Purification of common solvents

Commonly used reagents from chemical suppliers are only sufficient for general chemical reactions. In order to ensure the smooth running of some organic synthesis reactions, further purification of the reagents is often required. A common solvent treatment is distillation. If the reaction requirements are only anhydrous, can be added to the condenser tube dry tube, oil seal or nitrogen ball can be filled, if you need to achieve anhydrous and oxygen-free conditions, the solvent needs to be deoxygenated. Generally under nitrogen atmosphere.

 

Purification of reagent grade solvents anhydrous reagent grade solvents often have sufficient purity, sometimes without distillation. To ensure adequate dryness, activated molecular sieves may be added to them during storage. To deoxidize a solvent, nitrogen can be drummed into it for about five minutes using a syringe or glass tube. Purification of general solvents Most solvents can be made sufficiently pure by distilling them from the desiccant in an inert atmosphere.

 

1. Alkanes

Alkanes such as hexane, pentane, etc. First wash several times with concentrated sulfuric acid to remove the olefins, wash with water, dry with CaCl2, dry with sodium wire or P2O5 if necessary, and distill. Store in a stoppered reagent bottle.

 

2. Aromatic hydrocarbons

For example, benzene, toluene, xylene, etc. Dry with CaCl2, dry with sodium or P2O5 if necessary, and distill. Store in stoppered reagent bottles. 3.

 

3. Chlorinated alkanes

Such as dichloromethane, chloroform, carbon tetrachloride, dichloroethane, etc. Wash to remove alcohols, etc.. Wash with water to remove alcohols, etc., dry with CaCl2, and evaporate by refluxing in P2O5, or CaH2. Never dry with sodium filaments, or it will explode. Long-term storage should be in airtight bottles and kept in the dark.

 

4. Ethers and furans

Such as ether and tetrahydrofuran. Many ethers, when in contact with air, slowly form peroxides which are non-volatile and of unknown structure. Peroxides decompose and explode when heated. Therefore, ethers and furans that have been stored for a long time should be checked for the presence of peroxides before use, especially before distillation. Test method: 1 mL of 10% KI solution containing a drop of starch indicator mixed with 10 mL of ether solution, there is no color change, then there is no peroxide. Alternatively, test with 1% ammonium ferrous sulfate solution, ferrous sulfate and potassium thiocyanide solution. If present, add 5% FeSO4 or sodium bisulfite solution to the ether and shake to decompose the peroxide. pre-dry with CaCl2 and evaporate by refluxing in sodium filament or LiAlH4. store in an airtight bottle. Store in airtight bottles and keep in a cool dark place.

 

Purification of commonly used organic solvents – ether Boiling point 34.51 ℃, refractive index 1.3526, relative density 0.71378. Common ether often contains 2% ethanol and 0.5% water. The ether often contains a small amount of peroxide.

Test and removal of peroxides: put 2~3 drops of concentrated sulfuric acid, 1mL2% potassium iodide solution (if the potassium iodide solution has been oxidized by air, dilute sodium sulfite solution can be used until the yellow color disappeared) and 1~2 drops of starch solution, mix well and add the ether, the appearance of blue that is, that is, the presence of peroxides. Remove the peroxide can be used to prepare a new dilute solution of ferrous sulfate (preparation method is FeSO47H2O60g, 100mL water and 6mL concentrated sulfuric acid). Wash 100mL of ether and 10mL of freshly prepared ferrous sulfate solution in a dispensing funnel several times until no peroxide is present.

Test and removal of alcohols and water: a little powdered potassium permanganate and a grain of sodium hydroxide were placed in the ether. The presence of alcohol is proved by the presence of brown particles on the surface of the sodium hydroxide when left to stand. The presence of water is tested by anhydrous copper sulfate. Most of the water is first removed by anhydrous calcium chloride and then dried by metallic sodium. The method is: 100mL of ether in a dry conical flask, add 20~25g of anhydrous calcium chloride, the mouth of the bottle with a cork tightly corked, placed for more than a day, and intermittent shaking, and then distilled to collect 33~37 ℃ fractions. With a sodium press, 1g of sodium metal is pressed directly into sodium filaments placed in a bottle containing ether and corked with a cork with a calcium chloride drying tube. Or insert a glass tube with the end pulled into a capillary tube in the cork, so as to prevent moisture from soaking in, but also to allow the gas produced to escape. Place until no bubbles occur can be used; after placing, if the surface of the sodium filament has become yellow and coarsened, it must be steamed again, and then pressed into the sodium filament.

 

Purification of commonly used organic solvents – tetrahydrofuran (THF) boiling point 67 ℃ (64.5 ℃), refractive index 1.4050, relative density 0.8892.

Tetrahydrofuran is miscible with water and often contains a small amount of water and peroxide. If you want to make anhydrous tetrahydrofuran, can be used to reflux lithium aluminium hydride in the isolation of moisture (usually 1000mL about 2 ~ 4g lithium aluminium hydride) to remove the water and peroxides, and then distillation, collection of 66 ° C fractions (do not steam distillation, the remaining small amount of residual liquid that is poured out). The refined liquid is added to the sodium filament and should be stored in a nitrogen atmosphere.

When treating tetrahydrofuran, it should be tested in small quantities and purified only when it is certain that there are only small amounts of water and peroxides in it and that the action is not too intense. ！！！！！ Peroxides in tetrahydrofuran can be tested by acidified potassium iodide solution. If there is a large amount of peroxide, separate treatment is appropriate. Large amounts of peroxide can be removed by refluxing with CuCl now.

 

Purification of commonly used organic solvents – dioxane Boiling point 101.5 ° C, melting point 12 ° C, refractive index 1.4424, relative density 1.0336.

Dioxane can be mixed arbitrarily with water, often containing a small amount of diethanol acetal and water, long-stored dioxane may contain peroxide (identification and removal of reference to ether). Dioxane purification method, in 500mL dioxane add 8mL concentrated hydrochloric acid and 50mL water solution, reflux 6~10h, in the reflux process, slowly pass nitrogen to remove the generated acetaldehyde. After cooling, solid potassium hydroxide was added until it could not be dissolved any more, the aqueous layer was separated, and then dried with solid potassium hydroxide for 24 h. Then it was filtered, heated and refluxed in the presence of sodium metal for 8~12 h. Finally, it was distilled in the presence of sodium metal and pressed into starvation wire for sealing and preservation. Refined 1,4-dioxane should be protected from contact with air.

 

5. Amides

Such as dimethylformamide, dimethylacetamide, HMPT. Add CaH2 and reflux, evaporate under pressure, otherwise it will decompose easily. Add freshly activated molecular sieves and store in bottle with date.

 

Purification of commonly used organic solvents, N, N-dimethylformamide -DMF N, N-dimethylformamide Boiling point 149 ~ 156 ℃, refractive index 1.430 5, relative density 0.948 7. Colorless liquid, and most organic solvents and water can be mixed arbitrarily, the solubility of organic and inorganic compounds is good.

N,N-dimethylformamide contains a small amount of water. Some decomposition during distillation at atmospheric pressure, yielding dimethylamine and carbon monoxide. Decomposition is accelerated in the presence of acids or bases. Therefore, some decomposition occurs when solid potassium (sodium) hydroxide is added and left at room temperature for several hours. Therefore, calcium sulfate, magnesium sulfate, barium oxide, silica gel or molecular sieve are most commonly used to dry the product, which is then distilled under reduced pressure to collect the 76°C/4800 Pa (36 mmHg) fraction. Which, if it contains more water, can be added to its 1/10 volume of benzene, at atmospheric pressure and below 80 ℃ evaporate water and benzene, and then anhydrous magnesium sulfate or barium oxide drying, and finally for decompression distillation. The purified N,N-dimethylformamide should be stored away from light.

The presence of free amines in N,N-dimethylformamide, if any, can be checked with 2,4-dinitrofluorobenzene to produce color.

 

6. Dimethyl sulfoxide

Add CaH2 and stir overnight, then fractionate under reduced pressure. Add freshly activated molecular sieves Store in vials and date.

 

7. Pyridine

May be KOH, NaOH, CaO, BaO or sodium, then evaporate. Add freshly activated 5Å molecular sieves Store tightly closed and date.

 

Purification of Commonly Used Organic Solvents – Pyridine Boiling point 115.5°C, refractive index 1.509 5, relative density 0.9819.
Analytically pure pyridine contains a small amount of water, can be used for general experiments. If you want to make anhydrous pyridine, pyridine can be refluxed with potassium hydroxide (sodium), and then isolated from moisture vaporization standby. Dried pyridine is highly absorbent and should be stored in a container with the mouth sealed with paraffin.

 

8. Ethanol

The main impurities are heteroalcoholic oils, aldehydes, alcohols, ketones and water. It can be purified by refluxing magnesium chips and iodine, and then refluxing with CaO and evaporating. Add freshly activated 3A molecular sieves and store in vials.

 

Purification of common organic solvents – ethanol Boiling point 78.5 ℃, refractive index 1.3616, relative density 0.7893.
There are many ways to prepare anhydrous ethanol, and different methods are chosen according to the different requirements for the quality of anhydrous ethanol.

If 98%~99% ethanol is required, the following methods can be used:
(1) the use of benzene, water and ethanol to form a low azeotropic mixture of the nature of benzene, benzene added to ethanol, fractional distillation, at 64.9 ℃ when evaporated benzene, water, ethanol ternary constant boiling mixture, excess benzene in the formation of a binary constant boiling mixture with ethanol in the 68.3 was evaporated, and finally evaporated ethanol. Industry mostly use this method.

(2) dehydration with quicklime. In 100mL of 95% ethanol add fresh block quicklime 20g, reflux 3~5h, and then distillation.

If you want more than 99% ethanol, you can use the following methods:
(1) In 100mL of 99% ethanol, add 7g of sodium metal, wait until the reaction is complete, then add 27.5g of diethyl phthalate or 25g of diethyl oxalate, reflux for 2~3h, then distill.

Although sodium metal can act with water in ethanol to produce hydrogen and sodium hydroxide, but the generated sodium hydroxide and ethanol have an equilibrium reaction, so the use of sodium metal alone can not completely remove the water in ethanol, must be added to an excess of high-boiling-point esters, such as diethyl phthalate and the generated sodium hydroxide to inhibit the above reaction, so as to achieve the purpose of further dehydration.

(2) In 60mL of 99% ethanol, add 5g of magnesium and 0.5g of iodine, and when the magnesium is dissolved to generate alcohol magnesium, then add 900mL of 99% ethanol, reflux for 5h, and then distill, which can obtain 99.9% ethanol.

Because ethanol has a very strong hygroscopicity, so in the operation, the action should be rapid, minimize the number of transfers in order to prevent the entry of moisture in the air, and at the same time the instrument used must be dried well beforehand.

 

Note: When applying metal compounds as purifying agents, the solvent in the distillation flask should be kept at least one quarter of the volume during distillation, and should never be allowed to evaporate to dryness, as this may be dangerous.