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After the Ullmann reaction in the o-dichlorobenzene system, the solvent was evaporated by rotary evaporation. The typical phenomena that occurred included difficulty in completely evaporating the solvent, material adhering to the vessel and causing it to stick, blackening and deterioration of the product at high temperatures, and easy boiling and material splashing during the vacuum reduction process. All of these are typical phenomena and not due to operational control errors.
The boiling point of o-dichlorobenzene is relatively high. The conventional vacuum at normal temperature for rotary evaporation has extremely low efficiency in removing the solvent. Moreover, the residual copper salts, alkaline additives and organic ligands in the reaction system will further increase the boiling point of the system. Long-term heating at high temperatures can also cause decomposition of the coupling product and an increase in impurities.
For rotary drying, a gradient temperature increase and high vacuum mode should be adopted. First, set the water bath at 90 to 110°C to remove most of the solvent. Then, increase the temperature to 120 to 130°C for final concentration. The temperature throughout the process should not exceed 140°C to avoid damage to the target structure. During the operation, release pressure slightly at appropriate times to break the boiling of the liquid surface and prevent material splashing. After the concentration is completed, dissolve the crude product with an appropriate solvent, filter out inorganic residues, and then carry out the subsequent purification steps. Controlling the vacuum degree and the heating rhythm can successfully remove o-dichlorobenzene and retain the complete product.
