Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Violagirl on November 11, 2011, 01:10:19 PM
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I had a few questions pertaining to doing vaccum filtration and recrystallization in the lab.
For vaccum filtration, after you've set up your apparatus, wetted your filter paper down in your funnel, and have turned it on, do you want to dump your solid/liquid product right away all at once? Also is it best to rinse the flask with your solvent to get any remaining product still in the flask and then dump that too after you've dumped your initial amount out?
For recrystallization, when you have hot solvent with your product dissolved in it, do you want to occasionally swirl your flask after adding your cold solvent with a pasteur pipet or to continuously swirl as you add it? Also, is it best to scratch the bottom of the flask to get crystals to form once you see cloudiness appear at the bottom of your flask before letting it cool? Any answers to these questions would be great. Thanks a bunch.
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You do want to dump it in all at once! You should swirl vigorously to get all solid suspended, then immediately dump the flask into the filter. Keep some cold solvent handy to rinse the flask down with, to capture any that sticks and wash off the liquid.
When you have hot solvent with your product completely dissolved, you don't add cold solvent: you simply set the hot flask aside in a quiet place to cool and crystallize. Do not swirl, scratch or anything else at first; undisturbed = better crystals. If your stuff reaches room temperature and sits there for awhile, you can scratch to try to induce crystallization or chill the flask (do not actually add cold solvent) to make it go. The cold solvent is for when crystallization is finished, you vacuum filter the product and wash it with cold solvent in the funnel to get impurities off. Hope this helps!
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For recrystallization, when you have hot solvent with your product dissolved in it, do you want to occasionally swirl your flask after adding your cold solvent with a pasteur pipet or to continuously swirl as you add it?
You can do either, depending on your skill level. Don't try to swirl a small flask consonantly, and end up sloshing out of the flask, or tipping the flask over. But otherwise, the better you swirl, the better you mix, and that's why an Erlenmeyer flask is shaped the way it is, for maximum ease of swirling.
Also, is it best to scratch the bottom of the flask to get crystals to form once you see cloudiness appear at the bottom of your flask before letting it cool?
That's not really necessary. Yes, some brand new flasks don't provide nucleation sites, but that isn't really a problem, in a university, or in industry, flasks get used so much, and washed so often, they're not that smooth. You don't want to use too much "english" with the glass rod, and launch the flask across the lab bench. Besides, the cloudiness is micro-crystals forming.
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Back in the dark ages, I was taught to rinse the flask to get the remaining crystals onto the filter, not with fresh solvent, but with the mother liquor that had already gone through the filter. That way, the solvent you are washing your crystals into the funnel with is already saturated with your compound. Once all of your crystals are actually in the funnel, then you can wash them with some fresh cold solvent, just to make sure you have washed away any solvent containing impurities that might be sticking to the crystals.
If you have the foresight, saving a few seed crystals is a far better method of seeding your flask than scratching the glass. Opsomath's advice on nucleation is good - just let it cool slowly to room temp. If the solution starts to turn cloudy and you don't see any crystallization, that would be a good time to add a couple of tiny seed crystals - that might mean your compound is starting to come out of solution but hasn't yet begun to nucleate.
Again, back in the dark ages, there was a story that went around about a famous chemist (the name changed depending on who was telling the story) who dipped the tip of his beard in any solution he was crystallizing. If no crystals formed when the solution cooled, he would shake the end of his beard over the flask, dislodging the seed crystals that would have formed as the solution evaporated from his beard and seeding the flask. You can do the same thing by stirring the hot solution with a glass rod, taking it out, and waving it around gently until the solvent evaporates. This will usually leave you with some useful seed crystals on the end of the glass rod.
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Thank you all for your input!! It's really helpful!
I just had one more question on recrystallization. For my lab this week when we have to recrystallize our produc t, we have to dissolve it in hot ethanol but to use water only if necessary. I'm confused as to why water would not be used as we've normally added it at a dropwise rate until we saw microcrystals forming at the bottom of the flask. ???
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Water can be a problem with organic compounds. In many compounds, once you've got water involved, it is very difficult to get rid of it again. Consequently, we tend to avoid using it during final purifications if possible.
And that will come with experience too - other purification techniques such as reverse-phase chromatography typically use lots of water. They just have to get rid of it later, by freeze-drying or vacuum drying for the easy cases, and by re-dissolving and chemically drying for harder cases.
So if you can get a good recrystallization with just ethanol, you're probably better off avoiding the water.