A study of mercury(II)-catalysed substitution of cyanide in hexacyanoferrate(II) by 2-methylpyrazine: effects of inhibitor on the catalysed reaction

Abstract

The kinetics and mechanism of the mercury(II)-catalysed ligand substitution reaction between potassium hexacyanoferrate(II) and 2-methylpyrazine (2-Mepz), in potassium hydrogen phthalate buffer of pH 3.50 ± 0.02 at 25.0 ± 0.1°C and constant ionic strength of 0.1 M, were studied spectrophotometrically by measuring the absorbance of the yellow-coloured product at 447 nm (k max ). The reaction was studied under pseudofirst-order conditions using excess of 2-Mepz. The reaction was found to obey first-order dependence in concentration of both [Fe(CN) 6 ] 4-and 2-Mepz. At higher concentrations of 2-Mepz, the effect of [2-Mepz] levels off. A complex behaviour is observed on varying [Hg 2? ]. The effect of pH on the reaction rate was also analysed. The effects of dielectric constant and water content of the reaction medium have been interpreted in terms of the formation of a polar activated complex and suggest an I d mechanism. The inhibitory effect of 2,3-dimercaptopropanol on the reaction was also studied and explained in terms of binding of this species to the catalyst.

A kinetic method for the determination of organosulfur compounds by inhibition: determination of cysteine, 2,3-dimercaptopropanol and thioglycolic acid

“…= 1 9 10 -7 -20 9 10 -7 M and 1 9 10 -6 -10 9 10 -6 M, after this the rate further increased on [Hg 2? ] up to 7.5 9 10 -5 M and then it fell [26].…”

“…The logarithmic plot (not shown) showed a complicated relationship between [Fe(CN) 6 4-] and reaction rate, but no tendency toward zero order. The relationship between rate and [Fe(CN) 6 4-] has been examined in detail in a previous paper [26]. For further work, a concentration 2 9 10 -3 M of [Fe(CN) 6 4-] was chosen.…”

“…Hg(II) complexes with sulfur-containing ligands are generally very stable [25]. I have previously reported that replacement of cyanide ion in hexacyanoferrate(II) by 2-Mepz is catalyzed by Hg(II) [26].…”