Specific chelating extraction of gallium(III) with H-N and H-CCl2 hydrogen atoms of dichlorinated secondary amide

Three secondary amides with different numbers of chlorine atom(s)—N-(2-ethylhexyl)monochloroacetamide (MCAA), N-(2-ethylhexyl)dichloroacetamide (DCAA), and N-(2-ethylhexyl)-2,2,2-trichloroacetamide (TCAA)—were synthesised for Ga(III) extraction. These three amides exhibited a preference for extracting Ga(III) compared to other metals at higher HCl concentrations, with the stoichiometries of Ga(III) extraction using the three reagents investigated via slope analysis. Spectroscopic studies were performed to elucidate the extraction mechanism of GaCl₄⁻. The hydronium ion extraction with (O)=C oxygen atoms of amides did not occur in lower HCl concentration media ([HCl] < 8.46 mol dm⁻³), indicating that (H)-N hydrogen atom displayed a distinctive interaction with GaCl₄⁻. The DCAA exhibited the highest Ga(III) extraction efficiency among the three secondary amides. The proton nuclear magnetic resonance spectroscopic analysis and density functional theory calculations indicated that the δ⁺ value of the hydrogen atom in (H)-C-Cl₂ of DCAA was enhanced to assist GaCl₄⁻ extraction with the (H)-N hydrogen atom. Stable intermolecular hydrogen bonding between (H)-N hydrogen and the partially negatively charged (O)=C oxygen atoms of MCAA inhibited GaCl₄⁻ extraction. The complete stripping of Ga(III) from the Ga-loaded MCAA and TCAA was achieved using water, while chelation extraction with DCAA suppressed the efficiency of Ga(III) stripping with water.