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@KangLee about your comments / subsequent questions below
6) Just now, searching for oil bubbler, I found the text that if there is underpressure, oil can be sucked into the flask. During reaction, if pressure becomes too low (Does this correspond to infrequent bubbles?), then should I supply inert gas again? —
Krang Lee yesterday
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I am searching for reactor set-up that can maintain inert atmosphere, and found that many organic synthesis labs use Schlenk line. But organic synthesis is not main thing of my lab, so there is some difficulties (especially for vacuum pump) for setting up Schlenk line. Therefore I have searched f...
A1) The preparation of large cycles / macrocycles indeed is an example where a low concentration is needed to favour intramolecular reactions over intermolecular reactions.
So if you have to work at 0.03 M concentration, an increase of the reaction volume may be justified. The suggestion to scale down to a $\pu{25 mL}$ flask was not aware about the reaction / type of reaction you were running.
A2) The classical Schlenk line consists of two lines, one for the inert gas (essential), one for the vacuum (very much helpful, becomes essential if you want to heat out the mounted setup under reduced pressure).
The two-line approach is a more costly (one time) investment than the single line, because of additional glass tubing, other types of valves. On the other hand, it may pay dividends because some chemicals will in part decompose if filled in «only air dry» glass ware.
Invisible to the air, not recognized by the touching by with hand, the surface of glass is capable to adsorb a thin film of water. And this one may disturb the intended reactions.
A3) Heating the mounted setup eases to reduce this film of water, in this respect like cooking and boiling, you /gradually/ increase the temperature to evaporate the liquid water into gaseous water. If the setup is flushed by inert gas, then this water is carried out of the setup.
If the setup is evacuated while heating the walls of the flask, condenser, etc. then the either a) the temperature required to obtain the same effect is lower; however b) using the same temperature at reduced pressure dramatically increases the efficiency of getting rid of this impurity.
In both modes, though, you have to wait a little to let the setup cool to room temperature.
A4) Air is not saturated by water (except rain, heavy fog); and inert gas is well capable to pick up water (keyword relative humidity, partial [equilibrium] pressure above a liquid/solid). Of course, this applies not only to water. Inert gas equally may pick up and saturate with THF, EtOAc, or any other vapour of the solvent you are using.
And because a constant flow of fresh inert gas (without molecules THF, EtOAc, or what ever solvent you used), thermodynamics enters again and again the scene: to establish this equilibrium between solid and liquid on one side, and the gas phase, on the other side, molecules pass from the liquid into the gas phase.
As a result, the volume of your liquid decreases over time if the flow of inert gas across your reaction setup. Depending on the reaction to run, one may counter this by eventually mounting a balloon on top of the reaction (e.g., of the reflux condenser) instead of a blubberer.
But this requires some care, namely, solvent vapours must not reach the balloon (and even less, liquid solvent). And the material of the balloon may not allow water to pass. The balloons from the toy shop thus may not be adequate here. (But the time to keep the reaction under inert conditions, and the reaction conditions need to be part of the equation, too.)
The small constant flow only intends to prevent air and humidity to enter the setup, thus does not be strong; the oil-filled bubblerer typically seals the setup well enough.
(If the gas tank gets empty during the night, then the reaction wasn't well enough prepared. One may put the gas tank on a balance in advance and estimate how much gas they (still) contain. Either a) by difference from the mass when these were delivered from the supplier.
Or b) because their tara (mass of the empty gas cylinder) is marked on the flask, next to the neck.
A5) As stated, if you have a flowmeter than you do not need to setup the pneumatic trough to get an estimate about the volume of gas by unit of time available.
A6) True, a pressure lower than the external pressure may push the oil of the blubberer into the inner of the setup. But you anticipate such situations:
a) you intentionally evacuate(d) the setup with a pump.
b) the slow flow of inert gas into the reaction setup is too low to compensate for a contraction of gas in the reaction setup; typically if you heat-dried your reaction with the heat gun, allowed for cooling to room temperature (first potential contraction causing this trouble) before charging with solvent and reagents.
Then you closed the reaction setup and intentionally lowered the temperature in the setup even lower (e.g., dry ice -78 C cooling bath).
Both a) and b) require some additional pressure of the inert gas. However, add enough safety volume between the blubberer and the reaction flask which may contain all of the oil from the blubberer.
In this respect, it is very similar to using gas washing flasks and vacuum filtration, where you put a safety flask between the pump or gas cylinder (on one hand) and the reaction flask (on the other hand).
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