Equilibrium and Kinetics Studies of Reactions of Manganese Acetate, Cobalt Acetate, and Bromide Salts in Acetic Acid Solutions

摘要

The oxidation of hydrogen bromide and alkali metal bromide salts to bromine in acetic acid by cobalt(III) acetate has been studied. The oxidation is inhibited by Mn(OAc)_2 and Co(OAc)_2, which lower the bromide concentration through complexation. Stability constants for Co~(II)Br_n were redetermined in acetic acid containing 0.1% water as a function of temperature. This amount of water lowers the stability constant values as compared to glacial acetic acid. Mn~(II)Br_n complexes were identified by UV-visible spectroscopy, and the stability constants for Mn~(II)Br_n were determined by electrochemical methods. The kinetics of HBr oxidation shows that there is a new pathway in the presence of M~(II)Br_n. Analysis of the cincentration dependences shows that CoBr_2 and MnBr_2 are the principal and perhaps sole forms of the divalent metals that react with Co(III) and Mn(III). The interpretation of these data is in terms of this step (M,N = Mn or Co): M(OAc)_3 + N~(II)Br_2 + HOAc -> M(OAc)_2 + N~(III)Br_2OAc. The second-order rate constants (L mol~(-1)s~(-1)) for different M,N pairs in glacial acetic acid are 4.8 (Co,Co at 40 deg C), 0.96 (Mn,Co at 20 deg C), 0.15 (Mn(III) centre dot Co(II), Co at 20 deg C), and 0.07 (Mn,Mn at 20 deg C). Following that, reductive elimination of the dibromide radical is proposed to occur: N~(III)Br_2OAc + HOAc -> N(OAc)_2 + HBr_2~(centre dot). This finding implicates the dibromide radical as a key intermediate in this chemistry, and indeed in the cobalt-bromide catalyzed autoxidation of methylarenes, for which some form of zerovalent bromine has been identified. The selectivity for CoBr_2 and MnBr_2 is consistent with a pathway that forms this radical rather than bromine atoms which are at a considerably higher Gibbs energy. Mn(OAc)_3 oxidizes PhCH_2Br, k = 1.3 L mol~(-1)s~(-1) at 50.0 deg C in HOAc.