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Journal of the Chilean Chemical Society
versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.52 n.1 Concepción mar. 2007
http://dx.doi.org/10.4067/S0717-97072007000100011
J. Chil. Chem. Soc, 52, Nº 1 (2007)
KINETICS AND MECHANISM OF OXIDATION OF 6-AMINOCAPROIC ACID BY DIHYDROXYDIPERIODATOA RGENTATE (III) IN ALKALINE MEDIUM
SHUYING HUO*, CHANGYING SONG, SHANJIN HUAN, SHIGANG SHEN, HANWEN SUN
Key Laboratory of Analytical Science and Technology of Hebei Province, College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China
ABSTRACT
The oxidation of 6-Aminocaproic acid by Ag(III) complex, dihydroxydiperiodatoargentate(III) (DPA), was studied in alkaline medium. The reaction was first order with respect to Ag(III) complex, and the order with respect to 6-aminocaproic acid was from one to two. A plausible mechanism involving a pre-equilibrium of adduct formation between the complex and reductant was proposed form the kinetics study. The rate equations derived from mechanism can explain all experimental phenomena, and the activation parameters along with rate constants of the rate-determining step were calculated.
Keywords: dihydroxydiperiodatoargentate (III), 6-aminocaproic acid, redox reaction, kinetics and mechanism
INTRODUCTION
6-Aminohexanoic acid(EACA) is a kind of hemostasis drugs.[1] It can restrain the plasminogens activeted enzyme, then the plasminogen cannt be converted to fibrinolytic enzyme, and the fibrin can not be dissolved easily. So 6-aminohexanoic acid has a function of hemostasis, and it can be used as a hemostasis drugs in the medicine. When the drug was used to the human body, the side-effect must be study, and the drugs metabolizability must be studied in the human body, because the metabolizable product of the drugs can cause the side-effect to the human. So 6-aminohexanoic acid was used as a hemostasis drug to human, its metabolizability in the human body must be researched. 6-aminohexanoic acid has similar structure to amino acid, just the -NH2 group has different position to the COOH group, so the NH2 can break away from 6-Aminohexanoic Acid through the metabolizable proceed in the body, which is similar to the metabolizability of amino acid in the body.
So oxidation deamination reaction would be occurred in the body. To these thinking above, the paper studied the kinetics of oxidation of 6-Aminohexanoic Acid.
Ag(III), Cu(III), Ni(IV) complexes can be used as an oxidation reagents in organic chemistry and analytical chemistry.[2,3] As a kind of oxidation reagents, those complexes have been used widely in kinetic study.[4]The oxidation of glutamic acid by Ag(III) complexes has been studied in our previus work, through the kinetics study the mechanism of the oxidation reaction has been proposed.[5] In the reaction, the NH2 break away from the glutamic acid, and the RCH2NH2COO was oxidated to RCOCOO . Then it just like the metabolizability of the amino acid in the human body.
So it can be used as an oxidable model in this kind of oxidation reaction, which is similar to the metabolizability of the amino acid in the human body.[6] In this paper, 6-aminohexanoic acid was oxidated by Ag(III) complex, and the kinetics and mechanism of the oxidation reaction was studied.
EXPERIMENTAL SECTION
Materials
All the reagents used were of A.R. grade. All solutions were prepared with doubly distilled water. 6-aminohexanoic acid was changed to Potassium Acid 6-Aminohexanoic by adding KOH solution, molar rato of 6-Aminohexanoic acid and KOH is 1:1, Solution of [Ag(OH)2(H2IO6)2]5-(DPA) was prepared and standardized by the method reported earlier.[7] Its UV spectrum has a characteristic absorption band at 362nm, which was found to be consistent with the reported. The concentration of DPA was derived by its absorption at ?=362nm. Solution of DPA was always freshly prepared before use with solution and doublydistilled water. The ionic strength µ was maintained by adding KNO3 solution and the pH value of the reaction mixture was regulated with KOH solution.
Apparatus and Kinetics Measurements
All kinetics measurements were carried out under pseudo-first order conditions. Solution (2 mL) containing definite [Ag (III)] [OH ][IO4 ] and ionic strength µ and reductant solution (2mL) of appropriate concentration were transferred separately to the upper and lower branch tubes, it is a type two-cell reactor. After it was thermally equilibrated at desired temperature in thermobath (made in China), the two solutions were mixed well and immediately transferred to a 1 cm thick rectangular cell quartz with a constant temperature cell-holder (±0.1K). The reaction process was monitored automatically by recording the disappearance of Ag(III) with time (t) at 362 nm with a TU-1901 spectrophotometer (made in China). All other species absorb significantly at this wavelength.
Product Analysis
-Solution having known concentrations of [Ag (III)] and [OH ] was mixed with an excess of 6-aminohexanoic acid, The completion of the reaction was marked by the complete discharge of Ag (III) color.[8] After completion of the reaction, the products of oxidation were identified as ammonia and the corresponding aldehyde acid by their characteristic spot test.[9]
RESULTS AND DISCUSSION
Evaluation of Pseudo-First Order Rate Constants
Under the conditions of [Reductant]0>>[Ag(III)]0, the plots of ln(At-A8) versus time are linear, indicating the reaction is first order with respect to [Ag(III)], where At and A8 are the absorbance at time t and at infinite time respectively. The pseudo-first-order rate constants kobs were calculated by the method of least squares (r=0.999). To calculate kobs generally 8-10 At values within three times the half-life were used. kobs values were at least averaged values of three independent experiments and reproducibility is within ±5%.
Rate Dependence on 6-Aminohexanoic Acid
At fixed [Ag(III)] [OH ] [IO4 ] ionic strength µ and temperature. k obs values increased with the increase in [EACA] and the order in the concentration of [EACA] was found to be 1< nap <2, here nap means the apparent reaction order. The plots of [EACA]/kobs versus1/ [EACA] are straight line with a positive intercept (Table 1,
-Rate Dependence on [OH ]
-At constant [Ag(III)], [EACA], [IO4 ], ionic strength µ and temperature, kobs values decreased with the increase in [OH ], as the [OH ]=0.06mol/L, -the rate is the lowest, after this, the rate increased with the increase in [OH ]. ( Table 2, Figure 2)
Rate Dependence on [IO4 ] and Ionic Strength \i
At constant [Ag(III)], [EACA], [OH ], ionic strength µ and temperature, ^obs values decreased with the increase in [IO4 ] and the plots of 1/£obs versus [IO4 ] were lines(Table 3, Figure 3). The rate changed slightly with the addition of KNO3 solution, so it indicated that there was no salt effect. (Table 4)
DISCUSSION OF THE REACTION MECHANISM
In aqueous periodate solution, equilibria (1)-(3) were detected and the corresponding equilibrium constants at 273.2K were determined by Aveston.[10]
The distribution of all species of periodate in alkaline solution can be calculated from equilibria (1)-(3). In the [OH ] range used in this work the dimer and IO4 species of periodate can be neglected (H2IO63-: H3IO62-:
Here eand t mean the equilibrium concentration of the all kinds of Ag(III) complexes and the total concentration of all kinds of Ag(III) complexes respectively The reaction (8) is the rate-determining step then the rate equation can be expressed as below:
In the paper, Equation (11), (18) was given, which can explain &0hs values decreased rapidly with the increase in [OH 1 up to 0.06M. After the point it increased gradually with the continuous increase in [OH 1. Equation(20) show that the plots of l/&0bs versus [IO4 ] should also be linear. The Equation(18) and Equation(19) show that the order in 6-Aminohexanoic Acid should be l<ncr\<2 and [EACA] /Avyhc versus 1/TEACA] should be linear The rate equations derived from the reaction mechanisms are consistent with our experimental results and the rate-determining step constants (k) at different temperatures were evaluated from Equation(20) and the activation energy and the thermodynamic parameters were evaluated by the method given earlier.[11]
ACKNOWLEDGE:
The Project Supported by Science Foundation of Hebei University (2005Q12).
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(Received: june 23, 2006 - Accepted: December 18, 2006)