Nucleophilic displacement in polyhalogenoaromatic compounds. Part 11. Kinetics of protiodeiodination of iodoarenes in dimethyl sulphoxide–methanol

Abstract

The rates of methoxide-ion induced protiodeiodination of a number of polychloroiodobenzenes and their derivatives have been measured in dimethyl sulphoxide–methanol (9 : 1 v/v; 323.2 K). The true reagent under these conditions appears to be the dimethyl sulphoxide anion, and the rates of reaction in some cases appear to approach that expected of a diffusion controlled process. This corresponds to a major decrease in the efficacy of further activating substituents in the aromatic system, although deactivating groups such as p-OMe still show large effects. Chlorine promotes protiodeiodination in the order of efficiency o-Cl > m-Cl > p-Cl; the trifluoromethyl group activates displacement in the order o-CF₃ > p-CF₃ > m-CF₃, although with much less difference between isomeric sites. o-Nitro-groups promote protiodeiodination whereas the p-nitro-group encourages methoxydeiodination. No evidence of methoxydeiodination was found in attack of the polychloroiodobenzenes, although the rates of methoxydechlorination of the corresponding polychlorobenzenes suggest that in some cases this might occur. Evidence rejecting the possible S_RN1 mechanism and supporting nucleophilic attack by a carbanionic species upon iodine is presented.