ELSEVIER Journal of Electroanalytical Chemistry 444 (1998) 261-269 JOURNAL OF A comparative kinetic investigation of ethylcinnamate electrohydrodimerization on mercury and liquid gallium Giovanni Pezzatini, Silvia Becagli, Massimo Innocenti, Rolando Guidelli * Department of Chemistry, University o] Florence, Via Gino Capponi 9, 50121 Florence, haly Received 21 October 1997 Abstract Electrohydrodimerization (EHD) of ethylcinnamate (EC) on mercury takes place both from hydrotropic solutions of tetraethylammonium-p-toluenesulphonate (TEA-PTS) and from dilute aqueous solutions of the strong surfactant Triton X-100. In both cases the one-electron reduction wave due to hydrodimer formation satisfies the requirements for a rate-determining homogeneous coupling of the electrochemically generated anion radicals. The kinetics of EC EHD on liquid gallium is identical with that on mercury at the same temperature of 31°C in hydrotropic solutions of TEA-PTS; conversely, in aqueous Triton X-100 it is faster than on mercury and satisfies the requirements for a rate-determining heterogeneous radical-radical coupling. This behaviour is explained by a stronger adsorptivity of EC and its intermediate reduction products on gallium than on mercury, so that Triton X-100 does not succeed in displacing these species completely from the electrode surface. The general trend consisting in a higher adsorptivity of organic compounds with conjugated double bonds and/or aromatic rings on gallium than on mercury is explained on the basis of the difference in hydrophilicity between these two metals. ,~) 1998 Elsevier Science S.A. All rights reserved. Keywords: Kinetic investigation; Ethylcinnamate; Electrohydrodimerization; Mercury; Gallium I. Introduction Electrocatalysis is mainly concerned with the influ- ence of the nature of the metal upon the kinetics of electrode processes. The electrode material does not have a primary effect upon the kinetics of a given electrode process if, in passing from reactant to product, no chemical bonds are formed or broken, and if all reacting species are nonspecifically adsorbed on the electrode surface [1-3]. This situation is exemplified by the Fe(II)/Fe(III) redox couple on several sp and transition metals [2,3]. However, by far the majority of' electrode processes involve breaking or making of chemical bonds and a specific adsorption of the various reacting species whose extent depends on the nature of * Corresponding author. Tel.: +39 55 2757540; fax: + 39 55 244102; e-mail: guidelIi@CESIT1.UNIFL.IT 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII $0022-0728 (97)00592-.5 the metal. In this case, if the electrode process is controlled by an electrochemical step, its rate may vary by several orders of magnitude in passing from one metal to another, and no general rule can be estab- lished, since electrocatalytic effects depend drastically upon the particular electrode process investigated. On the other hand, if the electrode process is controlled by a chemical step (e.g., a protonation or a coupling step), then the changes in its rate with varying the electrode material are less drastic and are amenable to a more straightforward interpretation. Small changes in the rate of an electrode process require accurate measurements, which are more easily achieved with liquid metals, since they allow a continu- ous renewal of the electrode surface by the use of dropping electrodes. In nature only two metals are liquid in the proximity of room temperature, namely mercury and gallium, the latter melting at 30°C. These