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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: OrganicDan96 on August 17, 2019, 01:40:53 PM

Title: trifluoroacetic anydride purity.
Post by: OrganicDan96 on August 17, 2019, 01:40:53 PM

I have a reaction that requires exactly 1 equivalent of TFAA (more than an equivalent causes side reactions) i am aware that TFAA will hydrolyse in storage this means when I add one equivalent the reaction doesn't go to completion as it has hydrolysed to some extent and i am really adding less than 1 equiv. so i would like to know if there is a simple way I could titrate the TFAA.  i have a couple ideas add TFAA to one equiv of MeOH then getting an NMR of the mixture determine the ratio of methyl trifluoroacetate to unreacted methanol. the other idea is to take an ¹⁹F NMR of the TFAA in a dry NMR tube with DRY THF solvent.

do these ideas sound good?

Title: Re: trifluoroacetic anydride purity.
Post by: rolnor on August 17, 2019, 03:46:23 PM

Can you not keep the reagent under argon and use a schlenk type bottle with septum and take material with a syringe? It should be possible to avoid hydrolysis.

Title: Re: trifluoroacetic anydride purity.
Post by: OrganicDan96 on August 17, 2019, 04:45:39 PM

Quote from: rolnor on August 17, 2019, 03:46:23 PM

Can you not keep the reagent under argon and use a schlenk type bottle with septum and take material with a syringe? It should be possible to avoid hydrolysis.

I would if I bought some new stuff, i'm just thinking about the 2 or 3 bottles we have in the fridge, have no idea how old it is.

Title: Re: trifluoroacetic anydride purity.
Post by: wildfyr on August 17, 2019, 09:30:04 PM

Fractionally distill it to ensure purity. Anhydride boils 80C lower.

Title: Re: trifluoroacetic anydride purity.
Post by: BobfromNC on September 04, 2019, 10:08:49 AM

Just run a F19 NMR of the TFAA.   That should allow you to determine the ratio of anhydride to acid.    It will be hard to find TFAA without some acid, even in a new bottle.

I have actually seen the opposite problem, some bottle of TFA acid that we purchased had substantial levels of the anhydride in them, which was reacting with our material.   if the TFA is used as solvent for in excess, even a small amount of the anhydride can provide lots of it for reaction with certain compounds. 

Title: Re: trifluoroacetic anydride purity.
Post by: OrganicDan96 on September 04, 2019, 10:37:51 AM

Quote from: BobfromNC on September 04, 2019, 10:08:49 AM

Just run a F19 NMR of the TFAA.   That should allow you to determine the ratio of anhydride to acid.    It will be hard to find TFAA without some acid, even in a new bottle.

I have actually seen the opposite problem, some bottle of TFA acid that we purchased had substantial levels of the anhydride in them, which was reacting with our material.   if the TFA is used as solvent for in excess, even a small amount of the anhydride can provide lots of it for reaction with certain compounds.

that is probably the best idea, my concern was mostly regarding hydrolysis in the nmr tube, but I could just use dry THF for that purpose. i know of techniques to prepare nmrs under inert atmosphere. I don't mind if there is some TFA in it, i would just like to know how much so i can compensate for it. 

Title: Re: trifluoroacetic anydride purity.
Post by: wildfyr on September 04, 2019, 04:51:51 PM

Fractional distillation is definitely a good place to start. If done correctly, your material will be quite pure. Then you can check it by 19F-NMR.