Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 2(7), 7-15, July (2012) Res.J.Chem.Sci. International Science Congress Association 7 Oxidation of Tranexamic Acid by Bromamine – T in HCl Medium Catalyzed by RuCl: A Kinetic and Mechanistic ApproachDiwya, Iyengar Pushpa1 and Ramachandrappa R.2*Acharya institute of Technology, Soldevanahalli, Hesarghatta Road, Bangalore-560090, Karnataka, INDIA Department of Chemistry, Jyoti Nivas College, Koramangala Industrial Layout, Bangalore - 560095, Karnataka, INDIAAvailable online at: www.isca.in Received 20th March 2012, revised 24th March 2012, accepted 30th April 2012Abstract Kinetics and oxidation of tranexamic acid (TX) [trans -4-(aminomethyl) cyclohexanecarboxylic acid] by sodium -N- bromo - p- toluenesulphonamide (bromamine – T or BAT) in hydrochloric acid medium using RuCl as catalyst at 303K have been studied. The rate was first order in [BAT]o, fractional order in [TX], first order in RuCl, fractional order in [H] and [PTS]. Addition of NaCl and NaBr did not affect the rate of the reaction which indicates that the rate of the reaction depends only on [H]. Variation of ionic strength did not affect the rate of the reaction indicating that non – ionic species are involved in the rate limiting step. Dielectric effect is positive. Rate increased with increase in temperature from 293K to 323K. From the linear Arrhenius plot, activation parameters were computed. Addition of reaction mixture to aqueous acryl amide solution did not initiate polymerization, showing the absence of free radical species. Oxidation products were identified. Protonated oxidant HBr is the reactive species which reacts with the substrate. Based on kinetic results, reaction stoichiometry and oxidation products, a suitable mechanism have been proposed. Key Words: Reaction kinetics, mechanism, tranexamic acid, bromamine – T. IntroductionAromatic N- halosulphonamides behave as mild oxidants and halogenating agents containing a strong polarized N- linked halogen which is in +1 state. Bromamine – T (sodium N- bromo- p- toluenesulphonamide or BAT; p – CHSONBrNa.3HO) can be easily prepared by bromination of Chloramine – T (CAT) and is found to be a better oxidizing agent than CAT. Literature survey reveals that several work have been carried out with BAT as oxidant 1-8. Kinetic study has also been carried out using other oxidant. Tranexamic acid, (TX) [trans – 4 – (aminomethyl) cyclohexanecarboxylic acid] is an antifibrinolytic agent and has applications in pharmaceuticals10,11. Its kinetics and oxidation study is very important. Kinetics and mechanistic study of tranexamic acid by Bromamine - B in alkaline medium catalyzed by Palladium (II) has been studied12. Literature survey revealed no information in the kinetics of oxidation of this drug by this oxidant. Hence, kinetics and mechanism of oxidation of tranexamic acid with BAT in HCl medium catalyzed by RuCl3 at 303K have been studied. Material and MethodsBromamine – T was obtainedby partial debromination of dibromamine – T (DBT). To a solution containing 20g of CAT in 400ml of water, about 4 ml of liquid bromine was added drop-wise from a micro burette with constant stirring at room temperature. Separated dibromamine-T (DBT) was filtered under suction, washed thoroughly with ice cold water until all the absorbed bromine was completely eliminated and then vacuum dried for 24 h. About 20g of DBT was dissolved with stirring in 30ml of 4 mol dm-3 NaOH at room temperature and the resultant aqueous solution was cooled in ice, pale yellow crystals of BAT formed were filtered under suction, washed quickly with minimum amount of ice cold water and dried over . The product was confirmed to be bromamine – T by qualitative analysis and then by its mass spectrum, UV, IR, H and 13C- spectral data13. An aqueous solution of BAT was prepared afresh, standardized by iodometric method and preserved in brown bottles to prevent its photochemical deterioration. Tranexamic acid (Biocon, India) and p – toluenesulphonamide (E. Merck) was used without further purification and aqueous solution of desired strength was prepared freshly each time. All other chemicals used were of analytical grade. Permittivity of the reaction medium was altered by the addition of methanol in varying proportions (v/v) and values of permittivity of methanol – water mixtures reported in literature14 were employed. Double distilled water was used in preparing all aqueous solutions throughout the studies. Kinetic procedure:All reactions were carried out under pseudo – first order conditions ([TX] �� [BAT]) at constant temperature 303K, in a glass – stoppered pyrex boiling tubes coated black on outside to eliminate photochemical effects. Oxidant and the requisite amounts of substrate, HCl, RuClsolutions and water (for constant total volume) taken in separate boiling tubes were thermally equilibrated at 303K. Reaction was Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 7-15, July (2012) Res.J.Chem.SciInternational Science Congress Association 8 initiated by rapid addition of measured amount of BAT to the mixture and was shaken intermittently. Progress of the reaction was monitored by iodometric determination of unconsumed BAT in known aliquots (5ml each) of the reaction mixture at regular intervals of time. Reaction was studied for more than two half- lives. Pseudo – first order rate constants (k) calculated from the linear plots of log [BAT] versus time were reproducible within ± 3-4 %. Stoichiometry: Various ratios of substrate ie Tranexamic acid (10x10-2 mol dm-3) and bromamine – T (1x 10-2 ), in presence of HCl (10x 10-2 mol dm-3 ) and RuCl (2x10-5 mol dm-3) catalyst at 303K were equilibrated for 24 h. Unreacted BAT was determined and showed 1:1 stoichiometry. 15NO + TsNBrNa + HO C12 + TsNH + NH + Na+ + Br (1) Product Analysis: Reduction product of BAT, p – toluenesulphonamide was detectedby thin layer chromatographyusing light petroleum – chloroform – 1 butanol (2:2:1 v/v/v) as the solvent and iodine as the reducing agent15. Further it was confirmed by its MP (138-143°), IR and NMR spectra. Oxidation product 4 – formylcyclohexane – 1 – carboxylic acid was detected by spot testsie. Schiff’s reagent test, 2, 4- dinitrophenyl hydrazine testand was confirmed by IR spectroscopy16,17. Strong peak for -CHO group was observed at 1725cm-1. Results and DiscussionEffect of Reactants: With substrate in excess, at constant (TX), (H+), (RuCl) and temperature, plots of log (BAT) vs time were linear, indicating a first order dependence of rate on (BAT). Rate constant k/ was unaffected by variation in (BAT)as shown in table 1, confirming first order dependence on (BAT)o . In table 1, increase in concentration of TX, lead to increase in k/ under identical experimental conditions. Plot of log k/ vs log (TX) was linear with a positive slope of 0.633 indicating a fractional order dependence on (TX). Increase in (HCl), lead to increase in reaction rate, as reported in table 1. Plot of log k/ vs log (HCl) was linear with a positive slope of 0.433 indicating a fractional order dependence on (). Concentration of RuCl3 was varied keeping other parameters constant, lead to increase in rate of the reaction as shown in table 1. Plot of log kvs log [RuCl] was linear with a positive slope of 1.233 indicating a fractional order dependence on [RuCl]. Catalytic constant Kc was found to be 1.99x10. From the above discussion figure 1 and figure 2 were drawn. Effect of halide ions and p- Toluene sulphonamide: Under identical experimental conditions, addition of NaCl and NaBr (2x10-3 mol dm-3 to 10x10-3 mol dm-3) did not affect the reaction rate as reported in table 2. Hence it reflected that rate depends only on (H). Addition of reduction product of the oxidant PTS (1x10-3 mol dm-3 to 10x10-3 mol dm-3) decreased the rate of the reaction. The data have been summarized in table 2. In figure 3, plot of log k/ vs log PTS is linear with a negative slope of 0.438 showing fractional order on (PTS) which indicates that PTS is involved in fast pre - equilibrium to the rate limiting step. Effect of ionic strength and dielectric constant: Variation of ionic strength using NaClO solution(2x10-3 mol dm-3 to 50x10 mol dm-3) did not affect the reaction rate, indicating that non – ionic species are involved in rate limiting step as seen in table 2. A plot of (1/D) vs k is a straight line18. Dielectric constant (D) of the medium was varied by adding methanol (0 – 30% v/v) to the reaction mixture. Addition of MeOH lead to decrease in reaction rate as reported in table 3. In figure 4, plot of (1/D) vs log k is linearwith a negative slope of 1.05. Values of D for MeOH- OH mixtures were taken from the literature14. Blank experiments with methanol showed that there was slight decomposition (~3%) of solvent under the experimental conditions. This was corrected for the calculations of net reaction for the rate constant. Effect of temperature: Reaction was studied over a range of temperature (293K to 323K) by varying the concentration of TX at constant experimental conditions. Rate increased with increase in temperature as shown in figure 5. From figure 6 which is the linear Arrhenius plot of log k/ vs 1/T, values of activation parameters like energy of activation(Ea) , enthalpy of activation(), entropy of activation(), free energy of activation (() were computed. Results obtained from the above discussion are tabulated in table 4 and table 5. Test for free radicals: Addition of reaction mixture to aqueous acrylamide solution did not initiate polymerization, showing the absence of free radicals species. Catalytic activity: The relationship between catalyzed and uncatalyzed rate constants is k = k + K[Ru(III)]x where k is the observed rate constant in presence of Ru(III), k is the rate constant in absence of catalyst, Kc is the catalytic constant, x is the order with respect to Ru(III)19. Catalytic constant was found to be 1.99x10. Values of K were evaluated at different temperature. Plot of logK vs 1/T (plot not shown) was linear and thermodynamical parameters were calculated which is reported in table 5. Existence of similar equilibrium in acid and alkaline solutions of N- metallo – N – haloarylsulphonamides was reported20-22. Bromamine-T is analogous to CATin aqueous solution behaving as strong electrolyte forming different species of the following types (2 – 7) TsNBrNa TsNBr +Na+ (2) (Here Ts=p-CHSO) TsNBr + H TsNHBr (3) 2 TsNHBr TsNH + TsNBr2 (4) Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 7-15, July (2012) Res.J.Chem.SciInternational Science Congress Association 9 TsNHBr + HO TsNH+HOBr (5) TsNHBr+ H Ts N Br (6) TsN Br + HO TsNH2 + HBr (7) Table-1 Effect of varying [BAT], [TX], [RuCl] and [HCl] on the reaction rate at 303K10 3 [BAT] o 10 3 [TX] o 10 3 [HCl] 10 6 [RuCl 3 ] 10 4 k / mol dm - 3 mol dm - 3 mol dm - 3 mol dm - 3 s - 1 5.0 10.0 20.0 20.0 3.30 10.0 10.0 20.0 20.0 3.29 15.0 10.0 20.0 20.0 3.31 20.0 10.0 20.0 20.0 3.32 10.0 5.0 20.0 20.0 2.93 10.0 10.0 20.0 20.0 3.33 10.0 20.0 20.0 20.0 5.19 10.0 30.0 20.0 20.0 6.30 10.0 10.0 5.0 20.0 1.33 10.0 10.0 10.0 20.0 2.44 10.0 10.0 20.0 20.0 3.34 10.0 10.0 30.0 20.0 4.44 10.0 10.0 20.0 10.0 1.66 10.0 10.0 20.0 20.0 3.33 10.0 10.0 20.0 30.0 5.93 10.0 10.0 20.0 40.0 8.66 Table-2 Effect of concentration of halide ions, PTS, NaClO on the oxidation [TX] by BAT10 3 [conc] PTS NaCl NaClO 4 NaBr mol dm 3 10 4 k / (s - 1 ) 10 4 k / (s - 1 ) 10 4 k / (s - 1 ) 10 4 k / (s - 1 ) 5.0 10.96 3.31 3.34 3.21 10.0 7.585 3.32 3.32 3.21 50.0 6.025 3.30 3.33 3.42 100.0 5.011 3.34 3.13 3.12 Table-3 Effect of Dielectric constant of the medium on the reaction rateMeOH (% v/v) D 10 2 /D 10 4 k / (s - 1 ) 0 76.93 1.30 3.33 10 72.37 1.38 2.75 20 67.38 1.48 2.21 30 62.71 1.60 1.72 Table-4 Effect of [TX] on different temperatures in acidic medium10 3 [TX] mol dm10 4 k / (s - 1 ) 293K 303K 313K 5.0 1.66 2.93 2.77 10.0 2.22 3.33 4.66 20.0 3.73 5.19 7.04 30.0 5.01 6.30 9.77 Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 7-15, July (2012) Res.J.Chem.SciInternational Science Congress Association 10 Table-5 Effect of temperature and values of Activation parameters for the oxidation of TX by BATTemperature (K) 10 4 k / (s - / ) Activation parameters 293 2.22 Ea = 40.687(39.060) kJ mol - 1 303 3.33  = 38.125(36.498) kJ mol - 1 313 4.66  = 53.520(50.267) kJ mol - 1 323 10.66  = - 49.986(- 44.697) kJ mol - 1 logA = 3.554(3.278) Note: Values in the parenthesis were calculated by plotting logK vs 1/T. 0.60.81.01.21.41.60.10.20.30.40.50.60.7 4 + logk3 + log[HFigure-1 Effect of [HCl] on the rate of the reaction 0.00.10.20.30.40.50.60.70.20.30.40.50.60.70.80.91.0 4 + logk5 + log[RuClFigure-2 Effect of [RuCl] on the rate of the reaction Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 7-15, July (2012) Res.J.Chem.SciInternational Science Congress Association 11 1.71.81.92.02.12.22.30.650.700.750.800.850.900.951.001.05 4 + logk4 + log[PTS]Figure-3 Effect of [PTS] on the rate of the reaction 1.301.351.401.451.501.551.600.200.250.300.350.400.450.500.55 4 + log k10/DFigure-4 Effect of dielectric constant on the rate of the reaction Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 7-15, July (2012) Res.J.Chem.SciInternational Science Congress Association 12 0.60.81.01.21.41.60.10.20.30.40.50.60.70.80.91.0 4030204 + logk3 + log[TX]Figure-5 Effect of [TX] on the rate of the reaction at different temperatures 3.053.103.153.203.253.303.353.403.453.500.20.40.60.81.01.2 4 + logk10/TFigure-6 Effect of Temperature on the reaction rate in acidic medium Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 7-15, July (2012) Res.J.Chem.SciInternational Science Congress Association 13 possible oxidizing species in acid medium are TsNHBr, TsNBr, HOBr, H+ Br. If TsNBr were to be the reactive species, then the rate law predicts a second order dependence of rate on [BAT]o, which is contrary to the experimental observations. First order retardation of rate was observed by adding PTS. Hence, equations (6) and (7) play a dominant role in the oxidation of tranexamic acid by BAT. In aqueous solution of [RuCl], Ru(III)can exist as [RuCl3—, [RuCl(HO)]2—, [RuCl(HO) , [RuCl(HO)], [RuCl(HO), [RuCl(HO)2+ and [Ru(HO)3+ 23-26. Literature survey reveals that the active catalyst species may be assumed to be either [RuCl3— or [RuCl(HO)]2— ion as shown in the following equilibria (equations. (8) and (9)): RuCl + 3HCl [RuCl3— + 3 H+ (8) [RuCl3— + HO [RuCl(HO)]2— + Cl— (9) The above equilibria in reactions catalysed by Ru(III) was used27,28. In present case since there was no effect of chloride ion on the reaction rate, equation 9 does not play a role in the reaction and hence the complex ion, [RuCl3— is assumed to be the reactive catalyst species. pH dependent relative concentration of the species present in acidified CAT solutions of comparable molarities have been studied which shows that its acid form is likely oxidizing species in acid medium29. Based on the preceding discussion and observed kinetic results, a mechanism (scheme 2) is proposed for the oxidation of TX by BAT in acid medium. TsNHBr+ H Ts NH + H+ Br fast (i) + Br + TX X fast (ii) X + [Ru(III)] X fast (iii) + H products slow and rds (iv) Scheme-1 In scheme 1, X and X/ represents the intermediate species, whose structures are shown in scheme 2 in which a detailed mechanistic interpretation of TX oxidation by BAT in acid medium is proposed. The reactive HBr species interacts with the substrate, tranexamic acid in a fast step (ii) to form the bromo intermediate X. Furthermore, X reacts with the active catalyst species, [Ru(III)] , in a fast pre – equilibrium step (iii) to form the metal complex X. Fractional order in [Ru(III)] supports the fast pre – equilibrium, step(iii), in the mechanism. Finally X/ hydrolyses in the slow- rate limiting step forming end products. If [BAT]t represents total BAT concentration in solution, then [BAT]t = [TsNHBr] + [H Br]+ [X] + [X] (10) Rate law equation 11 is in agreement with the observed kinetic orders. Proposed schemes and derived the rate law are also supported by following observations: Addition of methanol decreases the reaction rate. Limiting case of zero angle approach between two dipoles or an ion-dipole system has shown that plot of logk vs 1/D is linear with a negative slope for a reaction between a negative ion and a dipole or between two dipoles, while a positive slope results for a positive ion – dipole interaction reported30. Negative dielectric effect observed in this experiment reveals the presence of dipole-dipole interaction. Reduction product TsNH2 (PTS) does not influence the rate showing that it is not involved in pre-equilibrium. Change in ionic strength of the medium does not alter the rate indicating that involvement of non-ionic species in the rate limiting step. Addition of halide ions had no effect on the rate indicating that no interhalogen or free bromine is formed. Proposed mechanism is further supported by moderate values of energy of activation and other activation parameters. Fairly high positive values of free energy of activation and enthalpy of activation indicate that the transition state is highly solvated, while the large negative entropy of activation suggests the formation of the compact activated complex with less degrees of freedom. Conclusion Kinetics and oxidation tranexamic acid by sodium – N – bromo – p – toluenesulphonamide (bromamine-T or BAT) in aqueous HCl in presence of RuCl as catalyst have been studied at 303K. Activation parameters were computed. Major oxidation product was identified as 4 – formylcyclohexane – 1 – carboxylic acid. Br is the reactive species which reacts with the substrate. Based on kinetic results and reaction stoichiometry, a suitable mechanism has been proposed. From the above, following rate law can be derived: Rate=[BAT][TX][H][Ru(III)][TsNH] + K[H]+ K[TX][H] (1+ K[Ru(III)])-d[BAT] dt (11) [BAT][TX][H][Ru(III)] TsNH[BAT][TX][Ru(III)] [BAT][Ru(III)] [BAT] (12) Research Journal of Chemical Sciences __________________________________________________________ ISSN 2231-606X Vol. 2(7), 7-15, July (2012) Res.J.Chem.SciInternational Science Congress Association 14 Scheme-2 Mechanism of Oxidation of [TX] with [BAT] in presence of RuCl as catalyst AcknowledgementOne of us is grateful to the Principal and management of Jyoti Nivas College, Bangalore, for permission to carry out research at Jyoti Nivas College. We thank Prof. Mohammad Afzal Pasha and Prof. Puttaswamy, Dept of Chemistry, Bangalore University, Bangalore for helpful discussions. 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