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 (pKa(DMSO)=24.2 and hexamethyldisilazane (HMDS, pKa(DMSO)=26), has been compared in DME+HMPA+Et4NBF4 solution using the sacrificial magnesium anode method in an undivided cell under constant current. Both acids are reduced at transition metal cathodes to form dihydrogen and their conjugate base (as evidenced by their deprotonation abilities) whereas no gas evolution is observed at non-catalytic surfaces such as glassy carbon. Kinetic studies of dihydrogen evolution at Pt, stainless steel, Fe and Ni, throughout the electrolysis showed different behavior: a theoretical current efficiency (CE=1) characterizes 2-pyrrolidone, but HMDS needs a latency period with slow gas evolution mainly corresponding to reduction of HMPA, followed by a faster and linear evolution period during which HMDS is predominantly reduced (51<CE<64%). All these data are consistent with a direct dissociative discharge mechanism for the NH bond reduction giving an adsorbed hydrogen atom on the metal surface. But 2-pyrrolidone verifies Evans’ peak potential correlation with a pKa value for ‘category II acids’, which corresponds to a diffusion controlled process, while HMDS behaves differently without any observable reduction peak, probably showing a charge transfer controlled mechanism.