Studies on the breakdown of organoselenium compounds in a hydrobromic acid–bromine digestion system

Abstract

The chemical role played by bromine in HBr solution was investigated in the breakdown of both the trimethylselenonium ion and selenomethionine. Hydride generation and gas chromatography, both coupled with atomic fluorescence detection, polarography, and ¹H and ⁷⁷Se nuclear magnetic resonance spectrometry, were employed in order to collect information on the intermediate and final selenium compounds. Trimethylselenonium and selenomethionine were quantitatively converted into inorganic Se^IV in hot concentrated HBr, but only in the presence of a large excess of bromine with respect to organoselenium. In the absence of additional bromine and under a stream of inert gas, the digestion of trimethylselenonium resulted in a 95% loss of selenium from the solution. Dimethyl diselenide and dimethyl selenide were detected in the headspace gases of the digestion apparatus. Under the same conditions, selenomethionine gave a result of about 70% of selenium caught in the solution. In this instance experimental evidence indicated that the major selenium component in the headspace gases of the digestion apparatus was methaneselenol. The roles played by bromine and HBr in the conversion of organoselenium into inorganic Se^IV were identified as (i) the oxidative addition of bromine to divalent selenium to form bromoselenonium intermediates and (ii) the dealkylation of selenonium compounds by the bromide ion.