Two classes of kinetics are exhibited in the y-radiation-induced radical-chain reduction of toluene-p-diazonium ions with the above reducing agents in aqueous oxygen-free solution, the overall reaction being ArNzf + RH2 --bArH + Nz + R + H i . In class A, G(-ArN,+) is proportional to [RH,]/[ArN,+] and independent of dose rate; and in class B, proportional to [ArN,+]/(dose rate)) and independent of [RH,].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 ArNz+ 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 RH, is in the order hypophosphite and phosphite > formate > methanol > formic acid.

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