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Conclusive Evidence for Delayed Autocatalytic Behavior of Mn(II) Ions at a Critical Concentration

H. Bahrami^a,b and M. Zahedi^a,*

^aDepartment of Chemistry, Faculty of Sciences, Shahid Beheshti University, Evin, Tehran, Iran, 19839
^bInstitute of Biochemistry and Biophysics, University of Tehran, Tehran, Iran

The kinetics of the permanganic oxidation process of glycine, L-alanine and L-leucine in strong acid media were investigated using a spectrophotometric technique. Conclusive evidence has proven that the autocatalytic activity of Mn(II) in these reactions in strong acidic media is analogous to that of weak acid media, but in the former, Mn(II) ions should acquire a critical concentration for them to show autocatalytic characteristics. This critical concentration depends on the nature of the amino acid used. Considering the delayed autocatalytic behavior of Mn(II) ions, we herein present the rate equations and mechanisms satisfying observations for both catalytic and noncatalytic routes. The correspondence of the pseudo-order rate constants of the catalytic and noncatalytic pathways to Eyring law verify both the critical concentration as well as the delayed autocatalytic behavior concepts. In general, the onset of delayed behavior can be attributed to the concentration ratio of Mn(II) to amino acid which can be of a certain value for any particular amino acid.

Keywords: Glycine, L-alanine and L-leucine, Permanganate oxidation, Concentrated acidic medium, Delayed autocatalysis, Free radical intermediate