Abstract
Reversible cleavages of thiamine mononitrate (I), dimethialium mononitrate (II), 3, 4-dimethyl-5-(2'-hydroxyethyl) thiazolium iodide (III), thiamic acid (IV) and N-methylbenzothiazolium iodide (V) were automatically followed by anodic waves of the thiol forms. Pseudo-first-order rate constants for the cleavage, (kN) or the recyclization (ks), are proportional to (OH-) or (H+), respectively. The pH where kN is equal to kS, corresponds to pKav: 9.3 (I), 9.4 (II), 10.3 (III), 11.2 (IV) and 6.5 (V). Electron donating groups seem to stabilize the thiazolium rings for OH- and to decrease the acidities.
