Mechanistic deduction of gallium(III) extraction using halogenated secondary amides: Spectroscopic interpretation

Abstract

Secondary amides act as bifunctional compounds to extract metal anionic species due to the presence of N–H hydrogen and protonated carbonyl oxygen atoms. To exclude the latter function with stronger interaction and less extraction selectivity for metal anionic species, two secondary amide compounds with halogen atoms, namely N-(2-ethylhexyl)-2,2,2-trichloroacetamide as TCAA and N-(2-ethylhexyl)-2,2,2-tribromoacetamide as TBAA, have been synthesised to investigate the efficiency and mechanism of Ga(III) extraction for comparison with N-(2-ethylhexyl)-2,2,2-trifluoroacetamide (TFAA). The 1H NMR spectroscopy results indicated the absence of proton extraction from the carboxylic oxygen atoms of both halogen-containing amides at sufficiently high concentrations of HCl. The three amides exhibited preferential extraction of Ga(III) compared to other metals in highly acidic media. A slope analysis was conducted to assess the stoichiometry for the extraction of Ga(III). The FT-IR and 1H NMR spectroscopies were employed to elucidate the extraction mechanism of GaCl4– using TCAA and TBAA. Therefore, the hydrogen atoms of the N–H group of both amides, with extremely weak δ+ nature, exhibited a distinct extractive preference for anionic GaCl4–. Following the forward extraction, Ga(III) was effectively separated from the organic phase through stripping with distilled water. The extraction efficiencies of GaCl4– among the three amides are not followed by the strength of electron-withdrawing nature of halogen atoms, but the strength of intra- or intermolecular interactions of the amides. This argument was substantiated by calculations using density functional theory (DFT).