[Babcock_Hall]

We are synthesizing two amino acids via a multistep synthesis, one has a methyl group and the other has a benzyl group.  We purified the amino acid with the methyl group using Dowex-50 and eluting the amino acid with 3 column volumes of 4 M HCl.  We saw adequate recovery and an increase in purity as judged by NMR.  For the compound with a benzyl group we tried 6 column volumes of 2 M HCl.  The very preliminary indication is that we obtained poor recovery.  This was not unexpected inasmuch as there are indications that Dowex like hydrophobic molecules.  IIRC we have tried ethanolic hydrochloric acid in the past, but we did not see much recovery.  What else should we try to recovery as much material as possible from this column?

[Babcock_Hall]

We continued to elute with a second portion of 2 M HCl, and we obtained a solid material, and I took a third fraction using 4 M HCl.  We are in the process of characterizing the various fractions.

[rolnor]

What is the benzylgroup, ester? Is sounds harsch with strong HCl with ester.

[Babcock_Hall]

The benzyl group is on the sulfur atom of a vinyl sulfone.  We chose HCl over 2 M ammonia to elute our compound.

[rolnor]

Sulfides are usually stable to acid compared to ether or ester but still its not nice for the product. I am not so used to ion excange resins, can you use amberlite with carboxy residues, then you could use weaker acid maybe 0.1% TFA or something? I guess you have to evaporate significant volumes 2-4M HCl also, not very good.

[Babcock_Hall]

Dowex-1 in the acetate form is another option for us.  It has been some time since I used it, but IIRC either HCl or HOAc can be used to elute one's compound.  I will look into this further.  The sulfur is part of a sulfone, as opposed to a sulfide.

[rolnor]

Sorry, sulfone. I guess they are even more stable.

[Babcock_Hall]

We have nearly completed our analysis of the Dowex-50 fractions.  The presence of a benzyl group versus a methyl group on our product did retard elution, but not so much as to make recovery impossible.  We used a larger volume of 2 M HCl to elute.  At some point we may try a phenyl group.

[rolnor]

Great! How do you evaporate the fractions? I guess aspirator vacuum? A pump would not last long.

[Babcock_Hall]

We used a Welch (diaphragm?) pump, but we tried to catch as much of the liquid as possible by chilling the receiving flask on ice.  We also rotovapped our water fraction last.  Should we try to clean the pump in some fashion?  If we have to do this kind of operation again (and that seems likely), I will try to get our old aspirator pump working. 

[rolnor]

I dont know if your pump contains metal parts but its likely so, HCl is probably the worst thing you can have. I dont know if its possible to clean, maybe you need to take ut appart? I have ruined expensive pumps with HCl by accident not very fun.

[Babcock_Hall]

Based on our conversation, I decided to pull an old aspirator pump out of storage to use as the vacuum source for a fraction from a different synthesis, that eluted with hydrochloric acid.  When I used to use this more regularly, I used a slow feed of water from a tap to replace the water in the pump.  If one left this out, the water would gradually get warmer.  However, my present laboratory layout does not allow for such a set-up, and simply putting ice into the water reservoir of the pump means that ice is drawn into the jets.  Therefore, I filled a nylon fine washables bag with ice and put the bag into the water.  It seemed to work well.  I then add two portions of water to the sample to remove residual acid from the sample.

[rolnor]

I think you have another type of aspirator then mine, mine is attached directly to the tap and sucks the vapors down in the drain.

[Babcock_Hall]

https://www.amazon.com/Circulating-Water-Vacuum-Anti-Corrosion-Chemistry/dp/B07V2GMYTB/ref=sr_1_4?_encoding=UTF8&c=ts&dchild=1&keywords=Lab+Aspirator+Pumps&qid=1624281707&s=industrial&sr=1-4&ts_id=7656068011

The model in the link is a bit fancier than mine, having gauges.  Most of the water is recirculated.  However, there are two outlets on the side, which I believe are for a slow flow of water from a tap and to waste.  It seems to me that if the water becomes warmer, the vacuum must become weaker; that is why my model has a provision for a slow feed of cooling water.  The ice bag trick seemed to work OK, also.

[rolnor]

OK, have never seen this type. I guess the water becomes acidic then if you get HCl in the vapors? Maybe the extra flow of water washes this out. The type I use consumes more water so its maybe less economic.