Kinetics andMechanistic Studies on Oxidation of Levocarnitine by Bromamine-T in HClMedium Catalyzed by Ru ( III )

A kinetic study on RuCl3-catalysed oxidation of levocarnitine (LC) by sodiumN-bromo-p-toluenesulphonamide or bromamine-T (BAT) has been carried out in HCl medium at 303K. e reaction rate shows a �rst order dependence on [BAT]0 and fractional order with respect to both [LC]0 and [H ]. Addition of the reaction product, p-toluenesulphonamide, retards the rate.e addition of RuCl3 and chloride ions to the reaction mixture shows an increase in the rate of the reaction. e dielectric effect is positive. e variation of ionic strength of the medium has no signi�cant effect on the rate of the reaction. e reaction fails to initiate polymerization of acrylamide. Michaelis-Menten type of kinetics has been proposed. ermodynamic parameters have been computed from Arrhenius plot by studying the reaction at different temperatures. e reaction stoichiometry and oxidation products were identi�ed. Based on the experimental observations a suitable mechanism was proposed and rate law deduced.


Introduction
Levocarnitine ((R)-(-3-carboxy-2-hydroxypropyl)trimethylammonium chloride) is a carrier molecule in the transport of long chain fatty acid across the inner mitochondrial membrane.It is a naturally occurring substance required in mammalian energy metabolism.It has been shown to facilitate long chain fatty acid entry into cellular mitochondria, thereby delivering substrate for oxidation and subsequent energy production.
Levocarnitine may also alleviate the metabolic abnormalities of patients with inborn errors that result in accumulation of toxic organic acids [1,2].
N-haloamines are mild oxidants and generally undergo a two electron change per mole in its reactions.ey act as sources of halonium cations, hypohalite species, and Nanions, which act both as bases and nucleophiles [3].ere are many reports on N-halocompounds behaving as oxidizing agents [4][5][6][7].However, a review of literature shows that kinetic studies with Bromamine-T (BAT) are meagre [8,9].
Literature survey shows that there is no information available on the kinetics and oxidation of levocarnitine by any oxidizing agents from the mechanistic view point.ere was a need for understanding the mechanism of oxidation of this drug, so that this study may throw some light on the metabolic conversions in the biological system.e reaction of LC with BAT in HCl medium was found to be sluggish but the reaction was found to be facile in the presence of RuCl 3 catalyst.erefore, in the present communication, we report the kinetics and mechanism of oxidation of levocarnitine by bromamime-T in HCl medium catalyzed by RuCl 3 at 303 K.

Experimental
Bromamine-T was prepared [10] by partial debromination of dibromamine-T (DBT).Its purity was checked by iodometry for its active bromine content.An aqueous solution of BAT was standardized iodometrically and stored in brown bottles to prevent any of its photochemical deterioration.Levocarnitine (Biocon Ltd.) was used as received.e aqueous solution of the substrate was prepared freshly each time.All the other chemicals used were of analytical grade of purity.Doubly distilled water was used for all the measurements.A constant ionic strength of the medium was maintained (  0 mol dm −3 ) using concentrated solution of NaClO 4 .
2.1.Kinetic Procedure.e reactions were carried out under pseudo-�rst order conditions by keeping an excess of levocarnitine over BAT in glass-stoppered pyrex boiling tubes coated black on outside to eliminate photochemical effects.Oxidant and the requisite amounts of substrate, HCl, RuCl 3 solutions, and water (for constant total volume) taken in separate boiling tubes were thermally equilibrated at 303 K. e reaction was initiated by rapid addition of measured amount of BAT to the mixture and was shaken intermittently for uniform concentration.e progress of the reaction was monitored by iodometric estimation of unreacted BAT in a measured aliquot (5 mL) of the mixture at regular time intervals.e course of the reaction was studied up to 75 to 80% completion.e rate constants were evaluated from the plots of log [BAT] against time.e pseudo-�rst order rate constants ( ′ ) calculated were reproducible within ±4%.

Results and Discussion
e oxidation of LC by BAT in the presence of RuCl 3 catalyst has been kinetically investigated at different initial concentrations of the reactants in HCl medium at 303 K.

Effect of Reactant Concentrations on the Reaction
Rate.Under pseudo-�rst order conditions, with the substrate in excess, at constant [LC] 0 , [HCl], [RuCl 3 ], and temperature, plots of log [BAT] 0 versus time were linear indicating a �rst order dependence of the reaction rate on [BAT] 0 .Further, these values are unaffected by a variation of [BAT] 0 (Table 1), con�rming the �rst order dependence on [oxidant] 0 .Under similar experimental conditions, an increase in [LC] 0 increased the  ′ values (Table 1).A plot of log  ′ versus log [LC] was linear (Figure 1;   0.) with a slope of 0.22, showing a fractional-order dependence of the rate on [LC] 0 .e rate of the reaction increases with increase in [HCl] (Table 1) and a plot of log  ′ versus log [H + ] was linear (Figure 1;   0.) with a slope of 0.62 indicating a fractional-order dependence of rate on [HCl].

Effect of Catalyst on the Reaction Rate and Catalytic Activity. e reaction rate increased with increase in [Ru(III)]
(Table 1).A plot of log  ′ versus log [Ru(III)] was linear (Figure 2) having a slope of 0.39, indicating a fractional-order dependence on [Ru(III)].Catalytic constant values were calculated [12] at different temperatures using the relationship between catalyzed and uncatalyzed rate constants: where  1 is the observed rate constant in presence of Ru(III),  0 is the rate constant in absence of catalyst,   is the catalytic constant, and  is the order of the reaction with respect to Ru(III).Plot of log   versus 1/T was linear and thermodynamical parameters were calculated with respect to the catalyst.e data are summarized in Table 2.

Effect of Halide Ions on the Reaction Rate.
At constant [H + ], addition of NaCl resulted in positive effect on the rate of the reaction (Table 3).Plots of log  ′ versus log [Cl − ] were linear with a positive fractional slope (0.28).Addition of NaBr showed negligible effect on the rate of the reaction.

Effect of Added p-Toluenesulphonamide on the Rate.
Addition of reduction product of the oxidant, p-toluenesulphonamide had a retardation effect on the reaction rate (Table 3).A plot of log  ′ versus log [PTS] was linear with negative fractional slope (−0.45), indicating a negative fractionalorder dependence on [PTS] and thus suggesting that TsNH 2 is involved in a pre-equilibrium to the rate determining step.

Effect of Ionic Strength and Dielectric
Permittivity on the Rate.Variation of ionic strength of the medium using  NaClO 4 (1.0 × 10 −3 mol dm −3 to 10.0 × 10 −3 mol dm −3 ) solution did not affect the rate of the reaction.e dielectric permittivity of the medium was varied by adding different proportions of methanol to the reaction mixture.e rate of the reaction increased with increase in methanol content (Table 4).e plot of log  ′ versus 1/D, where D is the dielectric permittivity of the medium (D values are taken from the literature [13]), gave a straight line with a positive slope.Blank experiments with methanol indicated that oxidation of methanol by BAT was negligible under the experimental conditions employed.

3.7.
Test for Free Radicals.Addition of acrylamide to the reaction mixtures did not initiate polymerization which indicates the absence of free radical species.Bromamine-T (TsNBrNa, Ts  p-CH 3 C 6 H 4 SO 2 − ) like chloramine-T behaves as a strong electrolyte [14,15] in aqueous solutions and in acid medium forms different types of reactive species such as TsNHBr, TsNBr 2 , and HOBr.If TsNBr 2 were to be the reactive species, the rate law predicts a second order dependence of rate on [BAT]  which is contrary to the experimental observations.e rate increases with increase in [H + ] but is retarded by the added PTS.Further, it is well known that with aqueous haloamine solutions, TsNHBr is the likely oxidizing species in acid medium.Protonation of monohalomines at pH 2 has been reported [16,17] and in the present case, the fractionalorder dependence on [H + ] indicates that the protonation of TsNHBr results in the formation of TsNH 2 Br + which is likely to be the active oxidizing species involved in the mechanism.
RuCl  is also known to exist in solution in various aquaforms.UV spectral studies of aqueous ruthenium(III) complexes have shown that the octahedral complex species such as and [RuCl(H 2 O) 5 ] 2+ may not exist in aqueous solution of RuCl  [18,19].Other studies [20][21][22] have shown that in acid medium the following equilibrium exists for RuCl  : In the present study, positive effect with respect to Cl − suggests that the following equilibrium is shied to the right in acidic ruthenium (III) chloride solution [12,23].It may, therefore, be assumed that [RuCl 6 ] − is the reactive catalysing species in the present case.Ultraviolet spectral measurements showed that Ru(III) and BAT solutions exhibit absorption  bands at 204 nm and 224 nm, respectively, in the presence of 10.0 × 10 −3 mol dm −3 HCl and a mixture of both at 233 nm indicating the formation of complex between BAT and Ru(III) [23].support steps (ii) and (iii), respectively, in the mechanism.Finally, X ′ in a slow step rearranges to give the product.Based on Scheme 2 the total effective concentration of BAT is: By substituting the values of [TsNHBr], [TsNH 2 Br + ], and [X] from steps (i), (ii), and (iii) of Scheme 2 in (3), we get From the slow and rds of Scheme 2, Rate =  4 X ′  .
Substituting ( 4) in ( 5), we can obtain the rate law as: e rate law (6) is in good agreement with the observed kinetic results.From the intercept of linear double reciprocal plot of 1/ ′ versus 1/[LC] (Figure 4), decomposition constant ( 4 ) was calculated.e decomposition constant was found to be 4.3 × 10 −4 s −1 .e reduction product of oxidant, PTS, had a retardation effect on the reaction rate indicating that TsNH 2 is involved in a pre-equilibrium to the rate determining step.e change in the ionic strength of medium does not alter the rate indicating that nonionic species are involved in the rate limiting step.e effect of varying solvent composition and dielectric constant on the rate has been described in several studies.For the limiting case of zero angle of approach between two dipoles or an ion-dipole system, Amis [25] has shown that a plot of log  ′ versus 1/D is linear.e positive dielectric effect observed in the present studies clearly supports the suggested mechanism.e proposed mechanism is further supported by the moderate values of energy of activation and other activation parameters.e 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 rigid associative transition states with less degrees of freedom.

Conclusions
Oxidation kinetics of LC with BAT in HCl medium catalysed by RuCl 3 has been studied at 303 K. e stoichiometry of the reaction was found to be 1 : 1. e oxidation product of LC was identi�ed as ((R)-(-3-carboxy-2-oxopropyl)trimethylammonium chloride).[RuCl 6 ] 3− and TsNH 2 Br + were assumed to be the active reactive species.Activation parameters were computed.e observed results have been explained by a plausible mechanism and the related rate equation has been deduced.
T 5: Effect of temperature on the reaction rate and activation parameters.
[24] .isstep(i) supports the fractional-order dependence on [H + ] which was observed experimentally.In the next step, TsNH 2 Br + coordinates to the metal centre of [RuCl 6 ] 3− to form a loosely bound metal ion complex (X).is kind of loose metal ion complex formation has been used as an intermediate in some studies involving Ru(III) catalyst[24].e metal complex further reacts in a fast step with the substrate to form another intermediate species (X ′ ) and also TsNH 2 .e fractionalorder in [Ru(III)] and negative fractional-order in [TsNH 2 ]