Demonstration of a Runaway Exothermic Reaction: Diels–Alder Reaction of (2E,4E)-2,4-Hexadien-1-ol and Maleic Anhydride

Abstract

In a demonstration that involves a solvent-free Diels–Alder reaction of (2E,4E)-2,4-hexadien-1-ol and maleic anhydride, one can use relatively small quantities of reactants to illustrate the process of scaling up a solvent-free reaction, including consideration of reactivity of the starting substances, scale of the reaction, and size and shape of the reaction vessel. Upon being mixed, the two solid compounds form a melt and react in the liquid phase to form a solid product. In a 50 mL beaker on a 5.00 mmol scale, the reaction was relatively uneventful. However, an increase to a 10.00 mmol scale in the same size beaker resulted in a vigorous exothermic reaction, which produced dark insoluble syrupy material. When water was added to the reaction mixture, the reaction was faster, with only a modest increase in the reaction temperature and 1,3,3a,4,5,7a-hexahydro-5-methyl-3-oxo-4-isobenzofurancarboxylic acid was obtained as a pure white solid. The demonstration may help students understand the role of solvents in organic reactions, why most reactions are carried out in solvents, as well as potential problems that may be encountered when developing solvent-free reactions.