Sciencemadness Discussion Board

small scale solvent stripping

poisoninthestain - 19-3-2009 at 23:11

I have 300ml of IPA as my solvent and about 20ml of a desired freebase amine suspended in it. I'm trying to strip off the IPA but since I'm new to vac distillation this is an alien area for me.

I have a feeling that after about 250ml or so of IPA are stripped under vac(which can pull about 55mmHg) that the rest won't be able to make it out of the flask.

I understand that short path distillation would be best but the apparatus is too expensive. I only have a 1000ml flask so I'm somewhat concerned. Perhaps It could be done with just substituting a smaller flask.

watson.fawkes - 20-3-2009 at 06:44

Quote:
Originally posted by poisoninthestain
I have a feeling that after about 250ml or so of IPA are stripped under vac(which can pull about 55mmHg) that the rest won't be able to make it out of the flask.
What would be different at that concentration to change the partial pressure of the IPA? Do you have an azeotrope to deal with?

panziandi - 20-3-2009 at 07:17

Are you sure it is the free base suspended in it? I would expect an amine to dissolve in IPA. If you have a solid suspended in the IPA then it could be filtered. If you have two liquids immisible separate them in a funnel. If you mean the amine is DISSOLVED in the IPA you have a couple of options. Pass in dry HCl to precipitate the amine HCl. Distill the IPA off (bp 82*C) I am assuming that the amine boils much higher since its likely to be a substituted aromatic compound of some description. Once you obtain your oil of amine then you can apply a vacuum to that and drive off any remaining IPA.

poisoninthestain - 20-3-2009 at 11:32

sorry, i meant to say dissolved in IPA not suspended. wouldn't titrating the IPA/amine solution also yield isopropyl chloride?

i was under the impression that distilling small amounts in a 24/40 rig wouldn't work for some reason. Now that I think about it, I think the only problem would be if I were to distill the amine freebase which might result in mechanical loss since it's such a small amount. Again, I don't have much experience with these two things:vac distillation and small amounts.

DJF90 - 20-3-2009 at 12:01

The acid base reaction between the HCl and the amine is very fast, whereas the nucleophilic subtitution of the alcohol is a much slower reaction. So long as no more HCl is passed in once precipitation has ceased then there should be no 2-chloropropane formed.

Once your amine hydrochloride crystallises out, you could use vacuum filtration to remove the solvent? This would eliminate the need for you to have to struggle with distillation.

Jor - 20-3-2009 at 14:17

I think (but I'm not sure!) no isopropyl chloride will form at all, as HCl is not acidic enough to protonate the -OH group, and thus no water can split off, and no attack of the chloride on the resulting carbocation.

You need a Lewis acid catalyst such as ZnCl2 to make it work.

HBr and HI are strong enough though.

turd - 20-3-2009 at 14:28

Quote:
Originally posted by poisoninthestain
i was under the impression that distilling small amounts in a 24/40 rig wouldn't work for some reason. Now that I think about it, I think the only problem would be if I were to distill the amine freebase which might result in mechanical loss since it's such a small amount. Again, I don't have much experience with these two things:vac distillation and small amounts.

???
You're not distilling, you're stripping a solvent. Just leave it on the pump for a few hours while heating it on a water bath. Stir if your freebase is a liquid, add a cooling trap if using a rotary vane pump. Simple.

Or just isolate the hydrochloride as described in the literature a bazillion of times.