Figure 1. Acetophenone conversion (mol%) and 1-phenelethanol yields (mol%) under 15V of positive external electric field for 24 h with different organic solvents and stainless-steel electrodes.
One possible reason for the significantly different performance may be their respective solubilities in water. Compared to the other solvents, 1-butanol has the highest solubility in water, which initially promotes most of the interaction between the reactants at the interface, resulting in the highest conversion and reaction rate (more than 2 fold faster than that with 1-octanol as solvent). On the other hand, these alcohols are protic solvents, which possess the ability to generate protons by dissociation. With the largest dielectric constant among the studied solvents, 1-butanol has the greatest potential to polarize and generate protons under an external electric field. These protons could further promote the dissociation of the neutral Ru compound (RuCl(p-cymene)[(S-S)-Ts-DPEN]) into the cationic Ru complex (species 2 in Scheme 1) by removing the chloride ion,31,32 therefore accelerating and promoting the generation of the catalysts and ultimately increasing the production rate. While the dipole moment and electrical conductivity might be important in electrostatically promoted reactions according to our previous investigation25 and Shaik and coworkers,2 the differences of these properties between the five solvents are small. Accordingly, it is difficult to attribute the differences in reaction performance to dipole moment and conductivity in the present study.
Table 1. Properties of the studied organic solvents.