Formation of diazohydroxides ArN 2 OH in aqueous acid solution: polarographic determination of the equilibrium constant K R for the reaction of 4‐substituted arenediazonium ions with H 2 O

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 2 O and OH − at the terminal nitrogen to give azo adducts of the type ArN 2 OH 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 2 O − . The equilibrium constant for reaction with OH − has been determined for a limited number of ArN 2 + from kinetic measurements but not with H 2 O ( 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 2 OH (XH, 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 2 O − . The technique applied was differential pulse polarography, which is very selective and sensitive. The determined p K R values are 5–6, and they are somewhat higher than those obtained for the reaction of ArN 2 + with alcohols ROH (p K 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. Copyright © 2013 John Wiley & Sons, Ltd.