an overall view on the oxidation of reducing sugars. Earlier 1 Capon’s review recorded the complex reaction mechanism OH H2O H2O 2 in carbohydrate chemistry. Varela in his review updated the β-furanose α-furanose treatment of the subject in a review by Green3. Kinetic studies CH(OH) 2 on the oxidation of reducing sugars by various metal ions in acidic and alkaline medium have been reported in an another OH 4 5 review by Bhatnagar et al. and a review by Rangappa included Acyclic hydrate the kinetic and mechanistic studies on the oxidation of monosac- Scheme-I charides (such as erythrose and therose series, pentoses and hexoses, 6-deoxyhexoses, aminosugars and uronic acid) with The kinetics and mechanism of the oxidation reaction all available N-metallo-N-halo aryl sulfonamides in alkaline depends on the nature of reducing sugar in solution, the active medium. A review of the subject by Odebunmi and Ogunlaja6 species of the oxidant used and the pH of the medium. The focusing on homogeneous catalytic oxidation of some sugars kinetic study of carbohydrates have been carried out both in using different metal and metal complex as catalyst has appeared alkaline as well as in acidic medium. recently. Kinetic and mechanistic studies on oxidation of reducing Oxidation of reducing sugars in alkaline medium: The sugars reported other than in these reviews except some studies reducing sugar molecule isomerises in alkaline solution to an in Varela's review2 are included herein this review article. intermediate enediol. 596 Singh et al. Asian J. Chem. H - K1 - S E H O + HO + 2 H CO H C OH H COH fast C OH C HO C H O K - - 2 E X C + RNBr H C OH H OH H C OH fast Aldose Enediol Ketose - k3 X + HO X1 Slow, Rate In most of the oxidation of reducing sugars in alkaline determining step medium, a mechanism involving enolization step and the k4 intermediate enediol has been proposed. products X1 The kinetics of oxidation of D-xylose, L-arabinose, fast D-mannose and D-galactose8 and sorbose9 by chloramine-T Scheme-III (CAT) led to the rate law: [d CAT ] − 2 zero order kinetics in [oxidant], fractional order in [fructose/ − = [k CAT [] substrate [] OH ] – dt glucose] and nearly first order in [OH ]. A rate-determining reaction of hypochlorite ion and the Nine sodium salts of mono and di-substituated N-chloro- anion of the aldose has been proposed (Scheme-II). arylsulfonamides have been employed as an oxidant for studying the kinetics of oxidation of D-fructose and D-glucose 16 R H in alkaline medium . The reactions show first order kinetics R O O each in [oxidant], [substrtate] and [OH–]. OH OCl O O + + O 17 H H Kinetic studies on the oxidation of galactose and mannose , 18 19 β-pyranose anion O arabinose and xylose , maltose and lactose and melibiose and cellobiose20 by iodine in alkaline solution have indicated Cl that the active oxidizing species is hypoiodus acid. The study of non-catalyzed molecular reaction of reducing sugars21 (galactose and fructose) with iodine in an alkaline R OH medium indicate that the reaction follows first order kinetics + OH + Cl O with respect to sugars, hypoiodous acid and zero order kinetics O with respect to iodine concentration. The proposed reaction aldonic acid anion mechanism involves the formation of intermediate complex Scheme-II (Scheme-IV). The oxidation of fructose and dulcitol10,11 by chloramine-T shows a first order dependence in [alkali]. From the zero order HOI HI I2 H2O + dependence in [bromamine-T] (BAT) and first order in [sugar] + - and [alkali] at lower concentrations and zero order in [alkali] O at higher concentrations in the oxidations of some hexoses k1 R C HOI like D-glucose, D-fructose, D-galactose and D-mannose by R CO HOI 12 + bromamine-T in alkaline solution, Singh and Prakash k- 1 H proposed that the rate determining step is the formation of H enediol anion which reacts rapidly with the oxidant. complex Based on the first order dependence of rate on [oxidant] – and [substrate] and second order on [OH ] in the oxidation of O D-galactose, L-sorbose and D-xylose13 by bromamine-T and k 2 + - bromamine-B in alkaline medium, the following reaction C OH +H + I sequence (Scheme-III) has been proposed. The kinetics of oxidation of L-fructose, D-fructose and R L-rhamnose14 by bromamine-B is first order in [oxidant] Scheme-IV [sugar] and [OH–] in alkaline medium. A mechanism involving an enediol anion (E–) of sugar reacts with the oxidant in the The kinetics of oxidation of maltose and lactose by Cu(II) rate determining step has been proposed. in presence of NH4OH revealed that the reaction proceed after N-Bromo-arylsulfonamides of different oxidizing a slight induction period and the rate of reaction is zero order strengths have been used for studying the oxidation kinetics in [oxidant] and first order in [sugar] and [OH–]. The rate of D-fructose and D-glucose in aqueous alkaline medium and determining step involves the reaction between hydroxyl ion the results have been analyzed and compared with those from and reducing sugar, leading to an intermediate active product the sodium salts of N-bromo-benzenesulfonamides and N- which is rapidly oxidized by Cu(II) complex through an bromo-4-methyl-benzenesulfonamide15. The reactions show electron transfer process22. Vol. 25, No. 2 (2013) Kinetics and Mechanism of Oxidation of Reducing Sugars: A Review 597 R' Kinetic studies have also been reported for the reaction O of Cu in presence of ammonia with maltose, lactose, melibiose O.. H 23 H - and cellobiose . O OH R' The results of comparative studies on kinetics of cataly- Cl O zed oxidation of glucose and galactose by hexacynoferrate(III) O Cl H ion and CuSO4 in alkaline medium show that the rate of oxi- OH dation increased with increase in substrate concentration, temperature, ionic strength as well as pH24. The positive effect 2+ + of catalyst was observed with Cu in the reaction of galactose Na R' R' - R' - + but not in glucose with hexacynoferrate(III) as oxidant. Oxida- OH O O Na O tion of both sugars with CuSO4 as oxidant was found not to O -NaCl C O Cl - - + Na 3+ O OH OH enhance by Cr . Na -H 2O H The rate of the oxidation reaction of D-xylose, D-ribose R'=H OH and L-arabinose by 1-chlorobenzotriazole (CBT) in NaOH is first order in [CBT] and fractional order each in [sugar] and Scheme-V [OH–]25. From reaction product, salt effect, dielectric constant effect, solvent isotope effect, a mechanism having C1-O-Cl Oxidation kinetics of D-glucose, D-galactose, D-mannose intermediate has been proposed (Scheme-V). and D-fructose by t-butyl hypochlorite (t-BuOCl) in alkaline K 1 - [Ir(H 2O) 3Cl 3] + OH [IrCl 3(H 2O) 2OH] + H2O (i) - 2 K 2 O O - H O (ii) IO + 2 [IrCl 3(H 2O) 2OH] + 3 I O IrCl 3(H 2O)OH - - 2 2 O k O H2O (iii) IrCl (OH) + slow and rate O I 3 O I O IrCl 3(H 2O)OH determining step O - - 2 H OH - O O - 2OH 2 O + Fast R C C + [IrCl 3(H 2O) 2OH 3] IrCl (OH) C OH O I 3 + OH H O Cl - (iv) + IO 2 Where R stands for C 3H7O3 - O O - - Ir(III) - - IO HO - IO (v) R C C + 3 + HCOO + RCOO + 2 + H2O OH H - (vi) -2 [IrCl 3(H 2O) 3] 2 HO [IrCl 3(H 2O)(OH) 2] + 2 H2O + Scheme-VI 598 Singh et al. Asian J. Chem. – medium follows both zero and first order pathways in of Ru(III) and [IO4 ] leading to the formation of a complex, [t-BuOCl] and fractional order kinetics each in [hexose] and which in the rate-determining step decomposes into another [OH–]26.