OXIDATION OF DIMETHYLSELENIDE BY DELTA-MNO2 - OXIDATION-PRODUCT AND FACTORS AFFECTING OXIDATION RATE

Volatile dimethylselenide (DMSe) was transformed to a nonvolatile Se c ompound in a delta-MnO2 suspension. The nonvolatile product was a sing le compound identified as dimethylselenoxide based on its mass spectra pattern. After 24 h, 100% of the DMSe added to a delta-MnO2 suspensio n was converted to nonpurgable Se as opposed to 20%, 18%, and 4% conve rsion for chromate, permanganate, and the filtrate from the suspension , respectively. Manganese was found in solution after reaction. These results imply that the reaction between manganese oxide and DMSe was a heterogeneous redox reaction involving solid phase delta-MnO2 and sol ution phase DMSe. Oxidation of DMSe to dimethylselenoxide [OSe(CH3)(2) ] by a delta-MnO2 suspension appears to be first order with respect to delta-MnO2, to DMSe, and to hydrogen ion with an overall rate law of d[OSe(CH3)(2)]/dt = 95 M(-2) min(-1) [MnO2](1)[DMSe](1)[H+](1) for the MnO2 concentration range of 0.89 x 10(-3) - 2.46 x 10(-3) M, the DMSe concentration range of 3.9 x 10(-7) - 15.5 x 10(-7) M Se, and a hydro gen ion concentation range of 7.4 x 10(-6) - 9.5 x 10(-8) M. A general surface site adsorption model is consistent with this rate equation i f the uncharged \ OMnOH is the surface adsorption site. DMSe acts as a Lewis base, and the manganese oxide surface acts as a Lewis acid. DMS e adsorption to \ OMnOH can be viewed as a Lewis acid/base complex bet ween the largely p orbitals of the DMSe lone pair and the unoccupied e (g) orbitals on manganese oxide. For such a complex, frontier molecula r orbital theory predicts electron transfer to occur via an inner-sphe re complex between the DMSe and the manganese oxide.