Influence of Cholesteric Solvents on the Kinetics of Schiff Base Formation from Enantiomeric Amines

Abstract

In order to examine the influences of both the macroscopic and microscopic chirality of cholesteric media on a chemical process, the kinetics of the bimolecular reaction between p-ethoxybenzaldehyde (EBA) and enantiomeric S-(-)- or R-(+)-l-phenylethylamine (PEA) were studied in right-and left-handed cholesteric liquid crystals. The reaction was followed using two methods: (1) continuous monitoring of changes in pitch, and (2) monitoring changes in the intensity of IR absorption peaks, related to C=O and CH=N bonds. Differences between rates and activation parameters were observed when reactions were performed in steroidal cholesteric or chiral nematic solvents, and the effective helical twisting powers (HTP) of S-(-)- and R-(+)-PEA are not equivalent in the employed systems. That is, the reaction rate in the steroidal cholesteric solvent is lower for the enantiomer exhibiting the higher value of HTP. The value of HTP appears to be a guide to the strength of the solute-solvent interaction.