Nucleophilic aromatic substitution: transmission of substituent effects in alkaline hydrolysis of 4-aminophenylazobenzothiazolium dyes

Abstract

Alkaline fading of 2′-, 5-, and 6-substituted 2-(4′-aminophenyl)azobenzothiazolium dyes (I) possesses [graphic omitted] a second-order rate constant (k_OH). The value of k_OH is shown to predicted by equation (i) where σ_N is log k_OH= 0.76σ_N+ 0.24σ_S+ 1.69σ⁺+ log[10^{–0.15(pK–9.5})/(1 + 10^{0.25(pK–9.5)})]+ 0.74 (i)σ⁺ for 6- and σ for 5-substituents, σ_s is σ^– for 5- and σ for 6-substituents, σ⁺ is for 2′-substituents, and the pK is that of the leaving 4′-amine. The dye fading is not very sensitive to substituents in the benzothiazole phenyl ring but the charge is transmitted more effectively through nitrogen than through sulphur.