Kinetics and Mechanistic Investigation of Decarboxylation for the

Article pubs.acs.org/IECR

Kinetics and Mechanistic Investigation of Decarboxylation for the Oxidation of Levofloxacin by Chloroamine-T in Acidic Medium Aftab Aslam Parwaz Khan,†,‡,* Abdullah M. Asiri,†,‡ Naved Azum,†,‡ Malik Abdul Rub,†,‡ Anish Khan,†,‡ and Abdulrahman O. Al-Youbi†,‡ †

Chemistry Department, Faculty of Science, and ‡Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia S Supporting Information *

ABSTRACT: The mechanistic investigation of oxidation of levofloxacin (LF) has been studied by chloramine-T(CAT) in aqueous perchloric acid medium at 25 °C. The reaction followed first-order kinetics with respect to [CAT], [LF], and [H+] in their lower concentrations range, tending to zero-order at their higher concentrations. First-order kinetics with respect to [CAT] was observed for the oxidation of levofloxacin. An increase in perchloric acid concentration increased the rate of reaction. The effect of added products, ionic strength, and dielectric constant of the medium was studied on the rate of reaction. The main products were identified by spot test, FT-IR, and NMR. A mechanism was proposed on the basis of experimental results. The activation parameters with respect to the slow step of the mechanism was evaluated, and the thermodynamic parameters were also determined and discussed. their preliminary extraction with organic solvents.6 A method was proposed for determining these antibiotics in biological fluids using a mixed-ligand complex formed by terbium and triphenylphosphine oxide.7 The interaction of fluoroquinolones with metal ions is of interest not only for the development of analytical techniques but also to afford information about the mechanism of action of the pharmaceutical preparation.8 Since the metal ions cause fluorescence quenching of the drug, the spectrofluorimetric method for quantitative determination of the quinolone type drugs has been developed9 along with titrimetric,10 spectrophotometeric,11,12 electrochemical,13 electrophoretic14 and chromatographic15 techniques. The increase of fluoroquinolone in aquatic environments, even in low concentrations, may cause intimidation to the ecosystem and human health by inducing the multiplication of drug resistant bacteria owing to long-term exposure. Chemical oxidation of pollutants in drinking water and wastewater by chloramine-T has been widely done. The title reaction was extensively investigated to clarify the behavior of the fluoroquinolone antibacterial agent during wastewater treatment by chloramine-T. Sodium-N-chloro-4-methyl benzenesulfonamide, p-CH3− C6H4SO2NClNa·3H2O, commonly known as chloramine-T (CAT) is a very significant member of the organic N-halogenoamines.16 The redox potential of chloramine-T decreases with an increase of the pH of the medium and the reaction conditions.17 Chloramine-T is a source of a positive halogen and the most potent oxidant in acid as well as in alkaline media,18,19 and the mechanistic aspects of many of its reactions have been well accepted.20−29 This study is concerned with the identity of the redox reaction and to explore a suitable mechanism for oxidation of

2. INTRODUCTION Levofloxacin (−)-(S)-9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl1-piperazinyl)-7-oxo-7H pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acid hemihydrate (Figure 1) is the active S-isomer isolated

Figure 1. Structure of levoflaxacin (LF).

from the racemic ofloxacin. Levofloxacin is a broad spectrum drug of activity against various bacteria, including gram-positive and gram-negative microorganisms.1,2 Levofloxacin has been widely used for the cure of infectious diseases, such as communityacquired pneumonia and acute exacerbation of chronic bronchitis.3 The antibacterial activity of the quinolones is not linearly proportional to their concentration. As the concentration of the drug in the human body falls the bacterial population increases. However, an optimum concentration of the drug must be maintained before the surviving bacteria start regrowing.4 In the case of fluoroquinolones the number of the surviving bacteria is lower, but the unusual dependence is retained. A mainly limiting concentration of fluoroquinolones has been determined. The pharmacopoeia method is based on a microbiological test involving the diffusion of the antibiotic into agar (nutrient medium) and the growth inhibiting effect of known concentrations of the drug on the microorganism with reference to samples.5 The intrinsic fluorescence of quinolones is used for their determination in biological preparations after © 2012 American Chemical Society

Received: Revised: Accepted: Published: 4819

October 30, 2011 January 15, 2012 March 16, 2012 March 17, 2012 dx.doi.org/10.1021/ie202483c | Ind. Eng. Chem. Res. 2012, 51, 4819−4824

Industrial & Engineering Chemistry Research

Article

4.2. Stoichiometry and Product Analysis. Different sets containing LF, HClO4, and NaClO4 with an excess of CAT were kept for closed vessels under nitrogen atmosphere at 37 °C for 48 h. After 48 h an estimation of unreacted CAT at an absorbance at 520 nm showed that four mole of oxidant was sufficient to oxidize one mole of LF leading to the oxidation product. The results indicated that 1:4 stoichiometry equation may be formulated:

levofloxacin by chloramine-T in acidic medium on the basis of kinetics parameters.

3. EXPERIMENTAL DETAILS 3.1. Chemicals and Solutions. The solution of Levofloxacin (Sigma-Aldrich) was prepared by dissolving a known amount of its hydrochloride salt in distilled water. ChloramineT (E. Merck) was purified by the method of Morris et al.30 An aqueous solution of CAT was prepared, standardized periodically by the iodometric titration, and stored in brown bottles to prevent its photochemical deterioration. All other reagents were of analytical grade, and their solutions were prepared by dissolving the requisite amounts in doubly distilled water. HClO4 and NaClO4 were employed to maintain the pH and ionic strength, respectively. 3.2. Kinetic Studies. The oxidation of [LF] by CAT was followed under pseudo-first-order conditions at 25 °C. The reaction was initiated by mixing previously thermostatted solutions of CAT and [LF] which also contained the required quantities of perchloric acid and sodium perchlorate to maintain acidity and ionic strength, respectively. The reaction medium of pH < 2.0 was maintained throughout the experiments. The reaction was monitored by a decrease in absorbance of CAT at its absorption maximum of 520 nm. The kinetic runs were followed for more than 75% completion of the reaction and good first-order-kinetics were observed. The pseudo-firstorder rate constants kobs were obtained from the slope of the tangent drawn at a fixed time of the plot of unconsumed [CAT] versus time (Figure 2). The effect of dissolved oxygen

C18H20FN3O4 + 4TsNHCl + 4H2O → C16H20FN3O4 + 4TsNH2 + 4HCl + 2CO2

(1)

The main reaction products [(3S)-7-fluoro-3-methyl-8-(4methylpiperazin-1-yl)-3,4-dihydro-2H-1,4-benzoxazin-5-yl](oxo)acetic acid were identified and isolated with the help of TLC and characterized by FT-IR and NMR. The IR spectrum shows a peak at 1624 and 1718 cm−1 due to acidic ν (CO) stretching, respectively: 2830−2753 cm−1 is due to CH3 stretching of the piperazine moiety, and a broad peak at 3407 cm−1 is due to ν (OH) stretching. 1H NMR (DMSO) shows a singlet at 5.8 ppm due to acidic OH. The NH of the piperzine moiety singlet appears in the region of 2.3 ppm, and another singlet of NH of the benzoxazine moiety appears at 3.96 ppm, which disappears on D2O exchange and confirms the formation of product. The liberated CO2 was qualitatively detected31 by bubbling N2 gas through the acidified reaction mixture and passing the gas into limewater. 4.3. Effect of Oxidant. With invariable concentration of [LF], 3.0 × 10−3 mol dm−3, and [H+] 3.0 × 10−3 mol dm−3, at constant ionic strength, 0.10 mol dm−3, the CAT concentration was varied in the range of 1.0 × 10−4 to 1.0 × 10−3 mol dm−3. All kinetic runs exhibited identical characteristics. The linearity of plots of log (absorbance) vs time, for different concentrations of CAT, indicates order in CAT concentration as unity. This was also confirmed by the constant values of pseudo-firstorder rate constants, kobs, for variable [CAT] (Table 1). Table 1. Effect of Variation of [CAT], [LF], and [H+] on the Rate of Oxidation of Levofloxacin at 25 °C

Figure 2. First-order plots for the oxidation of LF by CAT in acidic medium at 25 °C.

on the rate of the reaction was checked by following the reaction in nitrogen atmosphere. No significant change difference between the results obtained under nitrogen and in the presence of air was observed. Added carbonate had also no effect on the reaction rate. Fresh solutions were used while conducting the experiments. Regression analysis of experimental data to obtain the regression coefficient r and the standard deviation S of points from the regression line was performed using a Microsoft Excel-2010 program.

4. RESULTS AND DISCUSSION 4.1. Test of Free Radicals. The intervention of free radicals was examined as follows. The reaction mixture, to which a known quantity of acrylonitrile scavenger had been added initially, was kept in an inert atmosphere for 1 h. When the reaction mixture was diluted with methanol, no precipitate resulted, indicating the absence of free radicals in the reaction mixture.

[LF] × 103 (mol·dm−3)

[CAT] 104 (mol·dm−3)

[HClO4]

kobs × 10−3 (s−1)

Kcal × 10−3 (s−1)

0.5 1.0 2.0 3.0 5.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 1.0 2.5 5.0 7.5 10.0

1.5 1.5 1.5 1.5 1.5 0.5 1.0 2.0 3.0 5.0 1.5 1.5 1.5 1.5 1.5

4.23 5.25 6.76 7.12 8.20 4.23 6.29 7.12 8.21 8.24 8.14 8.16 8.18 8.18 8.20

4.28 5.29 7.18 8.21 8.28 4.26 6.34 7.15 8.21 8.26 8.21 8.21 8.21 8.21 8.21

4.4. Effect of Concentration of LF. The LF concentration was varied in the range of 5.0 × 10−4 to 5.0 × 10−3 at constant acidity and CAT concentrations and constant ionic strength of 0.10 mol dm−3 at 25 °C. The kobs values increased with increase in LF concentration range shown in (Table 1). At low concentration of LF, the reaction was first order, and at high concentration of LF the reaction was independent. 4820

dx.doi.org/10.1021/ie202483c | Ind. Eng. Chem. Res. 2012, 51, 4819−4824


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4.5. Effect of Acidity. The oxidation of LF by CAT was studied at varying concentrations of HClO4 [0.5−3.0 mol dm−3] by keeping other conditions constant. The rate of oxidation decreased with increase in the concentration of HClO4 (Table1). 4.6. Effect of Ionic Strength and Dielectric Constant. The effect of ionic strength was studied by varying the potassium nitrate concentration from 0.1 −0.9 mol dm−3 at constant concentrations of CAT, LF, and acidity. It was noted that increasing ionic strength had no effect on the rate constant. The effect of the dielectric constant was studied in different compositions of the mixture of acetic acid and water. The rate constant decreased with increasing acetic acid content in the reaction medium (Table 3). The plot of log kobs versus 1/D was linear with positive slope (Figure 4) 4.7. Effect of Temperature. The effect of temperature on the reaction rate was studied by performing the kinetic runs at four different temperatures (25−37 °C) with keeping other experimental conditions constant. The activation energy corresponding to these rate constants was evaluated from the linear Arrhenius plot of log k versus 1/T from which other activation parameters (Ea, ΔH⧧, ΔS⧧ and ΔG⧧) for the overall reaction were evaluated. (Table 2).

Thus, CAT exists as a free acid (TsNHCl) in acidic media. The dissociation constant of TsNHCl at a pH of ca. 4.5 is 2.8 × 10−5 reported by Morris et al.30 Further, TsNHCl can undergo disproportion or hydrolysis according to the following reactions.35 2TsNHCl ⇌ TsNCl2 + TsNH2

(iii)

TsNHCl + H2O ⇌ TsNH2 + HOCl

(iv)

TsNHCl + H+ ⇌ TsNH2Cl+

(v)

TsNH2Cl++ H2O ⇌ TsNH2 + (H2OCl)+

(vi)

It appears from the equilibrium i−vi, that various probable oxidizing species of CAT exist in acidic media such as TsNHCl, TsNCl2, TsNH2Cl, TsNH2, HOCl, and H2OCl+. In the present work, the rate depends on [H+] in that the deprotonation of TsNH2Cl+ results in the formation of TsNHCl, which is a probable oxidizing species involved in the oxidation of LF in acid medium (Scheme 1). A detailed

Table 2. Activation and Thermodynamics Parameters for the Oxidation of Levofloxacin

Scheme 1

(a) Effect of Temperature with Respect to the Slow Step of Scheme 1 k × 103 (s−1)

temperature (K) 293 298 303 308

log k

1/T × 103

8.21 0.914 9.31 0.969 10.12 1.005 11.23 1.050 (b) Activation Parameters parameters

Ea (kJ mol ) ΔH⧧ (kJ mol−1) ΔS⧧ (J K−1 mol−1) ΔG⧧ (kJ mol−1) (c) Rate Constants 293 298 303 308

mechanistic explanation of LF−CAT in acid medium is given in Scheme 2. 4.9. Mechanism and Derivation of Rate Law. According to the reaction in Scheme 1 and considering the fact that one mole of LF is oxidized by four moles of CAT, on the basis of the preceding discussion the following is proposed to account the observed kinetics for the oxidation of LF by CAT in acid medium. In Scheme 1, TsNH2Cl+ first generates the conjugate free acid TsNHCl. Then, TsNHCl reacts with LF and forms a complex. The complex was put on a slow step which reduced into an intermediate TsNH2 and a product of LF. Furthermore, to check whether the reactions in the slow step are in parallel with those in the fast step to form the final product of LF, the structures of complex intermediate species are shown in Scheme 2, where a detailed plausible mechanism of the oxidation of LF with CAT in acidic medium is illustrated.

values

−1

temp (K)

3.413 3.356 3.300 3.246

K1 dm6 mol−2

22.4 42.6 −123.4 92.12 K2 × 10−4 dm3 mol−1

2.64 8.23 16.13 18.12 (d) Thermodynamic Values

4.21 2.35 0.24 0.15

thermodynamic values

using K1 values

using K2 parameters

ΔH⧧ (kJ mol−1) ΔS⧧ (J K−1 mol−1) ΔG⧧ (kJ mol−1)

85.4 217 4.8

−45.5 −24.3 −11.6

rate = − d[CAT]/dt = k[complex]

4.8. Reactive Species of CAT. In general, CAT undergoes a two-electron change in its reactions forming the reduction products, TsNH2, and sodium chloride. The oxidation potential of chloramine-T−sulfonamide system is pH dependent and decreases with an increase in pH of the system (1.14 V at pH 0.65 and 0.5 V at pH 12).32 CAT behaves as different types of reactive species in solution.33,34 −

+

TsNClNa ⇌ TsNCl + Na

(i)

Ka TsNCl− + H+ HooI TsNHCl

(ii)

(2)

The total concentration of CAT is given by [CAT]t = [TsNH2Cl+] + [TsNHCl] + [complex]

(3)

Solving for [TsNH2Cl+], [TsNHCl] from equilibrium (i) and (ii) of Scheme 1 can be derived as follows: [TsNH2Cl+] =

[TsNHCl] =

(CAT) (where Ts represents the CH3C6H4SO2-group). 4821

[TsNHCl][H+] K1

[complex] K2[LF]

(4)

(5)



Article

Scheme 2. A Detailed Mechanistic Interpretation of the Oxidation of LF by CAT in Acidic Medium

Substituting eq 4 and 5 in eq 3 we get [complex] =

Or

K1K2[CAT]t [LF] +

[H ] + K1{1 + K2[LF]}

1 K obs

(6)

K K k[CAT]t [LF] −d[CAT] = +1 2 dt [H ] + K1{1 + K2[LF]}

(7)

Equation 7 explains all the experimentally observed orders, wherein a first-order dependence of rate on [CAT], a fractional-order with [LF], and fractional-order inverse in [H+] have been observed. Since Rrate = kobs [CAT]t, the rate law eq 7 can be rearranged to eq 8, which is suitable for verification. K obs =

K1K2k[LF] +

[H ] + K1{1 + K2[LF]}

⎧ [H+] ⎫ 1 1 ⎨ ⎬+ K2k[LF] ⎩ K1 + 1 ⎭ k

(9)

Equation 9, indicates that the linear plots of 1/kobs vs 1/[LF] and 1/kobs vs [H+] were obtained with a straight line and positive intercept on the y-axis (Figure 3). This proves the validity of rate law 7, and the proposed reaction scheme has been derived. Similarly k, K1, and K2 were calculated at different temperatures (Table 2). Since the rate was fractional order in [LF], the MichalisMenten type kinetics36 was adopted to study the effect of [LF] on rate at different temperatures. Using the calculated kobs values, activation parameters for the rate-limiting step were computed from the Arrhenius plot (Table2). Van’t Hoff plots were made (log K1 versus 1/T and log K2 versus 1/T). The ΔH, ΔS, and ΔG were calculated for the first and second equilibrium steps (Scheme 1). The change in dielectric constant of the medium has been made by the addition of acetic acid in the reaction mixture. Before conducting experiments for the study of the effect of dielectric constant

Substituting eq 6 in eq 2, the following rate law can be obtained: rate =

=

(8) 4822



Article

Figure 3. Verification of rate law eq 9 at a different [H+] and [LF] for the oxidation of LF by CAT at 25 °C.

Figure 4. Effect of ionic strength and dielectric constant for the oxidation of LF by CAT in acidic medium at 25 °C.

of the medium on the rate of reaction, we performed experiments taking acetic acid as an organic substrate instead of LF in the usual manner and found that acetic acid under the condition of our experiments is not oxidized. It is clear from Table 3 that there is a decrease in dielectric constant (D) with

parameters (Table 2). The lower energy of activation and high free energy of activation support the formation of highly solvated transition state. The negative value of ΔS⧧ also supports the proposed mechanism and indicates the formation of a transition state fairly rapidly with a lower degree of freedom.40 Further, the experimental observation shows that there is no effect of TsNHCl and ionic strength on the reaction rate, which also substantiates the proposed mechanism.

Table 3. Effect of Ionic Strength and Dielectric Constant on the Oxidation of LF by CAT in Acidic Medium at 25 °C ionic strength I, (D = 70.0)

5. CONCLUSION The kinetics of oxidation of LF with CAT in HClO4 medium has been studied. The oxidation products were identified [(3S)7-fluoro-3-methyl-8-(4-methylpiperazin-1-yl)-3,4-dihydro-2H1,4-benzoxazin-5-yl](oxo)acetic acid by spot test, FT-IR, and NMR. The active oxidizing species involved in the acid medium is TsNHCl. The rate constant of the slowest step and other equilibrium constants involved in the mechanism are evaluated, and activation parameters with respect to slowest step were computed. The overall mechanistic sequence described here is consistent with product, mechanistic, and kinetic studies.

dielectric constant D, (I = 0.5 mol dm−3)

I (mol dm−3)

kobs × 103

D

kobs × 103

0.1 0.3 0.5 0.7 0.9

1.25 1.29 1.36 1.45 1.65

70.0 65.0 60.0 55.0 50.0

1.63 1.71 1.76 1.89 1.92

an increase of the value of the rate constant. The dependence of the rate constant on the dielectric constant of the medium is given by the following equation:

■

ZAZBe 2N 1 log k obs = log k0 − × 2.303(4πε0)dABRT D

ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra of the product of oxidation of LF by CAT. This material is available free of charge via the Internet at http://pubs.acs.org.

where ko is the rate constant in a medium of infinite dielectric constant, ZA and ZB are the charges of reacting ion, dAB refers to the size of activated complex, T is absolute temperature and D is dielectric constant of the medium. This equation shows that if a plot is made between log kobs versus 1/D, a straight line having slope equal to −ZAZB and e2N/(2.303(4πεo)dABRT) will be obtained. The effect of solvent on the reaction kinetics has been described.37,38 The decrease in first-order rate constant with the increase in dielectric constant (D) of the medium was also evident from the plot of log kobs vs 1/D (Figure 4). The plot of log kobs versus 1/D was linear, having positive slope. This clearly supports the involvement of an ion−dipole system in the rate limiting step in the proposed mechanism. The value of dAB has been evaluated with the help of a slope of a straight line and found to be 2.32. According to the rate determining step in Scheme 1, the change in the ionic strength of the medium does not alter the reaction rate, which suggests the involvement of nonionic species at the rate-determining step. The negative dielectric constant effect in the present system supports the rate determining step of the mechanism.39 The proposed mechanism is also supported by the moderate values of energy of activation and other thermodynamic

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

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