A theoretical study on catalyzed ethen-1,1-diol-acetic acid tautomerizations

Catalyzed ethen-1,1-diol-acetic acid tautomerizations in gas phase by H₂O, NH₃, H₂S, HCl, and HF were studied by ab initio calculations. Corresponding ethenol-acetaldehyde tautomerizations were also studied for comparison purposes. According to an isodesmic reaction calculated at the level of MP2/6-31G*, the second hydroxyl group on ethen-1,1-diol provides 4.5 kcal/mol more thermodynamic stability than ethenol. As acetic acid has a much higher stability than acetaldehyde, it makes pK(E) of ethen-1,1-diol bigger than that of ethenol. Transition states of catalyzed keto-enol tautomerizations were located and all the catalyzed tautomerizations found to involve a concerted mechanism. As far as catalyzed and uncatalyzed keto-enol tautomerizations were concerned, ketonization of ethen-1,1-diol has a lower activation energy than that of ethenol while enolization of acetic acid has a higher activation energy than that of acetaldehyde. Both H₂O and NH₃ function as base catalysts in the tautomerizations, while H₂S, HCl, and HF function as acid catalysts. Regardless of acidic or basic catalysts, the catalytic effect on ethen-1,1-diol-acetic acid tautomerization is more efficient than that on ethenol-acetaldehyde tautomerization by ca. 2 kcal/mol.