Selectivity of stationary phases for the resolution of some simple substances

Abstract

The relative molar heats and entropies of solution for methylbenzenes, chlorotoluenes, chlorobenzenes and chloroxylenes have been determined using the relative retentions on Apiezon L, poly(ethyleneglycoladipate), poly(phenyl ether) and methylphenylsilicone; the corresponding values for C 5-C 6 olefins were calculated on pentadecane and dinitriloadipate. It was shown that the semi-empirical theory of selectivity of the stationary phases being applied to such systems permits one to obtain reasonable results, within the limit of experimental error. The relative molar heats of solution give information about the number and nature of the substituents in the benzene ring. The difference between the relative molar heats of solution in polar and non-polar stationary phases is proposed as an analytical parameter for identification that is sensitive to the polarity of the solute. It was shown that the orientation forces in a polar stationary phase do not affect the selectivity for methyl and chlorobenzenes strongly. A “hyperconjugation” effect is discussed for the solutes under study in non-polar stationary phases and some semi-empirical equations are derived for calculating the relative molar heats of solution of methylbenzenes and olefins using this effect. The calculated values of the relative entropies of solution permit one to estimate the positions of substituents in the benzene ring. The possibility of calculating the relative molar entropy of solution of light olefins is discussed. Some semi-empirical equations are derived for calculating the enthalpic and entropic selectivities of non-polar stationary phases.