[Kate]

Hello.

So, the question is pretty straightforward: what's the difference between carrying out a reaction in a vacuum line as opposed to a nitrogen atmosphere?

Or are they the same thing?

I'm very confused about this, specially because I had a teacher that told me that synthesis can't be carried out in vacuum. 
Obviously, the reaction vessel has a vapor pressure, specially if it's kept in reflux.

[Corribus]

Given that a vacuum is going to pull off any solvent eventually, reactions - in solution anyway - aren't done under vacuum as far as I am aware. But a vacuum is often used to remove air prior to flooding a reaction chamber with nitrogen, or during freeze-pump-thaw cycles. So reactions are often done on a "vacuum line" (also called Schlenk line), which is the apparatus that allows you to do these kinds of air-free manipulations - pulling off air, flooding with nitrogen, etc. It doesn't mean the actual reaction is done under vacuum. The vacuum line consists of a vacuum pump, one or more solvent traps, a sophisticated piece of glassware called a manifold that has multiple valves and inlets, access to a nitrogen tank, and a bunch of hoses to connect to various reaction vessels simultaneously. You can find a picture of one here: http://en.wikipedia.org/wiki/Schlenk_line .

[Kate]

Understood. Thanks again.

[Babcock_Hall]

Just to expand a bit on what Corribus wrote, sometimes I have used a weak vacuum and a Firestone valve to achieve an inert atmosphere.  One alternates between the weak vacuum and filling with nitrogen several times to flush the system of oxygen.  The Firestone valve prevents the pressure of nitrogen in the apparatus from exceeding atmospheric.

[Kate]

But you can remove gases from the vessel just with the nitrogen, right? So using a vacuum pump gives more assurance that there's isn't any gas left?

[Corribus]

You can remove most oxygen by just bubbling nitrogen through your solution for 30 minutes or so and then sealing up the flask. (The bubbling is important to remove dissolved oxygen as well.) However, you don't get nearly as good of an oxygen-free environment this way, and also if your solvent is volatile you will likely lose a lot of it. It all depends on the sensitivity of your reaction to oxygen.

[Kate]

also if your solvent is volatile you will likely lose a lot of it.

Why though if you're increasing the pressure?

Also, how normal is it to have not only one teacher who doesn't know how to use a Schlenk line, but two? 

[Corribus]

Well, you have to have an outlet for oxygen to escape, along with solvent vapors. Net total pressure in your flask stays more or less constant, at least until you remove the outlet.

Using a Schlenk line isn't that difficult, but there is some technique involved. There are really only two important rules:

(A) Never pull air into your cold trap when it's submerged in liquid nitrogen.
(B) Always wear safety goggles.

The biggest hazard with a Schlenk line is explosion or implosion. I've experienced the latter firsthand, during a fpt cycle, and it's not particularly fun. This usually is only a real concern if you're pulling a vacuum.

[Kate]

Well, you have to have an outlet for oxygen to escape, along with solvent vapors. Net total pressure in your flask stays more or less constant, at least until you remove the outlet.

True, forgot that.

So I saw a guy operating a Schlenk line in the lab the other day. Didn't get a chance to operate it myself but I understood why he was switching the valves in the course of the experiment. It was pretty cool actually.

Why can the line explode though? I've read it has to do it impurities reaching the vacuum pump.

[Corribus]

Most common source of explosion is liquid oxygen trapped in the cold trap. This is why you usually don't want to cool down the trap with liquid nitrogen until the line is evacuated. (And warm the trap up before you release the vacuum.) Implosion usually happens due to structural flaw in glassware being subjected to vacuum. This is especially common during freeze-pump-thaw cycles. All that freezing and thawing is really hard on glass and can introduce microfractures. BANG! Glass shrapnel everywhere.