[Kai]

Hey all, my first post here

Here's my question:

Salicylic Acid + Ac2O, H2SO4(catalyst) -> Aspirin

Why is concentrated H2SO4 used as a catalyst and why is concentrated HCl or concentrated HNO3 not used?

Thank you for your answers

[Guitarmaniac86]

I don't know if what I am about to say is really correct, but here it goes.

HCl may react with acetic anhydride, namely, the chloride ion constituent, giving you the acyl chloride as a side product.

Very concentrated nitric acid (86%) will nitrate the benzene ring. I know, I have accidently done this in a lab to toluene (long story short, forgot I had toluene in a flask, put in 86% nitric, brown fumes, lots of heat and mess).

Sulfuric acid is perfect (as is phosphoric acid) as they do not have competing side reactions.

Also it iw worth noting, this reaction is an equilibrium. If you start with an acid like HCl which has a lot of water associated with it (conc HCl is 63% water) you have set the equilibrium up to favour the starting materials. You have so much water, any ester that is formed will react with the excess water until equilibrium is reached. Sulfuric acid is 2% water in conc form so you are already starting off favouring ester formation. At equilibrium you will have a more ester and less of the reverse reaction occuring.

[kriggy]

It doesnt matter that much that acetyl chloride will be formed because it can also react like the anyhdride.
I think, since you only add catalytical amount of acid the fact that HCl is 65% water shouldnt matter that much because you shift the equilibrium by using excess of anyhdride.

[Kai]

It doesnt matter that much that acetyl chloride will be formed because it can also react like the anyhdride.
I think, since you only add catalytical amount of acid the fact that HCl is 65% water shouldnt matter that much because you shift the equilibrium by using excess of anyhdride.

So what makes HCl an undesirable catalyst for this particular synthesis?

[kriggy]

I dont know. I always used H2SO4 for esterification, mainly because that is an acid I used for 1st time and it worked well so no need to change it.

[Babcock_Hall]

Hey all, my first post here

SNIP

Thank you for your answers

According to the Forum Rules, you should show your attempt or at least give your thoughts first.

[Guitarmaniac86]

It doesnt matter that much that acetyl chloride will be formed because it can also react like the anyhdride.
I think, since you only add catalytical amount of acid the fact that HCl is 65% water shouldnt matter that much because you shift the equilibrium by using excess of anyhdride.

True, I didn't really think HCl would matter much.

[Kai]

According to the Forum Rules, you should show your attempt or at least give your thoughts first.

Sorry about that, will keep that in mind for next post

[BRSM]

As has been pointed out, concentrated hydrochloric acid is a bit wetter than is ideal (~63% H₂O) versus concentrated sulfuric acid (2% H₂O), so that's definitely a factor.

Dry hydrochloric acid, for example generated from MeOH/SOCl₂ or AcCl/MeOH, or just purchased in an organic solvent, is used for esterification, which suggests to me that the acid is good for this reaction and it's the water that's the problem.

Nitric acid is both less acidic than hydrochloric acid or sulfuric acid, and much more reactive. It's really quite a strong oxidant, and can nitrate electron rich arenes (e.g. phenols).

Another good acid catalyst for acetylation with Ac₂O that hasn't been mentioned is perchloric acid (usually 70% aqueous). Peracetylation of glucose with Ac₂O/HClO₄ used to be a classic undergraduate experiment, although I've got to point out that the formation of perchlorate salts if it's mishandled can be dangerous.