The Hammett equation and Snyder theory as a criterion for adsorption of a functional group under liquid–solid chromatography

Abstract

The application of the Hammett equation in a suitable form to TLC separations on silica of aromatic aldehydes, 4-X–C₆H₄–Y, and related 5-arylidene thiazolidine-2, 4-diones has shown some substantial deviations to higher values from the correlation lines which are considered on the basis of Snyder theory as a proof for adsorption of the corresponding group X (from a substituent, it becomes a new adsorption centre Y′). Based on this criterion, the data of this paper and previous papers support the adsorption of group X when it is 3-OH, 4-OH, 3-NO₂, 4-NO₂ and 3-OCH₃. The adsorption of X seems to occur when its adsorptivity is similar to that of Y and the electronic interaction between X and Y is of limited importance.