Confirmation of organic compounds hydrates formation under the reversed-phase HPLC conditions

The confirmation of the correspondence of the detected forms of analytes to their molecular structures seems to be one of the principal problems of analytical chemistry. The previous analysis of the series of drugs by reversed-phase HPLC allowed revealing that the retention regularities of the complex polyfunctional organic compounds containing sulfonamide groups -SO2-N< differ from those of other compounds. It was explained by reversible formation of their hydrate forms. To verify this proposition, three model N-substituted arylsulfon­ami­des were synthesized, namely N,N-diethyl- (I), N-allyl- (II), and N-phenyl (III). The consideration of the dependencies of these analytes net retention times (tR) in reverse-phase HPLC on the content of orga­nic solvent (C, methanol) in an eluent did not permit us to reveal any regularities or anomalies due to the non-linearity of such depen­den­cies. However, to achieve this, the recurrent appro­ximation was re­com­mended, tR(C + DC) = atR(C ) + b (*), where DC = const – con­stant varia­tions of concentration of methanol (5% in our case), а and b – coefficients calculated by Least Squares Method. If the chemical origin of the analytes at the outlet of the chromatographic column remained the same within concentration range Сmin < С < Сmax, dependencies (*) were linear with correlation coefficients R > 0.999. Yet, if additional chemical transformations of analytes took place, na­me­ly rever­sible formation of hydrates (or variations of their composition) due to the presence of water in an eluent, it led to the deviations of recurrent dependencies (*) from linea­rity. Three sulfonamides under characterization corresponded to different kinds of such deviations: non-linearity within the whole range Сmin < С < Сmax (amide I), presence of two linear dependencies instead of one (amide II), and linearity within the whole range Сmin < С < Сmax (amide III). First two cases corres­ponded to the interconver­si­on of anhydrous forms and hydrates, or (less probable) different hydrates, while the latter meant the existence of analyte in the single form (pro­ba­b­ly hydrate). Thus, the ana­ly­sis of mo­del compounds confirmed that the presence of the polar functionality -SO2-N< was just the reason of hydrate formation in water soluti­ons.