Abstract 2, 3-Diphenylquinoxaline-N-oxide 1 with thionyl chloride gives only 5-chloro-and 6- chloro-quinoxaline and very little deoxygenation. Phenazine-N 5-oxide 2, in the same conditions, gives mainly 2-chlorophenazine and minor amounts of 1-chlorophenazine, but here deoxygenation is an important pathway. Freshly recrystallized para-toluenesulfonyl chloride gives a high chlorination-to-tosyloxylation ratio. All these results point to a ...