Solvent Selection and Experience

1. Solvent selection principles and experience

① Commonly used solvents: DMF, chlorobenzene, xylene, toluene, acetonitrile, ethanol, THF, Chloroform, ethyl acetate, cyclohexane, butanone, acetone, petroleum ether.
② Commonly used solvents: DMSO, hexamethylphosphoramide, N-methylpyrrolidone, benzene, cyclohexanone, butanone, cyclohexanone, dichlorobenzene, pyridine, acetic acid, dioxane, ethanol Glycol monomethyl ether, 1,2-dichloroethane, diethyl ether, n-octane.
③A good solvent has high solubility for crystallized substances near the boiling point but very little solubility at low temperatures. DMF, benzene, dioxane, and cyclohexane are close to the freezing point at low temperatures and have poor solubility, making them ideal solvents. Acetonitrile, chlorobenzene, xylene, toluene, methyl ethyl ketone, and ethanol are also ideal solvents.
④The boiling point of the solvent is preferably 50°C lower than the melting point of the substance to be crystallized. Otherwise, the phenomenon of solute liquefaction and stratification will easily occur.
⑤The higher the boiling point of the solvent, the stronger the dissolving power when boiling. For high melting point substances, it is best to choose a high boiling point solvent.
⑥Try not to use oxygen-containing solvents for substances that contain hydroxyl groups, amino groups and have low melting points. Because it is difficult to precipitate after the solute and solvent form intermolecular hydrogen bonds.
⑦ For substances containing oxygen and nitrogen, try not to use alcohol as a solvent for the same reason as above.

⑧Solute and solvent polarity Don't be too disparate. Water>Formic acid>Methanol>Acetic acid>Ethanol>Isopropanol>Acetonitrile>DMSO>DMF>Acetone>HMPA>CH2Cl2>Pyridine>Chloroform>Chlorobenzene>THF>Dioxane>Ether>Benzene>Toluene>CCl4>n-Octyl Alkane>cyclohexane>petroleum ether. 2. Recrystallization operation

3. Unconventional operation of hot suction filtration: the suction filter bottle cannot be preheated. Place the Buchner funnel and filter paper on the Erlenmeyer flask in which the solute is dissolved and moisten it with the rising steam. Place it on the suction filter bottle immediately. Suction filtration while it is hot. Pay attention to the suction pressure not to be too high to prevent the mother liquor in the suction filter bottle from boiling! Beginners often make mistakes: the filter paper is not tightly attached (double-layered ones can be used), slow movement causes crystallization to precipitate in the Buchner funnel, The pumping pressure is too high, causing the filtrate to be sucked into the pump, and the pressure is not released immediately after filtration, causing a large amount of solvent to be pumped into the pump. In short, it is the opposite of "like dissolves like". Use medium polarity for highly polar things. For crystallization using solvents; for things with low polarity, use solvents with high polarity. In this way, more than half of the cases are suitable.

1. Try first: petroleum ether (n-hexane), ether, ethyl acetate, ethanol, water, and try again : Acetone, methanol, acetonitrile, benzene, chloroform, acetic acid, pyridine, etc. If that doesn't work, just mix it. Diethyl ether can take advantage of its (1) volatility; (2) its ability to climb up the glass to precipitate solids. If acetone is not compatible with water, it should be dried.
2. Mixed solvent method: Dissolve with excess heat of good solvent, filter, heat, slowly add poor solvent until turbid, heat until clear. Wait. . .
3. Use fractional crystallization method. The accumulated mother liquor is passed through the column.
① Pre-purification through column, rough separation and recrystallization; ② Petroleum ether thermal extraction-cold precipitation method; ③ Choose a solvent with low boiling point such as diethyl ether; ④ To obtain the seed crystal, use a glass rod to dip a drop of the solution and evaporate to dryness . ⑤Do not freeze easily, let the solvent evaporate naturally. Regarding crystallization with diethyl ether. When refluxing ether, a condenser tube should be added. Keep adding ether from the top until the turbidity disappears. Sometimes it is because it dissolves slowly rather than because it cannot dissolve, so be patient. If a small amount of ether is added and a small amount of precipitate remains insoluble, filter it out, concentrate the filtrate until solid precipitates, heat again, and add a small amount of ether to clarify. Let it cool naturally to obtain crystals with better crystal form. filter. Wash the crystals with a small amount of ether. Combine the washing liquid into the mother liquor, cover the mouth of the bottle containing the mother liquor with a layer of filter paper, or plug a wad of toilet paper into it to allow the ether to evaporate naturally without falling into dust. Take a look at it every morning until a satisfactory number of crystals come out. Don't be too greedy. You will have to start over again when it evaporates:) I have successfully separated left-handed and right-handed alkaloids using this method. . The "petroleum ether hot extraction-cooling method" is also the method I use to deal with oily substances. Add petroleum ether, boil, pour out the supernatant, and continue to add petroleum ether to the bottom oil for hot extraction until the petroleum ether layer is colorless, then the extraction is basically complete. completely. Crystals usually precipitate after cooling. Another reason for oil formation is that the temperature drops too quickly.