On the stereochemical course of the chlorine-38 for chlorine substitution in diastereomeric and enantiomeric compounds

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Virginia Polytechnic Institute and State University

The stereochemistry of the energetic ³⁸Cl for Cl substitution was studied in diastereomeric 2,4-dichloropentane and 2,3-dichlorobutane as well as in enantiomeric 2-chloro-1-propanol in solution. It was found that the stereochemical course of the substitution reaction exhibits a strong dependence on the concentration and nature of the solvent added when the diastereomeric compounds were used as substrate. Similar results with a lesser degree of solvent dependence were observed in the 2-chloro-1-propanol system.

A conformational analysis of 2,4-dichloropentane was made. It showed that the relative conformer population is not affected to any significant degree by the nature of the solvent. Furthermore, an examination was made of the physical and chemical properties of the solvent which could be responsible for the drastic changes inflicted upon the stereochemistry of the ³⁸Cl for Cl substitution reaction.

The experimental results indicate that the stereochemical course of the substitution process is predominantly and directly controlled by the properties of the solvent molecules, most likely by the dielectric constant and molecular size which in turn are responsible for the magnitude of intermolecular interaction between reactants and solvents.

Although the experimental results seem to indicate that the radical-radical recombination mechanism is the most likely mechanism as compared to the hot one-step substitution process, they offer no conclusive distinction between these two mechanisms. It appears that strong intermolecular interaction favors substitution via retention of configuration. whereas in solvents having a low dielectric constant or large molecular diameter the retention/inversion ratio decreases.