A critical review of solvent influence on the performance of metal catalysts in the hydrogenation of carbonyl compounds, specifically unsaturated aldehydes, ketones and ketoesters, is provided. Additionally, the effects of solvent on the kinetics and mechanism of different reactions, stability, and sensitivity of the nanostructures are investigated. The uses of a large of variety of solvents varying from non-polar low basic solvents to polar strong basic solvents are discussed, showing that the properties of different solvents strongly influence the product composition of both studied reactions. The effects of solvent on the isomerization of monoterpenes epoxides as α- and β-pinene epoxide, limonene epoxide, verbenol epoxide, and nopol epoxide are explained from molecular and mechanistic points of view. The concluding study explores how heterolytic bond dissociation forms ionic species, while the corresponding homolytic bond dissociation results in the formation of neutral radical species.