| Pages:
1
2 |
Jor
National Hazard
Posts: 950
Registered: 21-11-2007
Member Is Offline
Mood: No Mood
|
|
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?
|
|
|
Nicodem
Super Moderator
Posts: 4230
Registered: 28-12-2004
Member Is Offline
Mood: No Mood
|
|
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.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
|
|
|
Jor
National Hazard
Posts: 950
Registered: 21-11-2007
Member Is Offline
Mood: No Mood
|
|
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!
|
|
|
not_important
International Hazard
Posts: 3873
Registered: 21-7-2006
Member Is Offline
Mood: No Mood
|
|
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]
|
|
|
Jor
National Hazard
Posts: 950
Registered: 21-11-2007
Member Is Offline
Mood: No Mood
|
|
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).
|
|
|
Nicodem
Super Moderator
Posts: 4230
Registered: 28-12-2004
Member Is Offline
Mood: No Mood
|
|
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]
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
|
|
|
bfesser
Resident Wikipedian
Posts: 2114
Registered: 29-1-2008
Member Is Offline
Mood: No Mood
|
|
| Quote: | | h.) dry it in an vacuum exicator and that's it. |
What's an "exicator"? Do you mean desiccator?
|
|
|
Jor
National Hazard
Posts: 950
Registered: 21-11-2007
Member Is Offline
Mood: No Mood
|
|
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
|
|
|
Nicodem
Super Moderator
Posts: 4230
Registered: 28-12-2004
Member Is Offline
Mood: No Mood
|
|
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.
|
|
|
Jor
National Hazard
Posts: 950
Registered: 21-11-2007
Member Is Offline
Mood: No Mood
|
|
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.
|
|
|
Jor
National Hazard
Posts: 950
Registered: 21-11-2007
Member Is Offline
Mood: No Mood
|
|
hs anyone retried yet? I would, but have no aniline, and besides, handling it in my homelab would be to hazardous.
|
|
|
| Pages:
1
2 |
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
