The present invention provides a reagent for organic syntheses 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 syntheses 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 developments. With the reagent, researches on and developments of, e.g., medicines through, e.g., compound library syntheses, etc. can be accelerated. It can hence contribute to a technical innovation in the biochemical industry and chemical industry.