Electronic structure from polarographic reduction

When a proton donor is added, the mechanism changes from two-electron reduction of half the amount of substrate (n = 1) to two-electron reduction of all the substrate (n = 2), because the added proton donor successfully competes with the substrate in proton transfer to the carbanion. In the case of the phosphonium ions the presence of the ylide was confirmed by the reduction potential of independently prepared ylide and by the isolation of the ylide after preparative electrolysis . The ylide corresponding to structure 2 was not isolated but from the height of the second polarographic wave the reduction of the supposed ylide is a two-electron process . 

From a polarographic point of view semi- and thiosemicarba-zones resemble the structurally related arylhydrazones. Earlier papers reported only isolated data on the reduction of aldehyde and ketone semicarbazones [145-147] and p-acetaminobenzaldehyde thio-semicarbazone [148, 149] at the dropping mercury electrode. The latter compound has been studied in more detail by Dusinsky [150], who observed a four-electron wave on the polarograms in acidic solutions at pH 7.0 this wave decreased almost to zero. A two-electron wave, the limiting current of which did not depend on pH, appeared in alkaline solution. These results were interpreted on the assumption that tautomeric equilibrium existed between the ionic form in acidic solutions and the thiol form in alkaline solutions. The opinions of the various authors differed on the question of the number of electrons involved in the electrode reactions with semi- and thiosemi-carbazones. 

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