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Electron Transfer from Aliphatic Radicals to Ring-Substituted Pyridine-N-Oxides and their Cobalt-Ammine Complexes in Aqueous Solution

The reduction of pyridine N-oxides by 1-hydroxy-1-methylethyl radicals, CMe2OH, is acid-dependent, in accord with the greater reducibility of protonated pyridine N-oxides. The respective rate constants for the unprotonated (kR0) and protonated (kR1) pyridine N-oxides have the following values (L mol-1 s-1): 8.1 x 107 and 2.8 x 109 (X ) 4-CN), 7.5 x 106 and 3.7 x 108 (4-Cl), 7.8 x 106 and 8.4 x 107 (H), and 1.7 x 106 and 4.1 x 107 (4-Me). The reduction of the Co(III) complexes (NH3)5Co(X-pyO)3+ by CMe2OH is rapid (k) 107-108 L mol-1 s-1) and acid-independent. On the basis of linear free energy relationships it is concluded that the reduction of the Co(III) complexes takes place by a chemical mechanism, whereby CMe2OH reduces the coordinated X-pyO followed by an intramolecular electron transfer to the cobalt center.

1-hydroxy-1-methylethyl radicals ; 2-hydroxy-2-propyl radicals ; kinetics ; reduction ; ions. ; series ; mechanism

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