A Study of the Liquid and Solvent Properties of Optically Active and Racemic α-Methylbenzylamine

Abstract

Speed-of-sound measurements are reported for RS (racemic) and S liquid alpha-methylbenzylamine (MBA) obtained using a modified design of previously published experimental geometry. After correcting for density changes, the resulting isentropic compressibility of the S liquid is found to be 2% larger than that of the RS racemic mixture. These data, along with proton NMR chemical shifts and published partial molar volumes, suggest that the structures of the racemic and optically active liquids are subtly different. The magnitude of the compressibilities and other data such as viscosity and the Kamlet-Taft parameters are consistent with the molecules in the liquids interacting via extensive hydrogen bonding. Cyclohexane, toluene, nitrobenzene, dimethyl sulfoxide, and methanol are completely miscible in MBA, as predicted from their corresponding Hildebrand solubility parameters. Proton NMR, optical rotation, and IR studies were carried out on solutions of the five solutes in MBA as a function of mole fraction. The asymmetric NH stretch of MBA was particularly informative in this regard. Solvation models were developed for the five solutes. The NH stretch frequency in cyclohexane changed only slightly up to a mole fraction of 0.7 in the hydrocarbon at which point the NH stretch greatly increased by about 200 cm(-1) at a mole fraction of 1. The hydrogen-bonding network does not dissipate until there is at least 70 mol % cyclohexane in the MBA. Gaussian calculations were also carried out to complement the ir studies.