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 .
Published Version
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