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Free Radicals: 1900-2000.

Canada's significant contribution to the history of free radical chemistry.

The year 1900 was marked by two events of great significance for the development of free radical chemistry, especially for Canada. The first was the discovery of the triphenylmethyl radical by Moses Gomberg at the University of Michigan in Ann Arbor, an event that marked the beginning of the modern age of free radical chemistry. The second was the birth of E. W. R. Steacie, on Christmas Day, in Montr[acute{e}]al, QC. Steacie [1] was the founder of free radical chemistry in Canada, and this field has been one in which Canada has enjoyed international prominence for 70 years.

Gomberg's discovery arose from the reaction of triphenylmethyl chloride with silver, and as in many examples in science Gomberg did not achieve the goal he was seeking, namely the preparation of tetraphenylmethane. Instead, Gomberg found something much more important, namely a reactive species that combined with oxygen, to which he assigned the triphenylmethyl radical structure 1, while the product from the reaction with oxygen was assigned as the peroxide 2 (eq 1). [1] This discovery will be celebrated from June 24-29, 2000, in Ann Arbor, at a symposium at which there will be significant Canadian participation.

[Ph.sub.3]CCl [Ag [above right arrow] -AgCl[below the same right arrow] [Ph.sub.3]C [dot] [[O.sub.2] [above right arrow]] [Ph.sub.3] COOCPh.sub.3] (1)

Gomberg's discovery attracted immediate worldwide attention, and progress in solution phase free radical chemistry up until 1929 concentrated on the preparation of a variety of other stable free radicals, primarily through the efforts of Heinrich Wieland, Wihelm Schlenk, and Karl Ziegler in Germany, and James Bryant Conant and Carl Marvel in the U.S. This interest did not, however, take root in Canada, and one indication of the low profile of Canadian chemistry at the time comes from the newsletter of the 1931 National Meeting of the American Chemical Society in Buffalo, NY, in 1931, when Gomberg was president of the Society. The main mention of Canada was an expression of thanks to the Liquor Control Board of Ontario, this being the time of Prohibition in the U.S.

At the same time, chemists studying photochemical and thermal reactions in the gas phase increasingly interpreted these processes as involving free radicals. The most influential of these developments came in 1929 when Frederich Paneth in Berlin conducted his famous metal mirror experiment, in which the thermal decomposition of tetramethyl lead flowing through a hot tube laid down a lead mirror, and when cool this mirror could subsequently be removed by methyl radicals generated in another such thermolysis. Thus, the reversible formation of methyl radicals and elemental lead from tetramethyllead (eq 2) was convincingly demonstrated, and this technique was quickly exploited by other chemists in Europe and North America.

Pb[([CH.sub.3]).sub.4] [right/left arrow] Pb + [4CH.sub.3] (2)

Steacie meanwhile received his PhD from McGill University in 1926 for research with professor Frederick Johnson on the solubility of gases in metals, but the greatest influence on him was due to professor Otto Maass, a pioneer in the development of physical chemistry in Canada. Steacie obtained a teaching position at McGill, and from about 1929 began the study of gas phase kinetics of free radical reactions, particularly on the pyrolysis of esters, ethers and alkyl nitrites, [2] the latter of which form alkoxyl radicals (eq 3). [2c] These studies were highly productive, and a particularly influential contribution was his book Atomic and Free Radical Reactions of 1946. [2d]

RONO [triangle above right arrow] RO + NO (3)

In 1939, Steacie moved to the National Research Council (NRC) in Ottawa as director of the Chemistry Division, and in 1943 he added some administrative responsibilities with the joint British-Canadian wartime project on nuclear energy based at the Universit[acute{e}]de Montr[acute{e}]al. The choice of this location was influenced both by the presence of a several eminent scientists from France on the team, and the availability of a just completed but as yet unoccupied medical school building, which was converted to other uses for the duration. Interestingly, the same Friedrich Paneth responsible for the demonstration of methyl radical formation was in charge of the chemistry group. The greater part of Paneth's career was devoted to the study of isotopes, and he had moved to Great Britain in 1933 and after the war he was head of the chemistry department at the University of Durham, and then director of the Max Planck Institute for Chemistry in Mainz, Germany.

After the war, Steacie not only continued his own world renowned work but also oversaw the major expansion of chemistry at the NRC, with a heavy emphasis on free radical chemistry. This included the appointment of Fred Lossing in 1946, who gained fame for the study of the formation of free radicals by mass spectrometry, as commemorated in 1990. [3a] In 1947, Steacie also worked for the appointment of Gerhard Herzberg in the physics division of NRC, and Herzberg was to win the Nobel Prize in Chemistry in 1971 for his photochemical studies, including the generation of carbenes, which can exist as diradicals. This concentration of expertise in free radicals prompted the Faraday Society of Great Britain to organize in 1952 an overseas meeting in Toronto on the subject, [3b,c] with more than 50 participants from Britain. There were no less than five future Nobel Prize winners in attendance (Gerhard Herzberg; John Polanyi, FCIC; R.G.W. Norrish; George Porter; and Linus Pauling). Polanyi was a postdoctoral coworker with Steacie from 1952-1954. Keith Ingold, FCIC was a postdoctoral fellow with Lossing from 1951-1953, and after a stay in British Columbia has remained at the NRC as a world leader in the study of free radicals in solution. Many of his outstanding coworkers have also had distinguished independent careers in free radical chemistry both at the NRC and in the academic world. Thus since the arrival of Steacie the NRC has been in the forefront of free radical chemistry, with major contributions in both gas phase and solution work, and it is fair to say that no other single institution in the world has made such a sustained contribution at the highest level to free radical chemistry. Unfortunately, there are clear signs that the government no longer values this leading position, and there is cause for concern that the preeminent position is being lost.

While the NRC has been the principal location for free radical work in Canada this subject has been of interest all across the country in locations and by individuals that are too numerous to describe. Among these may be noted photochemical and other radical studies by D.R. Arnold, FCIC at Dalhousie University in Halifax, NS, the pioneering work on spin trapping at the University of Guelph by E.G. Janzen, investigations at the University of Alberta of halogenation by Dennis Tanner and of sulfur radicals and other species by Harry Gunning, FCIC and Otto Strausz, FCIC, and early applications of electron spin resonance at the University of British Columbia by Charles McDowell, FCIC. Free radicals are now commonly used intermediates in organic synthesis, and are an integral part of both mechanistic and preparative chemistry. Chemistry in Canada has however become highly diversified, and for the 21st Century free radicals are no longer a Canadian specialty but remain an area of interest among many chemists and bi oscientists.

Thomas T. Tidwell, FCIC is a professor in the department of chemistry at the University of Toronto, Toronto, ON. His research interests are in mechanistic and synthetic organic chemistry. Tidwell was the 1995 winner of the R. U. Lemieux Award in Organic Chemistry. He has written an account of the initial development of free radical chemistry worldwide to appear in The Chemical Intelligencer.

References

(1.) King, M.C., E.W.R. Steacie and Science in Canada, University of Toronto Press, Toronto, 1989.

(2.) (a) Steacie, E.W.R. "The Kinetics of the Heterogeneous Thermal Decomposition of Methyl Formate', Proc. Roy. Soc. A 127:314, 1930. (b) Steacie, E.W.R., and G.T. Shaw, "The Homogeneous Decomposition of Alkyl Nitrites', J. Chem. Phys. 2:345, 1934. (c) Steacie, E.W.R., W.H. Hatcher, and S. Rosenberg, 'The Kinetics of Oxidation of Gaseous Propionaldehyde', J Phys. Chem. 38:1189, 1934. (d) Steacie, E.W.R., Atomic and Free Radical Reactions, ACS Monograph 102, Reinhold, NY, 1946.

(3.) (a) Special issue of J. Mass Spectrom. Ion Proc. 101:83-377, 1990. (b) Dis. Faraday Soc., 14:3-256, 1953. (c) Whittle E., Chem. Can. October, 1952, pp. 56-59.
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Author:Tidwell, Thomas T.
Publication:Canadian Chemical News
Date:May 1, 2000
Words:1445
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