Kinetics and Mechanism of Oxidation of L‐Cysteine by Bis‐3‐di‐2‐pyridylketone‐2‐thiophenylhydrazone‐iron(III) Complex in Acidic Medium

Abstract

The kinetics of oxidation ofL-cysteine by 3-di-2-pyridylketone-2-thiophenylhydrazone-iron(III), [Fe(DPKTH)2]3+complex in acidic medium was studied spectrophotometrically at 36 °C temperature. The molar ratios of DPKTH to iron(III) and iron(II) individually, were found to be [2:1] [DPKTH : iron(III)/(II)]. The reaction was stroked to be first-order with respect to iron(III) andL-cysteine, second-order with respected to DPKTH ligand and reversed second-order with respected to hydrogen ion concentration. Added salts did not affect the rate and no free radical was detected when radical detector was placed in the reaction mixture. Ethanol solvent ratio was found to effect both the initial rate and the maximum absorbance (λmax) of [Fe(DPKTH)2]2+complex. The initial rate rose when the temperature was increased which empowered to calculate the activation parameters. A suitable reaction mechanism was proposed.

DETERMINATION OF THE DISSOCIATION CONSTANTS OF SOME p-SUBSTITUTED AROMATIC HYDRAZONES

“…This band is a result of a low energy n→π * electron transition of the azometine group, and characterizes the neutral form of the compound. When the pH value of the solution increases (up to pH 9), the absorption band of the hydrazone H 4 shifts towards longer wavelengths (around 320 nm) and its intensity decrease ( Fig. 1).…”

“…The purity of the hydrazones is tested by measuring of melting point, as well as, by the data of elemental analysis. The other chemicals (NaOH, NaClO 4 and ethanol) were of analytical grade p.a. (Alkaloid), and they are used without further purification.…”

“…The spectra are recorded at ionic strength of 0.1, 0.25 and 0.5 mol dm -3 . The obtained spectra at ionic strength of 0.1 mol dm -3 for the hydrazones H 4 and H 5 are shown on the Fig. 1.…”

“…This dependence has sigmoidal form (,,S" curve) and it allows evaluating the pH region in which the dissociation process takes place. The ,,S" curves obtained for the hydrazones H 4 (reconstructed spectra)…”

“…Some hydrazones are used as quantitative analytical reagents in colorimetric, fluorimetric [1,2], and especially in spectrophotometric determinations of metal ions, because they act as multidentate ligands with transition metal ions forming colored chelates [3]. Hydrazone complexes have been of interest for many authors from aspect of coordination chemistry [4]. The knowledge of behavior of hydrazones in solution with different acidity is important in order to achieve higher sensitivity and selectivity for the metal determination [5].…”