Hantzsch, Arthur Rudolf

(b. Dresden, Germany, 7 March 1857; d. Dresden, 14 March 1935)

chemistry.

The son of Rudolf Georg Hantzsch, a Dresden wine merchant, Hantzsch attended the Dresden Polytechnic (now Technische Hochschule) from 1875 to 1879. He studied chemistry under Rudolf Schmitt, a pupil of Hermann Kolbe, and did his doctoral work under Schmitt’s direction. Dresden was not entitled to grant the doctor’s degree, however, and Hantzsch therefore obtained the degree after attending the University of Würzburg for one semester. He was an assistant at the Institute for Physical Chemistry in Leipzig before becoming a professor at the Zurich Polytechnic in 1885. He succeeded Emil Fischer at Würzburg in 1893 and ten years later succeeded Johannes Wislicenus at Leipzig. In 1883 Hantzsch married Katherine Schilling, by whom he had three children. She died in 1904 and in 1911 he married Hedwig Steiner. Hantzsch was a member of the academies of Göttingen, Halle, Leipzig, Vienna, and Zurich, and of the German and London chemical societies.

Hantzsch’s earliest work was in organic synthesis. In 1882 he announced a general method of synthesis for pyridine compounds from α-keto esters and aldehyde ammonia compounds. He then turned to the synthesis of other heterocyclic types. After Victor Meyer had noted the mimicry between benzene and thiophene, Hantzsch proposed a similar relationship between pyridine and thiazole.

He synthesized thiazole in 1887 and suggested that other aromatics may exist, arguing that imidazole, oxazole, and selenazole were analogues of thiazole. By new synthetic methods he prepared all of these aromatic types.

In 1890 Hantzsch and Alfred Werner, his student at Zurich, launched the stereochemistry of nitrogen compounds. They explained the existence of two monoximes and three dioximes of benzil by proposing that the valences of trivalent nitrogen were nonplanar and disposed along three sides of a tetrahedron. Hence, compounds with carbon-nitrogen double bonds should exhibit geometrical isomerism. There should be two benzilmonoximes, which they named syn and anti forms, and three dioximes:

The paper presented a new theory, although with very little experimental support. In one year of work Hantzsch provided overwhelming evidence for the stereochemistry of nitrogen. He also considered the assignment of configuration. He used the Beckmann rearrangement and various elimination reactions to determine oxime configurations, based on the assumption that cis groups rearrange and eliminate. All of Hantzsch’s syn and anti configurations subsequently had to be reversed with the realization that trans groups were involved in these changes.

Hantzch extended his theory to nitrogen-nitrogen double bonds in 1894 with his first paper on diazo compounds, which began a long, acrimonious controversy with Ludwig Bamberger. Three distinct diazo families existed: diazonium salts, normal diazotates, and isodiazotates. Bamberger argued that the isodiazo compounds were nitrosamines (Ar—NH—NO) and the isomeric normal diazotates were true diazo compounds (Ar—N=N—OH). Hantzsch established that they were not structural isomers but syn and anti forms.

The controversy with Bamberger, in which they exchanged many papers, was decisive in the development of organic chemistry. Hantzsch used physicochemical data from cryoscopic, conductivity, and absorption spectra studies. Bamberger used only reactions and syntheses for evidence of structure. He distrusted the physicochemical methods and arguments of Hantzsch, boasting that he used only pure organic chemical methods. Working with unstable compounds which changed into tautomeric forms in solution, he was at a disadvantage, whereas Hantzsch’s methods enabled him to elucidate the complex interrelations of diazo compounds.

His work on stable and labile tautomeric diazo forms led to the discovery in 1896 that phenylnitromethane forms a salt not in the neutral nitro form but in the tautomeric aci form:

He called the more stable nitro form a “pseudo acid.”

From 1899 he developed a general theory of pseudo acids and bases as neutral compounds which can undergo reversible isomeric change into acids and bases respectively. Hantzsch proved that the true acids or bases corresponding to the pseudo forms were often much stronger than the common organic acids and bases. Methylquniolinium hydroxide or the di-and triphenylmethane bases were colored, ionized bases which underwent isomerization into the colorless, nonionized pseudo bases. Hantzsch proved the existence of such isomeric forms in many cases. In the aromatic series the colored form always possessed a quinonoid structure. He contributed to the theory of indicators by proposing that indicator action was an intramolecular change of quinonoid and nonquinonoid forms.

From 1906 Hantzsch was largely concerned with the study of the absorption spectra of organic compounds and the relation of color and constitution. He found evidence of a new constitutional type, the conjugated aci form, utilizing Alfred Werner’s idea of partial valences and intramolecular complexes. Hantzsch’s views on conjugation changed several times. By 1919 he recognized the limitations of symbolism and structural formulas and acknowledged that the difference between quinonoid and non-quinonoid structures in a conjugated complex did not really exist, and his formulas were accordingly simplified. Hantzsch’s intent in eliminating the quinonoid character from his formulas was to show that all formulas were incomplete and that conjugation could not be expressed by any static formula or even as a dynamic equilibrium between different forms. His views came very near to those of the resonance theory, where forms are not expressible by ordinary structural formulas.

Hantzsch’s last important investigations began in 1917 with a paper on the absorption spectra of carboxylic acids. He detected the presence of two forms of acid, depending on the solvent used, and proposed two carboxylic acid structures:

These represented the true and pseudo acid forms respectively. The former possessed an ionizable hydrogen atom, whereas the pseudo acid in equilibrium with it did not.

Hantzsch thought that similar true and pseudo forms contributed to the nature of all acids, including the mineral acids. Furthermore, he obtained evidence that in aqueous solution the true acids were present as hydronium salts ([H₃ O] X); in every case the acidic function attributed to the hydrogen ion in the Arrhenius-Ostwald theory was that of the hydronium ion.

From 1917 to 1927 he investigated the whole range of organic and inorganic acids by means of their absorption spectra, molecular refractivity, electrical conductivity, and relative stability of their compounds. In 1927 he summarized his results and stated his final views. He abandoned his earlier hypothesis that acids exist in true and pseudo forms. He thus considered all acids to be pseudo acids only, ionizing by forming a hydronium salt with a suitable solvent, the degree of ionization being dependent on the extent to which these salts are formed. There was no single favorable observation for the existence of free true acids.

Regarding all acids as pseudo acids, Hantzsch adopted as the criterion of acidity the tendency to form salts and measured the relative strength of acids by several different methods. He proved that the “strong” acids differed greatly, perchloric and being the strongest, and established the now accepted sequence of mineral acid strength. Hantzsch’s investigations broadened the conception of acids, showing that their properties depended on reaction with a solvent.

BIBLIOGRAPHY

I. Original Works. Hantzsch wrote monographs on his major areas of investigation. His Grundriss der stereochemie (Leipzig, 1893; 2nd ed., 1904) appeared in French and English eds. as précis de stereochemie, Guye and Gautier, trans. (Paris, 1896) and The Elements of sterteochemistry, C. G. L. Wolf, trans. (Easton, Pa., 1901). His Die Diazoverbindungen was published at Stuttgart in 1902; the 2nd ed. (Berlin, 1921) was written with G. Reddelien. His theory of true and pseudo forms is found in Die Theorie der ionogenen Bindung als Grundlage der Ionentheorie (Leipzig, 1923).

Among his important papers are “⃜ber die synthese pyridinartiger Verbindungen aus Acetessigather and Aldehydammoniak,” in Annalen der Chemic, 215 (1882), 1-82; “Über Verbindungen des Thiazols,” in Berichte der Deutschen Chemischen Gesellschaft, 20 (1887), 3118-3132, written with J. H. Weber: “Uber raumliche Anordnung der Atome in stickstoffhaltigen Molekulen,” ibid., 23 (1890), 11-30, written with Alfred Werner; “Die Bestimmung der räumlichen Configuration stereoisomerer Oxime,” ibid., 24 (1891), 13-31; “Uber Stereoisomerie bei Diazoverbindungen und die Natur der Isodiazokorper,” ibid., 27 (1894), 1702-1725: “Über Isomerie beim Phenylnitromethan,” ibid., 29 (1896), 699-703, written with Otto Schultze; “Zur Constitutionsbestimmung von Körpern mit labilen Atomgruppen,” ibid., 32 (1899), 579-600; “Optische Untersuchungen von Diazo- und Azo-Verbinungen,” ibid., 45 (1912), 3011–3036, written with J. Lifschitz; “Die optischen und chemischen Veränderungen der organischen Nitroderivate und die stereochemische Erklürung ihrer Isomerien,” in Annalen der Chemie, 492 (1931), 65–104; “Ü die Konstitution der Carbonsäuren sowire über die optischen und chemischen Vorgänge bei der Bildung von Estern, Salzen, und Ionen,” in Berichte der Deutschen chemischen Gesellschaft, 50 (1917), 1422–1457; and “Reaktionskinetische Untersuchungen as starken Säuren,” in Zeitschrift fü physikalische Chemie, 125 (1927), 251–263, written with A. Weissberger.

II. Secondary Literature. The best accounts of Hantzsch’s work were written by his former students: Arnold Weissberger, in Eduard Farber, ed., Great Chemists (New York, 1961), pp. 1065-1083; T. S. Moore, “The Hantzsch Memorial Lecture,” in Journal of the Chemical Society (1936), 1051-1066; and Franz Hein, “Arthur Hantzsch 1857‘1935,” in Berichte der Deutschen chemischen Gesellschaft, 74 (1941), 147-163. See also A. Burawoy, “Arthur Hantzsch,” in Berichte der Deutschen chemischen Gesellschaft, 68 (1935), 65-68; F. Hein, “A. Hantzsch” in Zeitschrift für Elektrochemie und angewandte physikalische Chemie, 42 (1936), 1‘4; B. Helferich, “Nachruf auf Arthur Hantzsch,” in Berichte über the Verhandlungen der Sächsischen Akademie der Wissenschaften zu Leipzig, 87 (1935), 213-222; and C. Paal, “Arthur Hantzsch zum 70. Gebrtstage,” in Zeitschrift für angewandte Chemie, 40 (1927), 301‘303.

Albert B. Costa