Towards the electroreduction of very weak acids

In order to improve the understanding of the synthesis of electrogenerated bases (EGB), the electrochemical reduction of two weak acids, 2-pyrrolidone (pK(a)(DMSO) = 24.2 and hexamethyldisilazane (HMDS, pK(a)(DMSO) = 26), has been compared in DME + HMPA + Et4NBF4 solution using the sacrificial magne sium anode method in an undivided cell under constant current. Both acids a re reduced at transition metal cathodes to form dihydrogen and their conjug ate base las evidenced by their deprotonation abilities) whereas no gas evo lution is observed at non-catalytic surfaces such as glassy carbon. Kinetic studies of dihydrogen evolution at Pt, stainless steel, Fe and Ni, through out the electrolysis showed different behavior: a theoretical current effic iency (CE = 1) characterizes 2-pyrrolidone, but HMDS needs a latency period with slow gas evolution mainly corresponding to reduction of HMPA, followe d by a faster and linear evolution period during which HMDS is predominantl y reduced (51 < CE < 64%). All these data are consistent with a direct diss ociative discharge mechanism for the NH bond reduction giving an adsorbed h ydrogen atom on the metal surface. But 2-pyrrolidone verifies Evans' peak p otential correlation with a pK(a) value for 'category II acids', which corr esponds to a diffusion controlled process, while HMDS behaves differently w ithout any observable reduction peak, probably showing a charge transfer co ntrolled mechanism, (C) 2000 Elsevier Science S.A. All rights reserved.