Studies in decarboxylation. Part 14. The gas-phase decarboxylation of but-3-enoic acid and the intermediacy of isocrotonic (cis-but-2-enoic) acid in its isomerisation to crotonic (trans-but-2-enoic) acid

Abstract

At 593 K the complex system (A) is shown to be rapidly set up starting from any of the pure acids. Of particular crotonic acid ⇌ isocrotonic acid ⇌ but-3-enoic acid → propene + CO₂(A) interest is the intermediacy of isocrotonic acid(the thermodynamically least stable of the three) in the interconversion of crotonic and but-3-enoic acids. A mechanism involving the enolic form of a carboxylic acid is proposed to account for this feature. The interconversion of crotonic and isocrotonic acids is much more rapid than that of their ethyl esters and a basis is proposed for the difference. Activation parameters relating to the decarboxylation of but-3-enoic acid are isolated by the flow technique.