The “positive halogen” reaction between benzotrichlorides and potassium iodide: Relevant mechanistic and steric aspects

Abstract

The “positive halogen” reactions of 2:3:4:5-tetrachlorobenzotrichloride and perchlorotoluene with potassium iodide in aqueous acetic acid, at 100°, have been investigated. In the case of the perchlorotoluene the products are the two isomeric perchlorostilbenes along with some 2:3:4:5:6-pentachlorobenzylidene chloride. The chief product of the reaction of 2:3:4:5-tetrachlorobenzotrichloride with potassium iodide is α:α:2:3:4:5:α′:α′:2′:3′:4′:5′- dodecachlorobibenzyl; considerable amounts of the two isomeric α:2:3:4:5:α′:2′:3′:4′:5′-decachlorostilbenes have been isolated. Under the same conditions the α:α:2:3:4:5:α′:α′:2′:3′:4′:5′-dodecachlorobibenzyl reacts with potassium iodide to give the corresponding decachlorostilbenes. When the reaction of perchlorotoluene with potassium iodide is performed in aqueous acetic acid containing toluene the yield of 2:3:4:5:6-pentachlorobenzylidene chloride is greatly increased and benzyl iodide is formed. The mechanisms of these reactions are discussed. It is shown that the results are consistent with the occurence of the reaction through the intermediate α:α-dichlorobenzyl radicals and α:α:α′:α′-tetrachlorobibenzyls, the latter being formed by dimerization of the former. The ultra-violet absorption spectra of the geometric isomers of α:α′-dichlorostilbene, α:2:3:4:5:α′:2′:3′:4′:5′-decachlorostilbene and perchlorostilbene and recorded and interpreted. It is shown that the higher-melting isomerides have the trans configuration and that, as a result of the strong steric interaction between the alpha chloride atoms and the ortho atoms, there exists strong inhibition to resonance in the trans forms.