What’s the Diels? Part I

This year we preformed the Diels Alder reaction between maleic anhydride and furan. There are many Diels Alder reactions to choose from when planning an organic chemistry lab. I chose this one because it has an interesting history. Diels and Alder concluded that the endo product had been formed when they reported the reaction in 1931 in Justus Liebigs Annalen der Chemie. About twenty years later, the reknown synthetic chemist R. B. Woodward and his collaborator H. Baer looked more closely at this same reaction. They analyzed the product with a complicated bromination and hydrolytic process and reported that Diels and Alder had been wrong. The product of the addition, they claimed was the exo adduct: exo-cis-3,6-endoxo-Δ4-tetrahydrophthalic anhydride. Since then there have been many publications on this fascinating reaction. It is one of few known exceptions to the “endo rule” which states that Diels-Alder products prefer the endo stereochemistry. We investigated the reaction in three different solvents. We also attempted to detect the presence of the elusive “exo” isomer in the filtrate.


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