Volvagia
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Obtaining Acetic acid/Salicylic acid from aspirin
After spending till 3 AM at the biochem lab I work at playing with making and purifying esters, I've decided to try to make somewhat of a hobby out of
chemistry.
First thing I want to do is to replicate the ester experiments, and therefore the first step of that is getting my hands on the chemicals. I remember
reading somewhere that if you dissolve aspirin pills into water, and then add sulfuric acid it will cause the molecule to decomp into Acetic and
Salicylic acid.
Firstly, can anyone confirm if this is possible? I've learned to take random internet postings with a grain of salt.
Secondly, how to separate it:
Salicylic acid appears to be virtually insoluble in water, so it probably will precipitate out as I am pouring in the sulfuric acid. So use a vacuum
filtration with a cold water rinse to limit loss. Let that dry and bam, reasonably pure salicylic acid.
The Acetic acid seems to be the harder to extract. It is SLIGHTLY denser than water with an 18 degree increased boiling point. The sulfuric acid is
considerably denser than water (1.84) with a BP of 300 C. I would pour the filtrate into a seperatory funnel and pour out and (reuse?) the unused
portion of sulfuric acid that would be resting on the bottom. The remains will be a dilute acetic acid that can be concentrated by distillation by
water bath to stay as far as possible from the boiling point of acetic acid.
Is this procedure feasible? Is the amount of products made negligible? (I plan on doing this with the entire contents of a large bottle of Aspirin.)
I appreciate any input on the matter.
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not_important
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Water, acetic acid, and H2SO4 are miscible; there will be only a single aqueous layer.
You don't really need H@SO4 to do the hydrolysis, although a drop can speed things up. Simply boiling acetylsalicylic acid with water will result in
its falling apart.
Remember that aspirin has other stuff in the table, much of which is not water soluble. Use plain, uncoated, unbuffered aspirin.
Distillation is the most direct way to separate the acetic acid out, on a small scale. You'll want a fractionating column, though.
Other ways are reverse osmosis, pervaporation, solvent extraction, and azeotropic distallation. Of these solvent extraction might be the best choice,
a hydrocarbon solvent plus teriary amine with at least one long alkyl sidechain could pull the acetic acid out of the water (neutralize any H2SO4
first), then the the acetic acid or solvents could be distillecd off.
The other route is to neutralize with Na2CO3 or NaOH, evaporate to dryness, heat the salt to 150 to 200 C to drive off water of crystallization, cool
and powder it, add calculated amount of H2SO4 to free the acetic acid, filter, and distill off the acetic acid.
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Volvagia
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*smacks forehead about the single layer*
About the fractionating column though, would you mind explaining whats so special about it? Items online I have seen seem to be simply a tube with
material in it to put the surface area though the roof. Not even water cooled, so eventually the whole thing will heat up and lower BP materials will
start to rise higher in the column than you wanted. At least that is the impression I got.
Couldn't a double condensor system work better? You put West (or Liebig) condensor on the boiling flash to reflux, and have a thermometer between
that condesnor and the second one which condenses further vapors into your collection beaker. You watch the temprature of the vapor and alter
heat/water flow rate to make sure the temp of the vapor is where you want it.
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chromium
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If you use just condenser instead of filled column you will get somewhat better separation than with simple distillation but not that much. Quality of
separation will depend largely on speed of distilation. If you do it veeeeery slowly then results may be acceptable.
Condensation will take place with relatively fast rate only if vapours have lot of contact with condensate. This holds even if temperature in your
column is near to lowest bp. This is why filled column works much better than empty.
[Edited on 2-8-2006 by chromium]
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not_important
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That won't happen to a fractionating column unless you overdrive it. There should be a reflux above the top of the column, so condensate is always
flowing back into the column.
And that condensate plus the large surface area is what makes it work. Each small section is in effect performing a simple distillation, exchanging
between vapour and liquid. As you go up the column, the vapour is becoming richer in the more volatile substance. The column has a temperature
gradient along it, and will be at temperatures between the still pot's vapour and the vapour at the take off point.
Without that added area you are working with a simple distillation. There is a bit of fractionation along the neck of the flask, but not a lot.
Columns are rated in theoretical plates, the number of sequential simple distillations would need to do to obtain the same degree of separation.
Actual simple distillate setups run between 1 and 2 theoretical plates, columns will typically add 5 to 30 plates.
http://wulfenite.fandm.edu/labtech/fractdistill.htm
http://www.chemguide.co.uk/physical/phaseeqia/idealfract.htm...
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pantone159
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You can also split the acetic acid/salicylic acid apart with base.
I currently lack decent heat sources, so setting up reflux is inconvenient, so I avoided it. My procedure was:
1 - Purify acetylsalicylic acid, by dissolving ground aspirin tablets (generic brand) in as little hot (heated w/ water bath) ethanol as possible.
The starch binder does not dissolve, so you get a cloudy solution, not clear. Filter to remove the binder, then add hot water until the mix gets
slightly cloudy. Let cool, then put in freezer. Crystals of acetylsalicylic acid come out, separate by suction filtration, then let dry.
2 - Put acetylsalicylic acid into a flask. (I used c. 5 g). Add 2x molar equivalent of NaOH as c. 5% solution. The solid dissolves in the base.
3 - Let sit for 1 week. (I was in no hurry.) I have a reference that says the hydrolysis is complete at RT in 3 days using Na carbonate, so 1 week
is probably much more time than I needed, but I figured I get a purer product if I waited. Like I said, I wasn't in a hurry.
4 - Add c. 10% HCl until the mix is about pH 1. The clear solution has turned very cloudy by this point. Cool this in an ice bath, then suction
filter the crystals of salicylic acid.
5 - Purify the salicylic acid by dissolving the crystals in as little hot EtOH as possible, then add hot water until slightly cloudy, then cool,
suction filter, then let crystals dry.
I got about 85% yield. (Recrystallized salicylic acid per purified acetylsalicylic acid used in step 2)
Something else fun to do with salicylic acid is to add some ferric ion to it, you get a nice pretty purple solution from a complex.
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The_Davster
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I concur with Pantone, the base hydrolysis of esters is far superior. Acid hydrolysis is more of an equilibrium reaction so you will have to remove
something, like acetic via distillation. With base hydrolysis the reaction is complete in a 10% NaOH solution after boiling for 20 min, and after the
followup acidification, you get an easily isolated ppt of salicylic acid.
[Edited on 3-8-2006 by rogue chemist]
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hodges
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Have a look at this. Complete University lab instructions for preparing ethyl salicylate from aspirin and ethyl alcohol.
http://chem.okstate.edu/courses/chem3015/Experiments/Ethyl%2...
Hodges
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pantone159
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After I prepared salicylic acid, I tried making esters with methanol/ethanol/isopropanol/n-butanol.
I currently lack decent heat sources and glassware for proper reflux, so I just used test tubes immersed in hot water baths. (This probably limited
my results.)
I found that methyl salicylate was by FAR the easiest to prepare, i.e. it formed much faster than the others. Ethyl salicylate had a similar smell
('minty'), while isopropyl salicylate had a different smell (I guess 'fruity' but I can't really describe it) while I'm not sure I got any results for
n-butyl salicylate.
BTW - My 'isolation' procedure is to dump the reaction mixture into a beaker of water. The idea is that the ester, immicible in H2O, floats on top,
while the other stuff dissolves in the water. This makes it easier to smell the ester, independent of the reactants. This doesn't work so well with
n-butanol, which also floats on top, immicible with water.
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S.C. Wack
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| Quote: | Originally posted by pantone159
I have a reference that says the hydrolysis is complete at RT in 3 days using Na carbonate |
I'd like to hear more. In my hands it seems obvious that the reaction is instantaneous. My yields are much higher; by adding aspirin to Na2CO3,
filtering when it has dissolved, adding H2SO4, washing and recrystallizing with water, and drying over CaCl2 with vacuum.
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pantone159
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My reference for the time to hydrolyse aspirin:
J Chem Ed, Vol 82, No 4, Apr 2005, p. 542-544. A Salicylate sympathetic ink from consumer chemicals. Stephen W. Wright.
The author states that: 'Complete hydrolysis is rather slow and only occurs after three or four days, as determined by monitoring the reaction by 13-C
NMR.'
Of course, the aspirin will dissolve straight away, but that just indicates the free COOH part loosing its proton, not the separation of salicylic
acid from acetic acid, AFAIK.
[Edited on 6-8-2006 by pantone159]
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