Claus, Adolf Carl Ludwig

(b. Kassel, Germany, 6 June 1838; d. Gut Horheim, near Freiburg in Breisgau, Germany, 4 May 1900),

chemistry.

Claus was one of the first organic chemists to appreciate the importance of understanding the structural relationships of organic compounds, and in his books and papers he helped to make the Kekule Couper structural ideas known to all chemists. Claus entered the University of Marburg in 1858, intending to become a physician, but the lectures of Hermann Kolbe turned his interest to chemistry. He then studied under Wohler at Göttingen. In 1862 he accepted an assistantship at the University of Freiburg im Breisgau, remaining there for the rest of his life. He became the director of the chemical institute at Freiburg in 1883.

With his many students Claus published almost 400 papers in organic chemistry. His experimental researches included the study of alkaloids; the preparation and systematic investigation of quinoline compounds, especially the halogen and sulfonic acid derivatives; the preparation and study of mixed aliphatic-aromatic ketones; and syntheses in the naphthalene, anthracene, and phenanthrene series. His best-known experimental discovery was the synthesis of phenazine in 1872.

Claus was greatly interested in the theoretical problems of organic chemistry and contributed many papers on structural theory. He was a critic of van’t Hoff’s stereochemical views, maintaining that formulas depicting the arrangement of atoms in space were too hypothetical. During the 1890’s he wrote several papers against Hantzsch’s application of stereochemical ideas to the isomeric oximes. Claus argued that the oximes of benzil were not structurally identical and that there was no need to apply geometrical formulas to them. Of the two benzil monoximes, only one was truly an oxime.

the other being the isomeric nitroso alcohol,

Until the end of his life Claus fought for his structural formula of benzene against the rival representations of Kekule, Ladenburg, Dewar, Baeyer, and others. In 1865 Kekule proposed his hexagonal ring structure for benzene. In attempting to solve the problem of the disposition of the fourth valence of each carbon atom in the benzene ring. Kekule suggested an arrangement of alternate single and double bonds.

The presence of the double bond was questionable, since benzene did not display the typical addition reactions of double-bonded carbon compounds. Alternatives to Kekule’s structure appeared almost as soon as it was published, Claus advancing his diagonal formula in 1866. He presented two alternatives:

and

In both of these alternatives, of which he preferred the first, each carbon atom was bound to three others, thereby removing the need for double bonds.

Claus’s structure was a strong rival to Kekule’s. Kekule himself declared that Claus’s formula was an illuminating one, and Baeyer acknowledged that it agreed best with the facts, including those which Kekule’s formula did not explain. In the 1890’s Claus attempted to reconcile his ideas to those of Kekule by suggesting that the inner chemical bonds of benzene were labile and readily formed double or diagonal bonds, depending on the particular chemical change involved.

BIBLIOGRAPHY

I. Original Works. Claus wrote two books on theoretical organic chemistry: Theoretische Betrachntngen and deren Anwendung zur Svstematik der organischen Chemie (Freiburg, 1866) and Grundzuge der modernen Theorien in der organischen Chemie (Freiburg, 1871). His criticism of stereochemical explanations is found in “Zur Kenntnis der Oxime and der sogenannten Stereochemie,” in Journal für praktische Chemie, 44 (1891), 312–335, and 45 (1892), 1–20; and in “Zur Charakteristik der sogenannten Stereochemie des Stickstoffs,” ibid. 46 (1892), 546–559. Among his many papers on the structure of benzene are “Über die Constitution des Benzols and des Naphthalins,” in Berichthe der Deutschen chemischem Gesllschaft,15 (1882), 1405–1411; and Über die Constitution des Benzols,” in Journal für praktische Chemic 42 (1890), 260–267;43 (1891), 321–344; 48 (1893), 576–595; and 49 (1894), 505–524.

II. Secondary Literature. Two informative notices on Claus are Georg, W. A. Kahlbaum, in Biographisches Jahrbuchund deutscher Nekrolog, 9 (19060, 348–349; and G. N. Vis, “Adolf Claus,” in Journal für praktische Chemir, 62 (1900), 127–133,

Albert B. Costa