Hydrogen replacement reactions play an outstanding part in organic chemistry. They include such industrially important processes as halogenation, nitration, sulfona- tion, alkylation, and direct amination and metallation of organic substances. It is no accident that the data obtained in the investigation of these reactions have been used extensivel y in the solution of a series of fundamental problems in theoretical organ- ic chemistry and this tendency still exists. Isotopic exchanges of hydrogen also belong to this group of chemical conversions. They have attracted considerable attention as...