Sonochemistry in non-conventional, green solvents or solvent-free reactions

Few decades ago, the expertise gained from well-established extraction, processing and degradation techniques, paved the way for the use of ultrasounds as an alternative energy source in chemistry. Among other peculiar features, the locally extreme temperatures and pressures resulting from cavitation, proved to efficiently trigger reactions while maintaining mild average conditions, enhancing rates and yields and, hence, contributing to increase the popularity of sonochemistry up to the present level. The physical properties of the irradiated mixture are crucial for the effectiveness of cavitation, as well as for the proper transfer of acoustic energy to reactants. Therefore, the choice of a solvent that meets these requirements, while minimizing the environmental impact of the process is a fundamental one. Studies that combine sonochemistry with green, non-conventional solvents or with no solvents are surveyed in this review, evidencing how the most frequently investigated options are water phase, ionic liquids, followed by ethylene glycol and its oligomers, glycerol and few other biomass-based solvents. Numerous solvent-free, ultrasound-promoted procedures are also reported in the literature and are included in this contribution. The vast majority of the examples gathered here describe organic reactions, syntheses of nanosystems and polymerizations.