Equilibrium and kinetics studies of reactions of manganese acetate, cobalt acetate, and bromide salts in acetic acid solutions

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 (CoBrn)-Br-II 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. (MnBrn)-Br-II complexes were identified by UV-visible spectroscopy, and, the stability constants for (MnBrn)-Br-II, were determined by electrochemical methods. The kinetics of HBr oxidation shows that there is a new pathway in the presence of (MBrn)-Br-II, Analysis of he concentration dependences shows that CoBr2 and MnBr2 are the principal and perhaps Bole 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) + (NBr2)-Br-II + HOAc --> M(OAc)(2) + (NBr2OAc)-Br-III.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 degreesC), 0.96 (Mn, Co at 20 degreesC), 0.15 (Mn(III). Co(II), Co at 20 degreesC), and 0.07 (Mn, Mn at 20 degreesC). Following that, reductive elimination of the dibromide radical is proposed to occur: (NBr2OAc)-Br-III + HOAc --> N(OAc)(2) + HBr2.. 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 CoBr2 and MnBr2 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 PhCH2Br, k = 1-3 L mol(-1) s(-1) at 50.0 degreesC in HOAc.