For many (possibly most) organic reactions, the presence of oxygen in a reaction
detrimental than traces of H2O. If a reaction fails unexpectedly, it is a good
ensure an oxygen free environment, part of which entails degassing the reaction
For certain reactions, degassing is essential:
Any reaction heated above 120oC and many reactions heated for a prolonged
period over 80oC. These reactions are typically performed in a sealed tube
and are often run by heating over the boiling point of the solvent. Especially
in the case of intramolecular reactions, the formation of a yellowish or brown
color in the reaction is indicative of the presence of oxygen. High temperature
reactions are extremely sensitive to oxygen and even the act of opening the
reaction to take a sample for TLC analysis can introduce enough oxygen to
destroy or damage the reaction
Organometallic reactions. For many organometallic reactions, it is
provide an oxygen free (and sometimes N2 free) environment. It is not
uncommon to see strict procedures prescribing 10 cycles of freeze-pump-thaw
(see below) degassing for the use of sensitive catalysts.
Radical and photochemical reactions: unless oxidization is the desired
product, radical reactions, for obvious reasons, must be degassed.
Substrates containing thiols, thioethers, phosphines, electron-rich
etc.: Heating, solvents, or the presence of other reagents can often induce
oxidation of such substrates even when otherwise stable.
Methods of Degassing
This is the most effective methods for solvent degassing. A solvent in a sealed
Schlenk or heavy wall sealed tube is frozen by immersion of the flask in liquid
N2. When the solvent is completely frozen, the flask is opened to the vacuum
(high vacuum) and pumped 2-3 minutes, with the flask still immersed in liquid
N2. The flask is then closed and warmed until the solvent has completely melted.
This process is repeated (usually three times) and after the last cycle the flask
backfilled with an inert gas. Degassed solvent in a sealed Schlenk flask can
usually be kept for 1-2 days.
Atmosphere Exchange Under Sonication
Solvents can be roughly degassed by repeated sonication under light vacuum (i.e.
house vacuum) for 0.5-1 min and replenishing the atmosphere with an inert
solvent. By using 5-10 cycles, degassed solvents for HPLC and some reactions
can be obtained quickly.
Of the methods listed here, purging is the least effective way of degassing
however it is acceptable for some applications, particularly when large amounts of
solvent need to be roughly degassed. As it sounds, purging consists of bubbling
an inert gas (usually N2 or Ar) through the solvent for 30 min - 1 hour. Care
should be taken to prevent solvent evaporation and especially the condensation of
water in the solvent by using an appropriate setup.