Rates of reaction of singlet oxygen with some systemic pyrimidine fungicides and related compounds

Rate constants have been measured for the reaction in chloroform solution of singlet oxygen with the fungicides ethirimol (5‐butyl‐2‐ethylamino‐6‐methylpyrimidin‐4‐ol) and dimethirimol (5‐butyl‐2‐dimethylamino‐6‐methylpyrimidin‐4‐ol), and with the compounds 2‐dimethylamino‐5,6‐dimethylpyrimidin‐4‐ol, 2‐dimethylamino‐6‐methylpyrimidin‐4‐ol, 4‐benzyloxy‐2‐dimethylamino‐5,6‐dimethylpyrimidine and 2‐diethylamino‐6‐methylpyrimidin‐4‐ol. The values obtained show that singlet oxygen reacts readily with these compounds; the differences between the rate constant values have been rationalised in terms of the different structural features of the compounds studied. The possibility that singlet oxygen may react with agricultural chemicals under natural conditions is considered.