BENZYNE AS AN INTERMEDlATE IN NUCLEO - PHILIC AROMATIC SUBSTITUTION REACTIONS

IT sometimes happens in organic chemistry that a large body of related fact accumulates which seemingly cannot be rationalized on the basis of the currently available theoretical concepts. In such circumstances, substantial clarification may often be achieved by an approach which is novel only in application .to the system at band. The pieces of the puzzle may then fall into place and experimental verification of predictions may be thereby achieved. However, it almost invariably occurs that the initial success is later tempered by experimental findings which either require modification of the original ideas or indicate their limitations. In recent years, the interpretation of the rearrangements which sometimes occur in substitution reactions of non-activated aromatic halides with strong bases has gone through such a cycle. A rationalization of the pattern for this type of rearrangement was provided by postulation of an elimination-addition mechanism involving 'benzyne' iotermediates and there has been a rapid concomitant expansion of the chemistry of benzyne and related substances. At the same time, eTidence bas been accumulating that not all nucleophilic substitutions of non-activated aryl derivatives proceed by elimination-addition mechanisms.