Kinetics and Mechanism of the Permanganate-Induced Autocatalytic Dehydration Reaction of L-Y-Amino-N-Butyric Acid to Give 2-Pyrrolidone through a Radical Intermediate in Moderately Concentrated Acidic Medium

Abstract

The reaction kinetics and mechanism of the permanganate ion oxidation of L-y-amino-butyric acid (GABA) in moderately concentrated sulfuric acid medium has been investigated spectrophotometrically.The product of this reaction is 2-pyrrolidone, which is a 5-membered lactam structure compound (gamma-butyrolactam). Conclusive evidence indicates autocatalytic activity for Mn(II) in this reaction, analogous to that of some a- amino acids. It has been shown that such activity appears when a certain concentration ratio of Mn(II) to GABA is built up in the medium, termed the “critical ratio”. The magnitude of this ratio depends on the sulfuric acid concentration. In considering the “delayed autocatalytic behaviour” of Mn(II) ions, rate equations satisfying our observations for both the catalytic and non- catalytic routes have been presented. The reaction shows a first-order dependence on permanganate ion concentration both in the catalytic and non-catalytic pathways, and an apparent first-order dependence on Mn2+ions in the catalytic pathway. Also, it was found that the variation in the apparent rate constants of the catalytic and non-catalytic pathways versus the GABA concentration follows saturation curves. The correspondence of the pseudo-order rate constants of the catalytic and non-catalytic pathways to the Arrhenius and Eyring laws have verified the “critical ratio” as well as the “delayed autocatalytic behaviour” concepts.The activation parameters associated with both pathways have been computed and discussed. Mechanisms for both catalytic and non-catalytic routes involving radical intermediates, as well as a product having a lactam skeleton, are reported.