drying trifluoroacetic acid

[dudle...@gmail.com]

Does anyone know of an appropriate drying agent from which to distill
TFA? I would like to dry my TFA before using it in a strong acid (HBr
or TFMSA) peptide cleavage procedure. I can guess as well as the next
person, but I'm looking for a tried and true procedure.

Thanks.

[muha]

As many strong acids, TFA forms upper-boiling point azeotrope with
water. A simple atmospheric pressure distillation over a short column
should be sufficient for your purpose.

The TFA you can buy from Aldrich is good enough. TFMSA has drying
properties - especialy if you are add something like 10% of it into
your TFA clavage mix. So unless you have some old ugly TFA from
half-open bottle, drying your TFA is unnecesary.

[Ron Jones]

??? "Dry before adding to strong acid HBr" - Hydrobromic acid is 52%
water....

--
--
Ron Jones

Don't repeat history, see unreported near misses in chemical lab/plant
at http://www.crhf.org.uk
"The most exciting phrase to hear in science, the one that heralds new
discoveries, is not 'Eureka!' (I found it!) but 'That's funny ...'" :
Isaac Asimov:

[dudle...@gmail.com]

Thanks for your response. I do, in fact, have an old ugly bottle of
TFA from Aldrich, so I'll go ahead and try the simple fractional
distillation. By "upper-boiling point azeotrope" I assume mean an
azeotrope that boild at a higher temperature than TFA or water?

[dudle...@gmail.com]

Than you for your response. Yes, dry before addition to HBr. You see,
the HBr used for peptide cleavage is a ~30% soln. in acetic acid and is
an alternative to HF or TFMSA cleavage. The levelling effect renders
HBr more acidic in acetic acid than in aqueous solution, and more
acidic still in acetic acid/TFA mixtures. However, as with HF
cleavage, the presence of water can lead to the hydrolysis of backbone
amides. If you're interested, I can try to dig up some references for
you.

[Chris Cooksey]

In article <1120421901....@o13g2000cwo.googlegroups.com>,
dudle...@gmail.com writes

Seems to me that the easiest way out is to add enough trifluoroacetic
anhydride (bp. as I recall 43 degrees C) to react with all the water.
Which it will do rapidly and get hot too !

--
Chris Cooksey

[Uncle Al]

Make your mix, add an adequate trace of TFA anhydride, stir, wait a
bit, do the reaction.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf