Formation of diazohydroxides ArN_2OH in aqueous acid solution: polarographic determination of the equilibrium constant K_R for the reaction of 4-substituted arenediazonium ions with H_2O

摘要

In aqueous acid (pH <4) solutions, in the dark, and in the absence of reductants, arenediazonium ions, ArN_2~+ decompose spontaneously through the rate-limiting formation of the extremely unstable aryl cation that reacts with any nucleophile present in its solvation shell (D_N+A_N mechanism). However, in weak acidic and alkaline solutions, ArN_2~+ react with H_2O and OH~- at the terminal nitrogen to give azo adducts of the type ArN_2OH that are in equilibrium with the parent ArN_2~+. The diazohydroxide, in this case an acid, is in equilibrium with its conjugate base, and diazoate ArN_2O~-. The equilibrium constant for reaction with OH~- has been determined for a limited number of ArN_2~+ from kinetic measurements but not with H_2O (K_R). Here, we have exploited the electrochemical properties of ArN_2~+ to determine, for the first time, the equilibrium constants K_R of formation of 4-substituted X–ArN_2OH (X=H, Me, MeO, Br, and NO_2), which can decompose in several ways including Z–E isomerization or further reaction with OH~- to give diazoate ArN_2O~-. The technique applied was differential pulse polarography, which is very selective and sensitive. The determined pK_R values are 5–6, and they are somewhat higher than those obtained for the reaction of ArN_2~+ with alcohols ROH (pK_(DE)=3–5) under similar acidic conditions. The K_R values are not very sensitive to changes in the nature of the substituent in the aromatic ring and a linear Hammett plot with a slope of ρ = 0.58 was obtained.