Kinetic Study of an Esterification by Monitoring Pitch Changes in a Cholesteric Liquid Crystal Solvent

Abstract

To evaluate critically the importance of cholesteric ordering on the kinetics of a bimolecular reaction, a high degree of orientation must be maintained, and chiral reactants and products should be chosen which show a high helical twisting power and stability in the liquid crystal solvent. In this study, the phase can be maintained in an ordered planar texture by utilizing wall effects on a thin film sample, and the kinetics can be monitored continuously by observing the changes in pitch of the cholesteric solvent. After screening fourteen compounds, including chiral acid anhydrides, phenols and esters, appropriate soluble reactants and products with high helical twisting power were found. Esterification of p-(n-heptyloxy)phenol by (+) p-(2-methylbutyl) benzoic anhydride was studied in detail. The reaction rate shows no discontinuity at the transition from cholesteric to isotropic phases and the activation parameters remain unchanged (δE_a = 15.8 ± 0.7 Kcal mole^–1, δH = 15.1 ± 0.7 Kcal mole^–1, and δS ^# = -11.5 ± 1.5 cal mol^–1 deg^–1).