Asian Journal of Chemistry Asian Journal of Chemistry
Total Page：16
File Type：pdf, Size：1020Kb
Asian Journal of Chemistry; Vol. 30, No. 9 (2018), 2103-2106 ASIAN JOURNAL OF CHEMISTRY https://doi.org/10.14233/ajchem.2018.21463 Kinetics of Oxidation of Atropine by K2Cr2O7 in Acidic Aqueous Solutions 1 1,2,* 1 HAMZEH M. ABDEL-HALIM , ABDULLAH I. SALEH and SANAD K. AL-GHREIZAT 1Department of Chemistry, Faculty of Science, The Hashemite University, Az-Zarqa, Jordan 2College of Pharmacy, Al Ain University of Science & Technology, Al Ain, UAE *Corresponding author: E-mail: [email protected] Received: 21 May 2018; Accepted: 16 July 2018; Published online: 31 July 2018; AJC-19028 Oxidation of atropine (ATN) by K2Cr2O7 has been studied kinetically in aqueous strongly acidic solutions. The kinetics of this reaction was investigated at different temperatures. A constant pH and constant ionic strength were maintained constant throughout all measurements. The concentration of atropine was around an order of magnitude greater than that of K2Cr2O7, that is under pseudo first order kinetics. Rate constants were obtained by monitoring change in absorbance of K2Cr2O7 with time at its predetermined maximum wavelength. Overall rate constant and the orders of the reaction in terms of concentrations of both atropine and K2Cr2O7 were determined. Reaction runs at different temperature allows activation energy of the process to be convoluted from temperature effect on the rate of the reaction. A resonable mechanism for the reaction was proposed in accordance with kinetics results. Keywords: Kinetics, Atropine, K2Cr2O7, H2CrO4. INTRODUCTION Chromium(VI) is known to be mutagenic and carcino- genic, hence, posing a serious environmental hazard [15]. It is Reaction kinetics of transition metal complexes by various used for many applications [16-25]. Consequently, it is of crucial reducing agents has been widely studied. Different methods importance gaining as much information as possible regarding were employed for kinetics measurements. Methods including: its various forms and their interaction with organic substrates. stopped-flow spectrophotometry, chemical analysis of products and the use of radioactive and stable isotope tracers. Details of these methods along with much of data produced are given H3C elsewhere [1]. Of a special interest and relation to the present N work, several papers have been published on kinetics study OH of oxidation by potassium dichromate in its different forms. · H SO .H O In particular, kinetics of oxidation of various substrates by O 2 4 2 chromic acid has been reported [2-12]. Reaction kinetics of Ph oxidation of atropine by alkaline potassium permanganate has O been investigated recently [13]. However, detailed mechanistic 2 study of oxidation of atropine by powerful oxidizing reagents Fig. 1. Atropine sulfate monohydrate including potassium dichromate in appropriate pH’s are still limited. Potassium dichromate in strongly acidic media can be 2– There are several medicinal uses of atropine. In the warfare, present in the form of chromate ion (CrO4 ), bichromate ion – atropine in the salt form as atropine sulfate monohydrate (ASM), (HCrO4 ) and chromic acid (H2CrO4). Which species presumed Fig. 1, mixed with pralidoxime chloride, 2-PAM chloride, is present in aqueous media is a function of pH and ionic strength commonly used as an antidote for poisoning by organophosphate and largely still an open discussion subject [21]. However at insecticides and nerve gases [14]. the Cr(VI) concentration used herein, the chromate ion is This is an open access journal, and articles are distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) License, which allows others to copy and redistribute the material in any medium or format, remix, transform, and build upon the material, as long as appropriate credit is given and the new creations are licensed under the identical terms. 2104 Abdel-Halim et al. Asian J. Chem. – absent [22]. It is also demonstrated that the HCrO4 species is In a strongly acidic solution, a condition of this study, atropine spectrophotometrically undetectable in the UV-visible region. sulfate monohydrate is converted to atropine and K2Cr2O7 is That will leave chromic acid as the predominant and might be converted to H2CrO4. Diode Array Spectrophotometer model the only detectable species under the highly acidic conditions 8453E from HP Agilent Technologies was used. Reactions of this study. were monitored by measuring the absorbance of chromic acid In the present work and as an extension of our previous with time at its wavelength of maximum absorbance difference, kinetics studies on biologically important molecules [13], a λmax. This wavelength was predetermined from the absorption mechanistic aspect of oxidation reaction of atropine by chromic spectra for chromic acid alone and for its mixture with atropine acid has been investigated. The kinetics of the reaction has sulfate monohydrate after completion of the reaction. Fig. 2 been determined spectrophotometrically. Reaction rates have shows a scan at different time intervals for reactants and products, been determined by monitoring the decrease of absorption of from which λmax was determined to be at 350 nm. Absorbance Cr(VI) at its absorption maximum, λmax = 350 nm. It was verified at this wavelength is mainly due to chromic acid. However that at this wavelength there are no interferences from either absorbance at this wavelength of organic compounds, reactant the reactants, products or any other reagent involved. Scheme-I or products is negligible. represents the overall reactions involved in this study. 1.000 KCrO EXPERIMENTAL 227 0.900 2 min 0.800 4 min The following set of preliminary experimental runs was 6 min designed to sort out the different reactions taking place under 0.700 8 min the strongly acidic conditions used herein. 0.600 10 min 12 min 0.500 Acidic hydrolysis: Atropine sulfate monohydrate (0.10 14 min M) was treated with concentrated H2SO4 (1.0 M) solution for 0.400 16 min Absorbance 25 min. Products were then isolated, purified and characterized 0.300 18 min 20 min by various NMR and IR spectroscopic techniques. There is 0.200 22 min apparently no hydrolysis to any measurable extent under reaction 0.100 24 min conditions used in this study. 0 Cr(VI) in strongly acidic medium: A blank run of mixing 300 400 500 600 700 800 Wavelength (nm) all ingredients [K2Cr2O7 (0.001M), H2SO4 0.10M)] except Fig. 2. Absorbance vs. λ for K2Cr2O7 and for the reaction mixture; λmax = atropine sulfate monohydrate, has shown that the change in -3 -2 350 nm [K2Cr2O7] = 1.00 × 10 M, [ATN] = 1.00 × 10 M absorption by H2CrO4 is negligible throughout the measuring period. No spectrophotometrical evidence was obtained for the Reaction rates were determined under pseudo-first order presence of any other species apart from chromic acid. condition, The concentrations of atropine [10–2 to 10–1 mol Acidic K2Cr2O7 oxidation: Approximately 10 folds in –3 dm ] were an order of magnitude greater than those of K2Cr2O7 excess of atropine sulfate monohydrate solution (0.01 M) was [10–4 to 10–2 mol dm–3]. All runs were done at constant pH and treated with the acidic K2Cr2O7 (0.001 M) solution for 25 min. ionic strength (ionic strength was maintained using NaClO4). Spectroscopic data of the products have shown the presence of Rates were also determined at various temperatures. The temp- Scheme-I excess atropine and the oxidized product ( ). A radical erature for each run was thermstatically controlled within ± involvement in this reaction was ruled out based on the tests 0.1 ºC. of adding mercuric chloride or acrylonitrile monomer to the reaction. Neither induced a white precipitate formation or RESULTS AND DISCUSSION polymerization, respectively [17-19]. As a result, it is presumed that the reaction under investi- Chromium(VI) in strongly acidic medium exists in the gation is not accompanied by hydrolysis of atropine. No species form of chromic acid H2CrO4 [17]. Atropine on the other hand other than chromic acid has an absorbance at the selected will exist in the hydrated form. The UV/visible absorption wavelength throughout the course of the reaction. spectrum taken directly after mixing atropine with Cr(VI) in a Kinetics measurements: Different concentrations of atro- H2SO4/H2O medium shows an absorbance band at 350 nm and a shoulder at 420–500 nm, characteristic of Cr(VI) in acidic pine sulfate monohydrate and K2Cr2O7 were used in this study. medium (Fig. 2). The absorbencies at 350 nm and 420–470 + - Fast H O + Cr O 3 2 7 H2O + 2H2CrO4 OH OH CHO - H C 3 H3C H3C O O O O N H2CrO4 N N III + Cr H2O O O + O + 1 equivalant HO OH Atropine excess atropine ~ 10 equivalents Scheme-I: Overall reactions involved Vol. 30, No. 9 (2018) Kinetics of Oxidation of Atropine by K2Cr2O7 in Acidic Aqueous Solutions 2105 nm decay with time, while the absorbance at 570 nm increases. were carried out for each concentration and the average value The observation of isosbestic point (centered at λ = 520 nm) of the observed rate is reported. Experimental errors in value indicates that the stoichiometry of the reaction remains un- of rate constant are estimated to be ~ ± 10 %. changed during the chemical reaction and that no competing The observed rates constant of the reaction (kobs) at various 2– reactions occur during the considered time range. At the end concentrations of Cr2O7 and constant [ATN], with [ATN] 〉〉 2– of the redox reaction, the bands from 520-700 nm indicates [Cr2O7 ] are shown in Table-1. the formation of Cr(III) product [20]. It is well-established that both Cr(V) and Cr(IV) species absorb at 350 nm, consequently, TABLE-1 Cr(V)/Cr(IV) contributions have to be taken into account when KINETICS RESULT FOR THE OXIDATION OF ATROPINE BY K2Cr2O7 AT CONSTANT [ATN]; interpreting the absorbance decay [21].