Historical profile The iconic curly arrow Robert Robinson played an integral part in building the foundations needed to understand reaction mechanisms, and pioneered the use of curly arrows to show electron movement. David O’Hagan and Douglas Lloyd report on the work, and friends and foes, of this eminent historical figure

Sir Robert Robinson (1886–1975) was based there from 1921 to 1923. Gaining mechanistic insight was one of the giants of early In short During this two year period he Together these two papers initiated 20th century organic chemistry.  While at the University published 24 papers, two of which a major transition in organic He played a seminal role in our of St Andrews, Robert remain widely cited today. The chemistry, moving the international understanding of chemical Robinson wrote the first papers presented fundamental focus from structure elucidation to reactivity and made outstanding paper to use the curly contributions to developing understanding reaction mechanism. and extensive contributions arrow to demonstrate ideas on ‘electronic theory’. One, Like many transitions in science to the synthesis and structure electron movement published with James Wilson Armit there was much to evaluate determination of natural products,  He was close friends (a local baker’s son), had the first and fierce rivalries ensued in particularly the alkaloids. In his with Arthur Lapworth, illustration of an aromatic ring with establishing the ground rules in later career he held the most senior another eminent physical a circle in the centre of a hexagon what would subsequently be called positions in UK science as president organic chemist of the to represent delocalisation; a ‘physical organic chemistry’. in turn of the Chemical Society, the same era paper which has been widely In the 1922 paper, Robinson Society of Chemical Industry and  Christopher Ingold, acknowledged in discussions used the curly arrow to describe of the Royal Society. His eminent however, became a fierce on the history of benzene and the reactivity of the molecule work also earned him international professional rival aromaticity. The second paper 1,3,5-hexatriene. His interest in recognition in the form of the 1947 published in 1922 – and the focus of hexatriene lay in the fact it is closely Nobel prize in chemistry. this feature article – contained the related to butadiene – a molecule Born in Rufford, Derbyshire, first illustration of the curly arrow that was somewhat of a hot topic on 13 September 1886, Robinson as a device to represent electron at the time. It was well recognised was the son of a surgical dressing movement in conjugated organic that if butadiene is treated with manufacturer. He carried out his molecules. Robinson coauthored bromine, a mixture of 1,2- undergraduate and PhD research this second paper with William and 1,4-dibromobutenes at the University of Manchester, Ogilvy Kermack (see box p57). formed. But an the latter under the supervision of understanding of why was William Perkin Jr. He then followed yet to be developed. an academic career path, holding In 1899, the German a number of short professorial – chemist Johannes Thiele residencies prior to moving to 2e had put forward a ‘theory of the University of in 1932. partial valencies’ to describe He remained at Oxford until his double bonds. He proposed retirement in 1955. that the double bond in ethene The consisted of a full covalent bond was one of the stepping stones en with a second partial bond, allowing route to Oxford, and Robinson each carbon atom to retain a partial 54 | Chemistry World | April 2010 www.chemistryworld.org

0410CW-ROBINSON.indd 54 25/03/2010 10:45:52 MARY EVANS PICTURE LIBRARY PICTURE EVANS MARY

www.chemistryworld.org Chemistry World | April 2010 | 55

0410CW-ROBINSON.indd 55 24/03/2010 10:41:10 Historical profile

valency pointing into space. The that oxygen polarised the C=O partial valencies were believed bond and rendered the carbon to be predisposed to react with electropositive. It followed that bromine, for example, to make the next carbon would be negative 1,2-dibromoethane. By 1922, Thiele’s and the next hydrogen positive ideas had been extended to try ie induced alternate polarities. and understand the structure and Robinson stated in Lapworth’s reactivity of benzene, forming a Royal Society obituary: ‘when in useful working hypothesis given the the early Manchester days, one information available at the time. discussed synthetical projects with Rationalising this for butadiene, Lapworth, it was quite clear that he however, was less satisfactory as had some unusual private way of the two dibromo isomers were deciding whether they would “go” or generated. not. It turned out to be a scheme of

ROYAL SOCIETY OF CHEMISTRY LIBRARY AND INFORMATION SERVICE INFORMATION AND LIBRARY CHEMISTRY OF SOCIETY ROYAL In the 1922 paper, Robinson “alternating polarities” in a chain of offered a more satisfactory atoms and the theory was published hypothesis much closer to a modern in 1920.’ understanding of conjugation. In Despite their long friendship, developing his ideas, he drew on in 1922 Robinson and Lapworth the theories of the US chemists published separately, but Gilbert Lewis and Irving Langmuir simultaneously. In each case this – concerning two electron covalent independence was tempered with bonds and the stability of octets of generous acknowledgement of each electrons – and those of UK chemist other in their respective papers. Arthur Lapworth on alternating However they did not read each polarities. others manuscripts in advance, indicating that they were working in Robinson and Lapworth of reactions and changes in organic Lapworth (above) and a fast moving competitive field. Lapworth was a close colleague chemistry to necessary variations Robinson had a close and confidant of Robinson. Their of at most one or two simple laws’. friendship All in a muddle relationship began in 1909 when In 1904, he had recognised that In Lapworth’s paper, curly arrows Lapworth arrived as a teacher at the cyanide attacks a ketone to generate also appear; however they represent University of Manchester, where a cyanohydrin, and had proven the movement of partial valencies. Robinson was carrying out graduate It is difficult now to appreciate this research. Robinson stated that they model, as it doesn’t follow modern had, ‘unlimited opportunity for convention. A covalent bond was discussion from that time until 1912’. assumed to have three ‘partial Talking about his friend’s research valencies’ and his arrows move two at that time, Robinson said: ‘[he] was of these, thus the connection that obsessed by his theory of alternate an arrow moves two electrons is polarities, though not a reliable obscured, although Lapworth was guide in all circumstances, [it] still very close to describing the modern provides a useful mnemonic for convention. Lapworth’s focus was the behaviour of molecules of many this by the isolation of crystalline Curly arrows were used in on aliphatic carbonyl chemistry, carbon compounds in reactions.’ salts of cyanohydrins. He had also both Robinson’s (above) and he was developing a model to In 1912, Robinson moved away demonstrated that the process was and Lapworth’s (below) encompass cyanohydrin formation from Manchester to take a chair at acid catalysed and reversible. He 1922 papers as well as enolate formation. the University of Sydney in Australia. rationalised that this reactivity The Robinson and Kermack He returned to the University of was due to the polarity of the paper, on the other hand, focused on Liverpool in 1915, and then, after a carbonyl and the anionic nature of conjugated and aromatic systems, short period with the British Dye cyanide, leading to his recognition and was the easier to assimilate. Stuffs Corporation, arrived at St That’s not to say it was immediately Andrews in 1921. understood, and these new This was a busy ideas proved difficult for eight years, during the chemical community which time Robinson to grasp. An unseemly and Lapworth had scramble then ensued to both continued to present a more clearly develop their ideas on accessible rationale. electronic theory. Others such as Bernard Lapworth had Flürscheim, who had deeper roots in the studied under Thiele, subject. As early as and the fast emerging 1898, he alluded to Christopher Ingold at finding a unifying Imperial College London concept of reactivity also became involved. In the to ‘demonstrate few years following the 1922 that it is possible to papers, the controversy was refer the majority so intense and so confusing 56 | Chemistry World | April 2010 www.chemistryworld.org

0410CW-ROBINSON.indd 56 25/03/2010 10:46:26 that the Chemical Society refused Articulate Ingold to receive any more papers on Ingold was particularly energetic the subject from these leading in developing an understanding participants. of aromatic reactivity, and was the A 1925 private correspondence better articulator of the concepts from Martin Lowry, a professor of the new electronic theory. of physical chemistry at the Subsequent years would see University of Cambridge, to Robinson and Ingold go head to Robinson illustrates the extent head for priority, both on theory of the community’s confusion: ‘I and development of terminology. have not yet got used to your new Ingold’s notation gained widespread system of arrows and want a little acceptance, with his nucleophile practice in order to think in terms and electrophile terminology of them. Even now I am not quite successfully displacing Lapworth’s sure whether they represent a anionoid and kationoid (the transfer of one electron or two. My terms used by Robinson). The GIFT OF JOHN D ROBERTS, CHEMICAL HERITAGE FOUNDATION COLLECTIONS FOUNDATION HERITAGE CHEMICAL ROBERTS, D JOHN OF GIFT rather vague impression is that in pair also presented contradictory the earlier papers you generally conventions for the positive and transferred only one, while in negative signs attached to electron your last C and I [Chemistry and donating and withdrawing groups, Industry] paper you appear to with Ingold’s winning out. transfer two. It may not matter Robinson was rather bitter in much which scheme is used, but that Ingold took the foreground I want to get a clear picture in my and states in his memoir of 1976: own mind of your present view of ‘I am touching on this question of this mechanism. I hope you don’t acknowledgement as [Ingold] was mind being bothered like this apt to include a necessary reference with elementary questions; but hexatriene structure. However in Ingold (above) and to Lapworth or myself in a large in view of the fact that so many 1924 and 1925, Robinson used the Robinson were number of references so that any people misunderstand, more or less curly arrow notation widely, and professional rivals idea that our contributions are willingly, the Manchester doctrine, confidently illustrated two electron original or applicable to the matter it seems worth while to make sure movement in a manner akin to in hand was well and truly buried.’ that there are at least a few people contemporary usage (below). By contrast it is interesting to about who willingly understand it.’ note that Lapworth’s last published Indeed, in the Robinson and output in 1931 was a collaboration Kermack 1922 illustration it with Ingold on directing groups appears that only a one electron and reactivity in aromatic nitration, transfer is represented by the and clearly they had reconciled curly arrow. The electron count any differences they may have had. is clearly illustrated in the before Lapworth was restrained and gentle and after structures of butadiene, by character and as Robinson states: and by implication extends to the ‘there must have been occasions when Lapworth was angry, but I cannot recall one, and I am quite William Ogilvy Kermack sure he never did a mean action.’ Kermack was a Scotsman born statistical analysis of fertility The curly arrow was of minor in Kirriemuir in 1898. He studied and death rates in urban and significance in the 1922 St Andrews maths, natural philosophy and country populations and he paper, however its simplicity and

ROYAL SOCIETY ROYAL chemistry at the University of made seminal and enduring power as a symbol of electron (graduating in 1918) contributions to the emerging movement resulted in its widespread and then moved to the Dyson science of epidemiology where and rapid uptake, to the point now Perrins lab at the University he and his colleague Anderson where it holds an iconic status in of Oxford for two years with McKendrick modelled epidemic organic chemistry. William Perkin Jr to work on the episodes of infectious diseases alkaloid harmaline. He returned now known as the Kermack- David O’Hagan is head of organic to to the chemicals McKendrick theory. chemistry, and Douglas Lloyd is section of the Royal College of He was appointed the professor emeritus, at the University Physicians in where first professor of biological of St Andrews, UK he remained until 1949. Kermack was blinded aged 26 chemistry at the University of In 1922 Kermack collaborated Aberdeen in 1949. Despite his Further reading with Robinson but it appears caustic alkali exploded in his total blindness he rose through W O Kermack and R Robinson, J. Chem. Soc. that they never actually worked eyes. Showing considerable the academic ranks, with papers Trans., 1922, 121, 427 A Lapworth, J. Chem. Soc. Trans., 1922, 121, 416 in the same laboratory, as one fortitude he was married one being read daily by his secretary R Robinson, Memoirs of a minor prophet, was in Edinburgh and the other year later and carried on as a and colleagues. He became Elsevier, Amsterdam, the Netherlands, 1976 in St Andrews. research chemist in alkaloid dean of science in 1961. He T I Williams, Robert Robinson: chemist extra- At age 26, two years after and carbohydrate chemistry for remained at Aberdeen until his ordinary, Oxford University Press, US, 1990 P rofile of Christopher Ingold: Chemistry World, the curly arrow paper, Kermack many years. formal retirement in 1968, and December 2008, p50 was tragically blinded when His interests extended to the died working at his desk in 1970. P rofile of the Perkin family: Chemistry World, March 2010, p54

www.chemistryworld.org Chemistry World | April 2010 | 57

0410CW-ROBINSON.indd 57 25/03/2010 10:46:59