Nucleophilic displacement in polyhalogenoaromatic compounds. Part 3. Kinetics of protiodeiodination of lodoarenes

Abstract

Protiodeiodination occurs readily when polyhalogenoaryl iodides are heated with solutions of sodium methoxide in butanone–methanol (3 : 1 v/v). Under these conditions d[I^–]/dt=k₂[Arl][NaOMe]; in methanol, protiodeiodination of iodopentachlorobenzene shows the form d[I^–]/dt=k₃[Arl][NaOMe][butanone]. The deiodination agent is therefore butanone anion or its kinetic equivalent. In butanone–methanol (3 : 1 v/v), the deiodination of iodopentachlorobenzene at 293 K is not slowed, nor is the corresponding reaction of 2,6-dichloroiodobenzene accelerated, by added pentachlorobenzene. Although equilibrium concentrations of pentachlorophenyl anion are present, these do not interfere with deiodination; all stages between this and the formation of iodide ion must be irreversible. In the deiodination of a number of iodoarenes at 323 K, halogen substituents accelerate the process in the order orthometa > para; however, such loss of iodine is always much slower than the reduction of elemental iodine to iodide ion under the same conditions, and so only the transference of halogen from iodoarene to butanone anion is kinetically significant. The subsequent formation of iodide ion and the formation of the carbanionic reagent are not rate-determining processes in these conditions.