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Author: Subject: acetylsalicylic acid
Jor
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[*] posted on 14-2-2008 at 07:03


hmm, I do not want it to fail, because tomorrow is my last chance to try purify it.
Is there anyone who knows if acetanilide will get absorbed?
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Nicodem
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[*] posted on 14-2-2008 at 07:23


Don't waste your time. Every aniline is brown or otherwise colored unless freshly prepared. The amount of crap required to give it a color is in the ppm range and is generally irrelevant.
Just recrystallize the crude product from water, possibly doing it properly (with active charcoal and hot filtering). The crap will remain absorbed on charcoal. Then measure the mp.
How many TLC spots were in the crude product after all the washes?

PS: The main products can't get absorbed on charcoal. Only trace products, especially crap-like products stick to it. You would need hundreds of grams of charcoal to absorb all your acetanilide, while you only use a couple of spatulas. That's the whole point of using it (it is actually of no use as a true purifying method, since all it does is discolor your product, while it does not remove any real, "non-ppm" side products). It is one of those cosmetic operations.




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[*] posted on 14-2-2008 at 08:49


I havent done TLC. Only the chemistry teacher was at school today, and he doesnt know where the biology teacher has put the TLC paper.

Tomorrow I really have a small amount of time, so I'm just going to dissolve all the crystals in an as small as possible amount of hot water, and add some charcoal, then I will maintain temp, filter charchoal after few minutes, and let solution cool down.

But what is the solubility of actanilide in hot water. I dont have access to all these kind of
solubility datas and such. Im sorry for that!
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not_important
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[*] posted on 14-2-2008 at 08:50


Yellow, reddish, orange-brown, on tobrown-black junk is usually polymeric or at least condensation products. However it's more that that, as being polymeric doesn't mean being dark coloured; think of polystyrene, poly-acrylics like Plexyglass, and so on.

The colour comes from double and triple bonds, usually involving hetero atoms C=O, C=N, and so on. Often these are conjugated, alternating single and double bonds.

These large, unsaturated molecules interact with the surface of adsorbents, such as activated charcoal, much more strongly than do the simpler, smaller molecules that are the typical products we want. A trace of your product will be lost from the cleaning, most through mechanical entrapment in the charcoal. That's why treating a not too concentrated solution is best, so that the drop or two stuck in the charcoal doesn't contain a lot of product.

Separating product from simple side products and unreacted starting materials, all of which are usually present in much higher concentrations than the coloured stuff, needs other means. Acid and alkaline extractions are common, as has been suggested. Distillation is another, crystallising is common for solids. The trick with crystallisation is getting the right solvent choice, one that is a mediocre to poor solvent for the product when cold, but a decent solvent when hot.

Water works will in your case, small more polar compounds are likely to stay in solution in the cold water. The other choice would be a fairly non-polar solvent, again a poor one when cold, that would tend to keep less polar compounds in solution as the product crystallised out. In your case the reactants and other products are generally fairly polar and reasonably hydrophilic, so water is the best choice.

Decolourising can help crystallisation, as in some cases the polymeric material interferes with the crystallising of the product.

And Nicodem was quite right - TLC is very useful in workups as it gives fairly quick confirmation of purity. Never leave home without it.


[Edited on 14-2-2008 by not_important]
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Jor
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[*] posted on 14-2-2008 at 10:02


Ok, thanks for the imformation not_important, I will use that in my work.
I Will treat the satured solution of acetanilide in hot water with the activated charcoal, because I think (altho I dont have the exact solubility) the solution wont be very concentrated (based on the fact that in the cold, it hardly dissolves).
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[*] posted on 14-2-2008 at 10:36


The treatment with charcoal is part of the recrystallization process. Check a book on how recrystallizations are done in practice. Due to your questions about solubility and other things you wrote, I have an impression this will be your first recrystallization ever.
Essentially what you do step by step is (but really rather read it from a book where it is better explained):

a.) you put your crude product in a beaker, add a little water and start heating to a slow simmering
b.) you add more and more water while keeping the solution boiling until all is dissolved (actually, not always everything dissolves, sometimes you have impurities that don't - hence why it is good to do a TLC first)
c.) remove from the hotplate and add some charcoal (a spatula or two, depends on the batch size) - carefully, over boiling can occur!
d.) you let it slowly simmer for another few minutes and then rapidly filter it while still hot (acetanilide likes to crystallize and might start to do that on the filter paper - in such case you have to apply more boiling water to redissolve that).
e.) then you let the warm solution stand until it cools down and the crystals grow no more (some compounds just don't like to crystalize out - in such a case keeping it in the refrigerator and scratching with a spatula helps - but acetanilide is not one such and crystallizes fast)
f.) you vacuum filter and wash with some water.
h.) dry it in an vacuum exicator and that's it. (<-Edit: Yes I meant desiccator - English is not my native tongue)

[Edited on 15/2/2008 by Nicodem]




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[*] posted on 15-2-2008 at 10:31


Quote:
h.) dry it in an vacuum exicator and that's it.


What's an "exicator"? Do you mean desiccator?




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[*] posted on 19-2-2008 at 10:06


Ok, thank you all for the help. My acetanilide is pure (concluded from the melting point test). I got a 77,3% yeild, wich is not so bad.

Joris
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[*] posted on 19-2-2008 at 11:26


Nice job!

A 77% yield after recrystallization indicates the conversion is pretty much quantitative. Too bad as you did not check the purity before recrystallization.

Since nothing much is published about acetylsalicylic acid as an N-acetylating reagent, your work has quite some relevance - at least for the hobby chemists - given the availability of such a reagent. Could you write down the entire experimental? It would be nice if you would post it in the Prepublication forum section.
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[*] posted on 19-2-2008 at 13:45


Oops, to be honest, I had the 77,3% before I did recrystallisation. However in cold water, acetanilide hardly dissolves, so you won't losse to much product after recrystallisation. Also, recrystallisation is not really needed, as there wont be much impurities, at most some ethyl acetate trapped between the crystals. However a quik rinse with some cold water (after crushing the crystals), should get rid of that.

Yea ,i will definatly post in the republications section, but I sure want to be 100% sure I have the acetanilide. Although I concluded the melting point of the crystals was 113C (approx), Im not completely sure. I had 5 minutes left (next I had maths , and had no time after), So I heated pretty quikly, and used an alcohol thermometer, so maybe the there was a delay... If one can please retry the following method to be sure we are talkign about acetanilide (I dont have aniline at home).

1: take 1 equivalents of acetylsalicylic acid and 1,5 equivalents of aniline* (see note) in an erlenmeyer (100mL was used).
1 mol acetylsalicylic acid = 180,16 grams
1 mol aniline = 93,126 grams = 91,15 mL
In this case respectively 5 grams and 4mL were used.
2: Heat the contents on a hotplate at 100-120C for 20 minutes.

3. Dissolve the reaction mix in 40mL of ethylacetate.

4. Add ~30mL of (near) satured sodiumbicarbonate solution (to remove (acetyl)salicylic acid). Shake well for at least one minute (I used a magnetic stirrer).
Then, seperate the two layers. Pour the ethylacetate layer in a new erlenmeyer or beaker.

5. Add ~20-30mL 1M hydrochloric acid (to remove aniline) and shake well again. Seperate layers and pour the ethylacetate layer in a beaker.

6. evaporate the ethylacetate. At room temperature, this will last no longer than a few hours.

Shiny, long, hard crystals remain, wich are somewhat hard to get out. These are already fairly pure acetanilide, but you might want to perform one recrystallisation:

Add some water to the crystals, and boil the water, now add more water until all crystals are dissolved and you should have a satured boiling solution now. Now put teh solution in the freezer, and decant after no more crystals form.

Again, I am not 100% sure I had acetanilide , because I had not much time to perform a test, and the melting points of the substances are quite close to eachother (25C diff, I heated fast, so Im not sure how accurate it was). So I would be glad if anyone could test this method once more, to make sure it's acetanilide.

*(note):
The brown color of aniline can be removed by adding some activated charcoal to the aniline, let stand for a day, and filter off.
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[*] posted on 1-3-2008 at 16:22


hs anyone retried yet? I would, but have no aniline, and besides, handling it in my homelab would be to hazardous.
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