Free-radical reductions of arenediazonium ions in aqueous solution. Part II. Kinetics of reactions with formate, cysteine, methanol, hypophosphite, and phosphite as reducing agents

Abstract

Two classes of kinetics are exhibited in the γ-radiation-induced radical-chain reduction of toluene-p-diazonium ions with the above reducing agents in aqueous oxygen-free solution, the overall reaction being ArN₂⁺+ RH₂→ ArH + N₂+ R + H⁺. In class A, G(–ArN₂⁺) is proportional to [RH₂]/[ArN₂⁺] and independent of dose rate; and in class B, proportional to [ArN₂⁺]/(dose rate)^1/2 and independent of [RH₂]. A reaction sequence to account for these observations is proposed, the relative rates of the reactions involved determining the class of kinetics exhibited. By altering the concentration of ArN₂⁺ it is possible to change the behaviour of the system from one class to the other. The rate of hydrogen abstraction by the p-tolyl radical from RH₂ is in the order hypophosphite and phosphite > formate > methanol > formic acid. Addition of acid decreased G(–ArN₂⁺) in the case of the oxyanions, but not for methanol.