A Study of the Reduction Mechanism of p-Dimethylaminobenzaldehyde to 1,2-Bis(p-dimethylaminophenyl)-1,2-ethanediol by Rotating Disk Electrode Voltammetry

Abstract

Abstract Rotating disk electrode voltammetry was applied to study the reduction mechanism of p-dimethylaminobenzaldehyde (1) in water–ethanol alkaline solutions by using a mercury-plated platinum rotating disk electrode. In the logarithmic analysis of the wave, the plots of log [i2⁄3/(id–i)] vs. E showed good linearity, and the shifts of the half-wave potential with an increase in the rotation speed of the electrode (ω) and in the initial concentration of 1 (C) were ∂E1⁄2⁄∂logω=−22±2 mV and ∂E1⁄2⁄∂logC=19±2 mV, respectively. On the other hand, no appreciable change in E1⁄2 was observed with varying alkalinities of the solutions. These results and the large shift of E1⁄2 with an increase in the concentration of water (∂E1⁄2⁄∂log[H2O]=ca. 50 mV) are explained in terms of the following reactions: R+e\ightleftarrowsR\ewdot (solvated by water), (1) 2R\ewdot\oversetk→D2− (r.d.s.), (2) D2−+2H2O→DH2+2OH−, (3) where R and R\ewdot are the molecule and the radical of 1, respectively, D2− is the dimer of R\ewdot and DH2 is the hydro-dimer (2) of R. The rate of the dimerization reaction (k in Eq. 2) was estimated to be 2.5×104 M−1 s−1 from the shift of E1⁄2 with logω for the 50% water-ethanol solution containing 1 M NaOH, and this rate constant decreased with decreasing water concentration.