Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 2(11), 55-60, November (2012) Res.J.Chem. Sci. International Science Congress Association 55 Uncatalysed Oxidation of Dextrose by Cerium(IV) in Aqueous Acidic Medium-A Kinetic and Mechanistic StudyGhosh Manoj Kumar* and Rajput Surendra K. Department of Chemistry, Govt. Nagarjuna P.G. College of Science, Raipur-492010, Chhattisgarh, INDIA Available online at: www.isca.in Received 6th August 2012, revised 13th August 2012, accepted 14th August 2012Abstract A Kinetics investigation of uncatalysed oxidation of dextrose by cerium(IV) have been studied in acidic medium in the temperature range 303-328K. The reaction has been found to be first order with respect to dextrose in an uncatalysed reactions. The effect of [HSO] has also been observed. The increase in ionic strength of the medium decreases the rate of uncatalysed reaction. A 1:2 stoichiometry is observed in the oxidation reaction.On the basis of the experimental results, a reasonable mechanism has been proposed. Rate equation derived from this mechanism can explain all the experimental results. From the effect of temperature on the reaction rate, the Arrhenius equation and various activation parameters have been computed.Keywords: Kinetics, uncatalysed, dextrose, cerium (IV), arrhenius equation. IntroductionKinetic studies have been used as a tool to know the mechanism of a reaction. Chemistry of cerium is a very broad area which has received considerable attention through the years, resulting in substantial advance both in the synthetic1-2and mechanistic categories. Cerium(IV) is a well known oxidant in acidic media3-5 having reduction potential6-7of the couple CeIV/CeIII (1.70V) and is stable only in high acid concentration. In sulphuric acid and sulphate media, several sulphate complexes6,9,10 of cerium (IV) form,but their role has not received much attention so far. Kinetic provides the most important indirect evidence in the mechanism. The metal ion oxidants have been widely employed in the synthetic chemistry11,12 including carbohydrates chemistry13-16. These are stable, in expensive and can readily be stored and handled. The kinetic method of analysis have been widely developed and accepted in chemical analysis of different samples17and kinetic study of Ru (III) catalysed oxidation of sucrose by sodium periodate in acidic medium have been studied by various scientists. Kinetic study has also been carried out using other oxidant20. Material and MethodsChemicals: Chemicals of pure quality were used without further purification. Stock solution of dextrose and potassium bisulphate is prepared in double distilled water. Ce (IV) stock solution were prepared by dissolving ceric ammonium sulphate in aqueous sulphuric acid. sodium thiosulphate solution was standardized with standard iodine solution. All chemicals were purchased from E. Merck. Kinetic procedure: Kinetic runs were performed in stopper glass vessels in a controlled temperature ± 0.1C on water bath. Requisite volume (90cm) of all reagents, including substrate, were introduced into a reaction vessel and thermally equilibrated to 308K. A measured volume (10cm) of cerium solution, also at 308K was rapidly poured into the reaction vessel. The kinetics of the reaction was studied under the conditions [substrate]��[oxidant] in the case of Ce(IV).The kinetics were followed by estimating aliquots of the reaction mixture for cerium(IV) iodometrically, using starch indicator. Doubly recrystallised sugars (E.Merck) were used for the kinetic studies. Cerium solution was always made up and stored in black coated flask to prevent photochemical reaction. The solution was then standarised with sodium thiosulphate solution using starch indicator. Aqueous solution of dextrose were prepared fresh each day. All other reagent were of analytical grade. Conductivity water used throughout the studyResults and Discussion The order of reaction with respect to [oxidant] was determined in table 1. Result shows that the rate constant is directly proportional to the concentration of cerium (IV). Linear line is obtained in figure 1, when we plot of k1 v/s Ce(IV) concentration. This indicates the first order kinetics with respect to oxidant. The order of reaction was determined at different concentration of substrate and at fixed concentrations of other reactants in table 2. Plot of kv/s dextrose concentration is found to be a straight line figure 2(a), which indicates that rate of reaction is directly proportional to the concentration of the substrate. The plot of log k1 v/s log [dextrose]are linear. This indicates that the order with respect to [substrate] is one in figure 2(b). Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 2(11), 55-60, November (2012) Res. J. Chem. Sci. International Science Congress Association 56 Table-1 Effect of variation of [Cerium(IV)] on the reaction rate 10 3 x[Ce(IV)] K 1 x10 4 mol dm - 3 sec - 1 1.00 1.43 2.00 1.39 3.00 1.35 4.00 1.32 5.00 1.30 7.00 1.24 9.00 1.15 10[Dextrose] =5.00 mol dm-3; 10[HSO]=3.00 mol dm3;10[KHSO]= 5.00 mol dm-3; Temp.= 308K  104 sec-1 Figure-1 Plot of K v/s [Ce(IV)] The effect of H+ ion concentration on the reaction rate, the reaction has been carried out at various initial concentration of sulphuric acid tabulated in table 3. It has been observed that rate of reaction decreases with increase of sulphuric acid concentration as in figure 3. On the plot of k1 v/s 1/[H] and log v/s log [H] are found to be linear in nature as in figure 3 (a) and 3(b).This indicates that the order with respect to [H] is inverse first. The reactions were studied at different concentrations of salt [KHSO], while other reactants are constant. The observations of salt effect are given in the table 4. The graphical plot of k1 v/s [KHSO] is found to be a straight line in figure 4,which indicates that the rate of the reaction is inversely proportional to HSO- ion concentration. Thus the addition of salts viz. KHSO did not have much effect on the rate of reaction. To observe the effect of temperature on the reaction rate, the reaction was studied at different temperatures, while keeping all other reactants are constant in table 5. The kinetic data shows that the velocity of reaction increases with rise in temperature, showing the validity of the Arrhenius equation in figure 5. So an attempt has been made to correlate the various activation parameters on the reaction mechanism. Table-2 Effect of variation of [Dextrose] on the reaction rate 10 2 x [Dextrose] 2+log [Dextrose] x10 4 4+log [K 1 ] mol dm - 3 - sec - 1 - 1.00 2.00 1.13 4.05 2.00 2.30 1.26 4.10 3.00 2.47 1.42 4.15 4.00 2.60 1.56 4.19 5.00 2.69 1.74 4.24 6.00 2.77 1.88 4.27 10[Ce(IV)]=3.00 mol dm-3; 10[HSO]=3.00 mol dm-3;10[KHSO]= 5.00 mol dm-3; Temp.= 308K  \n   x 10sec- 10x [Dextrose] mol dm-3Figure-2(a) Plot of K v/s [Dextrose]  \n 4 + log k2 + log [Dextrose] Figure- 2(b) Plot of log K v/s log[Dextrose] Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 2(11), 55-60, November (2012) Res. J. Chem. Sci. International Science Congress Association 57 Table-3 Effect of variation of [H] on the reaction rate 10 2 [H 2 SO 4 2+log[H + ] K 1 x10 4 4+log[K 1 ] mol dm - 3 - sec - 1 - 1.00 2.00 1.99 4.29 1.50 2.17 1.63 4.21 2.00 2.30 1.49 4.17 3.00 2.47 1.34 4.12 4.00 2.60 1.22 4.08 6.00 2.77 1.08 4.03 10[Ce(IV)]=3.00 mol dm-3;10[Dextrose]=5.00mol dm-3; 103 [KHSO]=5.00 mol dm-3; Temp.= 308K       10sec- 10x [H-1Figure-3(a) Plot of k1 v/s [H-1   + log k2+log[H Figure-3(b) Plot of log K v/s log [HActivation parameters: The result shows the value of activation energy (E) was found to be 44.94 k J mol-1. The value of enthalpy of activation (H*) at 313k is 42.3kJmol-1, entropy(S*) at 313k is -17.9 J mol-1 , frequency factor (A) at 313k is 11.8 and free energy(G*) 47.9 kJmol-1. In order to seen that the high positive value of change in free energy(G*) indicates reaction is highly solvated transition state, while negative value of change in entropy (S*) suggested, the formation of an activated complex with reduction in the degree of freedom of molecules. Table-4 Effect of variation of [KHSO] on the reaction rate 10 3 x[KHSO 4 ] K 1 x10 4 mol dm - 3 sec - 1 0.50 1.88 1.00 1.60 2.00 1.38 3.00 1.00 4.00 0.65 5.00 0.25 10[Ce(IV)]=3.00 mol dm-3; 10[Dextrose]=5.00 mol dm-3, 10[HSO]= 3.00 mol dm-3; Temp.= 308K  104 sec-1 10x [KHSOFigure-4 Plot of K1 v/s [KHSO] Reaction Mechanism: The kinetics of the forgoing reactions were studied and showed that substrates and oxidant interact in an equilibrium step to form an intermediate complex which is assumed to disproportionate forming a free radical and reduced Ce(IV). It is believed that involvement of both Cand Chydroxyls in a complex formation. On the basis of above statement and observed first order dependence on [oxidant] as well as [substrate] a probable mechanism (scheme-1) is proposed for the oxidation D-glucose such complex Formation between oxidant and substrate was observed in earlier studies. Rate Law: The proposed mechanism involves the formation of complex in a reversible manner which reach with the substrate at rate determining steps to form [Ce(IV)-S] complex followed by a slow redox decomposition giving rise to aldoxide radical which oxidized by Ce(IV) rapidly. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 2(11), 55-60, November (2012) Res. J. Chem. Sci. International Science Congress Association 58 The oxidation of dextrose at different temperatures from 303K to 328K was studied. The rate of disappearance of cerium(IV) in this reactions increases sharply with increasing concentration of dextrose. The plots of k against T were linear for uncatalyzed oxidation. The Arrhenius activation energy E for the uncatalyzed oxidation of dextrose was 44.94 kJ mol-1. 104 sec-1Temprature in Kelvin Figure-5 Plot of K v/s T Table- 5 Effect of variation of Temperature on the reaction rate 10[Ce(IV)]=3.00 mol dm-3;10[Dextrose]=5.00 mol dm;10[HSO]=3.00 mol dm-3 ;10[KHSO]=5.00 mol dm-3Temperature in Kelvin 1/T x10 - x10 4 sec - 1 303 3.30 1.14 308 3.24 1.88 313 3.19 2.42 318 3.14 2.90 323 3.09 3.70 328 3.04 4.44 Kinetic and Activation parameters for uncatalysed reaction Parameter Dextrose * (kJ mol - 1 ) 44.94 H* (kJ mol - 1 ) 42.34 S* (J mol - 1 ) -17.90 G* (kJ mol - 1 ) 47.50 log A 11.88 Scheme-1 Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 2(11), 55-60, November (2012) Res. J. Chem. Sci. International Science Congress Association 59 The observed18 stoichiometry of the reaction corresponds to the reaction can be represented by the equation 1. 12 +2Ce(IV)+ HO = C10 + HCOOH + 2Ce (III) + 2H (1) Dextrose [S] Aldopentose Formic acid In this reaction one mole of substrate oxidized by two mole of cerium(IV).Formic acid formation and respective aldopentose were confirmed by spot test and also by paper chromatography and high performance liquid chromatographic method. Formation of intermediate carbon centered aldopentose free radicals were confirmed by induced polymerization reaction with acryle nitrile and EPR spin trapping method19. The rate of consumption of Ce(IV) is, [ ] [] ()dCeIVdt kcomplex (2) Based on mechanism as mentioned in the above, the rate law can be deduced as follows, [ ] 12 [()][][] dComplexdt CeIVSkcomplex (3) At steady state condition, [ ] 0 dComplexdt = (4) Hence, [][()][] s kSCeIVkcomplex (5) Therefore, the concentration of the complex becomes, [ ] 21 () [][] CeIV Complex (6) At steady state condition, the rate of disappearance of [Ce(IV)] as given as in equation (2) [ ] [] ()dCeIVdt kcomplex Putting the value of [Complex] form equation (6) we have, [ ] () 2[][()] CeIV SCeIV s d kkdt k = - (7) Now, the total Cerium(IV) may be considered as, [ Ce(IV)] = [ Ce(IV)]e + [complex] (8) Putting the value of [Complex] we have, 12 [][()] [()][()] SCeIV CeIVCeIV k =+ (9) 21 [()][[][()] [()] CeIVkkSCeIV CeIV (10) The value of [Ce(IV)] becomes as, since[Ce(IV)]e [Ce(IV)] 2 21 [()] [()] [] CeIVCeIV S k kk (11) From the equation (7) and (11), rate law becomes as, [ ] 12 22 [] ()2[][()]sT S dCeIVSCeIVdt kk kkk (12) [ ] 21[]() 2[][()] dCeIVSCeIVdt T (13) [ ] 121 [] ()/ 2[] [()]obs S dCeIVdt S CeIV k k kk == (14) 21 [] 1122 obsS kkksk=+ (15) On the plot of 1/kobs against 1/[S] is made from which the constants 1/k and k/k are determined form the slop and intercept respectively. According to the equation mentioned in the above, when plot between 1/kobs and 1/[S], a positive intercept would be observed which confirms the validity of the mechanism and also the rate law. Conclusion The proposed mechanism is well supported by the moderate values of energy of activation and thermodynamic parameters. The high positive value of the energy of activation(G*) and H*) indicate that the transition state is highly solvated where as the negative value of entropy of activation (S*) indicated that the activated complex is cyclic nature. AcknowledgementThe authors are thankful to Dr. K.N. Bapat, Principal, Dr. S. Nigam, Professor and Head, Department of Chemistry, Govt. NPG College of Science, Raipur for Lab facilities. We are also thankful to Dr. Sanjay Ghosh (Assistant Professor), for helpful discussions. References1.Mathew B., Narayana N.V., Sreekumar and vipin, 3-Thianaphthenoyltrifluoroacetone: A new reagent for the spectrophotometric determination of cerium(IV), Microchim Acta, 144, 291 (2005)2.Kolitsch V. and Shwendtner K., A new chromate of tetravalent cerium, Acta.Cryst60, 89(2004)3.Pol P.D., Katharic C.P. and Nandibewoorb S.T., Kinetics of oxidative degradation of pantothenic acid by cerium(IV) in aqueous perchloric acid, Transition met. Chem., 27, 807 (2002)4.Thabaj K.A., Chimatadar S.A and Nandibewoor S.T., Mechanistic Study of Oxidation of Palladium(II) by Cerium(IV) in Aqueous Acid, Transition met. Chem.,31,186(2006)5.Chimatadar S.A., Madawale S.V and Nandibewoor S.T., Mechanistic study of iodide catalysed oxidation of l-glutamic acid by cerium(IV) in aqueous sulphuric acid medium, Transition met. Chem., 32, 634 (2007) 6.Day M.C. and Selbin J., Theoritical Inorganic chemistry, Reinhold Pub.Corp., New York, 226 (1964) Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 2(11), 55-60, November (2012) Res. J. Chem. Sci. International Science Congress Association 60 7.Vogel A.I., A text Book of Quantitative Inorganic analysis, Longmans, London, 3rd Edition, 348 (1961) 8.Pottenger C.R. and Johnson D.C., Mechanism of cerium (IV) oxidation of glucose and cellulose, Polymer Science part A-1:Polymer Chem., 8(2), 301-318 (2003) 9.Patil R.K., Chimatadar S.A. and Nandibewoor S.T., Mechanistic study of cerium(IV) oxidation of antimony(III) in aqueous sulphuric acid in the presence of micro amounts of manganese(II) by stopped flow technique, Transition Met. Chem., 33, 625 (2008) 10.Kharzeoua S.E. and Serebrennikou V.V., Study of Sulphato Complexes of Cerium(IV) by Infrared Absoption Spectra, Russ. J. Inorg. Chem.,12, 1601 (1967) 11.Chinn L.J., Selection of oxidants in synthesis, oxid. at carbon atoms, Marcel Dekker, New York, (1971)12.Augustin R.L., Oxidation, I & II, Marcel Dekker, New York (1969)13.Evans W.L., Oxidation of Carbohydrate with alkaline permanganatc silver oxide, and copper acetate, Chem. Rev.,6, 281(1929)14.Butterworth R.F. and Haneesian S. Sythesis,70, 121-124 (1971) 15.Heyns K. and Paulsen H., The mechanism of carbohydrate oxidation, Adv. Crabohydr. Chem., 17, 169-176(1962) 16.Guthine R.D., Complex Sugars Research, Synthesis and Analysis, Adv. Crabohydr. Chem., 19, 109-116 (1962)17.Laidler K.J., Chemical Kinetics, McGraw Hill, New York (1965) 18.Singh R.B. and Siddhartha S.P., Kinetic Estimation of Dextrose from the Rate Data by Pseudo-First Order Reaction, Asian J. Exp. bio. Sci., 1(1), 204-207 (2010)19.Muller H., Catalytic methods of analysis: Characterization, classification and methodology, Pure Appl. Chem., 67(4) 601-613 (1995)20.Diwya Iyengar Pushpa and Ramachandrappa R., Oxidation of Tranexamic Acid by Bromamine-T in HCl Medium Catalyzed by RuCl: A Kinetic and Mechnistic Approach, Res.J.Chem.Sci., 2(7), 7-15 (2012)