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

Abstract

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 concentration 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(2)OAc. 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 degrees C), 0.96 (Mn, Co at 20 degrees C), 0.15 (Mn(III).Co(II), Co at 20 degrees C), and 0.07 (Mn, Mn at 20 degrees C). Following that, reductive elimination of the dibromide radical is proposed to occur: N(III)Br(2)OAc + HOAc --> N(OAc)(2) + HBr(2)(*). 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(2)Br, k = 1.3 L mol(-)(1) s(-)(1) at 50.0 degrees C in HOAc.

Kinetics and Mechanism of Reactions between Methyl Aromatic Compounds and the Dibromide Radical

“…In our previous studies in this area, we presented data on the kinetics of oxidation of cobalt(II) and manganese(II), M II , by the same or a different trivalent metal, N III . 5 Put another way, the divalent metal is a catalyst for the oxidation of free bromide by N III , to the point that the catalytic pathway is nearly dominant. The rates of these reactions are dependent on the HBr concentration.…”

Assessing the occurrence of the dibromide radical (Br2−) in natural waters: Measures of triplet-sensitised formation, reactivity, and modelling

“…The ion Br − is usually the main • OH scavenger in seawater, where it prevails over dissolved organic matter (DOM) that is the main • OH sink in most freshwaters (Brezonik and Fulkerson-Brekken, 1998;Nakatani et al, 2007). Bromide oxidation by • OH produces the radical Br • , which in solution would mainly react with a further bromide ion to yield the dibromide radical, Br 2 −• (Neta et al, 1988;Jiao et al, 2001). Less important reactions of Br • , as far as environmental conditions are concerned, would take place with H 2 O, OH − and BrO − (Neta et al, 1988).…”

NMSBA produced from NMST under the catalysis of supported H3PW12O40 and Co/Mn/Br catalytic system

“…In terms of the free radical chain reaction mechanism, the formation of free radicals is beneficial to the oxidation reaction. Nevertheless, excess cobalt concentration may cause the decomposition of carboxylic groups to carbon oxides and reduce the selectivity . Therefore, there exists a best cobalt concentration.…”

“…Nevertheless, excess cobalt concentration may cause the decomposition of carboxylic groups to carbon oxides and reduce the selectivity. [25,26] Therefore, there exists a best cobalt concentration.…”