Kinetics and mechanism of oxidation of DL-leucine by acid permanganate

Summary

The kinetics of oxidation of leucine (LCO₂H) by acid permanganate have been followed spectrophotometrically at 525 nm for the disappearance of Mn^VII and at 420 nm for the appearance of Mn^IV. The latter absorbance passes through a maximum, signifying the presence of a consecutive reaction involving a Mn^IV intermediate. The reaction is first order with respect to [MnO ⁻₄ ]. The rate constant, k₇, has been evaluated at different [LCO₂H], [H⁺] and temperatures, from plots of A ⁵²⁵_corr versus time. The overall rate expression satisfying the observed kinetic parameters is

$$\begin{gathered} - \frac{1}{{[Mn^{VII} ]_{tot.} }}\frac{{d[Mn^{VII} ]_{tot.} }}{{dt}} \hfill \\ = {{\{ k\prime _1 [LCO_2 H]_0^{1/2} + k\prime _2 [LCO_2 H]_0^{2} \} } \mathord{\left/ {\vphantom {{\{ k\prime _1 [LCO_2 H]_0^{1/2} + k\prime _2 [LCO_2 H]_0^{2} \} } {[H^ + ]}}} \right. \kern-\nulldelimiterspace} {[H^ + ]}} \hfill \\ \end{gathered} $$

It is found that 8 moles of CO₂ are produced and 8 moles of leucine are consumed per mole of permanganate consumed. The decarboxylation involves a cyclic chain reaction.