On the relationship between the rate constants for racemization and isotopic exchange

The relationship between the rate constants that describe the halide ion catalysis of the racemization (kr) of optically alkyl halides and the related isotopic halide exchange reaction (ke) is shown to be valid in two distinct cases. The first is when the racemization and exchange mechanisms follow the classical and conventional SN2 path characterized by a symmetrical trigonal (sp2) transition state. The second envisages the trigonal species characteristic of the SN2 reaction as an intermediate instead of a transition state. The latter mechanism is also shown to be characterized by the relationship kr = 2ke, and hence this diagnostic test is seen to be a necessary but not a sufficient condition for invoking the SN2 mechanism.Key words: racemization, isotopic exchange, kinetics, bimolecular substitution, inversion.