The Voltammetric Characteristics and Mechanism of Electrooxidation of Hydrazine (original) (raw)

Vol. 84 a partial decomposition of Schiff base. Possibly a higher activation energy is required to convert compound I to II than that for the Schiff base formation in the absence of metal, since the former requires splitting a nickel-oxygen bond that does not occur in the latter case. Reaction between Copper(II), Salicylaldehyde and Glycine.-The reactions between glycine and salicylaldehyde with copper(II) ion proved to be not so favorable for quantitative studies as those with nickel. Fig. 8-A, B, C and D are the spectra of copper ion and the copper complexes of salicyl-aldehyde, glycine and salicylaldehyde-glycine, respectively. The lower three curves of Fig. 9 represent the changes in optical density with time for the addition of salicylaldehyde to copper-glycine. As in the nickel system, there is a transition from the glycine complex spectrum to the Schiff base complex spectrum. The upper curve of Fig. 9 represents the optical density changes for the addition of copper(II) to salicylaldehyde-glycine. The initial optical density in this case, again as in the nickel system, is not that of the glycine complex but fairly close to the absorption of the Schiff base complex. Again the attainment of equilibrium is much faster when the Schiff base components are mixed initially than when the metal is permitted first to react with one of them. In fact, the initial optical density is equivalent to that obtained after more than 1.5 hr. when salicylaldehyde is added to copper-glycine. The behavior of copper differs from that of nickel in two respects: (1) copper increases the amount of Schiff base present at equilibrium, and (2) all of the reactions are very much slower than those with nickel, as a comparison of Figs. 2 and 4 with Fig. 9 will demonstrate. Conclusions These experiments demonstrate that the nature of the participation of metal ions in Schiff base formation is determined by the order in which the reactants are mixed; equilibrium is achieved most rapidly when the metal ion is added last. It is noteworthy that the thermodynamic stabilization of the product of a reaction by a metal ion can be accompanied by a retardation of the reaction with the metal. The instantaneous production of the spectrum characteristic of nickel Schiff base complexes upon the addition of nickel to the Schiff base components indicates that the Schiff base is formed in solution without the aid of metal. The addition of either copper or nickel ions to the premixed organic re-agents results in the immediate formation of Schiff base complex in concentrations not very different from those at equilibrium; further progress toward equilibrium produces somewhat more Schiff base with copper and less with nickel. The prior formation of a metal complex with gly-cine results in drastic retardation of Schiff base formation both with nickel and with copper. It is concluded that, even when the metal thermo-dynamically favors Schiff base formation, as is the case with copper, the metal tends to prevent rapid attainment of equilibrium. Ikawa and Snell21 and Christensen and Riggs'9 have found that salicylaldehyde does not participate in transamination and the other vitamin Be-catalyzed reactions. Salicylaldehyde is therefore not analogous to pyridoxal in the molecular rearrangements that follow Schiff base formation, since these rearrangements require either the pyridoxal nitrogen or another electron-attracting group.9·19 However, all of the ligands that bind metal in the pyridoxal-amino acid Schiff bases are also present when salicylaldehyde is substituted for pyridoxal. The conclusions drawn from the salicylaldehyde system are therefore also applicable to pyridoxal Schiff base formation. Since this reaction is retarded by metal ions, it would appear that the effect of metal ions on the vitamin Be-catalyzed reactions occurs after, and not before, Schiff base formation. Acknowledgments.-The authors wish to thank the Research Corporation for its generous financial support of a portion of this project. They are grateful to Mrs. Mary Ann Stevan and Miss Bar-bara Randall for technical assistance and to Drs. Jack Dunitz and Bernard Witkop for helpful discussion. (21) M. Ikawa and E. E. Snell, J. Am. Chem. Soc., 76, 653 (1954).