Electrohydimerization of cyclohex-2-en-1-one on liquid gallium from aqueous solutions

Cyclohex-2-en-1-one (R) electroreduction on gallium from unbuffered aqueous solutions yields an adsorption wave (wave I) whose behaviour is consistent with the mechanism: Rads + H2Oads → RHads+ + OH- (a) RHads+ + e- ⇄ RHads. (b) 2RHads. ⇄ R2H2,ads (c) and whose limiting current is observed even at pH values as high as 11.5. The diffusion-controlled one-electron wave (wave II) which is observed in buffered solutions of pH ≥ 7 is shown to be consistent with the mechanism: {A figure is presented} (a) {A figure is presented} (b) {A figure is presented} (c) over the pH range 7-10; here HA is a proton donor. The rate control passes from the protonation step to the subsequent coupling step with an increase in pH from 7 to 10. At pH values ≥ 11 wave II shows a behaviour which is consistent with the mechanism: {A figure is presented} (a) {A figure is presented} (b) R22- + 2H2O ⇄ R2H2 + 2OH- (c) The major differences in the mechanism of electrohydrodimerization of R from aqueous solutions in passing from a mercury to a gallium electrode stem from the much greater adsorptivity of both R and R2H2 on the latter electrode material.