Knorr, Ludwig

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(b. Munich, Germany, 2December 1859; d. Jena, Germany, 4 June 1921), chemistry.

Knorr, one of give sons of Angelo Knorr, a merchant, attended gymnasium in Munich. He began to Study chemistry at the university there in 1878. and also attended the universities of Erlangen and Heidelberg; at the latter he spent a term working under Robert Bunsen. In 1879 the organic chemist Emil Fisher accepted a professorship at Munich and offered knorr an assistant ship in the department of chemistry. In 1882 Knorr moved with Fischer to Erlangen and took his doctorate there with a dissertation on piperyl-hydrazine. He married the daughter of the artist C. Piloty before moving in 1885, again with Fischer, to the University of Würzburg, where he became assistant professor. In 1889 Knorr was elected to the chair of chemistry at the University of Jena.

During the last decades of the nineteenth century many of the “name reactions”of synthetic organic chemistry were developed and extended as German organic chemistry responded to the challenge of producing in the laboratory both natural and novel organic complexes, and contributed to the needs of a growing chemical industry for drugs and dyestuffs. Early in his career knorr began important work on heterocyclic compounds. especially the pyrazolones (1883–1911). He worked on several syntheses and by condensing acetoacetic acid and phenylhydrazine, he produced the compound 3-methyl, I-phenyl pyrazolone which when methylated, produced antipyrine, a chemical that was shown to reduce fever. He also synthesized by condensation reactions other complicated heterocyclic compounds, such as quinoline (1886), pyrazole (1902). (The pyrrole ring is the basic unit of the porphyrin system that occurs, for example. in hemoglobin and chlorophyll.) In the 1890’s Knorr also did much work on the complicated structure of the vegetable alkaloid morphine and discovered morpholine.

Besides his addition to synthetic organic chemistry, knorr contributed significantly to the theoretical problem of tautomerism. In 1863 acetoacetic ester (ethyl β-ketobutyrate) was isolated. It became evident that it reacts as though it possesses two different structural formulas. In fact it exhibits keto-enol tautomerism, in which the structures differ in the point of attachment of a hydrogen atom:

Along with other German chemists such as Ludwig Claisen and Johannes Wislicenus, Knorr became interested in the phenomenon, and in the 1890’s he worked on the keto-enol allelotropic mixtures of diacetylsuccinic ester, acetylacetone, and acetoacetic ester. In 1911 he prepared the two tautomeric forms of acetoacetic ester separately, each free of contamination by the other. The keto form was obtained by cooling a solution of the ordinary ester in alcohol and ether to – 78°C. The enol form was obtained by the action of dry hydrogen chloride gas on a suspension of sodium acetoacetic ester at the same temperature. By measuring the refractive indexes of the enol and keto forms, and also that of the equilibrium mixture, Knorr was able to calculate that the latter contains only 7 percent of the enol form. K. H. Meyer, working independently, succeeded in obtaining pure keto and enol isomers at about the same time.


I. Original Works. A list of Knorr’s scientific papers is in J. C. Poggendorff, Biographisch–literarisches Handwörterbuch der exakten Naturwissenschaften, IV, 768– 769, V, 645–646, and VI, 1345.

II. Secondary Literature. P. Duden and H. P. Kaufmann, “Ludwig Knorr zum Gedächtnis,” in Berichte der Deutschen Chemischen Gesellschaft, 60A (1927), I– 34; H. W. Flemming, ed., “Ludwig Knorr”, Dokumente aus Höchster Archiven no. 31 (Frankfurt, 1967); and R. Scholl, “Ludwing Knorr,” in Sitzungsberichte der Söchsischen Akademie der Wissenschaften zu Leipzig, 75 (1923), 155–165.

Neil Morgan