Nucleophilic aromatic substitution: effect of complexation on the alkaline hydrolysis of 4-aminophenylazobenzothiazolium dyes

Abstract

Nucleophilic attack by hydroxide ion on 2-(4-dimethylaminophenophenylazo)-3,6-dimethylbenzothiazolium iodide to expel the dimethylamino function is inhibited in aqueous solution by aromatic carboxylate, aromatic sulphonate, and phenolate ions. The inhibition is demonstrated to involve complexation of dye with solute to to form an unreactive species. The dissociation constant of the complex is relatively insensitive to structure except in the case of phenolate ions where electron-withdrawing substituents favour stability. Both charge transfer and electrostatic effects are necessary for efficient complexing ability of the additive. Complexation of cyclodextrin with the dye is followed by reaction to yield a dye–cyclodextrin covalent species; the intermediate slowly decomposes to give quinone imine, the regular product of dye hydrolysis.