[discodermolide]

I would be careful here. That's the lazy man's way of doing it. Evaporate the THF then add ether to the residue and extract. That way you will avoid losses due to THF being miscible with water.
If you want to do it the other way add as much ether as THF and extract at least twice.

[orgopete]

I weigh my flask, and put it back onto the rotary evaporator, and weigh it again a few minutes later.  If the solvent is all gone, the mass will be about the same.

I second this method. I used to do a little more also, depending on the solvent. If you are using a very volatile solvent, raise you rotovap out of the water bath and feel the flack. If it is  becoming cold, solvent is still evaporating. If the flask is as warm as the bath, you are near the end. This will have more meaning if you are evaporating chromatography fractions, especially those that will crystalize. If all of the solvent evaporates, you may experience some bumping.

My rule of thumb is never ever smell your flask. Although most chemicals are not that toxic, as Disco has proven, but I think it is a bad habit.

Water wash, depends. If sample is water soluble, as little as possible, if trying to wash out something with significant organic solubility, use more water.

Drying and assuming brine wash. With a reasonably dry solvent, add some drying agent, swirl, observe, caking, add more. If flows freely, stop. This should give you an idea for different solvents. You don't want to leave your sample on the drying agent either.

[Messi]

Evaporate the THF then add ether to the residue and extract.

Rotavap?

[discodermolide]

Yes, rotavap.

[Messi]

Just did that. Thanks sir! Let's see how my reaction looks like now!

[discodermolide]

Good luck. If it goes wrong it was not me 

[Messi]

Good luck. If it goes wrong it was not me 

Reaction worked very well Had an increase of yield and reaction had minimal impurity

[discodermolide]

Ha ha must have been my suggestion that did it

[Messi]

Another Lab technique question!

So, I made a compound and I believe it is pure... as I separated with a column and the NMR looked nice. From what I understand, pure compounds usually tend to crystallize, no? So how can I induce crystallization of my product to make sure it is pure? Can I leave it on the counter overnight and hope it crystallizes!?

[discodermolide]

You can certainly leave it standing. You could also dissolve it in a solvent and let it slowly evaporate, this may produce a few seed crystals for you to use for a proper crystallisation.

[fledarmus]

First, are you sure your compound is supposed to be a solid at room temperature? You wouldn't be the first person to try to crystallize a liquid...

There are all sorts of ways to try to induce crystallization. Usually, once you can get crystals to begin forming, the oil will solidify very quickly. All you need is a nucleation site to start the crystallization, if your compound is pure.

• Add a seed crystal. Of course, that means you have to have made the compound before, or have some commercially available.
• scrape a small spatula through your compound against the glass. This may give a place for your solid to nucleate and start crystallizing.
• hold a small piece of ice, or even better, dry ice against the glass. This may quick freeze a tiny portion of the material, and give you a nucleation site to crystallize the rest.
• take a tiny amount up on a small spatula and cool it rapidly by waving it through the air, or in extreme cases, resting the back of the spatula on a piece of dry ice. If the clear amount turns white or opaque, stir it back into your sample. This may give you enough seed crystals to start.
• if worse comes to worst, take a drop of your liquid, dissolve it into a tiny amount of chloroform or dichloromethane, and add a larger portion of hexane. If you get a flocculant, filter it through a very small filter and scrape the filter over your product to get the tiny seeds to drop in. You can also strip the flocculant down and add scrapings of the residue back to your compound.
• if all else fails, add a small amount of a solvent you know the compound is completely insoluble in, and triturate it with a glass rod until it begins to crystallize. For most of the compounds I worked with, hexane was a good choice for this. If it doesn't crystallize after about five minutes of trituration, pour off the hexane, add a fresh lot of hexane, and continue triturating.

That's my fairly standard list of ways to try to force crystallization. If none of those work, there are some other possibilities based on the structure of the compound you are working with, but they are pretty specific to the situation.

[Messi]

First, are you sure your compound is supposed to be a solid at room temperature? You wouldn't be the first person to try to crystallize a liquid...

Yes, I'm sure.  I'm looking at it now and it's actually a solid.

• Add a seed crystal. Of course, that means you have to have made the compound before, or have some commercially available.

Unfortunately this isn't possible

• scrape a small spatula through your compound against the glass. This may give a place for your solid to nucleate and start crystallizing.
• hold a small piece of ice, or even better, dry ice against the glass. This may quick freeze a tiny portion of the material, and give you a nucleation site to crystallize the rest.

I like these two options. I'll try this. How long will crystal formation take? Also, when I give my crystals to the crystal expert, how long will it take him to elucidate my structure?

Also, do I need to make sure my compound is completely dry? No solvent is present such as ethyl acetate or DCM for crystal formation to occur?

[fledarmus]

First, are you sure your compound is supposed to be a solid at room temperature? You wouldn't be the first person to try to crystallize a liquid...

Yes, I'm sure.  I'm looking at it now and it's actually a solid.

Okay, now I'm confused. If you already have a solid, what are you trying to crystallize? Or are you saying that you would like to RE-crystallize your material to make it more pure? Because that is a whole 'nother thesis...

[Messi]

I guess it's the lack of knowledge on my part that is confusing me. I thought a solid and a crystal were two different things? I thought you can still have a compound in a solid form that isn't pure while crystals are pure?

Also, is taking a crystal of my compound better than taking its NMR?

[discodermolide]

You can have pure solids which are amorphous, no defined crystal form. Fortunately a lot of solids are crystalline, but exist in various forms of the crystal.
To get a crystal for x-ray you need to grow a "perfect crystal" one that has no flaws in it, no solvent inclusions, no twinning, etc. etc. It's not so easy.
X-ray gives you the spacial arrangement of the atoms in the unit cell, i.e. the solid phase structure. Combine that with NMR which gives you the solution phase structure and you get a lot of information about your compound. Because x-ray is so very time consuming NMR is usually the way to go.