Steric Effect of Ortho Substituents of 1‐Phenyl‐3‐methyl‐4‐aroylpyrazol‐5‐ones on the Synergic Extraction of Scandium and Lanthanum with Tri‐n‐octylphosphine Oxide

Abstract

The substituent effect of ortho‐substituted 4‐aroyl derivatives of 1‐phenyl‐3‐methylpyrazol‐5‐one (HA) on the adduct formation reaction between their scandium or lanthanum (M) chelates and a neutral ligand, tri‐n‐octylphosphine oxide (TOPO, L), in benzene was studied using a liquid–liquid distribution system. The 15 extraction constants and 30 adduct formation constants were obtained for the extraction systems of trivalent scandium and lanthanum ions using eight kinds of ortho‐substituted 4‐aroylpyrazol‐5‐ones. Obvious steric hindrance by the ortho substituents was observed in the extraction reactions, especially in adduct formation reactions. The correlation between the steric hindrance and the size of the central metal ions was discussed on the basis of equilibrium constants, ionic radii and van der Waals volumes. The stepwise formation constants of their adducts indicate that the steric hindrance mainly occurs in the first step of the adduct formation, MA3L, for lanthanum and in the second step of the adduct formation, MA3L2, for scandium. The factors that cause the steric hindrance were considered to be the bulkiness of the ortho substituents, the proximity of the neutral ligand to the metal chelate and the crowdedness of the ligands around the central metal ion.