Polarographic investigation of the tautomeric transformations of β-formylacrylic acid and of the hydrolysis of its ethyl pseudoester. I

A polarographic method was used to study the cis (HA_c), trans (HA_t), and lactol (L) forms of formylacrylic acid (FAA) and its ethyl pseudoester (E) and their interconversions. It is shown that the slow step in the process E?L?HA_c?A_c ^? is hydrolysis of E. The pH of the solution has a substantial effect on the state of the equilibrium in all of the steps because of the shift in the equilibrium HA_c?A_c ^?. The equilibrium constants of all of the steps and the rate constant of the hydrolysis of E as a function of the pH of the solution were found. The most stable of the three forms of FAA is HA_t. The cis form exists only in solutions. The polarographic reduction of HA_c and HA_t at the ethylene bond proceeds at more positive potentials than the reduction of maleic and fumaric acids. In contrast to HA_c, HA_t does not form a polarographic dissociation curve; this is explained by the closeness of the potentials for the reduction of HA_t and A_t ^?.