Formation of diazohydroxides ArN2OH in aqueous acid solution: polarographic determination of the equilibrium constant KR for the reaction of 4‐substituted arenediazonium ions with H2O

Abstract

In aqueous acid (pH <4) solutions, in the dark, and in the absence of reductants, arenediazonium ions, ArN2+ decompose spontaneously through the rate‐limiting formation of the extremely unstable aryl cation that reacts with any nucleophile present in its solvation shell (DN + AN mechanism). However, in weak acidic and alkaline solutions, ArN2+ react with H2O and OH− at the terminal nitrogen to give azo adducts of the type ArN2OH that are in equilibrium with the parent ArN2+. The diazohydroxide, in this case an acid, is in equilibrium with its conjugate base, and diazoate ArN2O−. The equilibrium constant for reaction with OH− has been determined for a limited number of ArN2+ from kinetic measurements but not with H2O (KR). Here, we have exploited the electrochemical properties of ArN2+ to determine, for the first time, the equilibrium constants KR of formation of 4‐substituted X–ArN2OH (XH, Me, MeO, Br, and NO2), which can decompose in several ways including Z–E isomerization or further reaction with OH− to give diazoate ArN2O−. The technique applied was differential pulse polarography, which is very selective and sensitive. The determined pKR values are 5–6, and they are somewhat higher than those obtained for the reaction of ArN2+ with alcohols ROH (pKDE = 3–5) under similar acidic conditions. The KR 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. Copyright © 2013 John Wiley & Sons, Ltd.