Dangers of Peroxides
Peroxides have strong oxidizing properties and are flammable and explosive compounds. Peroxides all contain peroxy groups (-O-O-). Due to the weak binding force of the peroxy bond, the energy required for breakage is not large. The peroxy group is an extremely unstable structure and is resistant to heat, vibration, impact or friction. It is extremely sensitive and will decompose when exposed to slight external force. If the heat release rate of the reaction exceeds the heat dissipation rate of the surrounding environment, the temperature will rise under the action of the heat of decomposition reaction, and the reaction will accelerate and develop to an explosion.
The order of change in the stability of organic peroxides is: ketone peroxide < diacetyl peroxide < perether < dihydrocarbyl peroxide. Lower-level homologues of various types of peroxides are more sensitive to mechanical effects than higher-level homologues and have greater explosion risks. The interaction between peroxide and organic substances can form explosive mixtures under certain conditions. Under the action of variable-price metal salts and amines, concentrated peroxide will decompose rapidly when mixed with strong acid, causing an explosion. The interaction between hydrogen peroxide and formaldehyde has caused explosions. Acetone peroxide derivatives accumulated in the still kettle residue, which exploded in the presence of acid. There have been numerous explosions and fires when solutions of acetone peroxide derivatives containing polyester resins were mixed with cobalt naphthenate. Solid inorganic peroxides can also cause oxidation and ignite when they come into contact with organic matter. For example, spontaneous combustion accidents have occurred when barium peroxide comes into contact with sacks.
Detection and removal of peroxides in tetrahydrofuran
Tetrahydrofuran and diethyl ether are prone to produce peroxides when stored for a long time. Be careful when using them, and be sure to check the peroxide content in the solution first. If more than 0.05% peroxide is present, the peroxide must be removed before distillation. If the peroxide content is 1% or higher, THF must be disposed of by incineration and cannot be used again.
Peroxide detection:
Qualitative detection
Method 1. Prepare 10% KI (potassium iodide) aqueous solution, add a small amount of starch, then add 10 ml of tetrahydrofuran, shake, and leave for about 3~ If it turns yellow in 5 minutes, it means there is peroxide. The darker the color, the more peroxide. Otherwise, there is no peroxide and you can use it with confidence.
Method 2. Use starch-potassium iodide test paper to determine whether it changes color.
Method 3. Add 1 mL of 10% potassium iodide solution to 5 mL of tetrahydrofuran and shake for 1 min. If there is peroxide, free iodine will be released and the water layer will turn yellow-brown. Or add 4 drops of 0.5% starch solution and the water layer will turn blue. .
Method 4. Put 2 to 3 drops of concentrated sulfuric acid, 1 mL of 2% potassium iodide solution (if the potassium iodide solution has been oxidized by air, use dilute sodium sulfite solution until the yellow color disappears) and 1 to 2 drops of starch solution in a clean test tube. , mix evenly and then add tetrahydrofuran. The appearance of blue or purple indicates the presence of peroxide.
2. Quantitative detection:
Add 6 ml of acetic acid + 4 ml of chloroform + 1 g of potassium iodide to 50 ml of tetrahydrofuran, leave it in the dark for 5 minutes, and titrate with 0. 1 N sodium thiosulfate solution until it is gone. Color, peroxide percentage: NXV
Peroxides formed in the solution
Removal of peroxides in tetrahydrofuran
1. Tetrahydrofuran peroxides can be removed by mixing and stirring sodium hydroxide and tetrahydrofuran. If tablets are used, Alkali, use 5 grams of caustic soda per 100 grams of THF. If using 73% sodium hydroxide solution, use 15 grams per 100 grams of THF. However, if the peroxide content is greater than 0.5%, sodium hydroxide should be added slowly to prevent the trifluorobenzene boric acid from reacting violently and causing a sudden rise in temperature. The use of sodium hydroxide to destroy peroxide has been successfully used in practice. Small amounts of peroxide-free tetrahydrofuran for laboratory use can be prepared by adding ketone chloride, ferrous sulfate or other reducing agents, followed by distillation over lithium aluminum hydride.
Peroxide will decompose rapidly in the presence of alkali. This process is an elimination reaction that depends on the presence of a hydrogen atom at the α-carbon atom.
2. To remove peroxide, use a newly prepared dilute solution of ferrous sulfate (the preparation method is FeSO4 H2O60g, 100mL water and 6mL concentrated sulfuric acid). Place 100 mL of tetrahydrofuran and 10 mL of newly prepared ferrous sulfate solution in a separatory funnel and wash several times until there is no peroxide. (The dosage is 20% of the volume of tetrahydrofuran).
3. Pass THF through activated alumina to remove peroxide.
4. Use appropriate amount of 10% sodium sulfite to neutralize and reduce.
5. Once the barrel of THF is opened, the storage period will be shortened, even if it is refilled with nitrogen for protection. Therefore, THF left in the bucket should be used as soon as possible.
6. Adding antioxidants such as 2,6-di-tert-butyl-p-cresol to tetrahydrofuran can effectively inhibit the generation of peroxide. Its mechanism of action is that this type of antioxidant can combine with active groups to form more stable compounds, thereby terminating the chain reaction.
7. Containing peroxide in a container with a rough surface will accelerate its decomposition. For example, 38% hydrogen peroxide does not decompose in a polished platinum dish when heated to 60°C, but it will decompose at room temperature in a platinum dish with multiple scratches on its inner surface.
These are just a few ways to remove peroxide.
Preparation methods of several laboratory anhydrous solvents
Equipment for preparing anhydrous solvents in the laboratory
1. Preparation of anhydrous toluene, anhydrous tetrahydrofuran, and anhydrous dioxane
Required reagents: sodium (in addition to water), benzophenone (indicator, it shows blue under absolutely anhydrous conditions).
In terms of dosage: 1000ml of solvent requires up to 10 grams of sodium, and benzophenone requires about 5 grams.
Operation��: Dry, cool and set aside the required devices (usually 1000ml round-bottom bottle, spherical condenser tube, straight condenser tube, tail pipe, triangular flask, tee, glass stopper for holmium carbonate). Add toluene and benzophenone into the round-bottom bottle, use tweezers to add the sodium block, wipe off the kerosene on the surface of the sodium block with cotton, then use scissors to cut the sodium into small pieces, add it to the toluene through the addition funnel; then set it up Device, the air in the device is replaced with nitrogen, including the receiving bottle. After heating and refluxing for 2 to 3 hours, after the reflux turns blue, it is slightly cooled (no refluxing is enough). It is changed to a distillation device, and a small amount of the front fraction is collected and collected. part. When about 50 mL of liquid remains in the round-bottom flask, stop heating and post-process: Add absolute ethanol to the remaining sodium balls in the round-bottom flask and stir at room temperature until complete decomposition. Pour into waste bottle.
Note: THF must not be evaporated to dryness, as it will produce peroxide and is easy to explode!
2. Preparation of anhydrous DMF (N, N-dimethylformamide):
DMF is dried with anhydrous magnesium sulfate (25g/L) one day in advance; dry the required equipment (usually: 1000ml round-bottomed bottle, straight condenser tube, tail pipe, Erlenmeyer flask, tee, glass stopper, water pump). Dry, cool, and set aside. Add dried DMF to the round-bottom bottle, then set up the device and use a nitrogen balloon to check the air tightness of the device. Then carry out vacuum distillation at 60℃-70℃ to collect a small amount of the front fraction and collect the required part.
3. Preparation of anhydrous dichloromethane:
① Calcium hydride: remove water;
Dosage: 50g/L
Operation: After refluxing for 3-4 hours, distill and store in 4A molecular sieve.
Post-treatment: Add absolute ethanol to the remaining CaH2 in the round-bottom flask and stir at room temperature until complete decomposition. Pour into waste bottle.
② Anhydrous calcium chloride, molecular sieve;
Operation: Stir at room temperature overnight, distill and store with molecular sieves
4. Preparation of anhydrous ether:
Sodium: dry;
Dosage: 7-8g /500mL
Operation: Cut the sodium into small pieces and dry them for 24 hours before use. Plug the 500mL reagent bottle tightly with a 19# rubber stopper and insert a deflated balloon.
Note: When rotary evaporating ether, the temperature should not be too high and should not exceed 30°C; ether that has been left for too long cannot be heated because it contains peroxide and is prone to explosion.
5. Preparation of anhydrous methanol and ethanol:
If it is less than 1%, it can be dried with 4A molecular sieve;
Anhydrous calcium sulfate can be heated to reflux and distilled.
