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 Oxford 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 University of St Andrews 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.