Kinetics and mechanism of oxidation by metal ions: Part XV1―Oxidation of glyoxalate ion by alkaline Osmium tetroxide

Abstract

The kinetics of oxidation of glyoxalate ion by alkaline osmium (VIII) is studied with stopped-flow in the presence of excess of glyoxalate ion. A linear correlation between (a) <em>k</em><img src='http://www.niscair.res.in/jinfo/equation85.gif' border=0> and [OH^-], and (b) <em>k</em>_obs and [CHO.COO^-] is observed where Kobs is the observed pseudo- first order rate constant. The rate determining step is the decomposition of, probably, an inner sphere complex [OsO₃(OH)₃(OH)₃C.COO]^3- formed between [OsO₄(OH)₂]^2- and (OH)₂CH.COO^-. The formation of the complex is inferred by the rapid scanning of the spectrum of the reaction mixture. The kinetically deducted value of the equilibrium constant <em>K</em>_ha<em>, </em>for the deprotonation of the hydrated glyoxalate ion at 25ᵒC is 3.54 ± 0.34 dm³mol^-l. Using this value of <em>K</em>_ha<em>, </em>the <em>pK</em>_avalue of the gem-dial of glyoxalate ion is calculated to be 13.4 which is in good agreement with the <em>pK</em>_a value of 13.6 for formaldehyde. The thermodynamic parameters and activation enthalpy and entropy values are reported.