Comparison of Proton- and Electron-Transfer Equilibria and Rates for Tetraalkylhydrazines

Article abstract of DOI:10.1021/ja00414a002

Relative electron-transfer equilibria for 17 tetralkylhydrazines (R4N2) were determined by measurement of their standard potentials for oxidation (E<*>') and rates of electron loss by measurement of their standard heterogeneous electron-transfer rate constants (k_s) determined from their cyclic voltammograms.These data are compared with equilibrium and rate constant measurements for proton transfer between R4N2 and salicylic acid.All measurements were in dimethyl sulfoxide containing 0.1 M tetraethylammonium perchlorate.Similar trends are found for electron and proton transfer; the most basic R4N2 examples are the most easily oxidized and the fastest both to protonate and lose an electron.The proton transfers run under these conditions show anomalous Broensted behavior; the rate of protonation of R4N2 is about twice as sensitive to change of the R groups as is the equilibrium constant, and the rate of deprotonation consequently becomes slower as the reaction becomes more exothermic.A Broensted-like plot for electron transfers was constructed by estimating the electron-transfer rate constants for all R4N2 as the same potential (called k₀); electron transfer also gives anomalous Broensted behavior for R4N2.The electron-transfer rate has been previously shown to be far faster in anti lone pair conformations than in gauche ones, so that electron transfer from acrylic R4N2 involves a prior conformational equilibrium to the unstable but rapidly oxidized anti form.It is suggested that a similar gauche to anti conformational change precedes proton transfer under our conditions, and is a major contributing factor to the anomalous Broensted behavior of R4N2.