I J P A C Global Research Publications

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1 I J P A C Global Research Publications International Journal of Pure & Applied Chemistry Vol. 6 No. April-June 0 pp Mechanistic Investigation of Pd(II) Catalyzed Oxidation of D-Galactose and D- Lactose by Chloramine-T in Acidic Medium: A Kinetic Study Pushpanjali Singh * Feroze Gandhi College, Raebareli (U.P) India ABSTRACT: Kinetics of Pd(II) catalyzed oxidation of D-Galctose and Lactose by acidic solution of chloramine- T in the presence of mercuric acetate, as a scavenger have been made in the temperature range of C.The oxidation products are galactonic acid and lactobionic acid respectively. The reaction is of first order in case of chloramine-t and Pd(II) both and order of reaction is zero with respect to galactose and lactose.the reactin shows less than unit order in case of [Cl -, while rate of reactions shows zero effect in case of [H +. Negligible effect of mercuric acetate and ionic strength of the medium was observed. A transient complex, formed between PdCl and chloramine-t. [PdCl being the reactive species of Palladium(II) chloride, disproportionates in a slow and rate determining step. A suitable mechanism in agreement with observed kinetics has been proposed.various activation parameters with respect to slow step of the mechanism are computed and discussed. A suitable mechanism in agreement with observed kinetics has been proposed. Key words: Pd(II) chloride, Chloramine-T, D-Galactose, D- Lactose, acidic medium, catalytic oxidation. INTRODUCTION Catalysis by transition metal ions in trace amounts is of recent interest.pdcl acts as efficient catalyst both in acidic as well as basic medium because of the capacity of Palladium ion to exist in more than one oxidation state.kinetics and mechanism of palladium(ii) catalyzed oxidation of some compounds by chloramine-t in perchloric acid medium has been reported. Considerable attention has centered on the chemistry of organic chloramines with formulas R NCl and RNCl, (R) is an organic group,generally known as organic haloamines, because of their versatility and their behavior both as bases and nucleophiles. The prominent member of this group is chloramine-t (CAT) and the mechanistic aspects of many of its reactions have been documented -3. The catalytic as well as inhibition action of Pd(II) in various redox reactions has been reported over the past decade, there has been a considerable interest on the speciation of aqueous PdCl solutions and complexes 4-6 of Pd(II) with Cl - ions. The use of palladium(ii) chloride as a non toxic and homogeneous catalyst has been reported by * To whom correspondence be made: s: pushpanjali.singh@yahoo.com several workers 7-0. Scant work has been done for Pd (II) catalyzed oxidation by Chloramine-T which prompted us to undertake the kinetic study of Pd(II) catalyzed oxidation of Galctose and lactose by Chloramine-T in acidic medium. EXPERIMENTAL Aqueous solution of substrates (E. Merck), Chloramine-T (CDH grade), and mercuric acetate (E. Merck) were prepared by dissolving the weighed amount of sample in triple distilled water. Perchloric acid (60%) of E. Merck grade was used as a source of hydrogen ions. Palladous(II) chloride (Johnson Matthey) was prepared by dissolving the sample in hydrochloric acid of known strength. All other reagents of analytical grade were available. Sodium perchlorate (E. Merck) was used to maintain the ionic strength of the medium. The reaction stills were blackened from outside to prevent photochemical effects. KINETICS A thermostated water bath was used to maintain the desired temperature within ±0. C. Requisite volume of all reagents including substrate, were taken in reaction vessel and thermostated at 35 C for thermal equilibrium. A measured volume of

2 40 Pushpanjali Singh chloramine-t solution, which was also maintained separately at the same temperature, was rapidly poured into the reaction vessel. The kinetics was followed by examining aliquot portion of reaction mixture for chloramine-t iodometrically using starch as an indicator, after suitable time intervals. RESULT & DISCUSSION Reaction mixture containing excess of chloramine- T over sugars in different ratios were allowed to equilibrate at 35 C for about 4 h. The estimation of unconsumed oxidant showed that one moles of oxidant were consumed per moles of substrates, according to following stoichiometric equation- RCHO + H O + RNHCl RCOOH + RNH + HCl Where, R C 5 H 0 O 5 for Galactose R C 0 H O 0 for lactose Identification of the end product formed in the above reaction i.e corresponding acids was carried out as follows- Neutralised 5ml of acid with excess of ammonia in a boiling test tube.then boiled the solution to remove excess of ammonia cooled and added few drops of neutral FeCl 3 solution.a reddish brown colour ppt. is obtained, which confirms presence of carboxylic group. The kinetic results were collected at several initial concentrations of reactants (Table ). Firstorder rate constants i.e. (-dc/dt) were calculated from the plots of unconsumed chloramine-t Vs. time (Fig. ). It was observed that values of(-dc/dt) were doubled when the concentration of chloramine-t was made two times, showing thus first-order dependence on chloramine-t. The kinetic results recorded at various [Pd(II), ionic strengths of the medium along with kinetic effects on successive addition of mercuric acetate, potassium chloride and sodium perchlorate are given in (Table ). First order dependence on [Pd(II) is evident from close resemblance between the slope values (5.00 x 0 - for galactose and 8.57 x0 - for lactose at 35 respectively), of (-dc/dt) Vs. [Pd(II) (Fig. ) and average of experimental k ( -dc/dt/[pd(ii)) values (4.56 x 0 - for galactose and 8.80 x 0 - for lactose at 35 respectively). Table Effect of Variation of Oxidant, Substrate, Catalyst & Perchloric Acid at 35 C Oxidantx0 3 M [Sub. x 0 M [Pd(II)x 0 6 M [HClO 4 x0 3 M (-dc/dt)x0 7 ML - s - Gal./ Lac. Gal. Lac / / / / / / / / / / / / / / / / / /

3 Mechanistic Investigation of Pd(II) Catalyzed Oxidation of D-Galactose and D-Lactose by chloramine-t... 4 Table Effect of Variation of [KCl, Mercury(II) Acetate and Sodium Perchlorate at 35 C [KCl x 0 3 M [NaClO 4 x0 3 M [Hg(OAc) x0 3 M (-dc/dt)x0 7 ML - s [Chloramine-T-.00 X 0-3 M [Substrate-.00 X 0 - M [Hg(OAc) -.5 X 0-3 M [KCl-.00 X 0-3 M [HClO X 0-3 M [Pd(II)-.5 X 0-6 M for lactose [Pd(II) X 0-6 M for Galactose Gal Lac. This can also justified by least square method. (Fig. 3). Negligible effect of variation of ionic strength of the medium, addition of mercuric acetate and positive effect of chloride ions on reaction rate was obvious from the kinetic data in (Table ).Positive effect of chloride ion has shown in (Fig. 4). Change in ionic strength has a negligible effect. Kinetic results obtained on varying concentrations of hydrogen ions indicate negligible effect of Figure : Plot between (-dc/dt) x 0 7 M L - s - and [RNHCl x 0 3 at 35 C Figure : Plot between (-dc/dt) x 0 7 M L - s - and [Pd(II) x 0 6 at 35 C for oxidation of D-Galactose and D- Lactose Figure 3: Plot between (a+ bx) and [Pd(II) x 0 6 at 35 C for Oxidation of D-Galactose and D-Lactose

4 4 Pushpanjali Singh range in the present investigation [PdCl has been proposed and confirmed as the reactive species dominant in the ph range.00 to EQ () RNClNa RNCl - + Na+ EQ () RNCl - + H + RNHCl EQ (3) RNHCl + H O RNH + HOCl Figure 4: Plot between (-dc/dt) x 0 7 M L - s - and [Cl - x 0 3 at 35 C for oxidation of D-Galactose and D-Lactose hydrogen ion variation, which means rate of reaction is not affected by increase or decrease of [H + concentrations. The value of energy of activation ( E*), Arrhenius factor (A), entropy of activation ( S*) and free energy of activation ( G*) were calculated from rate measurement at 30 º, 35º, 40º, 45ºC, and these values have been recorded in (Table 3). Negligible effect of mercuric acetate excludes the possibility of its involvement either as a catalyst or as an oxidant because it does not help the reaction proceed without chloramine-t. Hence the function of mercuric acetate is to act as a scavenger for any Cl - ion formed in the reaction. Table 3 Activation parameters for D-Gal and Lac Rate constant (-dc/dt)x0 7 ML - s - atdifferent T ( C ) D-Gal. D-Lac Arrhenius parameters E*, kj mol log A S*, JK - mol G *, kj mol H*, kj mol At 35 C Pd(II) chloride has been reported to give a number of possible chloro species dependent on ph of the solution. Under the experimental ph EQ (4) RNHCl RNH + RNCl EQ (5) HOCl + H + H O + Cl However, there was If RNCl were to be the reactive species, then the rate law predicts the second-order dependence on [CAT, according to EQ. (4), which is not in agreement with the experimental observation. If HOCl acts as a reactive oxidant species a first-order retardation of rate. Hardy and Johnston, who studied the ph dependent relative concentrations of the species present in acidified bromamine-b solutions of comparable molarities, have shown that RNHBr is the likely oxidizing species in acid médium. Chloramine-T an analogue to chloramine-b behaves as a strong electrolyte in aqueous solutions and furnishes different types of reactive species in acidic solutions. To confirm this hypothesis, conductometric and ph-titrations between aqueous solutions of CAT and HClO4 were performed. The conductometric behavior of CAT is identical with that of CAB 3, 4, while the ph titration curves observed are similar to those noted by Morris et al. 5 In acidic solution of chloramine-t quick formation of RNHCl 6 has been reported. Firstorder dependence on [chloramine-t suggests that RNHClis itself involved in slow step as an oxidant. The kinetic results reported in Table, and 3 and the above statements lead us to suggest the following reaction scheme which gives the details of various steps in title reaction. RATE LAW Cl - exist as the following equilibrium in acidic solution of palladium(ii) chloride- () + k + k PdCl Cl PdCl [C [C Positive effect with respect to Cl - in the present investigation suggests that the equilibrium would shift to the right. Therefore [PdCl is the active species of palladium(ii) chloride in acidic media.

5 Mechanistic Investigation of Pd(II) Catalyzed Oxidation of D-Galactose and D-Lactose by chloramine-t () k [PdCl + RNHCl [RNHCl PdCl slowstep [C [Cl 3 [RNHCl...PdCl + RCHO + H O RCOOH + PdCl + RNH + HCl (3) [C 3 Now considering the above steps and applying the steady-state treatment with reasonable approximation, the rate law may be written in term as- d[rnhcl k [C [RNHCl dt (4) Rate k [C [RNHCl (5) [Pd(II) T [C + [C (6) d[cl dt (7) [C k - [C - k [C [Cl - k [C k [Cl By putting the value of [C in equation (5): [Pd(II) T [C + [C [Pd(II) T [Pd(II) T (8) [Pd (II) T [C k [C [C [K k\k k [Cl + [C +[C K(Cl [C + K [Cl [C K [Cl [Pd(II) K [Cl + K [Cl T By putting the value of [C in equation (4):- k K [Pd(II) [Cl [RNHCl Rate T + K [Cl CONCLUSION The experimental results as shown reveal that the reaction rate doubles when the concentration of catalyst is doubled. The rate law equation is in conformity with all kinetic observations and proposed mechanistic steps are supported by the negligible effect of ionic strength. The high positive values of free energy of activation ( G*) indicates highly solvated transition state, while fairly high negative values of entropy of activation ( S*) suggest the formation of an activation complex with reduction in the degree of freedom. From the present investigation, it is concluded that RNHCl is the reactive species of chloramine-t in acidic medium and the reactive species of Pd(II) in acidic medium is [PdCl under the experimental ph range. References [ C. K. Mythily, K.s Rangppa, Int, J. Chem Kinet, 3(), 7-36, (004). [ Puttaswamy, R. V. Jagadeesh, Int., J. Chem Kinet., 37(4), 0-0, (005). [3 Anju Shukla, Santosh K. Upadhyaya, Int., J. Chem. Kinet., 3(4), 79, (99). [4 O. Hamed, P. M. Henry, and C. Thompson, J. Org. Chem. 7, , (999). [5 El-Qisairi and P. M. Henry, J. Orgmet. Chem. (603), 50-60, (000). [6 O. Hamed, P. M. Henry, J. Org. Chem., 66, 80-85, (00). [7 O. Hamed, P. M. Henry, C. Thompson, J. Org. Chem. 64, , (999). [8 Kunj Bihari, Roli Sriv., Rima Das, T. Mat. Chem., 9(3), , (994). [9 P. Kapitan and Tiber Gracza, J. ARKVOC, 8, 8-7, (008). [0 M. H. Kondarasaiah, S. Ananda, Puttaswamy et al., Inorg. Met. Org & Nano Met. Chem., 33(7), 45-56, (003). [ Neeti Grover, Neelo. Kambo et al., Int. J. Chem., 4(), , (00). [ F. F. Hardy, J. P. Johnston, J. Chem. Soc. Perkin Trans II 74, (973). [3 D. S. Mahadevappa, Rangaswamy, Rev. Roumaine de chimie, 33, (977). [4 Rangaswamy, D. S. Mahadevappa, Indian. J. Chem. 4A, 463, (976). [5 J. C. Morris, J. R. Salazar, M. A. Winemann, J. Am. Chem. Soc. 70, 036, (948). [6 Neeti Grover, Neelu Kambo et al., Ind. J. Chem. 4(), 485, (00).