[wilycoder]

Hello,

I am new to organic chemistry.

I have been searching for an answer for this question for a few days now:

Lets say I am doing atmospheric distillation of organic compounds. To be anal about safety, I grease all the joints properly. When I have collected my distillate in the receiving flask, do I need to remove the grease from the receiving flask before getting the distillate out of the flask? Or is silicone grease completely inert like teflon?

Thanks!

[FeLiXe]

grease is definitely a problem and you often find traces of it if you take an NMR spectrum

I think the best way to get your substance out of the flask is by using a pipet.

[alphahydroxy]

I tend not to bother greasing joints unless they are being subjected to vacuum or are to be under an inert atmosphere. In these cases I tend to just not worry too much about the grease, as it can usually just be stripped off when columning products. Having said all that, I do tend to see grease in my NMRs!

[Ψ×Ψ]

alphahydroxy is pretty much right.  I rarely worry about greasing ground glass joints because they seal fairly well in the first place.  Example: When I arrived in my current lab, there was a condenser stuck in a flask, left over from the last person who worked there.  Nothing I or my labmates did could dislodge it.  We had to take a flame to it.

[Mitch]

Just grease the very top of the joint, in a ring, and you won't have a grease problem.

[beheada]

I was wondering about that just today, actually. I was doing a distillation of xylene under vacuum and thought I should grease up the joints real good. Well, low and behold, when the few drops of water started to come over before the xylene, they were cloudy and I instantly knew that grease had got into the mix. I don't want this to happen whenever I do vacuum (especially when I need the distillate to be clean of contaminants), so I'm taking it that the best suggestion for not allowing grease into your set up is just rubbing a light coat around the top and then swiveling the ground glass joints until you see them solidly coated?

[alphahydroxy]

I was wondering about that just today, actually. I was doing a distillation of xylene under vacuum and thought I should grease up the joints real good. Well, low and behold, when the few drops of water started to come over before the xylene, they were cloudy and I instantly knew that grease had got into the mix. I don't want this to happen whenever I do vacuum (especially when I need the distillate to be clean of contaminants), so I'm taking it that the best suggestion for not allowing grease into your set up is just rubbing a light coat around the top and then swiveling the ground glass joints until you see them solidly coated?

Now I used to just grease the top half off the joints when doing distillations under vaccuum or N₂. However, I'm working in a lab with an inorganic chemist who does a lot of work under inert atmospheres and with very dry solvents etc. and he says it is better to grease the whole joint.

The reason for this is that, when doing a distialltion, greasing the entire joint provides a full seal. If only a portion of the joint is greased, the solvent can get into the joint and start mixing with the grease more readily, pulling it into the system.

That said, I reiterate that grease isn't usually really that much of a problem in organic chemistry, as the reaction mixtures can be stripped free of any grease that did get in there when the time comes.

[Custos]

I tend not to bother greasing joints unless they are being subjected to vacuum or are to be under an inert atmosphere. In these cases I tend to just not worry too much about the grease, as it can usually just be stripped off when columning products. Having said all that, I do tend to see grease in my NMRs!

Heh  , I think that says it all.

You can get teflon sleeves for ground glass joints if you're really worried about contamination.

[P]

Yes - PTFE tape works quite well also.  Why not wipe the greese off with some wipes before decanting your product?

[english]

You seem like the type that gets worrisome by breaking convention. 

But if not, grease really isn't needed, provided you don't lock the joints too tight.  There shouldn't be any problems.  In fact, the times I have used fractional or simple distillation I have never greased joints; they never froze either.

[wilycoder]

I'm just really anal about sealing the glassware (I think I will always grease). I know that for vacuum distillations grease is pretty much required.

I will wipe off the grease from the inside rim of the receiving flask, but I'm positive that method doesn't remove 100% of the grease though..

The use of a pipette to recover the distillate had completely escaped my train of thought (brain fart I guess).

Can anyone confirm the use of PTFE based grease in place of Corning's High Vacuum Silicone Grease?

I figured PTFE based grease would be ok since it is very unlikely to react with materials. Also, the thermal range is similar to corning's silicone grease..

Thanks for all the replies

[alphahydroxy]

I will wipe off the grease from the inside rim of the receiving flask, but I'm positive that method doesn't remove 100% of the grease though..

Just use a bit of soft tissue damp with petrol (40-60) or pentane and that should remove all visible traces from the joint. Stuff like acetone is less effective as it obviously has less affinity for long hydrocarbons in the grease

[beheada]

I had that happen to me a bunch of times and then I took a close peek at my glassware. If you have a condenser whose male end tapers (I guess all of them do) you have to be extremely careful not to put grease (vaseline, silicon grease, etc) underneath the tip of the taper. If you do, the distillate will run off and go backwards along the tip of the male end and pull off a small bit of grease which will inevitably trickle down to your receiver. I agree with whomever mentioned greasing only the top of the joints. Since I've done that, never had a problem.

[Sikhandar]

Teflon tapes RuLeZ!!! Anyway (general rule) you can remove grease very easily with diethyl ether I use tons of it (I know I'll die young...) 

[wilycoder]

Any inputs on SuperLube(TM) grease? Its cheaper than the industry standard Dow Corning Grease.

They offer a regular version (which has PTFE), and a high vacuum version. The high vacuum version is a silicone based grease that is mixed with PTFE...

I wonder what benefit the presence of PTFE has on the grease (if any at all)?