SN2 Reactions in Dipolar Aprotic Solvents. Chlorine Isotopic Exchange Reactions of 2-Arylethyl Chlorides, Chloromethyl Aryl Ethers, and Chloromethyl Aryl Sulfides in Acetonitrile

The SN2 reactions having a symmetrical transition state were studied in a dipolar aprotic solvent. A good Hammett correlation was found for the chlorine isotopic exchange reactions in acetonitrile between tetraethylammonium chloride and three types of substituted methyl chlorides, 2-arylethyl chlorides, chloromethyl aryl ethers, and chloromethyl aryl sulfides. The reaction constant was positive for 2-arylethyl chlorides and negative for the methyl chlorides possessing an α-heteroatom. The presence of oxygen and sulfur atom on the α-position to the reaction center resulted in a rate enhancement of about 105 times and 103 times, respectively. Features of the transition states of these SN2 reactions are discussed.