Collect. Czech. Chem. Commun. 1980, 45, 2481-2488

Mechanism of oxidation of Cr(II) ions by chloroform, bromoform and iodoform

Peter Ševčík and Peter Cifra

Department of Physical Chemistry, Comenius University, 816 31 Bratislava


The kinetics and mechanism of oxidation of Cr(II) ions by haloforms were studied in H2O-CH3OH-HClO4 system, in the case of bromoform also in H2O-CH3COOH-HClO4 system. Six mol of Cr2+ ions is consumed in the oxidation-reduction reaction per one mol of CHX3, the final organic product is methane. The primary Cr(III) products are CrX2+ ions and [CrCHX22+] organochromic complexes. The oxidation-reduction reactions are 1st order with respect to each reactant, the rate constants of the 2nd order reaction, independent of the H+ ion concentration and the ionic strength, decrease in the order kI > kBr > kCl. The inner-sphere mechanism is suggested for the electron transfer; the effect of the C-X bond cleavage is more significant than the effect of the Cr-X bond formation and controls the rate of the halogen atom transfer from the haloform to Cr(II).