A reagent for organic synthesis with which a chemical reaction can be conducted in a liquid phase and unnecessary compound(s) can be easily separated at low cost from the liquid phase after completion of the reaction. The reagent for organic synthesis reversibly changes from a liquid-phase state to a solid-phase state with changes in solution composition and/or solution temperature, and is for use in organic synthesis reactions. This reagent for organic syntheses facilitates process development. With the reagent, research on and development of, e.g., medicines through, e.g., compound library synthesis, etc. can be accelerated. It can hence contribute to technical innovations in the biochemical industry and chemical industry.