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Offline nox

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Removing acetic acid
« on: August 07, 2011, 01:30:48 PM »
I'll be running a reaction that uses acetic acid as the solvent, anywhere from 60-200 mL depending on the scale. The original prep simply rotavaps off the acid, but they only used 15 mL so I guess that's tolerable.

I was thinking of ways to get rid of acetic acid without resorting to rotavap, but neutralizing it doesn't seem to be a viable option, especially if I'm going to use 200 mL of it (that's 3 moles of the stuff!!).

Since DMF and DMSO can be extracted away with water, I'm wondering if I could do the same for acetic acid. Maybe dilute my stuff down with 1:1 Hex/EtOAc, then keep washing with water/brine until it no longer smells like vinegar. I'm just wary of having residual acid in my crude if I try to column it, seems like it could really mess up my separation.

Offline discodermolide

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Re: Removing acetic acid
« Reply #1 on: August 07, 2011, 01:49:38 PM »
I'll be running a reaction that uses acetic acid as the solvent, anywhere from 60-200 mL depending on the scale. The original prep simply rotavaps off the acid, but they only used 15 mL so I guess that's tolerable.

I was thinking of ways to get rid of acetic acid without resorting to rotavap, but neutralizing it doesn't seem to be a viable option, especially if I'm going to use 200 mL of it (that's 3 moles of the stuff!!).

Since DMF and DMSO can be extracted away with water, I'm wondering if I could do the same for acetic acid. Maybe dilute my stuff down with 1:1 Hex/EtOAc, then keep washing with water/brine until it no longer smells like vinegar. I'm just wary of having residual acid

in my crude if I try to column it, seems like it could really mess up my separation.

Just evaporate on the rotavap until the volume is around 20-50mL then wash the rest out with bicarbonate watch out for lots of CO2!
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Offline opsomath

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Re: Removing acetic acid
« Reply #2 on: August 07, 2011, 02:16:27 PM »
Ugh. I used to do nitrations in acetic acid all the time and never resorted to rotovap - after all, the stuff has a boiling point much higher than water. I would drown it, just like DMF.The best thing would be if you could precipitate out product. What's the MP of your product? If it's solid, just pour your reaction mixture over crushed ice when you're done, and recover crude solid by filtration.  If you have a liquid, for 200 mL acetic acid I'd partition with ca. 400 mL water and maybe 200 mL hexanes or hexanes:ethyl acetate, drain off aqueous layer. Repeat three times, then wash with saturated bicarb solution: beware of bubbling. This way you aren't adding bicarb to pure acetic acid, which is a real drag.

Offline nox

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Re: Removing acetic acid
« Reply #3 on: August 07, 2011, 03:11:24 PM »
Trying to avoid the rotavap since ours are crappy and can barely pull off toluene.

According to the prep, the product is (unfortunately) supposed to be an oil. So I guess I'll just have to drown it as much as I can with water and then kill whatever is left with bicarb.

Offline Doc Oc

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Re: Removing acetic acid
« Reply #4 on: August 07, 2011, 04:56:32 PM »
What reaction are you doing and are you certain you need to use acetic acid as the solvent?  It's been my experience that a lot of times these types of reactions are reported in much older papers and that there's often a more modern/easy way to do it.  I had a reaction call for pyridine as solvent, only to later discover I could use THF and an excess of pyridine with equal efficiency and avoid the headache of getting all that pyridine out.

Offline nox

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Re: Removing acetic acid
« Reply #5 on: August 07, 2011, 07:36:32 PM »
It's a regioselective monobromination of a pyrrole derivative, from a 2005 Org Lett paper. Reaction is tricky enough as it is, so I'll take their word for now. (truth be told though if it doesn't work the way as advertised I'll probably just look into another route)

Offline opsomath

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Re: Removing acetic acid
« Reply #6 on: August 07, 2011, 07:41:59 PM »
I did a regioselective monobromination of a thiophene derivative once that had to be done in acetic acid - but then we found out DMF worked too. Might be worth a try - although really the only difference is that DMF isn't as stinky, neither one comes off on the rotovap.

I remembered an old trick I read about for getting rid of last traces of HOAc - add toluene and rotovap. Apparently it azeotropes off. No good for 200 mL, but might be good for the last little bit.

Offline nox

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Re: Removing acetic acid
« Reply #7 on: August 07, 2011, 08:17:25 PM »
Thanks for the tip. I gotta admit, I hate DMF with a passion, streaky TLCs (yes I highvac my plates, barely makes a difference :( ), messes up my columns if not drowned properly, residual dimethylamine from old bottles messing up my reactions etc, those were good times.

Not like acetic acid is going to be much better, but at least it's from Org Lett so it's supposed to work; whether yields are reproducible is another story. Funny story: postdoc in lab tries to repeat Org Lett prep from our own lab, consistently obtains 20% lower yield than claimed in paper.

Offline Åke

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Re: Removing acetic acid
« Reply #8 on: August 09, 2011, 12:07:14 PM »
I used a rotoevaporator connected to a water aspirator (http://en.wikipedia.org/wiki/Aspirator) as a vacuum source. This set-up, with water bath set on ~50°C and water tap on maximum flow, could readily remove e.g. acetic acid. Any remaining acetic acid could be washed away by shaking with NaHCO3 (aq.), water and brine.

Offline 408

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Re: Removing acetic acid
« Reply #9 on: August 09, 2011, 02:34:52 PM »
Add heptane or benzene. Both form azeotropes that distill off around 70-80C.  Evaporating around 500mL of glacial acetic usually takes around 1.5 L of benzene.

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