For many organic reactions, the presence of oxygen is more harmful than the presence of trace amounts of water. If a reaction does not occur as expected, it is important to verify that oxygen has been removed from the system, including dissolved oxygen in the solvent. For certain reactions, solvent deoxygenation is necessary:
1. Reaction over 120 ° C or heating over 80 ° C for a long time. These reactions are usually carried out in sealed tubes and are usually heated to temperatures above the boiling point of the solvent, especially for intramolecular reactions. If the color turns yellow or brown during the reaction, it is usually attributed to the presence of oxygen. The reaction at high temperature is very sensitive to oxygen, and even the oxygen introduced when opening the reaction for TLC sampling of the sample may disrupt the reaction.
2. Organometallic reactions. For many organometallic reactions, an oxygen-free (and sometimes nitrogen-free) environment needs to be provided. It is not uncommon for some sensitive catalysts to undergo 10 cycles of lyophilizing, pumping and thawing.
3. Free radical reactions and photochemical reactions: Free radical species often react with oxygen, and it is obvious that oxygen must be deoxygenated.
4. The substrate contains mercaptan, thioether, phosphine, electron-rich aromatic compounds, etc. Heating a solvent usually results in the oxidation of such substrates, although the compounds are generally stable.
1. Articles (purging)
This is the simplest but the least effective method of degassing and is suitable for rough and large quantities of solvent degassing. As the name suggests, inert gas (nitrogen or argon) is continuously pumped into the solvent for 30min to 1h.
The inert gas is usually passed into the solvent through a long, clean needle. The syrtle is attached to an adapter or an empty syrtle with a trapezoid at the end, allowing it to be fitted with Schlenk Line’s flexible rubber or plastic tubing. The needle is inserted through a rubber plug into a container containing the solvent. The needle must reach to the bottom of the solvent. A short syringe is also inserted to relieve pressure.
Be careful that the gas does not flow so fast that the solvent does not evaporate too much. Usually 2-3 bubbles per second is sufficient, but for low boiling point solvents can be slowed down appropriately. When using this method, it is important to pay attention to prevent the evaporation of the solvent and the concentration of the water in the solvent.
2. The ultrasonic
This method is more effective than the bubbling method and is suitable for providing degassing solvents for high performance liquid chromatography (HLPC) and some rapid reactions. It includes sonic degradation of solvents and inert atmosphere refill in a microvacuum environment such as an indoor vacuum. This process must be repeated 5 to 10 times, and the degradation time of the solvent is about 1 minute each time.
3. Freezing – pumping – thawing method
This is the most effective degassing method. It is suitable for small volumes of solvents (usually used in standard size reaction flasks), including freezing, vacuuming and defrosting. Immerse a sealed Schlenk tube or thick-walled sealed test tube in liquid nitrogen. When the solvent is completely cooled, open the piston and vacuum the bottle for 2-3min(the reaction bottle is still immersed in liquid nitrogen). Close the bottle and thaw until solvent has completely melted. Repeat the process, usually three times. The final process will fill the system with inert gas. Solvents that remove oxygen in Schlenk bottles usually keep for 1-2 days.