Dartmouth Flourine Chemists present at the American Chemical Society Meeting

Posted 04/08/02

On April 8, the American Chemical Society, the largest scientific society in the world, presented Dartmouth Professor David Lemal with the ACS Award for Creative Work in Fluorine Chemistry at the society's 223rd national meeting in Orlando, Florida. Also at this meeting at a special symposium to honor Lemal, Russell Hughes, the Frank R. Mori Professor of Chemistry at Dartmouth, reported his latest research findings about converting carbon-fluorine bonds to carbon-hydrogen and carbon-carbon bonds.

Lemal, the Albert W. Smith Professor of Chemistry, has been studying fluorine chemistry for more than 30 years, and the ACS award recognizes his many contributions to the field. Not only has Lemal published numerous academic papers in various peer-reviewed journals, he has also been a role model for his fellow chemists. Hughes, in his nomination letter, wrote, "[Lemal's] deep thinking and the rigor of his approach to chemical principles have been an inspiration to his colleagues and to the international community of fluorine chemists. ... Lemal's published works on organofluorine chemistry are characterized not by their unusually large number, not by the volume of words or numbers of compounds reported, but by a truly rigorous approach to intellectually stimulating and seminal problems, often requiring truly challenging synthetic effort."

While the element fluorine has been known for about 120 years, explains Lemal, it wasn't until the early part of the twentieth century that it came into its own as part of the notorious chlorofluorocarbons. CFCs, chemically engineered inert molecules, became important in the 1930s when their cooling properties were exploited in refrigerators and air conditioners, and again later when their propellant capacity was used prolifically in aerosols like hair spray and deodorants.

"Now we know that chlorofluorocarbons were building up and slowly destroying the ozone layer," says Lemal, who's been at Dartmouth since 1965. "It's really the chlorine in them that does the ozone damage. Fluorine plays the role of making them so stable that the molecules survive to diffuse up into the stratosphere." Once the CFC molecules reach the upper atmosphere, the intense ultraviolet radiation causes them to break down, releasing chlorine atoms, which catalyze the transformation of ozone into ordinary oxygen.

"Now chemists are working to replace the chlorine," says Lemal.

This is where Lemal's colleague Hughes plays a role. Hughes' work focuses on the new molecules that take the place of and do the job of CFCs. He explains that the best candidates to date are HFCs, or hydrofluorocarbons. They contain only bonds from carbon to hydrogen and carbon to fluorine - no chlorine.

"Because HFCs are also inert like CFCs," says Hughes, "we hope they won't cause any atmospheric problems. But we don't know that - it took decades the last time to notice there was a problem - but we're hopeful that the HFCs are the good guys."

HFCs are not new, but chemists are working feverishly to find inexpensive ways of replacing the carbon-fluorine bond with a carbon-hydrogen bond. Some of these new HFC molecules are more expensive than the old ones, in part, because the technology to create the new ones starts with making the old ones.

"They know how to make CFCs - there are industrial plants set up to do that," says Hughes. "Right now, it's sometimes easier to use the technology to make the old CFCs and then convert them into HFCs. The extra steps can cost more money, so the final product may be more expensive."

Hughes' team, completely by accident, discovered a few years ago a way to use transition metal compounds, in particular iridium compounds, to replace carbon-fluorine bonds with carbon-hydrogen bonds at room temperature. Hughes' latest findings will be presented at the April ACS conference in Florida.

"My team is learning more about how to create these kinds of replacement reactions in bulk samples rather than in the small laboratory samples," Hughes says. "Dave Lemal likes to keep his fluorine bonds there, I like to knock them off," he jokes.

Lemal and the students in his lab join molecules to make new and interesting combinations. Ultimately, they want to understand the consequences of substituting fluorine for the ubitquitous hydrogen in organic molecules. According to Lemal, fluorine is the only element in the periodic table that you can subsitute on a wholesale basis for hydrogens in all types of organic molecules and still have stable structures.

"We build strange, new molecules that are unknown," he says. "We spend most of our time synthesizing them, and then we study them. We carry out spectroscopic studies, kinetic studies, thermodynamic studies or look at their chemical reactivity."

For example, Lemal's students build molecules that incorporate unusual bond angles and thus have a high energy content. Because of the high energy, they have great reactivity and in turn, they spawn more unusual molecular structures. Lemal's research contributes to a better understanding of basic fluorine chemistry. Fluorocarbons and their derivatives enjoy a broad spectrum of uses, from the inert slippery polymer Teflon to inhalation anesthetics to the oxygen carriers in blood subtitutes.

Lemal traces his interest in fluorine molecules to an undergraduate student's research. "About 1969, I worked with a Dartmouth undergraduate on our first experiment involving fluorocarbon chemistry - trying to make a particular highly strained flurocarbon," he said. "We were successful, and I was really struck by the crazy properties of what this student made. They were so different from those of the hydrocarbon analogue that we'd earlier made, that it just intrigued me. I've been mucking about in this field ever since, and I still find it as much fun as I did when I started out."