IDENTIFICATION OF STEREOISOMERS BASED ON DIELECTRIC STUDIES - DIPOLE-MOMENTS OF CHLOROALKENES AND CHLOROCUMULENES

The use of dielectric measurements to assign molecular configurations was examined with model compounds of the chloroalkane, chloroalkene and chlorocumulene type. A dielectric microcell is described which allows measurements of 0.35 mL samples under exclusion of air. Both benzene and cyclohexane were used as solvents. Different theories of dielectric constant are compared to evaluate dipole moments. Dipole moments are analyzed in terms of bond moments. The results can be summarized as follows: (i) dielectric measurements are feasible up to 1 M solutions without substantial deviation from linearity; (ii) in general, the dipole moments found in both solvents are similar; the aromatic solvent, however, causes a substantial decrease of the values obtained for chloroalkenes with two bulky substituents at the double bonds; (iii) the (E) isomers of chloroalkenes exhibit significantly higher dipole moments than the corresponding (Z) isomers; (iv) isomers of non-chlorinated alkenes or cumulenes with dipole moments below 0.6 D cannot be discriminated dielectrically; (v) on the other hand, if the dipole moments of the compounds exceed 1 D, configurations can be assigned unambiguously on the basis of dielectrical data even in cases where other approaches fail.