Tetramethylammonium iodide is widely used in organic synthesis as a phase-transfer catalyst, facilitating reactions between compounds in different phases. It is particularly effective in nucleophilic substitution reactions, where it enhances the transfer of anions from the aqueous phase to the organic phase. In the field of electrochemistry, it serves as an electrolyte additive in batteries and supercapacitors, improving ionic conductivity and overall performance. Additionally, it is employed in the preparation of quaternary ammonium compounds, which are essential in the production of surfactants, disinfectants, and fabric softeners. Its role in the synthesis of ionic liquids also makes it valuable in green chemistry applications.