Deiodination kinetics of water-soluble radiopaques

Deiodination of diatrizoic acid, an anionic radiopaque, was found to be catalyzed by Cu(II). Through a detailed study of o-iodobenzoic acid, a model compound, the copper-catalyzed SN1 mechanism was established based on observations of common ion, salt, and pH effects. Meta- and para-iodobenzoic acids were unreactive. Deiodination thus was facilitated by a neighboring carboxylate that attracted copper. Iopamidol, a nonionic radiopaque, also underwent deiodination. At pH 7 or above, the hydroxide-ion substitution predominated. At pH below 7, the reaction is in favor of the copper-catalyzed SN1 mechanism.