Nucleophilic displacement in polyhalogenoaromatic compounds. Part 2. Kinetics of halogen displacement from bromopolyfluoroaromatic compounds

Abstract

The nucleophilic displacement of fluorine and of bromine from m-dibromotetrafiuorobenzene and from bromopentafluorobenzene by sodium methoxide in butanone–methanol mixtures at 298 K involves competing second-order processes. The rate of nucleophilic displacement of fluorine is somewhat more sensitive to substituent effects (ρ 6.0) than that of the corresponding displacement of bromine (ρ 4.3 ± 0.6) in butanone–methanol (3 : 1 v/v) at 298 K. Debromination is much more affected by changes in the butanone concentration than is defluorination, and the depression of rate which results from lowering the amount of ketone in the solvent is far greater than that predicted on the basis of a simple acid–base equilibrium involving the ketone anion.