A Mechanistic Puzzle: Variations on Decarboxylative Elimination

Abstract

This experiment expands on a previous study of decarboxylative elimination. A number of cinnamic acids were subjected to alkene bromination. The resulting dibromides were given to pairs of students, who treated the acids with a weak base in either aqueous or organic solvent. The resulting products of decarboxylative elimination displayed different stereochemistries, depending not only on the conditions employed but also on the substituents present. The differences in stereochemistry of the alkenes formed can be understood in terms of carbocation stabilities as well as the electron-donating and electron-withdrawing effects of substituents on aromatic rings. In this application, the experiment was presented to the students as an open-ended investigation; that is, students did not know what the product of the reaction would be. A variety of instrumental techniques (¹H NMR, ¹³C NMR/DEPT, IR, and GC–MS) was used to arrive at product structures and subsequently propose a reaction mechanism.