A skillful organic chemist, Otto Diels is known primarily for the Diels-Alder reaction, which he and his student Kurt Alder developed in 1928, and for which they received the Nobel Prize for chemistry in 1950. This reaction involves a synthesis between a molecule containing a double bond and a second molecule containing two adjacent double bonds. Diels' work in determining the structure of compounds led to the discovery of carbon suboxide, and helped other scientists determine the structure of cholesterol. As a professor, he was a masterful lecturer and produced a popular organic chemistry textbook.
Otto Paul Hermann Diels was born on January 23, 1876, in Hamburg, Germany, the second of three sons to Hermann Diels, a noted classical philologist and professor at the Friedrich Wilhelm University in Berlin and Bertha (n?e D?bell) Diels.
At age 13 or 14, Diels decided to become a chemist as a result of carrying out chemical experiments at home with his older brother Ludwig.
Diels received his early education in a classical gymnasium. In 1895, determined to study chemistry, he entered Friedrich Wilhelm University, where his father was a professor. With time out for his year of military service in 1896, he received a doctorate in 1899 for his work on cyanuric compounds under the direction of the eminent German chemist Emil Fischer. Upon graduation, he accepted an offer to become Fischer's assistant at the university's Institute of Chemistry.
In 1903, upon the suggestion of Swiss chemist and physiologist Emil Abderhalden, Diels began a study of cholestrol in order to establish the structure of the compound. His work supplied an essential step in the final determination of the structure by fellow German chemists Heinrich Wieland and Adolf Windaus.
In 1904, Diels became a lecturer in organic chemistry and began overseeing a display of chemical apparatus that was part of Germany's chemistry exhibit at the Louisiana Purchase Exposition in St. Louis, Missouri. An international jury awarded it a gold medal.
At Fischer's suggestion, Diels' first independent research was an investigation of certain compounds that could be extracted in large quantities from coal tar, but for which there was no commercial demand. In 1906, Diels dehydrated diethyl malonate with phosphorus pentoxide, producing a foul-smelling gas instead of the products he'd expected. On examination, the gaseous material turned out to be a new oxide of carbon made up of three carbon atoms and two oxygen atoms. Diels named this new compound carbon suboxide.
On Fischer's recommendation, Diels was appointed assistant professor at Friedrich Wilhelm University in 1906, and assumed the post of division head in 1913. In 1916, he was called to Christian Albrecht University in Kiel as full professor and director of its Chemical Institute. Simplicity, exactness, and clarity were the hallmarks of his numerous lectures and of his textbook, Einf?hrung in die Organische Chemie. First published in 1907, it proved very popular and remained in print through 1966, having gone through 22 editions. He also wrote a lab manual for elementary inorganic chemistry, which appeared in 1922. His graduate students numbered more than 100, and went on to play important roles in the academic and industrial worlds.
The demands of supervising a growing number of doctoral students caused Diels to abandon work on cholesterol for a time, but he returned to it in the early 1920s to make his second major discovery, a new method for dehydrogenating compounds.
Although none of the standard dehydrogenation procedures worked satisfactorily with cholesterol, Diels thought that one of them, a reaction commonly produced by sulfur, might be suitable if the sulfur were replaced with selenium, an element just below sulfur in the periodic table, and less active. The process of dehydrogenation with selenium, which he and his co-workers developed, was published in 1927. When Diels applied it to cholesterol, he obtained a hydrocarbon, 3'methyl-1,2 cyclopentenophenanthrene, sometimes called the "Diels hydrocarbon." This compound is the basic structural unit not only of cholesterol, but also of many other important natural products, such as the steroids (including the sex and adrenocortical hormones), bile acids, and the active components of digitalis, which are used medically as a heart stimulant.
The work for which Diels is best known is the Diels-Alder reaction, the union of a dienophile, a molecule containing a double bond, with a second molecule containing two adjacent double bonds, called a conjugated diene. This versatile synthesis, which produces a six-membered ring compound, is one of the most useful processes in organic chemistry. It takes place under quite mild conditions, room temperature often sufficing, and without the need of catalysts or condensing agents. As a result, the formation of by-products is minimized, and yields of the desired product are maximized. Furthermore, the two reactants join together in a very specific way, so that the structure of the product is known with certainty. The structure of camphor, for example, was confirmed when it was synthesized by means of a Diels-Alder reaction. During World War II, both the Allies and Germany used the reaction of butadiene (the diene) with styrene (the dienophile) to make a synthetic rubber. Since suitable dienes occur widely in nature and the reaction conditions are so mild, Diels thought that many natural products were synthesized in nature through a Diels-Alder reaction. The resultant paper appeared in 1928, and Diels continued to work on diene reactions for the next 16 years, publishing 33 papers on the subject. The last experimental work he published, in 1944, was on this topic. Because of the theoretical and practical developments that the reaction made possible, the Nobel Foundation recognized Diels and Alder with the 1950 Nobel Prize in chemistry.
In the turmoil that accompanied the end of World War I in Germany, Diels played an important role in reviving the university. There, he carried out his own laboratory research, lectured, and oversaw the research of his graduate students until 1944, when the bombing raids on Kiel destroyed the institute (as well as Diels' home). This brought a halt to his research activity, and he was unable to resume it after the war's end. His request for retirement was granted early in 1945, but he remained on staff from 1946 to 1948, when his successor was named. He continued to lecture until 1950, at age 74. Diels published 180 papers in 46 years. He died of a heart attack March 7, 1954, at the age of 78
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