Abstract
The extraction equilibria of copper(II) with 2-bromoaliphatic carboxylic acids and with conjugated carboxylic acids in benzene were investigated using a curve-fitting method in order to grasp the bonding character of a functional carboxyl group. A monomeric species, CuL2(HL)a, together with dimeric copper(II) carboxylate Cu2L4(HL)2 was extracted in the concentration region of copper(II) in the organic phase, CCu,o>3×10-5 mol dm-3. A new linear relationship between the extraction constant (Kex) and the acid dissociation constant (Ka) was empirically obtained as log Kex(11)=1.74log Ka-0.75 (1) for CuL2·HL and log Kex(12)=1.37log Ka-1.97 (2) for CuL2(HL)2, as well as log Kex(22)=1.71log Ka-3.17 (3) for the dimer. Thus, we can obtain an approximate value of the dimerization constant of copper(II) (Kdim(a,2)) in the organic phase from Ka by log Kdim(a,2)=log Kex(22)-2log Kex(1a) (a=1 or 2), except for cases of conjugated carboxylates, which deviate from Eq. (3), due to a lower log Kex(22). The deviations would be correlated to the change in the dipole moment affected by electronic delocalization with resonance over the conjugated ligand in the bridging form. Therefore, the extraction of a monomer together with a dimer is easily found by using carboxylic acids of low pKa or of a conjugate.
