Sciencemadness Discussion Board - Salicylic Acid Hydrolysis

Salicylic Acid Hydrolysis

gdflp - 18-7-2014 at 20:06

In an attempt to make salicylic acid from aspirin tablets, I decided to use base hydrolysis. First, I crushed up the tablets as well as I could then added them to 500ml distilled water. This was stirred vigorously overnight and vacuum filtered resulting in a snow white powder with no chunks. I then added this slowly to a solution containing slightly more than 3 equivalents of sodium hydroxide. When the first bit was added, everything occured as expected, however after adding more, the solution turned a faint yellow. After adding the rest of the ASA, the solution was yellow and significantly more viscous than water. Any ideas what happened?

UnintentionalChaos - 18-7-2014 at 21:22

You do know that the tablets have lots of insoluble binders, right? I think you just washed the binders and recollected them.

If you want to use acid/base to purify the ASA from the tabs, add equimolar NaHCO3 to the crushed tabs. Filter the crap from the sodium acetylsalicylate solution, and then acidify to ppt the ASA as a crystalline powder. I prefer to recrystallize from acetone. The binders in the tabs are typically meant to make the pill swell and break down (aka: glop) when it gets wet in the stomach. Acetone doesn't do this.

As for hydrolysis, just use acid. Trust me, I've done this at least a dozen times and it's the best way. The following is an extract from my phenol writeup in prepublication:

The salicylic acid used was prepared from acetylsalicylic acid (aspirin) recrystallized from acetone. The one-pot cleavage of the phenyl acetate ester and crystallization of the product was carried out in the following manner:

Conversion of Aspirin to Salicylic Acid

A 1L RBF is charged with a stirbar, 45.04g of acetylsalicylic acid (250mmol), 600ml of distilled water, and 10ml of 31.45% hydrochloric acid. A vertical condenser is attached and the reaction mixture heated to reflux. After 30 minutes (This may be excessive, but I know it works), heating is stopped and stirring turned off. The solution, which should be a clear, colorless liquid, is very nearly saturated with salicylic acid and after cooling for a few minutes should be agitated to induce crystallization or seeded with a small quantity of salicylic acid crystals. The flask is left to stand undisturbed for several hours while crystallization occurs. Solubility at 20oC is nearly negligible (~2g/L), and cooling in an ice bath is unnecessary. The product is a mass of small, white needle-like crystals that fill the solution. The solution is agitated to break up the fragile crystal mass and vacuum filtered. A clean stopper is used to press the "fluffy" mass of crystals to aid in removal of the mother liquor. This cake is washed with two 50ml portions of distilled water. The product can be dried in a dessicator, at room temp, or at mildly elevated temperatures. The yield of salicylic acid is roughly 95% of the theoretical as glittering white needles melting at 156-157.5oC (uncorr, lit m.p. 158.6oC)

The filtrate can be reused several times to hydrolyze additional batches of salicylic acid. Discard 100ml of liquid, add 1-2ml of 31.45% hydrochloric acid, and an additional 45.04g of aspirin. The reflux procedure is repeated and the product collected may be obtained in nearly quantitative yield, as solubility losses are minimized and the only limiting factors are the purity of the aspirin and mechanical losses.

[Edited on 7-19-14 by UnintentionalChaos]

Texium - 18-7-2014 at 21:26

Wow, sounds like a great little procedure! I'll have to try that too sometime soon.

violet sin - 18-7-2014 at 21:51

UnintentionalChaos , I was just re-reading you phenol thread to comment here, and realized I just didn't have the energy for all the linking, copy and paste. still have to say I loved that thread

well done. https://www.sciencemadness.org/whisper/viewthread.php?tid=27...

this one comment by sciencesquirrel, second from bottom, not coppying go look:
https://www.sciencemadness.org/whisper/viewthread.php?tid=28...
has a nice round up of stuff to do with it too. not instructions, but suggestions and possibilities

the copper aspirinate thread has a bunch of useful info:
https://www.sciencemadness.org/whisper/viewthread.php?tid=99...

my 10g of ASA are still chilling out in MeOH waiting for me to filter the binders out

and whats worse, been that way for many moons now. no sign of degradation though. would be nice to play in the lab again.

arkoma - 18-7-2014 at 21:57

did this basic procedure last night. dissolved ASA in 91%IPA couple daze ago, filtered out crap last nite, glugged in approx 20-30ml HCl and refluxed about 45 mins. Turned off heat and went to bed. Lovely mass of "fiberglassy" xtals in flask today. Filtered out and redissolved in hot IPA and let recrystallize.

It has a lovely "pearly"look reminds of a certain tropane alkaloid from Erythroxylaceae genus plants.

gdflp - 19-7-2014 at 07:17

Quote: Originally posted by UnintentionalChaos

You do know that the tablets have lots of insoluble binders, right? I think you just washed the binders and recollected them.
[Edited on 7-19-14 by UnintentionalChaos]

Yes I am aware of that. However, since I was using 500 325mg aspirin tablets, I don't think that much aspirin was dissolved. My idea was to was the tablets with water then filter to remove soluble contaminants. Then I was going to hydrolyze with sodium hydroxide to avoid needing to reflux, then I was going to precipitate the salicylic acid with hydrochloric acid. I left it stirring overnight and the mixture of ASA, NaOH, and insoluble binders is now this extremely gooey yellow liquid which is barely able to be stirred with magnetic stirring(no movement is visible on the top of the liquid).

Loptr - 19-7-2014 at 09:23

Quote: Originally posted by arkoma

did this basic procedure last night. dissolved ASA in 91%IPA couple daze ago, filtered out crap last nite, glugged in approx 20-30ml HCl and refluxed about 45 mins. Turned off heat and went to bed. Lovely mass of "fiberglassy" xtals in flask today. Filtered out and redissolved in hot IPA and let recrystallize.

It has a lovely "pearly"look reminds of a certain tropane alkaloid from Erythroxylaceae genus plants.

I think a picric acid synthesis is in order.

arkoma - 19-7-2014 at 09:37

Phenol maybe, but have no nitrates