THE INSECTICIDAL ACTIVITY OF PARATHION, ITS ISOMERS AND SOME RELATED COMPOUNDS

Heat treatment of parathion at 140° C. and above resulted in isomerization and then thermal decomposition; the loss of toxicity to Calandra granaria being correlated with a reduction of the thiono‐sulphur content. Similar results were obtained with O:O‐dimethyl‐(4‐nitrophenyl)‐thiophosphate. Paraoxon, O:S‐diethyl‐(4‐nitrophenyl)‐thiophosphate, and O:O‐diethyl‐S‐(4‐nitrophenyl)‐thiophosphate, although all possessing considerable contact activity, were less insecticidal than parathion; O:O‐dimethyl‐(4‐nitrophenyl)‐thiophosphate, on the other hand, was considerably more effective than parathion. O:O‐ bis (2‐chloroethyl)‐O‐(4‐nitrophenyl)‐thiophosphate was considerably less toxic than parathion and appeared to have a different mode of action. Compounds, S‐ethyl‐ bis ‐(4‐nitrophenyl)‐thiophosphate, O‐ethyl‐ bis ‐(4‐nitrophenyl)‐thiophosphate, and triethyl thiophosphate were ineffective as contact insecticides. In the series of compounds examined, replacement of the group P = S by P = O or alteration in size of the groups attached to the central phosphorus atom caused a reduction of insecticidal activity.