ldanielrosa
Hazard to Others
Posts: 124
Registered: 25-4-2007
Member Is Offline
Mood: transparent
|
|
acetylsalicylic acid hydrolysis
Back story: I was inspired by a synthesis video involving aspirin and copper (you know who you are), so I purified more than I needed. With this
surplus, I decided to hydrolyze half back to salicylic acid.
I decided not to use a catalyst. The hydrolysis seemed to go well, and I ran it for four cycles (add water, heat, cool, filter- and no, I didn't
monitor the temperature or record the time, sorry). The last two cycles didn't have much smell that I recognize as acetic acid, but I'd like to have
another indication.
I tested the solvent pH after filtration, and the third and fourth read at 2.64. The first and second were lower, but I neglected to write them down.
Per the Ka of salicylic acid, I should get pH=2.46 (at 25°C, assuming that the saturation value of 2.0g/L is accurate).
Is there an indicator that will detect acetylsalicylic acid or acetic acid, but will ignore salicylic acid? Are reactions that involve salicylic acid
resilient to ASA contamination?
|
|
|
redox
Hazard to Others
Posts: 268
Registered: 22-2-2011
Location: The Land of Milk and Honey
Member Is Offline
Mood: Chalcogenetic
|
|
Was there a specific reason you didn't use a catalyst? I think if the hydrolysis proceeded correctly you would definitely be able to smell acetic
acid.
I suggest hydrolyzing again, but using an acid or base catalyst.
If it was me, I would add three equivs. of NaOH to the acetylsalicylic acid, reflux for a while, and then collect the sodium acetate and the disodium
salt of salicylic acid. Acidify with two equivs. of acid, and the salicylic acid should precipitate right out.
My quite small but growing Youtube Channel: http://www.youtube.com/user/RealChemLabs
Newest video: Synthesis of Chloroform
The difference between chemists and chemical engineers: Chemists use test tubes, chemical engineers use buckets. |
|
|
ldanielrosa
Hazard to Others
Posts: 124
Registered: 25-4-2007
Member Is Offline
Mood: transparent
|
|
"Was there a specific reason you didn't use a catalyst?"
Yes. I saw some data that indicated that the acetylsalicylic acid would decompose above 140°C anyway, and I didn't mind letting it simmer longer. I
also didn't want to deal with removing the catalyst contaminant- the aspirin started out as USP grade, and while I don't know what I'll use the
salicylic acid for I'd like to keep it clean.
"I think if the hydrolysis proceeded correctly you would definitely be able to smell acetic acid."
I did smell the acetic acid after the first two runs. My apartment reeked of it. My nose isn't particularly keen, so there may have been some after
the third and fourth run that I couldn't smell.
I don't have the equipment for refluxing just yet. After I purchase a house, I will set up a workshop in the basement then I'll get all the pretty
toys!
Since this reaction will go to equilibrium, removing the acetic acid product is the best way to push it to completion that I can think of.
Unfortunately, the ASA and SA have similar solubility in water so I'm left with one reactant and one product. If I convert to the sodium salt to
force completion, then I will need to need to deal with sodium and whatever else I use to reconvert to SA when the ASA is exhausted.
|
|
|
redox
Hazard to Others
Posts: 268
Registered: 22-2-2011
Location: The Land of Milk and Honey
Member Is Offline
Mood: Chalcogenetic
|
|
Quote: Originally posted by ldanielrosa  | "
Yes. I saw some data that indicated that the acetylsalicylic acid would decompose above 140°C anyway, and I didn't mind letting it simmer longer. I
also didn't want to deal with removing the catalyst contaminant- the aspirin started out as USP grade, and while I don't know what I'll use the
salicylic acid for I'd like to keep it clean. |
I'm not sure what you mean by this. You used water as the solvent, right? How would you get it above 140°C? The NaOH in the basic hydrolysis of the
aspirin isn't really a contaminant, you just need to acidify the product to get the salicylic acid. It's easy. Even if you use dirty technical NaOH,
the salicylic acid will precipitate, so your product will still be clean.
| Quote: Originally posted by ldanielrosa | | " I did smell the acetic acid after the first two runs. My apartment reeked of it. My nose isn't particularly keen, so there may have been some after
the third and fourth run that I couldn't smell. |
I'm just curious, how long did you heat it for? One hour? Two?
My quite small but growing Youtube Channel: http://www.youtube.com/user/RealChemLabs
Newest video: Synthesis of Chloroform
The difference between chemists and chemical engineers: Chemists use test tubes, chemical engineers use buckets. |
|
|
ldanielrosa
Hazard to Others
Posts: 124
Registered: 25-4-2007
Member Is Offline
Mood: transparent
|
|
"How would you get it above 140°C?"
Sorry I wasn't clear. The dry ASA decomposes at 140°C, not necessarily into salicylic acid or acetic acid- that data wasn't shown where I looked.
I understand that conversions are typically faster than non-catalyzed hydrolysis, but this one looked favorable for the path I used. Next time I
probably will employ conversion to sodium salts, but not entirely for reasons of efficiency- also to enjoy the experience of doing the same thing two
different ways.
"I'm just curious, how long did you heat it for?"
I didn't keep close watch on the time, but I think the first cycle was four hours and the second cycle was twelve hours. The third and fourth were
about two and half hours each.
As for the original question, I'll take the response to be 'no'. I'm not terribly surprised- anything that responds to ASA will probably respond to
SA as well.
|
|
|
ldanielrosa
Hazard to Others
Posts: 124
Registered: 25-4-2007
Member Is Offline
Mood: transparent
|
|
Okay, here's the update. I didn't dry my product, but I do have a fair idea how my turnout was.
I made a solution of 16% sodium hydroxide (by weight, I have balances but no volumetric flask). I used 263 grams of the solution to convert the
product to sodium salts. Then re-acidified with an equal molar quantity (1.05 moles) of hydrochloric acid. The salicylic acid crashed out and there
was no smell of acetic acid.
I expected about 1.2 moles from what began as 227 grams of aspirin. The mechanical losses should not have been this much. The extraction kept
reusing the solvent (isopropanol) with heating and recrystallizing cycles 175 tablets at a time, and crashing out the last bit on the last cycle with
water.
I looked up the solubility of salicylic acid with various solvents, and found that the solubility in acetic acid is significant. I think I lost some
of the product with the liquor after hydrolysis.
|
|
|
bbartlog
International Hazard
Posts: 1139
Registered: 27-8-2009
Location: Unmoored in time
Member Is Offline
Mood: No Mood
|
|
Ah, so how much did you get? You didn't include that detail...
Also, it's a little goofy to do a slow hydrolysis and then convert to sodium salts afterward anyway. Just add the sodium hydroxide at the beginning,
heat, and your hydrolysis is done in ten minutes max.
When I've done this I've never gotten more than about 85% of theoretical yield; mechanical losses and some acid remaining dissolved in the water I
guess. Could probably be improved but not worth a lot of effort given that aspirin is cheap.
|
|
|
ldanielrosa
Hazard to Others
Posts: 124
Registered: 25-4-2007
Member Is Offline
Mood: transparent
|
|
| Quote: | | Also, it's a little goofy to do a slow hydrolysis and then convert to sodium salts afterward anyway. Just add the sodium hydroxide at the beginning,
heat, and your hydrolysis is done in ten minutes max. |
Sorry I wasn't clear on this.
| Quote: | | I understand that conversions are typically faster than non-catalyzed hydrolysis, but this one looked favorable for the path I used. Next time I
probably will employ conversion to sodium salts, but not entirely for reasons of efficiency- also to enjoy the experience of doing the same thing two
different ways. |
I did this as a confirmation. I couldn't find means of detecting acetylsalicylic acid that would ignore salicylic acid. I didn't do enough research
to determine how far the equilibrium point is for non-catalyzed hydrolysis. So in effect, I titrated it.
I suppose if I wanted the data for the data's sake, I should have dried the intermediate product to properly weigh it.
The final product is not completely dry (still a little clumpy), but dry enough to weigh. I have 135 grams. That's about 7% loss from after adding
the sodium hydroxide, which is entirely believable from the washings that were needed to get rid of the soluble salts- four washings with very fine
particle size. Contaminants from the HCl became visible and required more wash cycles to remove.
The slow hydrolysis resulted in 16% loss, which was probably from the salicylic acid dissolved in acetic acid. Mechanical losses from washing was
minimal- large particle size and fewer repetitions.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
