Fumarodinitrile electrohydrodimerization and acrylonitrile electroreduction on liquid gallium: a comparison with mercury

Electrohydrodimerization (EHD) of fumarodinitrile (R) from dilute aqueous s olutions of the strong surfactant Triton X-100 takes place both on liquid g allium and on mercury, via a rate-determining homogeneous radical-radical c oupling step. However, the potential range over which EHD takes place is mu ch narrower on gallium than on mercury. This behaviour is due to the fact t hat adsorbed water molecules act as proton donors towards the radical anion R.-, giving rise to the neutral radical RH., which is immediately reduced to the saturated monomer RH,. Therefore, EHD requires the complete displace ment of water molecules from the adsorbed state by Triton X-100; this is mo re difficult on the hydrophilic gallium than on the hydrophobic mercury. Th is also explains why, even though fumarodinitrile is much more strongly ads orbed on gallium than on mercury, no appreciable radical-radical coupling t akes place in the adsorbed state on gallium. Acrylonitrile is reduced to th e saturated monomer on gallium under the same experimental conditions in wh ich it undergoes EHD on mercury. This is also due to the high hydrophilicit y of gallium. (C) 1999 Elsevier Science S.A. All rights reserved.