Mechanism of the Oxidation of a Cobaloxime by Bromine and Sodium Hypochlorite in Aqueous Media

Undergraduate #155
Discipline: Chemistry and Chemical Sciences
Subcategory: Chemistry (not Biochemistry)

Lorne S. Joseph - University of the Virgin Islands
Co-Author(s): Michael Celestine and Alvin Holder, Old Dominion University, Norfolk, VA

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A detailed investigation of the oxidation of [Co(dmgBF2)2(OH2)2] by NaOCl and Br2 has been carried out using stopped-flow spectrophotometry at 450 nm, and the reaction was monitored for the peak’s disappearance in the presence of the oxidants under pseudo-first order conditions (1.0 ≤ [NaOCl] ≤ 10.0 mM and 0.5 ≤ [Br2] ≤ 4.0 mM) with a constant ionic strength maintained with NaCl. The main reaction products are [CoIII (dmgBF2)2(OH)2]- and [CoIII(dmgBF2)2(OH2)(OH)] in the presence of Br2 and NaOCl, respectively. The reaction rate for the Br2 study has been observed to be dependent on pH which was observed to increase as the acidity increased. At 10 °C the rate was observed to be first order with respect to the Br2 concentration whereas it was observed to be second order with respect to the NaOCl concentration. An equilibrium constant and rate constant for the oxidation of the complex was observed in the Br2 studies where K = 3.45 x 102 M-1 and k = 6.63 x 102 s-1. For the NaOCl study two rate constants were observed, k1 = 7.56 s-1 which corresponds to a reaction pathway independent of the NaOCl concentration and k2 = 2.26 x 104 M-1s -1 which is dependent on the concentration of the oxidant. In conclusion it was observed that in the presence of the two oxidants there is a fast electron transfer that occurs followed by a very slow substitution of the axial ligands on the cobalt (III) metal center.

Funder Acknowledgement(s): MBRS-RISE (Minority Biomedical Research Support Research Initiative for Scientific Enhancement) Grant # 5R25GM061325 and the National Science Foundation under grant number CHE-1431172 (formerly CHE-1151832).

Faculty Advisor: Alvin A. Holder,