Low-temperature sonoelectrochemical processes: Part 2: Generation of solvated electrons and Birch reduction processes under high mass transport conditions in liquid ammonia

Solvated electrons in liquid ammonia at −60°C (0.03 M LiCl) can be electrogenerated in the presence of power ultrasound and are shown to be essentially inert under these conditions in the presence of 3-methylanisole (3-MA). The fast mass transport and mixing induced by power ultrasound is then used to monitor the homogeneous Birch reduction process facilitated by the addition of ethanol as a proton source. Pseudo first-order kinetics for both the reduction of 3-MA and the unwanted formation of hydrogen is observed and analysed quantitatively. The rate laws proposed by Greenfield and Schindewolf (A. Greenfield, U. Schindewolf, Ber. Bunsenges. Phys. Chem. 102 (1998) 1808) for the reduction of protons, −(d[e−]/dt)=kH[CH3CH2OH][e−][Li+], and for the Birch reduction of 3-MA, −(d[e−]/dt)=kA[CH3CH2OH][e−][Li+][3-MA], are confirmed by varying the concentrations [CH3CH2OH] and [3-MA]. The rate constants kH=2±0.2 M−2 s−1 and kA=700±100 M−3 s−1 at −60°C are determined and employed for the optimisation of the product yield and current efficiency. Fast mass transport and electrode depassivation induced by power ultrasound are shown to maximise the current efficiency observed for the electrochemical Birch reduction process and to allow the process to be conducted in the presence of a high ethanol concentration and therefore with high overall rate.