The synthesis of dialkyl carbonates, versatile compounds with applications in organic synthesis, pharmaceuticals, and polymers, has attracted considerable attention due to their environmentally benign nature. Here, we describe the selective bimolecular nucleophilic substitution (S_N2) reaction between primary and secondary alkyl iodides with 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)-based carbon dioxide-binding organic liquids. We show that TBD is a great candidate for bulk carbon dioxide and alcohol binding at 100°C. TBD based carbonate salts are selective for S_N2 processes, allowing them to work with highly reactive alkyl iodide while eliminating unwanted base quaternization either in acetonitrile or in bulk at both 21°C and 65°C. The high reactivity of these TBD-based carbon dioxide-binding organic liquids toward backside S_N2 processes at low temperature is explained by the presence of the TBD.H⁺ guanidinium, revealing a unique metal-free cation-assisted S_N2 ion-pair process.