Electron transfer. 102. Some oxidations by bromite (BrO2−)

✍ Scribed by Amrit K. Jhanji; E. S. Gould

• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 465 KB
• Volume: 23
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550230305

✦ Synopsis

Abstract

Bromite, BrO~2~^−^, in aqueous carbonate or borate buffers (pH 9–11), reacts rapidly with H~3~AsO~3~, with the tartrate complex of Sb(III), and with the EDTA complexes of Sn(II) and U(IV). Each reaction yields Br^−^, even when BrO~2~^−^ is taken in excess. Reaction rates with Sb(III) and Sn(II) are independent of pH, but the acidity pattern observed with As(III) points to competing bimolecular reductions by H~2~AsO~3~^−^ and H~3~AsO~3~ having specific rates 3.4 and 1.2 M^−1^ s^−1^ (25°C, μ = 0.5__M__). Reduction by U(IV) is strongly inhibited by carbonate; data for carbonate buffers are in accord with the operation of two contributing reaction paths (specific rates 0.45 and 6.3 M^−1^ s^−1^) with the activated complex for the slower featuring one more carbonate than that for the rapid. Reactions of BrO~2~^−^ with single electron reductants, or with reagents that can undergo both 1e and 2e changes, are generally complicated by variable induction periods, inhibition by O~2~, catalyzed decomposition of BrO~2~^−^, or combinations of these. The much smoother reductions by 2e^−^ reagents must pass through OBr^−^, which reacts much more rapidly than BrO~2~^−^ in each case. An inner‐sphere path, operating through an oxygen‐bridged transition state and culminating in oxygen atom transfer, is proposed for the 2e reductants. Hypophosphite, despite its highly negative formal potential, is not oxidized by BrO~2~^−^ under our conditions. Here it is likely that the substitution‐inert character of the coordinatively saturated P(I) center rules out formation of the precursor required for redox.