Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2014, Tutor: Joaquín F. Pérez de Benito
The reduction of permanganate ion to MnO2-Mn2O3 soluble colloidal mixed oxide by Lphenylalanine
in aqueous phosphate-buffered neutral solutions has been followed by a
spectrophotometric method, monitoring the decay of permanganate ion at 525 nm and the
formation of the colloidal oxide at 420 nm. The reaction is autocatalyzed by the manganese
product and three rate constants have been required to fit the experimental absorbance-time
kinetic data. The reaction shows base catalysis and the values of the activation parameters at
different pHs have been determined. A mechanism including both the non-autocatalytic and the
autocatalytic reaction pathways, and in agreement with the available experimental data, has
been proposed. Some key features of this mechanism are the following: (i) of the two
predominant forms of the amino acid, the anionic form exhibits a stronger reducing power than
the zwitterionic form; (ii) the non-autocatalytic reaction pathway starts with the transfer of the
hydrogen atom in the alpha position of the amino acid to permanganate ion; and (iii) the
autocatalytic reaction pathway involves the reduction of Mn(IV) to Mn(II) by the amino acid and the posterior re-oxidation of Mn(II) to Mn(IV) by permanganate ion.