Choice of a suitable hetero-aromatic nitrogen base as promoter for chromic acid oxidation of dl-mandelic acid in aqueous media at room temperature

摘要

Chromic acid oxidation of dl-mandelic acid in the presence and absence of different promoters has been studied in aqueous media under the kinetic conditions [mandelic acid]_T a‰ [Cr(VI)]_T and [promoter]_T a‰ [Cr(VI)]_T at 30 C. The promoters used in this oxidation reaction, picolinic acid (PA), 2,2′-bipyridine (bpy), and 1,10-phenanthroline (phen), are strong chelating ligands which form complexes with most transition metal ions. The reaction is first-order with regard to [H~+], [mandelic acid] _T, and [Cr(VI)]_T and also has first-order dependence on [promoter]_T. HCrO_4 ~-was found to be kinetically active in the absence of promoters; in the presence of promoters the Cr(VI)-promoter complexes were believed to be the active oxidants. In this path the Cr(VI)-promoter complex in each case undergoes nucleophilic attack by the mandelic acid to form a ternary complex which subsequently undergoes redox decomposition involving 3e transfer as the rate-determining step. Among the three promoters oxidation is much faster with 1,10-phenanthroline.