Determination of the Rotameric Stability of 1,2-Dihaloethanes Using Infrared Spectroscopy. A Combined Experimental and Computational Project for the Physical Chemistry Laboratory

Abstract

The effects of structure and environment on the rotameric energy difference between gauche and trans conformations of 1,2-disubstituted ethanes are determined quantitatively through the use of infrared spectroscopy. A series of three 1,2-dihaloethanes (chloro, bromo, and iodo) were analyzed in the condensed phase (pure liquid and carbon tetrachloride and acetonitrile solutions) as well as in the gas phase. Since the gauche conformation of these species has a nonzero dipole, dissolving the compounds in solvents with increasing dielectric constant causes a stabilization of the gauche conformation, thus decreasing the absolute energy difference between the rotamers. The major factor governing the intrinsic stability is steric hindrance. As the size of the halogen increases, more steric repulsion occurs, resulting in an increase in the absolute energy difference. Although in most cases steric hindrance was the dominant factor, a solution of 1,2-dichloroethane in highly polar acetonitrile actually favors the gauche conformation, illustrating the importance of solvation effects.