Formation of diazohydroxides ArN₂OH in aqueous acid solution: polarographic determination of the equilibrium constant K_R 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, 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, + to determine, for the first time, the equilibrium constants K R of formation of 4-substituted X-ArN 2 OH (X¼H, Me, MeO, Br, and NO 2 ), which can decompose in several ways including Z-E isomerization or further reaction with OH À to give diazoate ArN 2 O À . 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.

Some Theoretical and Experimental Insights on the Mechanistic Routes Leading to the Spontaneous Grafting of Gold Surfaces by Diazonium Salts

“…On the basis of the work of Bravo-Díaz and co-workers, we will briefly examine the different species that are present in a diazonium solution depending on the acidity of the medium. − In ACN, the diazonium cation is present, but in an aqueous medium, the diazonium cation, diazohydroxide, and diazoate species can be present depending on the pH. In aqueous acidic (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.…”

Spectroscopic studies of arenediazonium ion stability in surfactant solutions

“…In this context heterolytic dediazoniation has been reported to occur with ortho-, meta-and para-methylbenzenediazonium (2MBD, 3MBD, 4MBD) and para-nitrobenzenediazonium (4NBD) ions in an aqueous medium [10,22], while other authors interpreted their results as showing evidence of heterolytic and homolytic processes during the thermal and photochemical dediazoniation of several arenediazonium ions in trifluoroethanol and ethanol [19][20][21]. However, in our latest article, we demonstrated that, at moderate pH, water can act as nucleophile giving rise to a diazohydroxide, which eventually can decompose to give radical products [23].…”

“…Diazonium ions have proved to be very sensitive to their environment, [1,14,22,23] and thus proper understanding of their behaviour in the micellar systems is of great importance [14-16, 24,25]. Hydrophobicity of arenediazonium ion and charge of the micelle are believed to be the most important factors affecting the localization of ions in the micellar system.…”

Two-electron redox chemistry ofp-nitro- andp-cyanobenzene diazohydroxides

“…This extinction coefficient is lower than that of the anti -diazotate and corresponds more closely to that of the diazohydroxide 11 whose formation has been recently inferred by electrochemical means. 19 Aryl diazonium compounds with highly electron-withdrawing substituents on the benzene ring, such as the nitro and cyano groups, demonstrate a faster rate of aqueous hydroxide combination to produce a more acidic diazohydroxide. 16,20 This product is indeed almost absent under the conditions of purely aqueous reaction because of rapid proton loss 21 but is expected to be stabilized with respect to the anionic diazotate in the less polar 50% ACN solvent.…”