Konigs first synthesized quinoline in 1879 by passing allylaniline over heated litharge. Shortly after he prepared quinoline by heating the condensation product of aniline and acrolein, thus anticipating the classical Skraup synthesis. This synthesis involves a series of reactions brought about by heating a primary aromatic amine, in which at least one position ortho to the amino group is unsubstituted, with glycerol, sulfuric acid, and an oxidizing agent. The product is a quinoline containing only those substituents that were originally present in the aromatic amine. Quinolines substituted in the hetero ring may be obtained by a modified Skraup synthesis in which a substituted acrolein or a vinyl ketone is used in place of glycerol.