Equimolar quantities of carbonohydrazide and carbodi-imides react in dimethyl sulphoxide or dimethylformamide to yield 4-substituted 3-aryl(or alkyl)amino-1,2,4-triazolin-5-ones. The use of an excess of carbodi-imide produces, in addition, 3,5-bisaryl(or alkyl)amino-4-aryl(or alkyl)-1,2,4-triazoles.

1-Isopropylidenecarbonohydrazide and carbodi-imide also yield these two 1,2,4-triazole derivatives, but 1-arylidenecarbonohydrazides afford acyclic mono-adducts. These 1-arylidene-5(NN′-diarylamidino)carbonohydrazides are ring-closed by acid, or on thermolysis, to 4-aryl-3-arylamino-1,2,4-triazolin-5-ones. The probable addition–cyclisation mechanism of these reactions is discussed.