Steric Effect of Alkyl Substituent on the Extraction of Nickel(II) and Cobalt(II) by N-Alkylcarbonyl Substituted N-Phenylhydroxylamines

Abstract

Nickel(II) and cobalt(II) were extracted by N-alkylcarbonyl-N-phenylhydroxylamines (R-PHA or HL) having primary, secondary, or tertiary alkyl substituents. The extracted species were nonadduct (ML₂) or selfadduct (ML₂(HL)) depending on the kinds of metal ions and R-PHAs. The selectivity in the extraction of the two metal ions by R-PHAs was highly dependent upon the structure of each alkyl substituent and was decreased with an increase in the branching. The selectivity was explained in terms of the steric effect of the alkyl substituent. For this purpose, the selectivity was defined by a separation factor, the ratio of the two extraction constants between a 1:2 nickel(II) chelate (NiL₂) and 1:2 Cobalt(II) chelate (CoL₂). The separation factor was correlated inversely with a steric parameter, STERIMOL B₁, representing the steric effect of the alkyl substituent located at the α carbon neighboring the carbonyl group of R-PHA. Thus, the steric effect of alkyl substituent plays an important role in the extraction of the two metal ions with R-PHAs.