Sciencemadness Discussion Board - aspirin synthesis without acetyl anhydride

aspirin synthesis without acetyl anhydride

dmounger - 21-10-2013 at 18:26

Our school district just banned it. Anyone have an alternative procedure with acetic chloride or some other substitute?

UTFSE

teragon - 22-10-2013 at 01:25

The first hit on google for a search of "aspirin synthesis acetyl chloride" gives the following classroom procedure PDF (attached to this post).

Learn to use the resources at your fingertips.

Attachment: Synthesis of aspirin.PDF (256kB)
This file has been downloaded 1764 times

deltaH - 22-10-2013 at 01:43

Hi dmounger, acetylation reactions are important in some types of illegal drug manufacture, this is probably why they are banning it. Also the reagents are dangerous to work with. I would guess that they would ban all acetylation reagents, so you might not have any luck with acetyl chloride route either?

This might require some thinking out-of-the-box. Perhaps if you intend doing this for a project, you can start from a reagent that is already acetylated. For example, if you could order or obtain 2-methylphenyl acetate, otherwise known as o-tolyl acetate, then you could in theory convert it to aspirin by oxidation of the methyl group. I don't know if the acetate group on the side would survive such a step though? I am sure you can find on this forum how to oxidise toluene to benzoic acid somewhere... a similar sort of thing.

That is just one example, maybe the organic chemists on this site can advise you on other analogous round-about routes to preparing aspirin.

[Edited on 22-10-2013 by deltaH]

ScienceSquirrel - 22-10-2013 at 02:49

You are never going to be able to oxidise a methyl group in the presence of an O acetyl group without it falling off.
Aspirin hydrolyses easily if boiled in dilute hydrochloric acid or sodium carbonate solution.
The only practical approach that I can think of is via the acetylation of salicylic acid with something.

deltaH - 22-10-2013 at 04:06

Hmmm... ok, what about oxidising the o-tolyl acetate with KMnO4 solution without added acid and boiling in a simple phosphate buffer (KOH + H3PO4 mix) set to pH neutral? Surely hydrolysis should be slow under those conditions?

I'm not an organic chemist, but surely conditions might be found that could make this work?

bfesser - 22-10-2013 at 05:17

If they banned "acetyl anhydride," try acetic anhydride, ethanoic anhhydride, ethanoyl ethanoate, etc.

deltaH - 22-10-2013 at 06:02

Quote:

The only practical approach that I can think of is via the acetylation of salicylic acid with something.

Could transesterification with ethyl acetate be an option?

[Edited on 22-10-2013 by deltaH]

dmounger - 22-10-2013 at 09:04

Thanks for the suggestions. I actually did try Google with that exact word combination a few days ago, but that hit never came up. Strange.

The ban is over safety, apparently. My students (14-15 years old) do this lab and a purity test (residual salicylic acid with iron nitrate, followed by spectrophotometry) because it's such a good lab for getting across a lot of concepts (organic, good lab procedure, filtration, catalysts, spectrophotometry) at a level that kids in their early teens can handle. I hate dropping it because it's so successful and the materials aren't expensive. I'll try the acetyl chloride procedure and see how it goes. I'd be interested if anyone has any further info on Nicodem's ethyl acetate idea.

deltaH - 22-10-2013 at 09:31

Oh wow... so acetyl chloride is not banned ?!?!? That's nuts, I've always thought it was even more dangerous than acetic anhydride lol

Anyhow, I had no luck finding any transesterification preps using ethyl acetate for you, so chances are it probably doesn't work... but I think some people on SM have access to much more powerful academic chemistry search engines, for example Beilstein, which might just turn up something for you if you're lucky

DraconicAcid - 22-10-2013 at 09:37

Quote: Originally posted by dmounger

Thanks for the suggestions. I actually did try Google with that exact word combination a few days ago, but that hit never came up. Strange.

The ban is over safety, apparently. My students (14-15 years old) do this lab and a purity test (residual salicylic acid with iron nitrate, followed by spectrophotometry) because it's such a good lab for getting across a lot of concepts (organic, good lab procedure, filtration, catalysts, spectrophotometry) at a level that kids in their early teens can handle. I hate dropping it because it's so successful and the materials aren't expensive. I'll try the acetyl chloride procedure and see how it goes. I'd be interested if anyone has any further info on Nicodem's ethyl acetate idea.

Acetyl chloride is much more dangerous than acetic anhydride. Find out who decided upon banning acetic anhydride and get them to grow up. Otherwise you will, in a few years, find yourself unable to have anything more dangerous than sodium chloride or sucrose in your lab (and to use that, you'll need to fill out forms in triplicate and have every student's parent sign a notarized waiver).

Transesterification of Alcohols

teragon - 22-10-2013 at 13:09

Transesterification of ethyl acetate by the alcohol may be a viable option-

see the following reference.

MEIRELES, Bruno A. and PEREIRA, Vera Lúcia P.. Synthesis of bio-additives: transesterification of ethyl acetate with glycerol using homogeneous or heterogeneous acid catalysts. J. Braz. Chem. Soc. [online]. 2013, vol.24, n.1, pp. 17-57.

http://dx.doi.org/10.1590/S0103-50532013000100004

---------------------------------------------------------------

ABSTRACT

A new catalytic route with potential practical interest to sustainable production of bio-additives from glycerol is described. Ethyl acetate was transesterified with glycerol, in the ratio glycerol:EtOAc 1:10, at 25 or 90 ºC using 0.1 equiv. of H2SO4 or TsOH, as homogeneous catalysts. H2SO4 led to the total glycerol consumption in 2 h. In the equilibrium, attained in 9 h, 100% yield of a diacetin:triacetin (55:45) mixture was formed. Using AmberlystTM 15 dry and AmberlystTM 16 wet in 1:30 glycerol:EtOAc ratio and reflux at 90 ºC the total glycerol consumption was achieved in 2 and 10h, respectively. The lower reactivity of Amberlyst-16 wet was explained in terms of deactivation of acid sites and decrease in glycerol diffusion to the inner resin pores, both factors caused by adsorbed water. The kinetics of glycerol transformation and product distribution in the equilibrium in relation to the H2SO4, Amberlyst-15 (dry) and Amberlyst-16 (wet) catalyzed reactions were measured.

EXPERIMENTAL

All reactions were carried out in batch mode. Glycerol (5.0 g, 54.3 mmol, 1.0 equiv.), ethyl acetate (47.8 g, 60.6 mL, 543 mmol, 10 equiv.) and an acid catalyst (5.43 mmol, 0.1 equiv.) were added to a round bottom flask. Next, a Liebig condenser and a pressure equalizer were adapted to the system and the reaction medium was kept under strong magnetic stirring at room temperature or 90 ºC for 24 h. Initially, two phases were formed but at 45 min (90 ºC) and 180 min (room temperature) only one phase could be observed. Next, the reaction medium was diluted with ethyl acetate (500 mL) washed with saturated NaHCO3 (3 × 20.0 mL) and brine (1 × 15 mL). The organic phase was dried over Na2SO4 and the solvent removed in vacuum. The residue was constituted of a low viscosity yellow liquid consisting of an acetin mixture (100% conversion, 10.1 g of acetins). Acetins separation could be done by column chromatography on silica gel (hexane/EtOAc 4:1) or more easily by counter current chromatograph resulting in colorless liquids

Attachment: Transesterification of ethyl acetate with glycerol.pdf (729kB)
This file has been downloaded 558 times

deltaH - 22-10-2013 at 23:18

Nice, thanks teragon

dmounger, if you use dry Amberlyst 15 beads instead of H2SO4, you won't need to stress at all that the students will hurt themselves with acid. Those ion exchange beads are pretty darn harmless as far as I recall and better yet, they are reusable.

I think you might just have the safest aspirin synthesis in existence lol

2h is a bit of a long time to be refluxing though for a chem lab no? You will need some creative problem solving. If all else fails, perhaps a higher boiling ester like isopropyl acetate with a b.p of 89C compared to 77C for ethyl acetate. The reaction time should be at least halved... i.e. 1h, which I think would be more manageable for a chem lab.

When you become famous don't forget to acknowledge the SM community!

Oh and the idea wasn't nicodem, look again, he just moved the thread. Ordinarily I wouldn't care much, except that this moderator keeps lobbying insults against me on this forum for any out-of-the-box thinking, so I am... shall we say... not on the best of terms.

[Edited on 23-10-2013 by deltaH]

Nicodem - 23-10-2013 at 05:37

Quote: Originally posted by dmounger

The ban is over safety, apparently.
...
I'll try the acetyl chloride procedure and see how it goes.

Like it was already said, acetyl chloride is way more hazardous than acetic anhydride. Getting it on the skin can cause serious burns. Imagine what it does to the eyes. It makes the anhydride look benign.
Also, pyridine is more poisonous than acetic anhydride (and more volatile and has reproductive toxicity, which might not be so nice to the young students). Generally, I would not mind students working with such materials. I'm just trying to make you understand the nonsense of such a ban.

Quote:

I'd be interested if anyone has any further info on Nicodem's ethyl acetate idea.

I'm sorry, but I would never blatantly suggest something that already intuitively goes against the theory of organic synthesis unless I had some reasonable and referenced arguments. This suggestion goes against the law of thermodynamics and you can't ignore that (unless you call yourself deltaH).
The acid catalyzed transesterification between ethyl acetate and salicylic acid can only give ethyl salicylate. It can not give aspirin, as salicylic acid is a phenol.

deltaH - 23-10-2013 at 06:03

Quote:

This suggestion goes against the law of thermodynamics and you can't ignore that (unless you call yourself deltaH).
The acid catalyzed transesterification between ethyl acetate and salicylic acid can only give ethyl salicylate. It can not give aspirin, as salicylic acid is a phenol.

What a waste of black photons!

Time for some chem101: acid catalysed transesterification mechanism:

http://www.fsj.ualberta.ca/chimie/chem161/bacids/sld074.htm

As you can see, the ester is protonated and then attacked by the nucleophile, in this case an alcohol.

Salicylic acid's carboxylic acid group is NOT A NUCLEOPHILE

Forming ethyl salicylate is impossible mechanistically, the only chance for any reaction is by the phenolic group acting as a nucleophile.

Finally, most transesterifications are equilibrium limited to some degree, so the product needs to be favoured by using an excess of reagent, hence why the triacetin synthesis referenced uses a ten fold excess of the ethyl acetate.

[Edited on 23-10-2013 by deltaH]

Moderator Warning

bfesser - 23-10-2013 at 07:14

<img src="../scipics/_warn.png" /> Nicodem & deltaH, that's enough. <img src="../scipics/_warn.png" />

deltaH, don't carry baggage from other topics. Get over it.

Nicodem - 23-10-2013 at 08:49

Quote: Originally posted by deltaH

What a waste of black photons!

OK, since you are obviously uneducated about organic synthesis, I'll ignore your habit at ignoring the scientific method (but not forgive!) and politely explain you why you are utterly wrong.

Quote:

Time for some chem101: acid catalysed transesterification mechanism:http://www.fsj.ualberta.ca/chimie/chem161/bacids/sld074.htm

That is the mechanism for the transacylation type of transesterification which is not aplicable to salicylic acid because of thermodynamic reasons. Like I said, salicylic acid is a phenol (its hydroxy group is phenolic). The transesterification of phenols with ethyl acetate would formally give phenyl acetates, however the equilibrium point prohibits this being a viable route. The only way to make it go in this direction, would be by the removal of ethanol from the system, which is anything but easy and as far as I know was never accomplished. Even the direct O-acylation of simple phenols with carboxylic acids was only achieved under very special conditions even though it formally only takes the removal of water.

But then again, salicylic acid is also a carboxylic acid, so such a reaction with ethyl acetate, even if made to work on phenol, would not be applicable. The reason is because ethyl salicylate + acetic acid are thermodynamically very much closer to the reactants when compared to aspirin + ethanol which are uphill.

Quote:

Salicylic acid's carboxylic acid group is NOT A NUCLEOPHILE

Carboxylic acids can be both, nucleophiles or electrophiles. A good example of both roles is the mechanism of the Fischer esterification. In the first step (protonation) the carboxy group acts as a nucleophile. The protonated carboxy group then becomes electrophilic enough for the nucleophilic addition to occur. In fact in most reactions where the carboxy group is involved, it acts primarily as a nucleophile.The reason is in that the carboxy group is a relatively weak electrophile and thus requires activation by a stronger electrophile (this is usually a proton or a Lewis acid).

Quote:

Forming ethyl salicylate is impossible mechanistically, the only chance for any reaction is by the phenolic group acting as a nucleophile.

Just because you are unfamiliar with a theory, it does not mean you can outfool nature. For example, children are unfamiliar with the theory of the gravitational force, yet they fall when they stumble. The same goes with synthesis. Organic synthesis uses hundreds of theories describing reaction mechanisms. The point is not to evaluate only the one you wish would apply, but all of them (reality can only be described by a full model of the reality - you can not describe reality by formulating a wish).

The phenolic group can act as a nucleophile in respect to ethyl acetate, but this will not help in forming aspirin due to the aforementioned reasons. However, in addition to reaction thermodinamics, the same rule can be described also by using several other concepts that are reflection of the same cause. Here are some examples:
- nucleophilicity: aliphatic alcohols are more nucleophilic than the aromatic alcohols (phenols);
- leaving group ability: the phenolate is a better leaving group than the alcoholate;
- acidity: the pKa of phenols is lower than the pKa of simple aliphatic alcohols;
- reactivity: phenyl acetates can acetylate aliphatic alcohols under proper conditions.
Since all these things have the same origins, they can all be used to evaluate the position of the equilibrium. You can help yourself by using those concepts with which you are more familiar.

On the other hand, the carboxy group of benzoic acids is comparably nucleophilic to the alcanoic acids, hence an acid catalyzed transesterification of alkyl acetates to alkyl benzoates is possible (and is well known in the literature, with ethyl acetate included, but not on salicylic acid).

Quote:

Finally, most transesterifications are equilibrium limited to some degree, so the product needs to be favoured by using an excess of reagent, hence why the triacetin synthesis referenced uses a ten fold excess of the ethyl acetate.

Not most, all transesterifications are equilibrium reactions. But using excess of one reactant can only force the equilibrium point forward as long as the two reaction sides are thermodinamically close (like in the transacylation between two aliphatic alcohols). Reagent excess is of no practical use in the case of small equilibrium constants. For example, you can gas acetic acid with plenty of HCl under as much pressure as you want, but this will not help you in synthesizing acetyl chloride this way. Yet, you add a stoichiometric amount of water to acetyl chloride and you get the same reaction going the other direction (without any excess).

teragon - 23-10-2013 at 11:11

I had a sneaking suspicion that salicylic acid would not work - thanks for that clarification Nicodem.

I always enjoy reading your posts

deltaH - 23-10-2013 at 11:46

Quote:

OK, since you are obviously uneducated about organic synthesis

You repeatedly make personal attacks like this that goes against the guidelines set for this forum, ironically written by you. Whether you like it or not, I am formally educated in organic chemistry.

Quote:

I'll ignore your habit at ignoring the scientific method (but not forgive!)

The OP wanted alternatives for his reaction, I made suggestions on what might be possible alternatives for consideration and tried to assist with finding references, failed, then made the appeal for others who have more access to help if they wanted to, teragon was kind enough to respond with as he says, and I agree, "Transesterification of ethyl acetate by the alcohol may be a viable option" based on that reference.

Quote:

Do you know this for a fact or is it an opinion, because as written it is an opinion with no calculation or reference to back up? I tried to help you here and actually do the thermodynamic calculation, but I do not have access to entropy of formation for phenyl acetate (model compound for this reaction). I could however find data for everything else required, so if you can locate this then please state it and I can work this out for you, then we don't have to guess here.

[EDIT]Ideally if one could find the dGf for aspirin and salicylic acid, that would be even better, but if all else fails, the best one can do would be to calculate for phenol and phenyl acetate.

That said, in general transesterification are usually equilibrium limited which is why one needs to employ some means to make it work for you... which brings me to this point:

Quote:

The only way to make it go in this direction, would be by the removal of ethanol from the system,

That is not the only way... by the same principle, you can also employ an excess of reagent or operate such so as to precipitate the product. The former is possible in this case and in fact was done so with the referenced triacetin synthesis.

Quote:

Even the direct O-acylation of simple phenols with carboxylic acids was only achieved under very special conditions even though it formally only takes the removal of water.

Again, without a reference I don't know what you mean by under very special conditions, so please explain this in such a way that I can be given the opportunity to criticise it, after all, this is the basis of a scientific argument, if you cannot be allowed to falsify a statement, it means that it is not even wrong to quote Prof. Pauli.

Quote:

But then again, salicylic acid is also a carboxylic acid, so such a reaction with ethyl acetate, even if made to work on phenol, would not be applicable.

But is a basis from which to try something similar, or is doing anything new strictly verboten.

Quote:

The reason is because ethyl salicylate + acetic acid are thermodynamically very much closer to the reactants when compared to aspirin + ethanol which are uphill.

I'm growing tired of saying this, but thermodynamics as about the numbers, please provide proof, not just a claim. Then I can accept you arguments, right now I have to take it on belief only.

Quote:

A good example of both roles is the mechanism of the Fischer esterification. In the first step (protonation) the carboxy group acts as a nucleophile.

Nucleophilicity is a relative concept because it refers to kinetics and so it is the norm to compare rates of nucleophilic attack. I cannot find carboxylic acids on any nucleophilicity scales which suggest that they are REALLY poor nucleophiles, but again, if you can quote me something to back up this argument. Simply claiming that sulfuric acid or similar super acids can protonate it has no bearing on it's nucleophilicity. The question here is it's nucleophilicity compared to phenol's hydroxyl group. If a chemist with access to better data than me can volunteer true pKb values for phenol and benzoic acid, that would at least be a start on some kind of comparison.

Quote:

Just because you are unfamiliar with a theory, it does not mean you can outfool nature.

I see you went too long without a personal attack again, so here it is again.

Quote:

The phenolic group can act as a nucleophile in respect to ethyl acetate, but this will not help in forming aspirin due to the aforementioned reasons. However, in addition to reaction thermodinamics, the same rule can be described also by using several other concepts that are reflection of the same cause. Here are some examples:
- nucleophilicity: aliphatic alcohols are more nucleophilic than the aromatic alcohols (phenols);
- leaving group ability: the phenolate is a better leaving group than the alcoholate;
- acidity: the pKa of phenols is lower than the pKa of simple aliphatic alcohols;
- reactivity: phenyl acetates can acetylate aliphatic alcohols under proper conditions.
Since all these things have the same origins, they can all be used to evaluate the position of the equilibrium. You can help yourself by using those concepts with which you are more familiar.

I do not dispute some of these points, however, that doesn't necessarily mean that this might not proceed, it is still opinion for now and you are not specifically calculating the thermodynamics or quoting values for this system, I don't expect you have to, but don't expect me to then know for sure that nature will follow this. It is my opinion that it might and so if it was me, i would simple try such an experiment because it is so straight forward and because there are so many balancing factors that may or may not be a dead nail.

Quote:

True, but again, we don't have the calculated K value for this reaction, we are merely speculating at this point. If you told me it is 10-6 at 70C, I would say, forget it.

I will conclude with this thought, the OP posted an appeal for suggestion for alternatives for aspirin synthesis from benign ingredients.

I have made two that may or may not work (undetermined as of yet) and teragon has kindly furnished some conditional basis upon which this may be attempted.

Now care to put all this tensions of yours aside and make other suggestions for this teacher who is just trying to give his students a good education and work WITHIN the laws he's told (whether we might agree with them or not).

Besides, I think trying to find safer and greener alternatives to existing synthetic methodologies is a noble cause and a nice challenge!

[Edited on 23-10-2013 by deltaH]

bfesser - 23-10-2013 at 15:11

Quote: Originally posted by Nicodem

OK, since you are obviously uneducated about organic synthesis, I'll ignore your habit at ignoring the scientific method (but not forgive!) and politely explain you why you are utterly wrong.

Quote: Originally posted by deltaH

You repeatedly make personal attacks like this that goes against the guidelines set for this forum, ironically written by you. Whether you like it or not, I am formally educated in organic chemistry.

I had hoped that you two were mature enough to resolve this and that I wouldn't have intervene; disappointingly, I was wrong. If you can't discuss the science, without making your arguments personal, your posts will be removed. I hope that you two don't choose to carry on with this bullshit in other topics, but be warned that if you do, I will delete/Detritus your posts, science-content and all.

As for the topic at hand, I hope that the original poster's (OP) questions have been answered satisfactorily, as I am now forced to close this topic. The argumentative replies are too intertwined with the rest of them to be effectively pruned or split. If the OP would like more help with this, feel free to open a new topic, to which Nicodem and deltaH are not to reply.

[closed]