© 2021 JETIR April 2021, Volume 8, Issue 4 www.jetir.org (ISSN-2349-5162) JETIR2104143 Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org 337 Oxidation of DL-methionine by quinolinium dichromate: Kinetics and mechanistic study Namrata Vyas 1 , Ammilal Rao 2 , Priyanks Purohit 3 * and Vinita Sharma 4 1 Research Scholar, 2 Research Scholar, 3 Assistant Professor, 4 Assistant Professor & Research Supervisor, 5 Sr. Professor Chemical Kinetics Laboratory, Department of Chemistry, J.N.V. University, Jodhpur (Rajasthan) 2-Department of Chemistry, University of Rajasthan, Jipur (India) Abstract The oxidation of methionine (Met) by quinolinium dichromate (QDC) in dimethylsulphoxide (DMSO) leads to the formation of corresponding sulphoxide. The reaction is of first order with respect to QDC. Michaelis-Menten type kinetics was observed with respect to methionine. The reaction is catalysed by hydrogen ions. The hydrogen-ion dependence has the form: kobs = a + b [H + ]. The oxidation of methionine was studied in nineteen different organic solvents. The solvent effect was analyzed by Kamlet’s and Swain’s multiparametric equations. Solvent effect indicated the importance of the cation -solvating power of the solvent. A suitable mechanism has also been postulated. Keywords: Dichromate, Kinetics, Mechanism, Methionine, Oxidation. 1. INTRODUCTION Cr(VI) salts of have long been used as oxidizing reagents in synthetic organic chemistry. However these salts are rather drastic in nature and non-selective oxidants. Further, they are insoluble in most of the organic solvents. Thus miscibility is a problem. To overcome these limitations, a large number of organic derivatives of Cr(VI) have been prepared and used in organic synthesis as mild and selective oxidants in non-aqueous solvents[1-4]. One of such compounds is quinolinium dichromate[5]. We have been interested in the kinetic and mechanistic aspects of the oxidation by complex salts of Cr(VI) and several studies have already been reported on the oxidation by halochromates and dichromates[6-10]. It is, known however, that mode of oxidation depends upon the nature of counter-ion attached to the chromium anion. Methionine (Met), a sulphur-containing essential amino acid, is reported to behave differently from other amino acids, towards many oxidants[11,12], due to electron-rich sulphur center which is easily oxidizable. There seems to be no report on the oxidation aspects of QDC. Therefore, in continuation of our earlier work by halochromates, we report here the kinetics of oxidation of DL-methionine by QDC in dimethylsulphoxide (DMSO) as solvent. A suitable mechanism has also been proposed. 2. MATERIALS AND METHODS 2.1 Materials QDC was prepared by the reported method 5 and its purity checked by an iodometric method. Methionime (Merck) was used as supplied. Due to non-aqueous nature of the solvent, toluene-p-sulphonic acid (TsOH) was used as a source of hydrogen ions. Other solvents were purified by the usual methods[13]. 2.2 Product Analysis Product analysis was carried out under kinetic conditions. The oxidation of Met by QDC resulted in the formation of corresponding sulphoxide, which was determined by the reported method[14]. The yield of sulphoxide was 943%. The oxidation state of chromium in completely reduced reaction mixtures, as determined iodometrically, was +4. 2.3 Kinetic measurements The pseudo-first order conditions were attained by maintaining a large excess ( 15 or more) of the Met over QDC. The solvent was DMSO, unless specified otherwise. The reactions were followed, at constant temperatures (0.1 K), by monitoring the decrease in [QDC] spectrophotometrically at 354 nm. No other reactant or product has any significant absorption at this wavelength. The pseudo-first order rate constant, kobs, was evaluated from the linear (r = 0.990 - 0.999) plots of log [QDC] against time for up to 80% reaction. Duplicate kinetic runs showed that the rate constants were reproducible to with in 3%. All experiments, other than those for studying the effect of hydrogen ions, were carried out in the absence of TsOH. The second order rate constant, k2, was evaluated from the relation k2 = kobs/[Met]. Simple and multivariate linear regression analyses were carried out by the least-squares method on a personal computer.