Hofmann, August Wilhelm Von

(b. Giessen, Germany, 8 April 1818; d Berlin, Germany, 2 may 1892)

organic chemistry.

Hofmann’s influence, as a teacher and experimentalist, on British and German chemistry was profound. He was responsible for continuing the method of science teaching by laboratory instruction that had been established and popularized by Liebig at Giessen, and for transporting it to England and to Berlin. He created his own school of chemists who were interested primarily in experimental organic chemistry and the industrial applications of chemistry, rather than in theoretical problems. Among his distinguished pupils and assistants were the Englishmen F. A. Abel, W. Crookes, H. McLeod, C. B. Mansfield, J. A. R. Newlands, E. C. Nicholson, and W. H. Perkin, and the Germans J. P. Griess, C. A. Martius, and J. Volhard.

Hofmann was the son of Johann Philipp Hofmann, the architect who enlarged Liebig’s Giessen laboratories in 1839. He matriculated at Giessen in 1836, electing to study law and languages, but gradually became attracted by Liebig’s chemistry classes. He obtained his doctorate in 1841 for an investigation of coal tar but was prevented from becoming Liebig’s personal assistant until 1843 because of his devotion to his dying father. In the spring of 1845, through Liebig’s influence, he was made Privatdozent at the University of Bonn. In the autumn of 1845, following guarantees of tenure at Bonn arranged by Queen Victoria’s German consort, Albert, he agreed to direct the Royal College of Chemistry in London, founded by some of Liebig’s English pupils as a private school for the training of agricultural, pharmaceutical, geological, and industrial chemists. Despite the financial insecurity of the college until it was absorbed by the government-sponsored School of Mines in 1853, Hofmann remained in London for twenty years. Although offered a chair of chemistry at Bonn in 1863, he preferred to accept the more commanding post at Berlin when it was left vacant by the death of Mitscherlich in the same year. After designing new laboratories for both Bonn (taken by Kekulé) and Berlin, he returned to Germany in 1865. In 1867 he was a founder of the Deutsche Chemische Gesellschaft, on the model of the Chemical Society, of which he was a prominent member. Hofmann was ennobled on his seventieth birthday.

Despite his preeminence as a practical teacher, Hofmann was personally incompetent in the laboratory; consequently he looked for, and showed extraordinary acumen in finding, unusual skills in his assistants, who devotedly performed much of his experimental work. A humorous, lovable, cosmopolitan, and intelligent man, he made a considerable fortune from his scientific work in both England and Germany. He was a good speaker in several languages, although in formal lectures or in letters he often overindulged in literary embellishment and circumlocution. His sense of style is most apparent in a large number of biographical notices and other essays on the history of chemistry (for example, his study of Liebig) which are still consulted by historians.

Hofmann had a thoroughly pragmatic attitude toward the relationship between useful experiments and theory, arguing: “As a chemical theory expands and becomes more and more consolidated, the interest attached to the individual compounds used as scaffolding in raising the structure becomes less and less, diminishing... in the inverse ratio of the number of compounds which the theory suggests” (Proceedings of the Royal Society, 11 [1860–1862], 425). On the other hand, although undoubtedly guided in his work by certain theoretical principles—particularly those of Laurent, the type theory, and the principle of homology—he was never interested in devising new theories. He remained the practical chemist.

The theme and variations of Hofmann’s voluminous scientific publications were coal tar and its derivatives. In his first publication (1843) he clarified a confused situation by showing that many substances which were identified in contemporary chemical literature as obtainable from coal tar naphtha and its derivatives were all a single nitrogenous base, aniline. Both Hofmann and the iconoclastic Laurent (who visited Giessen briefly in 1843) suspected that aniline was related to phenol, which (in modern terms) Laurent correctly supposed to be a hydrate of the phenyl radical, C₆ H₅. This relationship was confirmed and elucidated when they successfully converted phenol into aniline by the action of ammonia. Progressive chlorination of aniline, which could be satisfactorily explained only by Laurent’s theory of direct chlorine-hydrogen substitution, gradually weakened its basicity. In this manner Hofmann found further evidence for the incredibility of Berzelius’ electrochemical theory, in which such substitutions were deemed impossible without the wholesale disruption of the molecule; and he opened a way for the later reconciliation of ideas of polarity with unitary views of molecular constitution. Nevertheless, Hofmann was at first inclined to accept Berzelius’ judgment that alkaloids and organic bases (like aniline) were formed by the conjugation of a hydrocarbon radical with ammonia. But by 1849 experiments on a variety of volatile nitrogenous bases led him to suspect that they were substituted ammonia compounds, as Liebig had suggested in 1837.

Wurtz’s innocent preparation of the first aliphatic amines, methylamine and ethylamine, by the action of caustic soda on isocyanates in 1849 was crucial to Hofmann’s change of position. In a classic paper, “The Molecular Constitution of the Volatile Organic Bases” (Philosophical Transactions140 [1850], 93–131), he showed how amines could be prepared directly from ammonia by the action of alkyl iodides; whence he concluded that organic bases were substituted ammonia in which hydrogen was replaced by hydrocarbon radicals:

The analogy of these primary, secondary, and tertiary amines (as Gerhardt later called them) with ammonia was completed in 1851, when Hofmann prepared crystallline quaternary salts that were analogous to ammonium salts (for instance, tetraethylammonium iodide, (C₂ H₅)₄NI). These discoveries made Hofmann’s reputation and subsequently formed one of the pillars of the type theory of Gerhardt and Willilamson, in which both organic and inorganic compounds were systematized and classified according to the model formula of one of four inorganic molecules—hydrogen, hydrogen chloride, water, or ammonia—by the substitution of one or more atoms of hydrogen for an equivalent atom or group. In his own researches Hofmann exploited only the ammonia type, although in his interesting and much translated textbook, An Introduction to Modern Chemistry (1865), he used all four types for pedagogic purposes.

Hofmann’s lifelong interest in the nitrogen bases, which included the development of methods for separating mixtures of amines and the preparation of large numbers of “polyammonias” (diamines and triamines such as ethylenediamine and diethylenediamine), was extended to phosphorus bases in joint work with Cahours between 1855 and 1857. In another collaboration with Cahours in 1857, Hofmann prepared the first aliphatic unsaturated alcohol, allyl alcohol, C₃ H₅OH. Subsequently, in 1868, Hofmann’s investigation of its derivative, allyl isothiocyanate (mustard oil), led to the preparation of many sulfur analogs of the isocyanates and the developmetn of an elegant and heroic method for the preparation of the disgusting isonitriles (isocyanides, or carbylamines) by the action of alkalinated chloroform on primary amines. Hofmann’s love of analogy and his tenacity and thoroughness are well illustrated by his intermittent investigation of these nauseous compounds and by his twenty-year search for the lower homologue of acetaldehyde. His conviction that a methyl aldehyde (formaldehyde) must exist was rewarded in 1867 when he passed an air stream of methyl alcohol over incandescent platinum.

In 1848 Hofmann’s eccentric student C. B. Mansfield devised the fundamental method of fractional distillation of coal tar for the separation of pure benzene, xylene, and toluene, thus laying the foundation for the coal tar products industries. Little was known of the detailed structures of organic compounds in the 1850’s; nevertheless, there were many rational attempts to synthesize important natural products. In 1856 another of his students, W. H. Perkin, privately attempted to synthesize quinine but was led instead to the preparation, and subsequent production, of the first artificial dyestuff, aniline purple, or mauve. In view of Perkin’s youth and inexperience, Hofmann tried, without success, to dissuade him from embarking on an uncertain industrial venture. Many of Hofmann’s other pupils also became involved in the British dyestuffs industry, notably E. C. Nicholson, G. Maule, and G. Simpson, whose firm progressed from the manufacture of nitrobenzene and aniline to that of aniline dyes.

Hofmann was fascinated by the chemistry of dyes; and although he was not interested in the problems of large-scale industrial research, he well understood what the ideal symbiotic relationship between pure and applied research should be. It was failure to understand this, and commercial ineptitude, which allowed the initial British advantage in dyestuffs to be lost to Germany following Hofmann’s return and the early retirement of his English pupils after they had made their fortunes. Hofmann’s German pupils (like Martius) proved of different mettle, while German manufacturers, unlike their British counterparts, grasped the idea that the secret of commercial success lay in scientific research. Although a more complex matter, some later commentators thus attributed the decline of the British chemical industry to Hofmann’s departure from London.

In 1862 Hofmann isolated from the French commercial dye fuchsine (or magenta) a triamine which was identical with the crimson solution he had obtained in 1858 when reacting carbon tetrachloride with commercially impure aniline. This dye, a derivative of triphenylmethane which Hofmann named rosaniline, could not be prepared from pure aniline, for orthotoluidine and paratoluidine had to be present, as in the commercial product. In 1863 he succeeded in displacing hydrogen in rosaniline with aniline (thus phenylating a compound for the first time) and preparing the beautiful diphenylrosaniline, or aniline blue. When alkyl groups were substituted in rosaniline, Hofmann found that an exciting range of colors from blue to violet was produced. He patented these “Hofmann’s violets” (trimethylrosaniline and triethylrosaniline) in 1863 and, despite their instability, their brilliant hues enjoyed a considerable commercial success.

In the controversies between the old and modern chemical notations, Hofmann sided with innovation. From 1860 on, he adopted Gerhardt’s, and subsequently Cannizzaro’s, atomic weights based on the value 16 for oxygen. He also played a significant role in the dissemination of the concept of valence—the word is derived from Hofmann’s term “quantivalence” (1865). As a teacher he devoted much time and skill to devising interesting lecture experiments, many of which are still used. He was the first chemist to popularize atomic models (in 1865). But such was the speed of chemical progress in the 1860’s that, to some extent, Hofmann was left behind. He continued to use type formulas long after younger chemists, like Kekule, had adopted the structural theory of carbon compounds. On the other hand, in 1865, inspired by Laurent, he suggested a systematic nomenclature for hydrocarbons and their derivatives which was adopted internationally, with modifications, by the Geneva Congress in 1892.

BIBLIOGRAPHY

I. Original Works. Over 300 of Hofmann’s papers are listed in the Royal Society Catalogue of Scientific Papers, III, VII, X, XV (London, 1867–1925), with obituaries listed in XV. Books and essays are included in British Museum General Catalogue of Printed Books, CV. An unpublished bibliography listing 377 items is Kathleen Mary Hammond, “August Wilhelm von Hofmann” (University of London, diploma in librarianship, 1967), copies of which are at University College, London, and Imperial College Archives, London. Hofmann wrote some fifty obituary notices, which are not listed in the aforementioned items but are indexed in Lepsius (see below).

MS collection include Imperial College Archives, London; Bayerische Staatsbibliothek, Munich (the Liebig-Hofmann correspondence, of which a German-English ed. by E. Wangermann and W. H. Brock is in progress); Chemische Gesellschaft in der D.D.R., Berlin; and Vieweg, Brunswick.

II. Secondary Literature. The basic studies are J. Volhard and E. Fischer, August Wilhelm von Hofmann, ein Lebensbild (Berlin, 1902), also pub. as a special no. of Berichte der Deutschen chemischen Gesellschaft, 35 (1902); and the Hofmann memorial lectures by F. A. Abel, H. E. Armstrong, W. H. Perkin, and L. Playfair, in Journal of the Chemical Society, 69 (1896), 575–732, repr, in Memorial lectures Delivered Before the Chemical Society, 1893–1900, I (London, 1901). Also useful is B. Lepsius, Festschrift zur Feier des 50 jährigen Bestehens der Deutschen chemischen Gesellschaft (Berlin, 1918), a special no. of Berichte der Deutschen chemischen Edutschem chemischen Gesellschaft, 50 (1918). All the important literature on Hofmann, together with an analysis of his work, is given in J. R. Partington, A History of Chemistry, IV (London-New York, 1964), 432–444; and in an unpublished thesis by J. R. F. Guy, “Life and Work of A. W. Hofmann,” B. S. thesis (Oxford, 1969). See also J. Bentley, “The Chemical Department of the Royal School of Mines. Its Origins and Development Under A. W. Hofmann,” in Ambix,17 (1970), 153–181 ; and, for Hofmann’s industrial influences, John J. Beer, The Emergence of the German Dye Industry (Urbana, Ill., 1959), passim; and E. R. Ward, “Charles Blatchford Mansfield, Coal Tar Chemist and Social Reformer,” in Chemistry and Industry (25 October 1969), pp. 1530–1537.

W. H. Brock

August Wilhelm von Hofmann

The German chemist August Wilhelm von Hofmann (1818-1892) was one of the most influential organic chemists and teachers of the century.

August Wilhelm von Hofmann was born on April 8, 1818, in the small university town of Giessen. In 1836 he entered the University of Giessen, where he studied law, philosophy, and mathematics. However, in 1843 he turned to the study of chemistry. He received his doctorate summa cum laude with a thesis entitled Chemical Investigation of the Organic Bases in Coal Tar, a field in which he was destined to achieve worldwide fame.

In 1845, while holding a professorship at the University of Bonn, Hofmann derived analine from benzene and thus laid one of the foundations of the synthetic dye industry. He also worked out the problem of substituting atoms of chlorine for the hydrogen atoms of the aromatic compounds. This work won the coveted Gold Medal of the Parisian Societé de Pharmacie and made Hofmann famous. He received and accepted an invitation from Queen Victoria in 1845 to transfer his scientific activities to London.

Working in England as researcher and teacher for 20 years, Hofmann trained a generation of brilliant chemists, including Frederick Abel, Warren de La Rue, E.C. Nicholson, Charles Mansfield, William H. Perkins, and Sir William Crookes, and a host of future leaders of the German chemical industry such as Peter Griess, George Merck, C.A. Martius, and Jacob Vilhard. In 1865 Hofmann became professor of chemistry at the University of Berlin.

Honored by the leading scientific societies of Europe, Hofmann worked in a spacious new laboratory built by a grateful government. During this period his researches were directly related to the meteoric rise of the German dye and pharmaceutical industries, which with coal and iron were the foundations of the industrial supremacy of Wilhelmine Germany. Critical to this success was what became known as the Hofmann degradation process, the successive reduction of the length of a carbon chain through treating the amides of fatty acids with bromine and alkali. Indigo was produced industrially by precisely these steps. Another of Hofmann's industrially significant accomplishments was the production of formaldehyde by passing vapors of methyl alcohol over hot platinum.

Though most of Hofmann's 360 major papers grew out of his work with the derivatives of coal tar and the synthesis of related organic compounds, he also contributed to the chemistry of cadmium, antimony, phosphorus, and titanium. He discovered the quaternary ammonium salts and was thus led to classify all amines as formal derivatives of ammonia—an idea which was the foundation of the later "theory of types" of Charles F. Gerhardt.

Hofmann was married four times and was the father of 11 children. In 1868 he helped found the German Chemical Society and served as its president 14 times. Never spoiled by fame or fortune, he continued his teaching and writing until the very end, on May 2, 1892, in Berlin.

Further Reading

Eduard Farber, ed., Great Chemists (1961), contains a section on Hofmann. See also Archibold Clow, The Chemical Revolution: A Contribution to Social Technology (1952); Eduard Farber, The Evolution of Chemistry: A History of Its Ideas, Methods, and Materials (1952; 2d ed. 1969); and James R. Partington, A History of Chemistry, vol. 4 (1964). □

August Wilhelm von Hofmann

1818-1892

German chemist who discovered several important chemical compounds, founded the aniline dye industry, and developed a method of determining molecular weights. His systematic studies of aniline resulted in its extensive use in dyeing textiles. He discovered formaldehyde and allyl alcohol and developed the Hofman reaction, the standard reaction for converting amides into amines. A major contribution to chemistry was the development of a method of determining molecular weights of liquid organic compounds by measuring vapor densities.