The Positional Effect of Solute Functional Group among Positional Isomers of Phenylpropanol in Hydroxyl Group-Solvent Specific Interactions in Methanol/Water Mixed Solvents Monitored by HPLC

Abstract

We have evaluated the hydroxyl group-solvent specific interactions by using a Lichrosorb RP18 stationary phase and by measuring the retention data of carefully selected solutes in 50/50, 60/40, 70/30, 80/20, and 90/ 10(v/v%) methanol/water eluents at 25,30,35,40,45, and 50 °C. The selected solutes are 3 positional isomers of phenylpropanol, that is, 1-phenyl-1-propanol, 1-phenyl-2-propanol, and 3-phenyl-1-propanol. There exist clear discrepancies in ΔH° (solute transfer enthalpy from the mobile to the stationary phase) and TAS° (solute transfer entropy) among positional isomers. The difference in ΔH° and TAS° between secondary alcohols (1-phenyl-1-propanol and 1-phenyl-2-propanol)is negligible compared to the difference between the primary alcohol (1-phenyl-3-propanol) and secondary alcohols. The TAS° values of 3-phenyl-1-propanol are close to those of butylbenzene while the TAS° values of secondary alcohols are close to those of propylbenzene. The difference in ΔΔH° (specific solute-mobile phase interaction enthalpy) between the primary alcohol and the secondary alcohol decreases with increase of methanol content in the mobile phase. A unique observation is an extremum for 1-phenyl-3-propanol in the plot of TAAS vs. methanol volume %. The positive sign of TΔΔS° of 3-phenyl- 1-propanol implies that the entropy of 3-phenyl- 1-propanol is greater than that of the hypothetical alkylbenzene (the same size and shape as phenylpropanol) in the mobile phase.