ELECTROHYDRODIMERIZATION OF ALPHA-UNSATURATED ALICYCLIC AND ALIPHATIC-KETONES IN HYDROETHANOLIC SOLUTIONS ON A MERCURY-ELECTRODE .2. REDUCTION OF 3-BUTEN-2-ONE

Electrohydrodimerization (EHD) of 3-buten-2-one in buffered hydroethanolic solutions containing 50% (v/v) ethanol over the pH range 2.1-12.5 on mercury has been studied by polarography, cyclic voltammetry, and controlled-potential coulometry. In solutions of pH lower than 6, a one-electron process is found corresponding to the reduction of its protonated species, whereas above pH 6.3 the reduction process of its unprotonated form takes place. At pH greater-than-or-equal-to 12.0, however, this last process is not observed due to the fast conversion of substrate into an electroinactive species which polymerizes in solution. From voltammetric results, the protonated 3-buten-2-one is proposed to be initially reduced in a one-electron step to generate a neutral radical. This species further either dimerizes to give hydrodimers (the corresponding diketone and pinacol) or reacts with a Hg atom of the electrode to originate an organomercuric radical, which subsequently couples with the neutral radical to yield the organomercuric Hg(CH2-CH2-CO-CH3)2. Voltammetric data in neutral and basic solutions establish that the reduction of the unprotonated 3-buten-2-one follows the same EHD mechanism in all tested media, yielding the diketone 2,7-octadione. This hydrodimer is formed by coupling of the neutral radical obtained by protonation of the initially electrogenerated radical anion via a one-electron reduction step of the substrate. Polymers obtained as reaction product in solutions of pH greater-than-or-equal-to 8.0, then, result from decomposition of the neutral radical. The rate-determining step of each EHD reaction depends on the voltammetric conditions employed.