David Markham Lemal (born 1934) is the Albert W. Smith Professor of Chemistry Emeritus and Research Professor of Chemistry at Dartmouth College. He received an A.B. degree (summa) from Amherst College in 1955 and a Ph.D. in chemistry from Harvard University in 1959. At Harvard he worked with R. B. Woodward on deoxy sugars and a synthesis of the alkaloid yohimbine.
Research and teaching
Lemal began his independent academic career at the University of Wisconsin in Madison, first as instructor (1958–60), and then as assistant professor (1960–65). In 1965 he joined the department of chemistry at Dartmouth College and became a full professor in 1969. He chaired the department from 1976 to 1979. In 1981, he was appointed the Albert W. Smith Professor of Chemistry. At Wisconsin, Lemal developed a career-long interest in highly strained molecules, short-lived species and concerted reactions. In the late 1960s he became intrigued with organofluorine chemistry, which has remained a major focus of his research. Lemal was an active participant of the Gordon Research Conferences. In 1970, he chaired the GRC conference on Hydrocarbon Chemistry, in 1971 the GRC on Heterocyclic Compounds; he was a member of the board of trustees (1973–79; chair, 1977–78). He co-chaired the international Winter Fluorine Conferences in 1987 and 1989, then in 1990 chaired the Fluorine Division of the American Chemical Society. From 1996 to 2004, he served on the Society's Committee on Science. In 2005, Lemal retired from teaching, and became a Research Professor of Chemistry, continuing work in organofluorine synthesis until 2016. During his career, Lemal mentored in research more than a hundred undergraduates, graduate students, and postdoctoral fellows, and taught chemistry courses for 50 years.
Honors and awards
Lemal was awarded honors of a National Science Foundation Fellow (1955–58) and an Alfred P. Sloan Foundation Research Fellow (1968–70). In 1987, he received the Chemical Manufacturers Association Catalyst Award. In 1989, he was named the New Hampshire Professor of the Year by the Council for Advancement and Support of Education. He was awarded the Dartmouth President’s Award for Outstanding Leadership and Achievement (1991), and the Robert A. Fish Memorial Prize for outstanding contributions to undergraduate teaching (1996). He won the 2002 American Chemical Society Award for Creative Work in Fluorine Chemistry. In 2011, Lemal became a Fellow of the American Chemical Society.
Selected research
Some Hydrocarbon-Fluorocarbon Contrasts
Having synthesized and studied hexamethylprismane in 1966, Lemal’s group was interested in how a prismane substituted with electron-acceptor instead of donor groups would differ in nature and reactivity. This led to vapor phase photolysis of perfluorohexamethylbenzene (1), which yielded not only the prismane (2), but the dewar benzene (3) and benzvalene (4) as well. An English group made the same discovery independently. These strained molecules were remarkably robust and thermally stable as compared with their rather fragile hydrocarbon counterparts, and that intriguing contrast led to further exploration of fluorocarbon chemistry.
They later synthesized perfluorohexamethylbicyclopropenyl (5) to flesh out the first complete set of benzene valence isomers. This compound rearranges thermally to 1 with a half-life of ≥ 2 h at 360 °C, as compared with the parent hydrocarbon that polymerizes at -10 °C. Lemal coined the term "perfluoroalkyl effect" to represent the composite of stabilizing influences, thermodynamic and kinetic, that perfluoroalkyl groups confer upon highly strained carbon frameworks. The effect has proved to be general, but if fluoro instead of perfluoroalkyl groups decorate the carbon skeleton, behavior can be very different, as the following example reveals.
Another case of contrasting chemistry emerged when Lemal’s group prepared perfluoroquadricyclane (6) by a photoreaction at -30 °C and allowed it to warm above 0 °C. Rearrangement to tricyclic isomer 7 occurred, and further warming to room temperature yielded perfluorotropilidene (8). The parent quadricyclane (9) rearranges instead to norbornadiene (10), under far more vigorous conditions.
Keto-enol equilibria are affected dramatically by fluorine substitution. In sharp contrast to hydrocarbon-derived ketones, whose enol tautomers are generally present in only trace quantities at equilibrium, fluorinated ketones are sometimes far less stable than their enols. Keto-enol equilibria for cyclopentanone (11) and heptafluorocyclopentanone (12) are a case in point.