INGOLD, CHRISTOPHER KELK

(b. Forest Gate, Essex, England, 28 October 1893; d. Edg-ware, London, 8 December 1970),

physical organic chemistry, structure of benzene, electronic theory of organic reactions, reaction mechanisms, systematizer of organic chemistry.

A titan of twentieth-century chemistry, Ingold was a pioneer in the field that, from 1940, was called physical organic chemistry. Organic chemistry had previously focused on classifying the compounds of carbon in terms of their properties and structures. Using the principles and tools of physical chemistry, Ingold sought instead to understand organic chemistry in terms of the types of reactions these compounds underwent. Not only a prolific and original experimentalist, who waged with his coworkers research “campaigns” to develop his theories by means of veritable onslaughts of publications, he was also an important systematizer of the field, who was able to stand back from research detail, drawing it together to forge a new orientation for his subject. As had been the case with previous chemical systematizers, part of his success and enduring reputation lay in his ability to communicate and convince through the creation of evocative new terminology that encapsulated his ideas.

Early Life and Education . Though born to the east of London, Ingold moved with his family when very young to the Isle of Wight off the central south coast of England. His father had been a silk mercer in the metropolis, but set up as a confectioner after the move, a business that his wife carried on after his early death. In this environment, the young Ingold enjoyed considerable independence to explore the island and developed early what would be lifelong enthusiasm for outdoor pursuits, such as bird-watching, to which he would later add others such as rock climbing. He was educated in a recently established local, county funded secondary school, where he was by all accounts well taught in a range of subjects. In his final years at school, he studied mathematics, physics, and

chemistry, and capped his school career by passing in 1911 the Intermediate BSc of the University of London, the most advanced level national examination for secondary school students at that time, thereby qualifying for university entrance.

As was usual at the time for a young man educated at a local secondary school who had the talent and the ambition to seek higher education, Ingold went on with a county scholarship to a local institution, the Hartley University College, which was on the mainland just across the water in Southampton. There students prepared for the external degree examinations of the University of London and Ingold gained in 1913 a second-class honors degree in chemistry. Reminiscing shortly after retirement, Ingold commented that he had chosen chemistry for his degree studies, rather than physics in which he was a stronger student, because of the inspiring teaching at Hartley of Professor David R. Boyd (1872–1955), who imbued his students with the “spirit of science.” Boyd, on his own, had to cover the whole of the chemistry syllabus. By incorporating recent research findings, he taught the subject as a “living” science with exciting frontiers. Ingold followed this pedagogic strategy in his own teaching.

Ingold went on, with a set of scholarships, to the Royal College of Science in South Kensington, London, a constituent institution of Imperial College of Science and Technology, which had been formed in 1908, to work with the new professor of organic chemistry, Jocelyn Field Thorpe (1872–1940), a classical organic chemist, trained in Germany. Ingold obtained in 1914 the Associateship of the Royal College of Science with first-class honors, the equivalent of a second BSc degree, and immediately went on to undertake postgraduate research under Thorpe on a structural problem, with a view to achieving a DIC (Diploma of Imperial College). However, World War I intervened, and Ingold became engaged in emergency production at the college first of an analgesic and then of a war gas, a teargas code-named SK—for South Kensington. In 1915 he sought deferral of a prestigious research fellowship in order to move to the Cassel Cyanide Company in Glasgow to oversee their manufacture of SK, work for which he later received the British Empire Medal. After the war, Ingold remained at this industrial post although, having continued to collaborate with Thorpe throughout his time in Glasgow—achieving the DIC in 1916 and then a University of London MSc and the Associateship of the Royal Institute of Chemistry in 1919—he had already decided that he wanted to follow an academic career. He returned to academic life at Imperial College early in 1920 as a demonstrator in industrial organic chemistry, taking a considerable cut in salary to do so.

Early Career: Imperial College London and Leeds . On returning to London, Ingold rapidly made up for lost time and began what would become his standard practice of publishing concerted batches of papers on various aspects of a problem. In 1921 he submitted successfully a group of published papers together with a summary of the work to the University of London for the research degree of DSc. An early problem resulted in a revision of the accepted view of a fixed geometry with uniform bond angles for compounds of tetrahedral carbon. Another was the structure of benzene, which would be a recurring topic during his career. In 1922 he was awarded the first ever Meldola Medal, administered by the Royal Institute of Chemistry, for the most outstanding research during the year by a chemist under thirty years of age. He became a fellow of the institute in 1923 and was awarded the Meldola Medal for a second time in that year. It was also the year of his marriage to (Edith) Hilda Usherwood (1898–1988).

The couple met at Imperial College, where the future Mrs. Ingold arrived early in 1921 to do chemical research toward what was then a new higher degree in Britain, a PhD. Hilda Ingold was born in London into a family that included distinguished schoolteachers on both sides. She attended the renowned North London Collegiate School for Girls, where she excelled, and then studied chemistry and botany at Royal Holloway College, graduating in 1920 with a first-class honors degree in chemistry from the University of London. She completed her PhD early in 1923. After marriage, Hilda Ingold moved with her husband’s career, though she continued her own research, earning in 1926 a University of London DSc, which had been redefined as a higher doctorate after the institution of the PhD. Three children followed, but Hilda Ingold managed to continue publishing occasionally, sometimes jointly with her husband, until 1947, working as a researcher for many years in an unpaid capacity. However, from the onset of World War II, she took on a primarily administrative role in her husband’s department, for which she proved to have a genuine ability, and did not retire until 1968.

By the end of 1924, Christopher Ingold’s tally of publications was fifty-four, including a five hundred–page book on synthetic dyestuffs coauthored with Thorpe. In that year, at the very early age of thirty, on the basis of work during the previous ten years according to the citation, he was elected a Fellow of the Royal Society. The year 1924 also saw his appointment to the chair of organic chemistry at Leeds University, where he remained for six years and assembled a large research group. He was to comment in the 1950s that it was during this period that he developed a pedagogy that aimed to present organic chemistry to students more rationally and less empirically than had been customary. This systematizing impulse, which he suggested was already present in his teaching in the 1920s, would also come to characterize his research program from that time.

The Royal Society Fellowship citation mentioned not only Ingold’s productive experimental work in organic chemistry and an innovative and promising mathematical approach to the subject, but also important papers published in the field of physical chemistry. At Leeds, he would expand his interest in physical chemistry, later crediting the then-professor of physical chemistry, Harry Medforth Dawson (1876–1939), with awakening his interest in kinetic studies. From this time, Ingold’s research began to concentrate on the mechanisms of organic reactions. Over the period 1923–1927, he was involved in a protracted debate among a number of organic chemists, which has since been much analyzed by historians. In the event, the chief antagonists were Ingold himself and Robert Robinson (1886–1975), who was then at the University of Manchester. The debate was essentially about the positions of electrons in molecules and their consequent behavior in chemical reactions. Summarizing the debate, the historian of chemistry John Shorter has suggested that, during the course of his experimental work done in connection with it, Ingold shifted his position from supporting initially Bernard Flürscheim’s (1874–1955) theory of alternating affinities along carbon chains, which were variable and not strictly quantitative, to favoring explicitly electronic explanations. Robinson had been developing an electronic explanation simultaneously (and previously) from the theories of his former mentor and, at the time of the debate, Manchester colleague, Arthur Lapworth (1872–1941). It was Ingold who went on to become identified with the electronic theory, an outcome that rankled with Robinson, who was primarily a natural products chemist, for the rest of his life.

Ingold would from this time onward devote his career to generalizing his study of reaction mechanisms in the light of the electronic theory using the methods of physical chemistry. This resulted in a new, powerful way of understanding organic chemistry that would come to form the basis of research and teaching in the field. His first general statement appeared in the American Chemical Society’s prestigious Chemical Reviews in 1934 and served to consolidate Ingold’s standing in the field. It had been prepared in 1932 during a leave of absence from his United Kingdom post, on health grounds, which he spent at Stanford University in California. “Principles of an Electronic Theory of Organic Reactions” ran to fifty pages and remained for many years the key treatment of the subject. Based, he suggested, on the initial part of a lecture course he delivered at Stanford, it had the explicitly generalizing aim of giving “a connected statement of principles” of the theory, which had “emerged piece-meal” in the literature in the course of various investigations of particular applications of it, such that the theory itself had been obscured. The paper set out a classification in physical terms of measurable electronic effects in molecules, introducing a broad distinction between permanent “polarization” effects (to do with the positions of charges in isolated molecules) and impermanent “polarizability” effects (to do with the mobility of charges in molecules in the presence of neighboring ions or molecules). An integral part of the argument was the development of a number of new terms, which would become standard in the field, thereby consolidating support for the theory itself.

University College London: The Early Years . In 1930 Ingold was appointed to the second chair of chemistry at University College London (UCL), perhaps ironically in succession to Robinson, who had held it as a chair of organic chemistry for two years en route from Manchester to Oxford. By this time, Ingold’s publications totaled 135. UCL provided an ideal “physicalist” environment for the development of Ingold’s work along the path on which he had set out in Leeds. The eminent physical chemist Frederick George Donnan (1870–1956) was head of the department. Donnan believed strongly that the future of chemistry as a whole lay in interpreting it in the light of the methods and theories of physical chemistry. He argued successfully through the administration the retitling of the organic chemistry chair to adjective-free “chemistry” on the grounds that subject subdivisions impeded the proper development of the science. This conception of the unity of chemical science based on physical methods was one that Ingold shared and would develop at UCL.

Ingold’s first task there was to be build up an independent research presence, as only one student had moved with him from Leeds. He was joined immediately by a postdoctoral fellow, Edward David Hughes (1906–1963), who had recently completed a PhD at University College Bangor for which Ingold had acted as external examiner. Thus began one of chemistry’s most remarkable research partnerships that would end only with Hughes’s early death in 1963. Apart from a brief spell from 1943 to 1948, when he held a chair at his alma mater, Hughes spent his entire career in the UCL department, albeit in various rather unsatisfactory junior capacities before his sojourn in Bangor, after which he returned to a UCL chair. The two men could not have been more contrasting in appearance, with Hughes on the short side and portly and Ingold rather tall and ascetic. They were also complementary in personality. Hughes was the one whom students felt more comfortable approaching, but had a quirky, less engaging lecture technique. By all accounts, Ingold was a masterly lecturer.

Structure and Mechanism in Organic Chemistry . The Bangor chemistry department, headed by Kennedy Joseph Previte Orton (1872–1930), was a pioneering British center for physical approaches to organic chemistry, known particularly for work on reaction mechanisms and kinetics. Hughes’s expertise in the methodology of chemical kinetics proved the ideal complement to Ingold’s interest in electronic theory. Indeed, most biographies that have considered the question are agreed that it is difficult to disentangle their mutual contributions. Together they published 138 papers, which comprised just under half of Ingold’s outputs in the period and just under 60 percent of Hughes’s. In his Royal Society memoir of Hughes, Ingold discussed with almost no mention of his own role their pathbreaking work in the 1930s. In this early collaboration, they developed a scheme for classifying a very large number of reactions of aliphatic compounds (consisting of nonring carbon chains) across what had previously been considered unrelated areas into two broad, interconnected families—nucleophilic (a term they coined to mean “nucleus seeking”) substitution and elimination reactions. Nucleophilic substitution reactions they designated as either S_N1 or S_N2, according to whether one or two molecules were involved in the rate-determining step (the slow step) of the reaction. That is, whether the reaction was unimolecular or bimolecular. Elimination reactions were similarly designated E2 or E1. They also coined the term electrophilic to denote electron-seeking reactants/reactions.

In their hands and the hands of their students, this fundamental conceptual framework provided a powerful tool for understanding how varying conditions and structures affected and altered the way that reactions progressed. It also enabled Hughes and Ingold to explain and generalize certain puzzles in classical organic chemistry, such as the problem known as the Walden inversion, which had exercised chemists for some forty years. Their framework was not accepted without considerable opposition from more traditional chemical quarters and, according to Ingold, was known as the “British ‘heresy’” in some places abroad. He suggested that this was because their work exemplified the view that physical chemistry was the basis of all chemistry, including organic chemistry and therefore required a considerable shift in orientation on the part of classically trained chemists. Securing its acceptance involved sustained campaigns in communications as well as in chemistry. Ingold’s famous systematizing textbook aimed at university students, Structure and Mechanism in Organic Chemistry, was published in 1953 although finished by the end of 1951. Based on the Baker Lectures that he gave at Cornell in 1950–1951, it was a magnum opus in every sense—a highly structured eight hundred–page work that laid out in great detail his entire scheme. It quickly became, as Ingold’s biographer Kenneth Leffek has called it, a “bible” for organic chemistry. In 1952 Ingold received the Royal Society’s highest honor, its Royal Medal, for the experimental depth and theoretical breadth of his contributions across the field of organic reaction mechanisms.

The Structure of Benzene . In the same remarkably fruitful period in the mid-1930s, Ingold returned to the problem of the structure of the benzene molecule (C₆ H₆—six carbon atoms joined in a ring, each with one atom of hydrogen attached). As was characteristic of all his work, he saw the potential of bringing together a number of new techniques to tackle this classic problem, which had engaged chemists from the mid-1860s. A new method for preparing deuterium, the isotope of hydrogen with twice its mass, for which Hughes set up a production plant, made possible a strategy of substituting deuterium atoms successively for the hydrogen atoms in the benzene molecule. Deploying the new Raman spectroscopy and new techniques in infrared spectroscopy provided a two-pronged approach to analyzing the molecular vibrations of the resulting compounds and comparing them with those of normal benzene. By mid-1935, Ingold and coworkers had synthesized fully deuterated (C₆ D₆) and mono- and di-deuterated (C₆ H₅ D and 1:4–C₆ H₄ D₂) benzene and measured their infrared and Raman, as well as their ultraviolet, spectra. Responding to an earlier letter from a Copenhagen group reporting for certain deuterated benzenes only Raman frequencies, they published preliminary results in the form of a letter to the editor of Nature (135 [1935]: 1033–1034). Drawing also on quantum mechanical calculations by Edward Teller (1908– 2003), who held a visiting post at UCL as a refugee during 1934–1935, this work led to the conclusion that the benzene molecule did have a plane regular hexagonal structure, which had recently been predicted theoretically by Edgar Bright Wilson (1908–1992) and Linus Pauling (1901–1994). The complete analysis of the structure required additionally the synthesis and spectroscopic analysis of the full set of deuterated benzene molecules. The experimental “campaign,” which started with an eight-paper series in the Journal of the Chemical Society in 1936, was completed after the war with publication of part twenty-one of the series in 1946. In recognition of this work, Ingold was awarded the Bakerian lectureship of the Royal Society in 1938 and its Davy Medal in 1946.

UCL: Head of Department and Director of Laboratories, 1937–1961 . Ingold succeeded Donnan as head of the department and director of laboratories in October 1937, posts that, formally, he held until his retirement in 1961. Particularly toward the end of his term, however, Ingold was much aided in these roles by Hughes, who formally became deputy head in 1957 and was appointed to the substantive posts on Ingold’s retirement. Over the period of Ingold’s headship, the number of academic staff in the department grew considerably; the establishment roughly doubled and there were numerous honorary members as well. There was also a constant stream of research visitors. In terms of funding, throughout Ingold’s headship, the chemistry department had the highest internal grant from UCL, a long way ahead of all other departments. However, despite a steady stream of successful applications to industrial and government sources, especially for equipping the department with the modern instruments that were becoming so essential for chemical work, the level of external income in 1960–1961 was similar to that in the mid-1930s under Donnan. The decade of the 1950s was financially difficult for the department and for the college as a whole. Ingold’s management of departmental finances was, however, so skillful that his staff were generally not aware of the difficulties. Student numbers, both undergraduate and postgraduate were similar at either end of Ingold’s headship.

This apparent continuity belies the very considerable disruption of the war years, when the chemistry department was evacuated in two sections to Wales in 1939; Ingold accompanied the special honors degree students and the postgraduates to Aberystwyth on the west coast. Some research continued during the war, but the rate of publication declined, as did student numbers. On returning to London in 1944, Ingold faced a considerable task in rebuilding his department, particularly in staffing, since a number of senior staff did not return, but pursued opportunities that had arisen during the war. This circumstance was, however, also an opportunity to reshape the staffing complement, as a number of those who left had been Donnan’s appointees. Ingold recruited heavily from his own research school. Adhering to his view of the unity of chemistry, his strategy was to appoint individuals with expertise in a range of techniques or fields who, he anticipated, would interact constructively to move chemistry based on physical principles forward across a broad front. For example, assessing crystallography as crucial to the development of chemistry, in 1947, he recruited Kathleen Lonsdale (1903–1971). Furthermore, he argued, without internal disciplinary boundaries, resources could be deployed much more efficiently.

This policy of mutual support was also evident in Ingold’s continuing a practice that had been in place since Donnan’s time. It was expected that all researchers would attend advanced lectures given by senior staff members, as well as the colloquia given by students and visitors at various stages in their research. Ingold’s pointed and precise interventions across a wide range of topics at colloquia were keenly anticipated by research students, sometimes with trepidation. On the undergraduate side, Donnan and Ingold had been active in the 1930s in discussions of the reform of the requirements for the BSc degree of the University of London. Changes very much along the lines that they had advocated were finally implemented after the war. The special honors degree in chemistry was extended from a two-year to a three-year course, with students required to take at least one year of ancillary mathematics and two years of physics. Ingold’s view of the nature of chemistry as a subject resting on physical principles that required mathematical understanding thus permeated the curriculum. Although he was very active on key university and college committees, especially when it was a case of arguing for his department or his subject, unlike Hughes, Ingold did not become heavily involved in chemical institutions. This was another aspect in which they were complementary. Ingold was, however, president of the Chemical Society in 1952 and 1953.

After the war, the research tempo picked up quickly and Ingold and his coworkers resumed their extraordinary productivity. Two further “incursions into molecular spectroscopy,” as Ingold’s Royal Society obituarist Charles W. Shoppee termed them, took place. Work using deuterated benzene continued with investigation of the excited states of benzene by means of ultraviolet spectroscopy, the first successful such analysis of a molecule consisting of more than two atoms. Work on the excited states of acetylene, using deuterated acetylene, followed, resulting in the first demonstration of a change of shape in the geometry of a molecule on excitation. In the same period, Ingold returned to the problem of aromatic nitration, which had been the focus of his studies of electrophilic reactions in the 1930s, proving in 1946 that the active agent in the nitration reaction of aromatic molecules by nitric acid was the nitronium ion, a species predicted in 1903. This was the subject of another of his famous publication campaigns: Ingold wrote twenty-two papers on aromatic nitration, which occupied nearly three hundred consecutive pages in the Journal of the Chemical Society in 1950. There was also a series of papers on the mechanisms of elimination reactions. During the 1950s and 1960s Ingold worked jointly with Robert S. Cahn (1899–1981) and Vladimir Prelog (1906–1998) on the sequence rule for specifying the positions of four different groups tetrahedrally bonded to a chiral carbon atom, that is, one that gives rise to optical activity. Their CIP (Cahn-Ingold-Prelog) sequence rule became the international standard to specify the R (rectus, or right) or S (sinister, or left) handedness of an optically active molecule.

Ingold retired formally in 1961, and became professor emeritus. He retained a role in the department as a special lecturer, and was reappointed annually. He greatly enjoyed having the freedom from departmental duties so that he could accept scientific invitations that took him on travels around the world. Ingold was knighted in 1958. In addition to recognition through awards from the Royal Society, he received the major awards and medals of the Chemical Society as well as a medal from the American Chemical Society and numerous honorary degrees from universities around the world. The one great honor that he did not receive was the Nobel Prize, which has occasioned speculation. Ingold was truly a titan in his field who not only made major, enduring discoveries, but also changed the very way that the field would be approached. Ingold’s assessment of Hughes’s significance in his Royal Society memoir applies equally well to the significance of Ingold’s own career: “this work has changed the aspect of organic chemistry, by progressively replacing empiricism by rationality and understanding, to a degree which is now manifest in the terminology and teaching of the subject, and in the research activity all along its advancing frontier” (1964, p. 165).

BIBLIOGRAPHY

A list of 443 publications by Ingold dating from 1915 to 1969 appears in C. W. Shoppee, “Christopher Kelk Ingold (1893–1970),” Biographical Memoirs of Fellows of the Royal Society 18 (1972): 349–411 (on 385–411). There is no Ingold archive per se; however, documents and correspondence by and relating to Ingold are located in a number repositories of collections of other scientists, including that of Charles Coulson (the Bodleian Library, Oxford) and Robert Robinson (the Royal Society, London, http://www.nationalarchives.gov.uk/nra/) and in institutions with which he was connected, including, of course, University College London (http://www.chem.ucl.ac.uk/) and also the University of London (http://www.shl.lon.ac.uk/) For other archival materials in England and Wales, see “Access to Archives; The English Strand of the UK Archives Network” (http://www.a2a.org.uk) For a listing of cataloged archives of British scientists contemporary to Ingold, see “National Cataloguing Unit for the Archives of Contemporary Scientists” http://www.bath.ac.uk/ncuacs/) Also see The Royal Society, “Ingold, Sir Christopher Kelk,” Certificates of Election and Candidature, GB 117 The Royal Society, RefNo EC/1924/06” (http://www.royalsoc.ac.uk).

WORKS BY INGOLD

“Principles of an Electronic Theory of Organic Reactions.”Chemical Reviews 15 (1934): 225–274.

“The Structure of Benzene” (Bakerian Lecture). Proceedings of the Royal Society of London 169, ser. A, no. 937 (1938): 149–173.

Structure and Mechanism in Organic Chemistry. Ithaca, NY: Cornell University Press, 1953; 2nd ed., 1969.

“Organic Chemistry Begins to Grow Up.” Journal of the Chemical and Physical Society (of UCL) 1 (1956): 147–151.

“The Education of a Scientist.” Nature 196, no. 4859 (1962): 1030–1034.

“Edward David Hughes (1906–1963).” Biographical Memoirs of Fellows of the Royal Society 10 (1964): 147–182.

OTHER SOURCES

Brock, William H. Fontana History of Chemistry. London: Fontana, 1992. Especially chapters 13 and 14, which provide a good general, contextual introduction to the field of physical organic chemistry.

Davenport, Derek A., and Paul R. Jones, eds. “C. K. Ingold: Master and Mandarin of Physical Organic Chemistry.” Bulletin for the History of Chemistry 19 (1996). A collection of papers from a symposium of the same title held at the American Chemical Society National Meeting in Chicago, 24–25 August 1993, in commemoration of the centennial of the birth of Professor Ingold.

Leffek, Kenneth T. Sir Christopher Ingold: A Major Prophet of Organic Chemistry. Victoria, BC, Canada: Nova Lion, 1996.

———. “Ingold, Sir Christopher Kelk (1893–1970).” In Oxford Dictionary of National Biography, edited by H. C. G. Matthew and Brian Harrison. Oxford: Oxford University Press, 2004.

Nye, Mary Jo. From Chemical Philosophy to Theoretical Chemistry: Dynamics of Matter and Dynamics of Disciplines, 1800–1950. Berkeley: University of California Press, 1993.

Ridd, John H., ed. Studies on Chemical Structure and Reactivity, Presented to Sir Christopher Ingold. London: Methuen, 1966. Roberts, Gerrylynn K. “C. K. Ingold at University College London: Educator and Department Head.” British Journal for the History of Science 29 (1996): 65–82.

Schofield, Kenneth. “The Development of Ingold’s System of Organic Chemistry.” Ambix 41 (1994): 87–107.

———. “The Growth of Physical Organic Chemistry.” N.p., 1996. Copies are deposited with the Royal Society in London and with the History of Chemistry Research Group, The Open University in Milton Keynes, England. This is a more technical treatment of the subject.

Shoppee, C. W. “Christopher Kelk Ingold (1893–1970).” Biographical Memoirs of Fellows of the Royal Society 18 (1972): 349–411.

Shorter, John. “Electronic Theories of Organic Chemistry.” Natural Product Reports 4 (1987): 61–66. This has become the standard analysis of the famous Robinson-Ingold controversy of the 1920s.

———. “Physical Organic Chemistry.” Chap. 7 in Chemical History: Reviews of the Recent Literature, edited by Colin A. Russell and Gerrylynn K. Roberts. Cambridge, U.K.: Royal Society of Chemistry, 2005. A bibliographic essay, which charts the history of the subject, this contains numerous references to studies of various aspects of Ingold’s work, including an important series of papers by Martin Saltzman, as well as related topics.

Gerrylynn K. Roberts