Evaluation of Triethylamine as a Counter Solvent in Totally Organic Biphasic Liquid–Liquid Partition Systems

Abstract

Partition coefficients for varied compounds were determined for the triethylamine–organic solvent biphasic partition systems where the organic solvent is dimethyl sulfoxide, ethanolamine or formamide. These partition coefficient databases are analyzed using the solvation parameter model facilitating a quantitative comparison of the triethylamine-based partition system with other totally organic partition systems. By comparison with the system constants for the biphasic systems formed between n-heptane and dimethyl sulfoxide, ethanolamine and formamide, triethylamine is a weakly cohesive solvent (slightly stronger than n-heptane), weakly dipolar/polarizable (only slightly larger than n-heptane), and non-hydrogen-bond acidic. Triethylamine is moderately hydrogen-bond basic, but much less so than ethanolamine, formamide and dimethyl sulfoxide, which are strong hydrogen-bond bases. The separation properties of triethylamine–dimethyl sulfoxide are more similar to the isopentyl ether-propylene carbonate biphasic system; the triethylamine-formamide system octan-1-ol-formamide biphaic system; and triethylamine–ethanolamine the 1,2-dichloroethane–formamide and 1,2-dichloroethane–ethylene glycol systems. None of these pairs of biphasic systems are selectivity equivalent. In the first case, the significant difference in cohesive properties favors the distribution of larger solutes to the triethylamine layer and the greater hydrogen-bond basicity of dimethyl sulfoxide the selective extraction of hydrogen-bond acids. For the triethylamine–formamide system larger molecules have a slight preference for transfer to the triethylamine layer compared with octan-1-ol, and hydrogen-bond bases will be selectively extracted to the formamide layer, since triethylamine is not competitive with octan-1-ol as a hydrogen-bond acid. There are few totally organic biphasic systems suitable for the determination of solute hydrogen-bond basicity (B descriptor) and the triethylamine–formamide biphasic system can be used to supplement these.