No Evidence of the Monomer in the Extraction of Copper(II) with Carboxylic Acids Having Bulky Substituent at [alpha]-Position

The extraction behavior of copper(II) with three carboxylic acids having a bulky substituent at the α-position was investigated in order to clarify the steric effect on the dimerization of copper(II) carboxylate extracted in benzene. Pivalate and 2-ethylbutyrate were found to be extracted only as a dimeric species Cu2L4(HL)2through the slope analyses, taking into account the formation constant of (1:1) copper carboxylate complex in the aqueous phase. A new computing method based on fixing the species to the composition, Cu2L4(HL)2, was proposed to estimate both the dimerization constant of carboxylic acid (K2, HL) and the extraction constant of the dimeric copper species (Kex(22)) for the case of carboxylic acid of higher carbon number in nonpolar solvents, and the extracted diphenylacetate was also found to be only a dimer as is usual. Consequently, under our experimental conditions, there was no evidence for the coextraction of monomeric copper(II) species caused by the steric hindrance as reported in other papers. Moreover, a linear relationship between log Kex(22) and log Ka was maintained by adding the three bulky substituent carboxylic acids.