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Couper, Archibald Scott

Couper, Archibald Scott

(b. Kirkintilloch, Dumbartonshire, Scotland, 31 March 1831; d. Kirkintilloch, 11 March 1892)

chemistry.

Couper, at the same time as F. A. Kekulé and independently of him, introduced principles that underlie the modern structural theory of organic chemistry. He also introduced into organic chemistry lines to represent valence bonds in chemical formulas. All his papers were published in barely over a year, after which ill health terminated his scientific career. Details of Couper’s life were brought to light by Richard Anschütz, Kekulé’s biographer and successor at Bonn.

Couper was the only surviving son of Archibald and Helen Dollar Couper. His father was proprietor of a large cotton-weaving business that had been owned by the family for several generations. Couper received most of his early education at home and then went to Glasgow University for study in the humanities and classical languages. The summers of 1851 and 1852 he spent in Germany, and in August 1852 he entered the University of Edinburgh, studying logic and metaphysics under Sir William Hamilton while continuing his language studies. During the year 1854–1855 Couper moved to Berlin, where, sometime before his departure for Paris in 1856, he chose chemistry as his field of major interest. He is known to have attended Sonnenschein’s lectures in analytical chemistry at the University of Berlin, and to have worked two months in his laboratory.

After moving to Paris in August 1856, Couper engaged in independent research in the laboratory of Charles Adolphe Wurtz. His first publication—on the bromination of benzene—appeared in August 1857. His other experimental paper, on salicylic acid, was published in 1858. Early in 1858 Couper asked Wurtz to have presented before the French Academy of Sciences a paper entitled “On a New Chemical Theory.” Wurtz, not a member of the Academy, procrastinated before finding someone to present it. In the intervening period Kekulé’s paper, dated 19 May 1858, appeared in Liebig’s Annalen.1 Couper’s paper—on essentially the same topic—was presented to the French Academy by J. B. A. Dumas in June and was published soon thereafter. Kekulé attacked the paper almost immediately, claiming priority and greater significance for his own work.2 He also questioned Couper’s researches on salicylic acid and, since Couper never answered, his name was long forgotten.

Couper complained to Wurtz about the delay in presenting his paper and was asked to leave the laboratory. He returned to Scotland in the fall of 1858 and accepted a position as second laboratory assistant to Lyon Playfair, professor of chemistry at Edinburgh University. Soon after beginning his duties he suffered a mental breakdown, underwent treatment, apparently recovered, and went on a fishing expedition, during which, reportedly due to extended exposure to the sun, his illness returned. He never fully recovered and lived in retirement for the remaining thirty-four years of his life.

In Couper’s first experimental paper3 he sought a way to convert benzene, C6H6, into the corresponding hydroxy addition compounds, probably C6H7OH and C6H6(OH)2. Wurtz had just prepared ethylene glycol, C2H4(OH)2, from ethylene via the iodide C2H4I2 reacted with silver acetate, followed by treatment with potassium hydroxide. Couper reacted benzene with bromine and isolated two new compounds, bromo-benzene, C6H5Br, and p-dibromobenzene, C6H4Br2. The first showed only very slight reactivity with silver acetate, while the second led to an explosion.

Couper’s other experimental paper dealt with the constitution of the benzene derivative salicylic acid, a subject that a number of chemists, including Kekulé, had been investigating. Treating the acid with phosphorus pentachloride, Couper claimed to have obtained the “trichlorophosphate de salicyle.” He proposed detailed structural formulas for the acid and its derivatives. This represented the first time that the relations between the individual carbon atoms of benzene had been depicted in a formula.

Kekulé repeated Couper’s experiment more than twenty times without success, but Anschütz later fully confirmed Couper’s results. Apparently Kekulé had kept the hot reaction mixture too long before distillation4. Couper’s work was further confirmed and extended by A. G. Pinkus.5

Coming to chemistry from a study of philosophy and classical languages, Couper viewed the complex chemical formulas of his time as if they were words of a foreign language: “To reach the structure of words we must go back, seek out the undecomposable elements, viz. the letters, and study carefully their powers and bearing. Having ascertained these, the composition and structure of every possible word is revealed.”6 By this mode of reasoning he deduced that carbon has a valence, or combining power, of two or four and that it has the power to form valence links with other carbon atoms, thus making possible carbon chains. These two principles are the foundation of the structural theory of organic chemistry. Kekulé had proposed the tetravalence of carbon the previous year. Couper then devised a pictorial representation of chemical compounds using dotted lines, solid lines, or brackets to show the linkings between atoms in the molecule. These formulas represent the first introduction of the valence line into organic formulas. Since Couper used an atomic weight of eight for oxygen, his formulas always contain a pair of oxygen atoms instead of the single atom used today.

Couper’s formulas for butyl alcohol, CH3CH2CH2CH2OH

Couper’s formulas for acetic acid,

In 1858 Couper introduced the first ring formula into organic chemistry, to represent the structure of cyanuric acid. Kekulé’s ring structure for benzene appeared in 1865. Couper’s formulas for salicylic and cyanuric acids show some of the carbons as having a valence of two. Couper maintained that several elements, such as carbon, notrogen, and phosphorous, exhibit multiple valence. Kekulé strongly opposed such a view.

A. Butlerow who visited Kekulé and Couper during 1858, first strongl critized Couper view’s views,7 but in the well-known paper in which he introduced tern “chemical structure,” he credited Couper as the originator of his central ideas.8

NOTES

1. “Uber doe Constitution and die Metamorphosen der chemischen Verbindungen and über die chemische Natur des Kohlenstoffs, in Justus Liebigs Annalen der Chemie106 (1858), 129–159.

2. “Remarques sur l’occasion de ‘Couper, sur une nouvelle théorie chimique,’” in Comptes rendus hebdomadaries des seances de l’Académie des science47 (1858), 378–380.

3. Formulas in this paragraph are those now in use.

4. R. Anschütz, “Ueber die Einwirkung von Phosphorpentachlorid auf Salicylsäure,” in Justus Liebigs Annalen der Chemie, 228 (1885), 308–321.

5. A.G. Pinkus, P.G. Walrep, and W.J. Collier, “On the Structure of Couper’s Compound,” in Journal of Organic Chemistry26 (1961), 682–686.

6. O. T. Benfey, ed., Classics in the Theory of Chemical Combination., p. 139.

7. “Ueber A. S. Couper’s neue chemische Theorie,” in Justus Liebigs Annalen der Chemie 110 (1859), 51–66

8. “Einiges über die chemische Structur der Körper, in” Zeitschrift fur Chemie, 4 (1861), 560

BIBLIOGRAPHY

I. Original Works. Couper published six papers: “Recherches sur la benszine,” in Comptes rendus hebdomadaires des seances de l’Academie des sciences, 45 (10 Aug. 1857), 230–232; “Sur une nouvelle theorie chimique,” ibid46 (14 June 1858), 1157–1160; “On a New Chemical Theory,” in London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 4th drt., 16 (1858), 104–116, a much more extensive treatment than the preceding article; “Sur une nouvelle theorie chimique,” in Annales de chimie et de physique 3rd ser., 53 (1858), 469–489, largely a translation of the preceding article but with a different treatment of chemical formulas and with a note on nitrogen compounds, including a ring formula for cyanuric acid; “Recherches sur l’acide salicylique,” in Comptes rendus hebdomadaires des séances de l’Académie des science46 (7 June 1858), 1107–1110; and “Researches on Salicylic Acid,” in The Edinburgh New Philosophical Journal, n.s. 8 (July-Oct. 1858), 213–217, similar to the preceding article but longer and, unlike it, containing structural formulas. R. Anschutz, ed., “Ueber eine neue chemische Theorie” von Archibald Scott Couper, no. 183 in Ostwald’s Klassiker der Exacten Wissenschaften (Leipzig, 1911), appeared in 1865. Couper’s formulas for salicylic and Klassiker der Exacten Wissenschaften (Leipzig, 1911), is a German translation of the shorter “Sur une nouvelle théorie chimique” and of the longer article of the same title, with commentary and notes. On a New Chemical Theory and Researches on Salicylic Acid, Alembic Club Reprint no. 21 (Edinburgh, 1953), contains English translations of the shorter “Sur une nouvelle théorie chimique” and of “Recherches sur l’acide salicylique,” the two papers published in English, and a short commentary.

II. Secondary Literature. On Couper or his work, see R. Anschutz, “Life and Chemical Work of Archibald Scott Couper,” in Proceedings of the Royal Society of Edinburgh, 29 (Apr. 1909), 193–273, which contains detailed biography and all Couper’s papers as originally published—the German version of the biography is R. Anschutz, “Archibald Scott Couper,” in Archiv für die Geschichte der Naturwissenschaften and der Technik, 1 (1909), 219–261; O. T. Benfey, “Archibald Scott Couper,” in Eduard Farber, ed. Great Chemists (New York, 1961), pp. 705–715; and O. T. Benfey, ed., Classics in the Theory of Chemical Combination (New York,1963), which includes Couper’s two theoretical papers in English as well as Kekulé’s 1858 paper; L. Dobbin, “The Couper Quest,” in Journal of Chemical Education, 11 (1934), 331–338, which details the search for information on Couper and contains further details of his life; and H. S. Mason,” History of the Use of Graphic Formulas in Organic Chemistry,” in Isis, 34 (1943), 346–354.

Otto Theodor Benfey

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Archibald Scott Couper

Archibald Scott Couper

The British chemist Archibald Scott Couper (1831-1892) shares with Kekulé the distinction of recognizing the tetravalency of carbon and the capacity of carbon atoms to combine to form chains, thereby providing the basis for structural organic chemistry.

Archibald Scott Couper was born on March 31, 1831, at Kirkintilloch in Dumbartonshire, Scotland, the son of a prosperous cotton weaver. He commenced his university studies at Glasgow mainly in classics, spent the summer semester of 1852 in Berlin, and returned to Scotland to complete his university course in logic and metaphysics at Edinburgh. He spent the period 1854-1856 in Berlin and during this time decided to study chemistry.

Couper entered the laboratory of Charles Wurtz in Paris in the autumn of 1856 and remained there until his return to Scotland in 1858; during these 2 years he made all his contributions to chemistry: two papers containing experimental contributions and his now famous memoir "On a New Chemical Theory." A few months after his return to Edinburgh to be assistant to Lyon Playfair, in the autumn of 1858, he suffered a severe nervous breakdown, followed by a general breakdown in health. He retired to Kirkintilloch and lived there incapable of intellectual work and completely lost to chemistry until his death 34 years later.

Work on the Element Carbon

The story of Couper's work, its subsequent disappearance from view, and its later recognition, largely through the efforts of Richard Anschütz, as a major piece of chemical history is one of the most remarkable in science. Early in 1858 Couper, then 27 and after only some 3 years' contact with chemistry, asked Wurtz to present Couper's manuscript "On a New Chemical Theory" to the French Academy. Wurtz, however, delayed taking any steps, and in the interim August Kekulé's paper "On the Constitution and Metamorphoses of Chemical Compounds and on the Chemical Nature of Carbon" appeared, containing essentially similar proposals. Couper protested to Wurtz about his procrastination but was, it is said, shown out of the laboratory.

Couper's paper was, however, finally presented by Jean Baptiste Dumas to the academy on June 14, 1858, and published in the Comptes rendus; fuller versions were subsequently published in English and French. After pointing out the inadequacy of current theories, Couper wrote in his paper: "I propose to consider the single element carbon. This body is found to have two highly distinguished characteristics: (1) It combines with equal numbers of equivalents of hydrogen, chlorine, oxygen, sulphur, etc. (2) It enters into chemical combination with itself. These two properties, in my opinion, explain all that is characteristic of organic chemistry. This will be rendered apparent as I advance. This second property is, so far as I am aware, here signalized for the first time."

Valence and Aromatic Compounds

Couper also introduced the use of a line to indicate the valence linkage between two atoms and, had he used 16 rather than 8 for the atomic weight of oxygen, his chemical formulas would have been almost identical with those used today. It is also remarkable that in his paper he represents cyanuric acid by a formula containing a ring of three carbon and three nitrogen atoms joined by valence lines—the first ring formula ever published. The introduction of ring formulas is often ascribed to Kekulé, who in 1865 used this concept to develop his formula for benzene. It is interesting to speculate whether Couper might have anticipated Kekulé's formulation of aromatic compounds had he been able to continue his chemical work. But Couper's paper "On a New Chemical Theory" remains a landmark in the history of organic chemistry.

Further Reading

Alexander Findlay, A Hundred Years of Chemistry (1937; 3d ed. 1965), discusses Couper's work and includes a short bibliography. See also Eduard Farber, The Evolution of Chemistry: A History of Its Ideas, Methods and Materials (1952; 2d ed. 1969). □

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