Polarographic reduction of aldehydes and ketones: Part XXIII. Electroreduction of 1-methylpyridiniumcarboxaldehydes

Abstract

All three isomeric 1-methylpyridiniumcarboxaldehydes are present in aqueous solutions predominantly in the hydrated form (PyCH(OH) 2). At pH smaller than about 7, the protonated form of the free aldehyde is reduced in two one-electron steps following the sequence H +, e, H +, e. At pH greater than about 8 the free aldehydic form is reduced in two one-electron steps following the sequence, e, H +, e, H +. Up to pH about 10 the reducible species is formed by dehydration of PyCH(OH) 2, at higher pH by elimination of hydroxide ion from PyCH(OH)O −. The increase in polarographic reduction current between pH about 8 and 10 is due to a base-catalyzed dehydration, involving a rate-determining formation of PyCH(OH)O −. The polarographic dissociation curve ( i 1 vs. pH) is in this case shifted to lower pH-values when compared with the conditional dissociation curve ([PyCH(OH) 2] vs. pH). Such behavior shows that the effective reduction of the conjugate base of an electroinactive acid (PyCH(OH) 2) occurs. The increase in polarographic reduction current at pH smaller than about 1 is due to an acid-catalyzed dehydration involving formation of PyCH(OH)(OH 2 +). The rate of protonation governs the limiting current, since the increase in current with increasing acidity occurs in a region one to two acidity units higher than p K 1 values obtained spectrophotometrically, using acidity functions H C(OH) 2 . The free carbonyl species produced by dehydration is reduced following a sequence H +, e, H +, e.