Normal Phase Separation of Chloroanilines and Nitroanilines with Silica, Amine, and Cobalt(III) Complex Bonded Phases

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Abstract
The separation of chloroaniline and nitroaniline isomers that are capable of forming hydrogen bonds was studied using silica, octadecyl-bonded silica (ODS or C-18), propylamine-bonded silica, propyldiamine-bonded silica, and two cobalt(III) metal complex bonded silica columns at high percentages of 2-propanol in n-hexane or n-heptane. The capacity factors were determined at various temperatures and the van't Hoff's plots gave the thermodynamic parameters for the retention process, i.e., ΔH and ΔS. These data made it possible to rank the relative strengths of the above stationary phase columns for the separation of polar compounds in the following order: [Co(en)3]CI3 >[Co(edda)(en)] Cl> diamine >amine> silica > C-18. Enthalpy-entropy compensation relationships were observed for most of the chromatographic processes of solutes using different columns. The same observations were made for a particular column with different solutes except for the ortho-isomers. Plots of values of – ΔH vs. – RT In k gave similar slopes for amino and complex bonded phase columns, which indicated similar interaction modes for these columns. On the other hand, ortho-isomers were different from meta- and para-isomers with respect to their behavior with the bonded phases, presumably because of some steric interactions at the ortho-position. Finally, a linear relationship of ΔH and pKb, values for both chloroanilines and nitroanilines implied that the directional interaction is likely to take place through the lone pair electrons of the aromatic amino group. The interaction modes between solutes and stationary phases were proposed.

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