Historical profile A forgotten triumph In 1858, a new theory revolutionised organic chemistry, but its originator was brushed aside. Mike Sutton revives the memory of Archibald Scott Couper

Today, we would be surprised structural ideas, chemistry’s senior 1831, in Kirkintilloch, near Glasgow to hear of a chemist who began In short professionals lacked a theoretical in the UK. He became a dedicated studying science at the age of 24  Archibald Scott consensus. Between August 1857 student, reading classical languages and started serious research only Couper was the first and June 1858, he announced three and philosophy at Glasgow and eighteen months later. If this chemist to use modern discoveries to the Paris Academy of Edinburgh, and from 1851 he spent research produced a significant structural formulae, 150 Sciences. His first paper – an account part of each year in Germany. After theoretical advance within two years ago of the synthesis of bromobenzene mastering the language, Couper years, we would be astonished. But  His publication of this and p-dibromobenzene – was attended lectures at Halle and one hundred and fifty years ago the new concept was largely uncontroversial. His second, on Berlin, but there is no evidence of his frontiers of knowledge were closer, ignored at the time, and salicylic acid, initially attracted little studying chemistry before 1855. and enterprising beginners could even derided by his main attention, though it later caused Even so, by the summer of that rapidly reach uncharted territory. rival August Kekulé some debate. But the third, Sur une year, he was following the lectures Nevertheless, when Archibald Scott  He died before nouvelle théorie chimique, proposed of Karl Rammelsberg and Franz Couper announced his brilliant his contribution the existence of carbon-carbon Sonnenschein at Berlin University, new idea in the spring of 1858, it was was recognised but chains – and that got him into serious and gaining practical experience greeted with a cool reception. his memory and the trouble. in Sonnenschein’s laboratory. In Couper’s story is one of quiet importance of his work Couper was born on 31 March August 1856, he moved to Paris, tragedy. Soon after the publication of has since been revived his groundbreaking paper, his fragile health collapsed, and long before he C OOH C OOH died, in 1892, his pioneering work 2 2 was forgotten. But just a few years H H later he was recognised as the first C H2 C H2 person to deploy structural chemical formulae in the modern manner. 3 2 Couper brought his ideas into CH CH a decade when chemistry was in Couper’s pictorial a confused and confrontational representations of CH3 state. By the late 1850s, when propanol (left) and the young chemist published his butanol 44 | Chemistry World | May 2008 www.chemistryworld.org and started serious research the correct number of oxygen atoms. Couper is now regarded judgement about the forces holding under the supervision of Charles- (This was soon to be resolved by as a pioneer of structural their constituents together. Adolphe Wurtz. It was there that Stanislao Cannizzaro’s revival of chemistry There were questions about he encountered both triumph and the Avogadro hypothesis, which how atoms and molecules were disaster in rapid succession. confirmed O = 16 as correct.) distributed in space. Berzelius had There was debate about the puzzled over pairs of compounds A numbers game forces binding atoms together. The with the same composition but In the 1850s many chemists Swedish chemist Jacob Berzelius different properties, introducing dismissed atomism as a speculative developed a theory of chemical the term ‘isomerism’ for this hypothesis – inferred from numerical combination based on the attraction phenomenon. Several chemists regularities in the combining weights between opposite electrical charges (including Joseph Gay-Lussac of various elements. (The last anti- on individual atoms (or on stable and Michel Chevreul) suggested atomic diehards conceded defeat groups of atoms, called radicals). This that the molecules of these pairs after Jean Perrin’s work on Brownian dualistic theory explained why many of compounds contained the motion was published in 1908.) Even inorganic molecules were split by same atoms, arranged in different those chemists who did believe in electrolysis. But it was less successful geometrical patterns. The nature atoms, disagreed about their weights, with organic substances – many of the patterns, however, remained their modes of combination, and their of which remained unaffected by mysterious. spatial arrangement. electricity. Some accepted the HO formula Crucially, dualism failed to Stirring up trouble for water, giving oxygen an atomic explain how a stable hydrocarbon The significance of Couper’s new weight of 8 units. Others preferred molecule could exchange one atom ‘His proposal of theory (which appeared in a fuller H2O, making oxygen’s atomic of (positive) hydrogen for one atom English version soon afterwards) weight 16. Carbon might be rated of (negative) chlorine, resulting in an the existence of resides in two assertions. The first as 6 or 12 – causing disagreement equally stable chloro-hydrocarbon was that the combining power of over the representation of organic molecule. Organic chemists like carbon–carbon carbon (later known as its valency) compounds. Though they generally Jean-Baptiste Dumas and Charles chains got him was normally four. However, compare well with modern values, Gerhardt preferred a theory of types Couper was aware that carbon could Couper’s formulae were based on which sorted similar compounds into serious sometimes exhibit a combining O = 8, and consequently had double into family groups, while reserving trouble’ power of two, for example in carbon www.chemistryworld.org Chemistry World | May 2008 | 45 Historical profile monoxide. He apparently assumed assuming that the carbon atoms that this also occurred in unsaturated could soak up each other’s surplus hydrocarbons like ethylene (ethene) ‘saturation capacity’ by joining – his diagrams contain no carbon– together in chains. Couper, by carbon double bonds. actually providing diagrams of his Couper’s second and more radical conjectured molecular arrangements, assertion was that carbon could went a step further than Kekulé. ‘enter into chemical union with But though he was posthumously itself’ – its atoms joining together acclaimed as the one of the founders in long chains. Examples of this of modern structural chemistry, his phenomenon which he cited were contribution was brushed aside at propyl and butyl alcohols (propanol the time. and butanol). Their formulae are It is not hard to find reasons for C3H8O and C4H10O in modern this negative response. Couper’s notation, but C3H8O2 and C4H9O2 paper made ambitious claims, while in his, taking 8 rather than 16 as the expressing scorn for earlier chemical atomic weight of oxygen (see box theories (and their adherents). This p44). This conveniently explained alone – given the author’s lowly the existence of numerous series professional status – might explain of carbon compounds where each its unfavourable reception. But member differs by CH2 from the next other factors may also have been one in the sequence. involved. In the spring of 1858 Kekulé Later in 1858, a revised version was not far above Couper on the of this paper appeared in another career ladder. As a privatdozent at French journal. It included another Heidelberg he was licensed to teach interesting speculation. Couper but unsalaried, living on the fees paid discussed some carbon-nitrogen by any students he could recruit to his compounds, including cyanuric acid classes. Not until later that year did (2,4,6-trihydroxy-1,3,5-triazine) to he gain the security of a professorship which he gave a ring structure (see at the University of Ghent in the below). From here it would have Netherlands. been a short step to the benzene ring Kekulé might well have seen – seven years before Kekulé got there Couper’s paper as a challenge to – but Couper never made it. the uniqueness of his discovery, and hence a threat to his career. Bad timing If so, he would naturally want to The first version of Couper’s new respond forcefully. In that era, theory was handed to his supervisor disputes between rival chemists early in 1858. But Wurtz was so slow often degenerated into exchanges in submitting it to the Academy of personal insults, and Kekulé that it did not appear in print until Couper’s house in if that was so, then the molecules of participated with zest in many 14 June. Meanwhile, on 19 May, the Townhead, Kirkintilloch, many organic compounds seemed to such battles of words. A recent German chemist August Kekulé near Glasgow, UK contain more carbon atoms than the biographical study explores several had published a paper arguing available atoms of other kinds were in which he was involved, while along similar lines. He responded able to ‘saturate’. also citing examples of Kekulé’s aggressively to Couper’s article, At this point, Couper and Kekulé reluctance to acknowledge dismissing it as deficient in both independently realised that the intellectual debts to others. clarity and originality, and claiming discrepancy could be explained by Couper did not reply exclusive priority for himself as the to Kekulé. Instead, he discoverer of carbon–carbon chains. complained to Wurtz about Most historians now believe this was HO OAz C AzO OH the delay in publishing unjustified. his paper, and Wurtz As Kekulé himself later admitted, C C responded by expelling the concept of valency was still ‘in Az OOH him from the laboratory the air’. Earlier in the 1850s William for his insolence. Odling had talked of the ‘replaceable Couper then returned to value’ of an atom or radical; Edward OH Edinburgh and worked Frankland and Herman Kolbe briefly as an assistant used ‘atomicity’; and Alexander to Professor Lyon Williamson ‘saturation capacity’, for N N Playfair, but within the same property. (It was not until a year he suffered the mid-1860s that the German word HO N OH a breakdown. valenz became current – along with Thereafter he valence in French and valency in remained an invalid, English.) Couper’s drawing of the structure of cyanuric acid (top). The use unable to work and But by 1858, many chemists agreed of brackets alongside the bond lines is confusing, and Couper gives nursed by his devoted mother. that this quantity – whatever they nitrogen as Az in the French manner, while once again doubling the Chemistry moved on, and soon forgot called it – normally had the value of oxygen atoms – but his picture has a recognisable relationship with him. four for the carbon atom. However, the structure now accepted for this compound (bottom) In 1860 Kekulé introduced his own 46 | Chemistry World | May 2008 www.chemistryworld.org SCIENCE AND SOCIETY PICTURE LIBRARY / SCIENCE LIBRARY SCIENCE / LIBRARY PICTURE SOCIETY AND SCIENCE

structural diagrams - the unwieldy Alexander Crum Brown. In 1874, A wide variety of his biography. While doing so, ‘sausage formulae’ (see below). a third dimension was added representations of the he rediscovered Couper’s long- His hexagonal formula for benzene to chemical structure by the structure of benzene neglected work and made a was unveiled in 1865, and thereafter, stereochemical discoveries of were proposed in the successful effort to rehabilitate him, structural chemistry advanced Jacobus Van’t Hoff and Joseph Le 19th century with assistance from Crum Brown. rapidly, with further notable Bel. One consequence was that in 1931, contributions from Alexander We now take it for granted that the centenary of Couper’s birth, a Butlerov and the structure of a molecule can be commemorative plaque was unveiled as significant as its composition in Kirkintilloch, in the presence of – particularly if it is a biologically many leading Scottish chemists. The important molecule like prophet was at last honoured in his DNA or haemoglobin own country. – but in 1858 this required a leap of the imagination. Michael Sutton is a lecturer in history That leap can fairly be at Northumbria University, UK described as a revolution, or a paradigm shift, and Couper Further reading was certainly one of those who A S Couper, On a New Chemical Theory, Phil. initiated it. If his new theory ‘He was one of Mag., 1858, 16, 104 – available at http://web. had received a better reception, lemoyne.edu/~giunta/couper/couper.html the founders O T Benfey, Archibald Scott Couper, in E Farber his physical and mental decline (ed), Great Chemists, Interscience, New York, might have been averted, and he of modern 1961 might then have played a larger D G Duff, A S Couper: The Forgotten Genius, part in the further development structural Chem. Brit., 1987, 23, 350 A J Rocke, Kekulé, Butlerov, and the Historio- of structural chemistry. Yet one chemistry but, graphy of the Theory of Chemical Structure, thing was belatedly salvaged from Br. J. Hist. Sci., 1981, 14, 32 the wreck of Couper’s career – his at the time, his C A Russell, The History of Valency, Leicester reputation. contribution University Press, Leicester, 1971 After Kekulé died in 1896, his J H Wotiz (ed), The Kekulé Riddle: a Challenge was brushed for Chemists and Psychiatrists, Cache River successor at Bonn University, Press, Vienna, Illinois, US, 1993 Richard Anschütz, began writing aside’ L Dobbin, J. Chem. Educ., 1934, 11, 331 www.chemistryworld.org Chemistry World | May 2008 | 47