BARTLETT, PAUL DOUGHTY

(b. Ann Arbor, Michigan, 14 August 1907; d. Lexington, Massachusetts, 11 October 1997)

physical organic chemistry, free radical polymerization.

Bartlett was the outstanding American physical organic chemist of the twentieth century and a world leader in the field. Physical organic chemistry focuses on the mechanisms of organic reactions, and its mainly British founders were bent on upholding its status as a “proper” science and were little concerned with applications. When Bartlett’s career began in the early 1930s, the great majority of reactions under study were polar, involving ionic reactants and/or intermediates. Within a decade he had begun studying free radical as well as polar reactions and had turned his attention to free radical polymerization, thereby demonstrating the relevance of mechanistic investigations to processes of industrial and even military importance. Bartlett notably took advantage of organic chemistry’s synthetic powers to synthesize compounds specifically tailored to test mechanistic hypotheses. He promoted mechanistic organic chemistry in both the academy and industry, and a sizable number of Bartlett’s graduate students and postdoctoral fellows secured important posts in both spheres, accounting in large part for the U.S. dominance of physical organic chemistry after World War II.

Early Years. Paul Doughty Bartlett was born on 14 August 1907 in Ann Arbor, Michigan, the only child of George M. Bartlett and Mary Louise (Doughty) Bartlett. Mary Louise Bartlett was a piano teacher, and Paul imbibed from her a lifelong love of music. His technical prowess can be traced to his father, who possessed unusual mechanical aptitude and inventiveness. George Bartlett had been forced to leave school by the age of ten due to his father’s early death, but at the urging of a prescient teacher he returned to high school at age twenty-one. After receiving his diploma, George enrolled at Amherst College, an elite liberal arts institution in Massachusetts. Upon graduation in 1901, he obtained a teaching position at the Case School of Applied Science in Cleveland, Ohio. Two years later George was appointed an instructor at the University of Michigan in Ann Arbor, where Paul was born. In 1910, George Bartlett took a position with the Diamond Chain Company of Indianapolis, Indiana. After seventeen years there he was appointed to a professorship in the school of mechanical engineering of Purdue University in West Lafayette, Indiana, despite his lack of formal training in engineering.

By his own account, Paul Bartlett received an exceptionally good primary and secondary education in Indianapolis, where he became fluent in German and fascinated by chemistry, both of which were to be formative occurrences. An exceptional student even during his early years in Indianapolis, in 1924 Bartlett followed his father to Amherst, from which he graduated summa cum laude in 1928. Guided by his professors, Bartlett entered Harvard University that fall specifically to take his PhD with James Bryant Conant. Conant was a leader in applying physical techniques to the study of organic reactions, the defining methodology of physical organic chemistry. Bartlett’s combination of a strong interest in organic chemistry with a marked aptitude for physics and mathematics made him an ideal collaborator in Conant’s research. The aim of that research was to understand reaction mechanisms—the entire progression of energetic and geometric changes that molecules undergo in their journey from reactants to products. Such studies were the core of the new subdiscipline eventually termed physical organic chemistry.

Postdoctoral and Academic Appointments. After obtaining his PhD in 1931 for work on the kinetically complex reaction of semicarbazide with carbonyl compounds, Bartlett received a National Research Council (NRC) Fellowship. At Conant’s urging, he went to the Rockefeller Institute in New York City and worked in the laboratory of Phoebus A. Levene during 1931–1932. Conant was convinced that the future of organic chemistry lay in its applications to biology, and Levene’s research encompassed stereochemistry and nucleic acids. Although influenced by Conant’s views in many ways, Bartlett did not share his mentor’s enthusiasm for biochemistry. So, on nights and Sundays, he went to Columbia University where he tied up some “loose ends” from his thesis. The end of his NRC Fellowship marked the end of Bartlett’s brief foray into biochemistry.

In 1932, Bartlett accepted an assistant professorship at the University of Minnesota during the depths of the Depression. The salary could barely support himself and his wife, Mary Lula Court, whom he had married in 1931, and with whom he subsequently had three children: Joanna, Geoffrey, and Sarah. Bartlett’s research program in reaction mechanisms had just gotten under way in Minnesota when he was called back to Harvard as an instructor in 1934 to continue Conant’s initiative in reaction mechanisms, because Conant had accepted the university’s presidency in 1933. Bartlett would remain at Harvard for forty years, becoming Erving Professor of Chemistry in 1948. There he would carry out most of the work that established his renown. Following his mandatory retirement in 1972, Bartlett remained at Harvard for an two additional years while his coworkers completed their projects. During that period Bartlett accepted the Welch Research Professorship of Chemistry at Texas Christian University (TCU), a post he took up in 1974 and held for the next eleven years, retiring completely from academic life in 1985.

Pursing Organic Reaction Mechanisms. Several American contemporaries of Conant’s were also doing significant studies of reaction mechanisms, a pursuit that held out the eventual promise of allowing chemists to predict and control the course of reactions a priori. However, the

dominant figure in the field was a Briton, Christopher K. Ingold of University College London, whose 1934 article, “Principles of an Electronic Theory of Organic Reactions,” in Chemical Reviews, was a landmark in establishing the new subdiscipline. Indeed, some of Bartlett’s first research problems built upon the prior investigations of major European figures such as Ingold. The genuine admiration that Bartlett and his American colleagues felt for Ingold’s great accomplishments was tempered by what many saw as the latter’s imperious ways. In 1936, Bartlett wrote, “In the electronic interpretation of organic reactions, certain English chemists have been pioneers. Their views might originally have been more cordially received in this country, if presented inductively and in terms whose meanings are well known” (1938b, p. 2278).

Ingold had been studying nucleophilic substitution, an important class of reactions both chemically and biologically (Figure 1). Based on extensive experimentation, he asserted that there were only two fundamental mechanisms operative in nucleophilic substitution: a two-step mechanism in which the leaving group departs first and the remaining carbocation subsequently combines with the nucleophile, and a one-step mechanism in which bonding of the nucleophile and departure of the leaving group, which take place on opposite faces of the molecule, are coordinated (Figure 2). Bartlett carried out a classic experiment by designing a molecule, 1-chloroapocamphane, whose structure prevented it from reacting by either mechanism. This compound proved to be remarkably inert to substitution even under strenuous conditions, providing very convincing support for Ingold’s mechanistic hypotheses. Bartlett’s approach—designing a molecule specifically to test a theory—was a hallmark that eventually became characteristic of American physical organic chemistry.

Even before he conceived of the apocamphane experiment, Bartlett had taken a keen interest in the role of carbocations as reaction intermediates. A number of his earliest papers as an independent investigator concerned several rearrangements in which carbocations had been shown by the German chemist Hans Meerwein to play a

crucial role. In common with many others, including Ingold, Bartlett was a great admirer of Meerwein’s work and his approach to mechanistic problems. Yet Meerwein was never able to nurture physical organic chemistry in Germany to anywhere near the extent that Ingold and Bartlett did in the United Kingdom and the United States, respectively. That failure can be traced largely to differences in reception among the national chemical communities, an issue that is explored at greater length below.

The apocamphane investigation also sheds light on Bartlett’s human qualities. The student who carried it out, Lawrence Knox, was one of a miniscule number of African Americans studying for a PhD in the sciences in the late 1930s. Although Knox got off to a rocky start— he was thirty years old when he entered Harvard and had health problems his first year—Bartlett supported his request for funds, describing him as having conducted outstanding research. Bartlett’s laboratory was also unusually hospitable to female and Jewish students at a time when they were not very welcome in graduate science programs.

Industry, War, and Mechanistic Chemistry. In 1938, after receiving the American Chemical Society’s Award in Pure Chemistry, Bartlett declared during an interview that his work had no practical application. The following years were to completely disprove that claim. During the mid-1930s, as war clouds gathered over Europe, Universal Oil Products (UOP) discovered a one-step process for synthesizing isooctane, a major component of high performance aviation fuel (Figure 3), that was quickly scaled up for production. This counterintuitive paraffin alkylation reaction intrigued Bartlett, and by 1940, he had conceived a mechanism for it involving a carbocation (the type of intermediate involved in two-step nucleophilic substitutions; see Figure 2). In the summer of 1941, he became a consultant to UOP and received a grant to test his mechanism. The experimental results strongly supported a carbocation mechanism in which the crucial step took place in milliseconds, and Bartlett’s work made it possible to improve the yield.

After the United States entered World War II in December 1941, Bartlett’s involvement in war-related research grew much more intense. For example, his group expended considerable effort synthesizing antimalarials and insect repellents. Those projects were almost completely empirical, but Bartlett’s investigations of vinyl and allyl acetate polymerization (Figure 4) were decidedly more fundamental in nature. His professional interest in polymers was first engaged in 1940 by a small consulting contract. Then, a year later, Pittsburgh Plate Glass (PPG), which was exploring glasslike polymers, established two graduate fellowships at Harvard to support polymer research under Bartlett’s direction. These investigations were scientifically challenging as well as commercially and militarily important. In 1943, Bartlett remarked that the work was the most interesting that he had ever been involved in, because it was pure research in support of a synthetic resin that was of great concern to the Army.

At first glance, paraffin alkylation and vinyl acetate polymerization seem to have little in common. However, they are both chain reactions that take place by way of reaction intermediates, transient species with short lifetimes. In paraffin alkylation, the intermediate is a carbocation, in vinyl polymerization a free radical. The general types of intermediate are small in number (Figure 5), and identifying their presence is a principal focus of mechanistic investigations and a key to understanding and controlling a wide variety of organic reactions.

Free Radicals and Funding. Bartlett’s continuing involvement with polymerization led to several new research areas. His interests focused on oxygen (O₂), sulfur (S₈), and a number of their derivatives, all of which have

marked accelerating or inhibiting effects, or both, on free radical polymerization. His group’s examination of S₆ and S₈ and their reactions with organic substrates opened an entirely new research field. Bartlett’s exploration of organic peroxides and peroxyesters, widely used as polymerization initiators, brought clarity to a previously clouded subject. In the 1960s, Bartlett also became very interested in singlet oxygen, an electronically excited state of ordinary O₂, and pursued its chemistry at both Harvard and TCU.

Important insights into molecular behavior can often be gleaned from those cases in which reactions do not follow the expected course. The enormously useful Diels-Alder reaction is a thermal cycloaddition that creates a new six-membered ring and usually takes place in one step, without an intermediate (Figure 6). Bartlett studied a series of thermal cycloadditions that yielded mainly four-membered rings and showed that biradical intermediates intervened (Figure 7). These results were important for the theory of cycloadditions, which were enshrined in the subsequent Woodward-Hoffmann rules.

Before World War II, support for fundamental research was in short supply in the United States, but the war changed that dramatically. In 1941, both UOP and PPG made graduate fellowships and material support available for Bartlett’s work, and he later became a consultant to PPG and several other firms. These consultancies brought major benefits to both parties. Bartlett grappled with challenging problems that blossomed into major research topics for him, while his sponsors obtained valuable research data as well as access to talented graduate students and postdoctoral fellows looking for industrial rather than academic positions. Because of Bartlett’s success in applying a mechanistic approach to commercially important chemistry, his students were increasingly sought after. Many found places in industry, where they altered the focus of much industrial research, while Bartlett himself had a major impact by conducting mini-courses in physical organic chemistry at the research laboratories where he consulted.

Although much of Bartlett’s wartime research was routine, some projects demanded just the kind of skills and insight he cultivated. An outstanding example was the mechanistic investigation of mustard gas and the related nitrogen mustards (which found commercial applications and chemotherapeutic uses). When the war ended, military research contracts immediately dried up. Fearing that the pool of scientific talent and innovation that played a decisive role in the war would be lost, several military agencies stepped into the breach. First and foremost was the Office of Naval Research (ONR). In 1946, Bartlett submitted a proposal to the ONR, and the following year he received a contract for a research proposal, titled “General Theory of Structure and Mechanism of Reaction of Free Radicals,” which reflected the ONR’s commitment to fundamental research. In late 1947, Bartlett joined the ONR’s Advisory Panel on Organic Chemistry and assumed a major role in distributing ONR research funds for the next two and a half years. In spite of the many burdens they imposed, World War II and the Cold War advanced Bartlett’s research, and the dicipline of physical organic chemistry, in significant ways.

Honors and Internationalism. Given his scientific prowess and strategic position at the intersection of influential academic, industrial, and government-military networks, Bartlett was the generally acknowledged dean of American physical organic chemistry. His stature may be inferred from the various honors he acquired. In addition to the Award in Pure Chemistry, recognition from the American Chemical Society included the Gibbs, Richards, and Nichols Medals, while the Gesellschaft Deutscher Chemiker gave him its August Wilhelm von Hofmann Medal. In 1968, Bartlett received the highest scientific honor in the United States, the National Medal of Science. He also held a number of honorary degrees, fellowships, and lectureships, along with honorary memberships in numerous chemical and scientific societies. As much in the forefront ethically as professionally, Bartlett was a committed scientific internationalist. During the war he took on a Japanese-American graduate student shortly

after the student’s release from an internment camp, and after the war’s end, he established contact with several of the German chemists he had long valued.

Bartlett’s reputation for fairness and probity landed him in the middle of a seething controversy between Herbert C. Brown, on the one hand, and virtually the entire physical organic establishment, on the other, over the existence of so-called nonclassical ions (a term Bartlett deprecated). Asked to adjudicate the controversy, Bartlett put together an annotated collection of most of the principal papers through 1964 (Nonclassical Icons, 1965). The selection and annotations were as scrupulous as Bartlett’s experimental work.

Fluent in German, Bartlett admired German culture and especially German science. He was intent on reestablishing contact with those German chemists whom he still respected. In 1951, Bartlett hosted Rudolf Criegee from the University of Karlsruhe and the following year took on one of Criegee’s students as a postdoctoral fellow. In 1954, under the auspices the Amerika Häuser (America Houses), Bartlett went on a two-month tour of West Germany, giving thirty lectures and seminars in German and encountering enthusiastic receptions everywhere. In Marburg, he met one of his heroes, Meerwein, whose path-breaking studies had inspired some of Bartlett’s earliest independent research efforts.

Bartlett and Meerwein. A cadre of German chemists that included Meerwein, Criegee, Walter Hückel, and Fritz Arndt had been carrying out first-rate mechanistic work in the 1920s and 1930s, yet Germany played but a minor role in the initial development of physical organic chemistry. A comparison of Bartlett and Meerwein’s careers illustrates the different trajectories of physical organic chemistry in Germany and the United States and how those trajectories were shaped by cognitive, institutional, personal, and historical factors.

A major institutional factor working against the establishment of the new subdiscipline in Germany was the geographic separation of the organic and physical chemistry institutes in German universities. Each institute was self-contained, and there was often little interchange between their reigning professors. The situation in American universities was entirely different, allowing and even encouraging cross-disciplinary interaction. The presence of serious cognitive barriers is illustrated by Meerwein’s mechanistic investigations related to terpenes, naturally occurring plant substances. Camphor and borneol were important terpenes of commerce whose derivatives exhibited confusing chemical behavior. Building on the work of the Russian chemist Egor Egorovich Vagner, Meerwein made sense of this behavior with the revolutionary proposal that the compounds were undergoing carbon-skeleton rearrangement via carbocation-type intermediates (Meerwein and van Emster, 1922). The notion that organic compounds (in contradistinction to inorganic ones) could dissociate into ions flew in the face of the conventional wisdom that was strongly embedded in German organic circles.

In 1923, Meerwein submitted a manuscript proposing a completely dissociated, stable carbanion as a reaction intermediate. This was too much for the editors of the Berichte, which had published Meerwein’s previous articles; they returned the manuscript and asked Meerwein to shorten it by leaving out the “speculative” material. Highly offended, Meerwein withdrew the paper and began directing his future publications to other journals. This incident reveals much about the contemporaneous climate in German organic chemistry, in which outmoded notions about the distinction between organic and inorganic chemistry still held sway.

From 1900 to 1939, German organic chemists were, on the one hand, enormously successful in classical research; on the other hand, they were very conservative theoretically: The Lewis electron pair bond took hold very slowly in their ranks. Their work in synthesis and natural products, which required little knowledge of bonding or mechanisms, earned them more Nobel Prizes than organic chemists of any other nation. (The Americans garnered none in organic chemistry.) The research stars in the fields of synthesis and natural products attracted a majority of the best students, and published copiously. By comparison with those stars, Meerwein’s output was rather modest— seventy-six papers over a fifty-seven-year period (which included the war years, in which students and material resources were scarce). In the United States, the Bartlett groups published 270 papers over a fifty-one-year period. The personal and national import of comparisons like these was captured by a 1966 obituary of Meerwein in Angewandte Chemie, which noted that “his work has only been fully appreciated in Germany after World War II, and then largely as a result of the high regard in which he was held by the Americans.” Chief among them was Paul Bartlett.

BIBLIOGRAPHY

The majority of Bartlett’s correspondence is in the Harvard University archives. A substantial minority, including mainly but not exclusively material from his post-Harvard years, is in the Chemical Heritage Foundation Archives, Philadelphia. P.D. and the Bartlett Group at Harvard, 1934–1974 (1975) and A. A. M. Roof, ed., P.D. and the Bartlett Group at TCU, 1974–1985 (1985) together contain a complete list of Bartlett’s publications.

WORKS BY BARTLETT

With Irving Pöckel. “The Wagner-Meerwein Rearrangement: A Kinetic Reinvestigation of the Isomerization of Camphene Hydrochloride.” Journal of the American Chemical Society 60 (1938a): 1585–1590. This reevaluation of seminal work by Meerwein exhibits Bartlett’s acumen in identifying underappreciated results of his peers and predecessors.

Review of H. B. Watson, “Modern Theories of Organic Chemistry.” Journal of the American Chemical Society 60 (1938b): 2278.

With Lawrence H. Knox. “Bicyclic Structures Prohibiting the Walden Inversion: Replacement Reactions in 1-Substituted 1-Apocamphanes.” Journal of the American Chemical Society 61 (1939): 3184–3192. Presents Bartlett’s highly original work with Knox on substitution reactions.

With Francis E. Condon and Abraham Schneider. “Exchanges of Halogen and H between Organic Halides and Isoparaffins in the Presence of Al Halides.” Journal of the American Chemical Society 66 (1944): 1531–1539. The investigation that nailed down the mechanism of paraffin alkylation.

With C. Gardner Swain. “The Absolute Rate Constants in the Polymerization of Vinyl Acetate.” Journal of the American Chemical Society 67 (1945): 2273–2274. Bartlett’s first major paper on the kinetics of free radical polymerization.

With C. Gardner Swain. “Kinetics of Hydrolysis and Displacement Reactions of B, B-Dichlorodiethyl Sulfide (Mustard Gas) and of B -Chloro- B’ -Hydroxydiethyl Sulfide (Mustard Chlorohydrin).” Journal of the American Chemical Society 71 (1949): 1406–1415. A direct application of mechanistic organic chemistry to an important military problem.

Ed. Nonclassical Icons: Reprints and Commentary. New York: W. A. Benjamin, 1965. The selection and annotations are as scrupulous as Bartlett’s experimental work

With David Mendenhall and Dana L. Durham. “Controlled Generation of Singlet Oxygen at Low Temperatures from Triphenyl Phosphite Ozonide.” Journal of Organic Chemistry45 (1980): 4269-4271. Based on his continuing studies at Texas Christian University of peroxides, ozonides, and the highly reactive singlet oxygen, which he, Mendenhall, and Durham were able to generate under mild conditions.

“James Bryant Conant.” Biographical Memoirs of the National Academy of Sciences 54 (1983): 91–124. This obituary is revealing about both author and subject.

OTHER SOURCES

Dimroth, K. “Hans Meerwein als Mensch und Lehrer.” Angewandte Chemie 78 (1966): 353–355; “Hans Meerwein, the Teacher and the Man.” Angewandte Chemie International Edition in English 5 (1966): 338–341.

Ingold, Christopher K. “Principles of an Electronic Theory of Organic Reactions.” Chemical Reviews 15 (1934): 225–274. One of the principal documents in the founding of physical organic chemistry, in which Ingold introduces his much contested but generally triumphant nomenclature.

McBride, J. Michael, ed. P.D. and the Bartlett Group at Harvard, 1934–1974. New Haven, CT: privately published, 1975. A collection of speeches, encomia, research histories, and letters from co-workers during Bartlett’s Harvard years. This volume and the Roof publication cited below give a good deal of insight into Bartlett’s character as a person and research supervisor.

Meerwein, Hans, and Konrad Van Emster. “Über Die Gleichgewichtsisomerie Zwischen Bornylchlorid, Isobornylchlorid und Camphenchlorhydrat.” Berichte der Deutschen Chemischen Gesellschaft 55 (1922): 2500–2528.

Roof, A. A. M., ed. P.D. and the Bartlett Group at TCU, 1974–1985. Fort Worth, TX: privately published, 1985. Similar to P.D. and the Bartlett Group at Harvard, 1934–1974 (1975) but much smaller, it covers his years at Texas Christian University.

Westheimer, Frank. “Paul Doughty Bartlett, August 14, 1907–October 11, 1997.” Biographical Memoirs of the National Academy of Sciences 75 (1998): 25–37. This the “official” obituary, by Bartlett’s Harvard colleague and close personal friend.

Stephen J. Weininger