The Effect of Heat on cis- and trans-Crotonic Acids: Alternatives to Direct cis–trans Isomerism

The thermal cis–trans equilibration of cis- and trans-crotonic acids (2-butenoic acids) has been examined at temperatures in the range 100–189°. The trans acid has been found to yield a mixture of at least five components, of which 9–11.6% was cis-crotonic acid, and 19.4–31.7% was a dimer of trans-crotonic acid. Heating cis-crotonic acid yielded similar final mixtures, but with an initial component of vinylacetic acid, not seen in the trans acid equilibrations. The rates of formation and identities of the components formed in these two sets of experiments, together with data for the thermal tautomerization of vinylacetic acid, are discussed with reference to suggested pathways for these transformations.