Mechanism of electrohydrodimerization of cyclohex-2-en-1-one on mercury from aqueous solutions. Part 1.—Results obtained in the absence of surfactants

Abstract

The diffusion-controlled one-electron wave (wave I) due to cyclohex-2-en-1-one (R) electroreduction on mercury from aqueous solutions in the range 2.5 < pH < 4 is shown to be consistent with the following mechanism: R + HA ⇌ RH⁺+ A^–(1 a) RH⁺+ e ⇌ RH ˙(1 b) [graphic omitted] (1 c) where HA is a proton donor and the rate-determining coupling step (1 c), denoted by rds, proceeds in the adsorbed state. Double-potential-step chronocoulometric measurements indicate that R adsorption lies below the limits of sensitivity of the method, whereas R₂H₂ adsorption is appreciable at all potentials positive to – 1.4 V vs. SCE. The limiting current of wave I in the range 4 < pH < 6 is controlled by the protonation step (1 b), which is shown to proceed both heterogeneously and homogeneously. The contribution of the various proton donors, including adsorbed water, is pointed out. The diffusion-controlled one-electron wave (wave II), which develops at the expense of wave I at pH > 7, is shown to be consistent with the following mechanism: R + e ⇌ R ˙^–(7 a) R ˙^–+ HA ⇌ RH ˙+ A^–(7 b) [graphic omitted] (7 c) for 7 < pH < 10, and with the mechanism R + e ⇌ R ˙^–(10 a) [graphic omitted] (10 b) R^2–₂+ 2H₂O ⇌ R₂H₂+ 2OH^–(10 c) for pH > 11. For mechanisms (7) and (10) the rate-determining coupling step is homogeneous. The mixtures of isomeric forms of the hydrodimer R₂H₂ obtained at pH 5 via the heterogeneous coupling step (1c) and that obtained at pH 9 via the homogeneous coupling step (7 c) have practically the same composition.