Effects of Ascorbic Acid on Arenediazonium Salts Reactivity: Kinetics and Mechanism of the Reaction

We have examined the kinetics and mechanism of dediazoniation of o‐, m‐ and p‐methylbenzenediazonium (ArN) tetrafluoroborate in the presence of ascorbic acid (H₂A) at different pHs by combining spectophotometric (VIS‐UV), high performance liquid chromatography (HPLC), and polarographic measurements. Kinetic data show that, at low pH, observed rate constants increase linearly with increasing ascorbic acid concentration, but the saturation kinetics observed at higher pH suggest the formation of a transient diazo‐ether complex preceding the slow step of the reaction. Experimental evidence for the formation of such a complex was obtained from a competitive coupling reaction with the Na salt of `2‐naphthol‐6‐sulfonic acid' and by titration of ascorbic acid (H<sub>2</sub>A) with the arenediazonium ions (electrochemical measurements). HPLC Analysis of dediazoniation products indicates that, in the absence of H<sub>2</sub>A, only the heterolytic phenol derivative, ArOH, is formed quantitatively, in keeping with the predictions of the D<sub>N</sub>+A<sub>N</sub> mechanism. In the pH 2 – 4 range and in the presence of H<sub>2</sub>A, reduction products (ArH) are obtained in addition to heterolytic products (ArOH), corroborating that certain biological reducing agents like ascorbate (HA<sup>−</sup>) are capable of inducing reductive fragmentation of ArN into aryl radicals. All evidence is consistent with two competitive reaction pathways, the thermal decomposition of ArN, and a rate‐limiting decomposition of the transient diazo ether `complex', formed during the reaction of ArN with HA⁻ in a rapid pre‐equilibrium step.