MECHANISTIC STUDIES ON THE EXTRACTION OF COPPER(II) BY HYDROPHOBIC REAGENTS

Abstract

A high speed stirring apparatus was constructed for following the kinetics of very rapid solvent extractions. Reactions with half-lives of 15 seconds can be followed with good precision. Experimental data obtained with the device are shown to be superior to kinetic data for the same reaction obtained with batch shakers and cruder stirring devices. The apparatus was used to determine the rate law for the extraction of copper by 2-hydroxyl-5-nonylbenzophenone oxime(I) and 5-dodecyl-2-hydroxylbenzophenone oxime(II). The rate law for the extraction of copper by I catalyzed by 5,8-diethyl-7-hydroxy-6-dodecanone oxime(III) was also determined. Equilibrium data are used to characterize the stoichiometry of the extracted complexes. Compound I, which is an aromatic β-hydroxyoxime, extracts copper as the 2:1 neutral chelate. Compound III, which is an aliphatic α-hydroxyoxime, has a much more complicated extraction chemistry. Experimental evidence indicates the existence of a conventional 2:1 neutral chelate, a neutral (possibly polymeric) complex, and a singly charged 1:1 complex which extracts as an ion pair with a monovalent anion. Distribution constants between chloroform and water were also determined for each ligand. The experimentally observed distribution constants for I and II are much lower than constants derived from theoretical calculations. Compound III has a partitioning constant which is in good agreement with its theoretically calculated value. The rate equations for the extraction of copper by I and II, and for the extraction of copper by I catalyzed by III, are all first order in metal ion, second order in ligand, and inverse first order in hydrogen ion. The catalytic rate law is first order in each ligand. In the case of the reactions which are second order in ligand, the rate determining step is proposed to be the aqueous phase reaction of a 1:1 intermediate complex with the second ligand molecule. Knowledge of the partitioning constants of the ligands, which in turn provides their aqueous phase concentrations, leads to the calculation of second order reaction rate constants of 1.2 x 10('7)M⁻¹s⁻¹ for I and 1.1 x 10('7)M⁻¹ for II. In the case of the catalytic mechanism, the rate law cannot be used to discern which ligand reacts in the rate determining step. If the reactive 1:1 intermediate is assumed to be with I, then a catalytic rate constant of 9.5 x 10('9)M⁻¹s⁻¹ is obtained. A catalytic pathway proceeding through a 1:1 intermediate with III is also possible, but this complex is too poorly defined for calculating a rate constant. A kinetic study of the back extraction of copper into water from an organic solution of its 2:1 complex with I indicates three pathways. The chelate partitions into water and releases its coordinated ligands in a solvolysis reaction. Both hydrogen ion and III catalyze the stripping reaction. In the case of catalysis by III, a mixed ligand complex is proposed to partition. In conclusion, the "homogeneous phase" mechanism is found to be the better mechanistic interpretation of these reaction systems. Unlike the interfacial model, this mechanism can account for all the observed phenomena, is supported by independent measurements, and conforms to the vast body of chemical data already acquired on extraction systems.