tsathoggua1
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Behavior of nonpolars during extraction of high salt content aq. phases
I looked this up but didn't find what I wanted to know.
We all probably know about the 'salting out' effect, whereby a polar solvent can be forced out of aqueous solution by means of adding salts to the
aqueous portion, causing E.g alcohols to separate out, containing E.g amine salts present previously in the aqueous phase.
Does a similar but inverse effect take place when attempting to extract such compounds (primary, secondary, tert. amines mainly) which have been
converted to the freebase form, but whilst there is still a ton of salts in the squeous layer, from reactions conditions involving an acidic medium
which then must be neutralized by addition of base.
In this case there is an arseload of NaOAc present after neutralizing GAA w/ NaOH. The solvent is CH2Cl2, and what I'd like to know is how this, if at
all, affects solubility of non-polar compounds in NP solvents when using them for extraction from sich medium. Does it drive the base (slightly
soluble in H2O but not very) into the NP layer? does it drive it out of the NP? or is there no significant effect?
Thanks,
Tsath'
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JJay
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I'm not sure if there is a general answer to this without getting deep into physical chemistry; it depends on what functional groups are on the base,
how large the base is, what salts are used, concentration, etc....
I've done acid base extractions on amines in formal settings a couple of times, and under no circumstance was I ever told to salt them out. I've also
followed a couple procedures that discussed purifying amines, and none of them mentioned salting out either. I've also experimented with it with a few
different substances.
In general, you want to try to minimize the number of steps required when doing an acid-base extraction because the purity doesn't get substantially
better with repeated steps, but the yield usually drops. Salting out does seem to help with some substances, but I don't remember it ever trying it or
being told to try it with an amine (although my experience here is not exactly extensive).
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tsathoggua1
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No, I think you may be misunderstanding quite what is being asked.
I am wanting to know, if there is a corresponding equal and opposite reaction, rather than salting something OUT of H2O into a polar solvent, like an
amine salt, I want to know if with a freebase amine (primary in this case) being extracted from aqueous phase, when the aq. phase contains a lot of
salt (theres absolutely masses of it in this case), in this instance sodium acetate, whether there can be any retention of the amine in the H2O (its
slightly soluble, but not very) but still not separating out as an oil as expected. And purification isn't something that can be skipped in this case
owing to the nature of the synthesis, which involved the use of an Hg (II) salt. Not really the kind of thing one wants free-floating wherever it
pleases. Concentration, its probably saturated by now or close to it.
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JJay
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You seem to be confused about what salting out is. In salting out, a nonpolar compound is chased by dissolved ions out of an aqueous solution, usually
into a nonpolar solvent.
I am going to suggest that people who distribute for human consumption anything made using mercury salts without distillation deserve lengthy prison
sentences.
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Melgar
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This sort of leads into a question I never have seen explained adequately. Most solvents aren't really 100% polar or 100% nonpolar, but are somewhere
in between; it's more of a spectrum. And yet, most substances are said to be either one or the other. Sometimes, it will seem from context that
"polar" means "any solvent with non-zero polarity" and other times, "nonpolar" seems to mean "miscible with hexane but not water" and "polar" seems to
mean "miscible with water but not hexane". In this case, you need a nonpolar solvent in the second sense, which will form a layer with water and not
dissolve any sodium acetate or acetic acid. To get your amine completely out of the aqueous layer, you need to saturate it not just with sodium
acetate, which has a high solubility in many less-polar solvents especially alcohols, but you should also make sure that the pH is very high, and that
it's more or less saturated with an alkali hydroxide. That way you're sure to form the base from any amine acetate in your aqueous layer. Do you
know the pH of your aqueous phase?
In any case, OP, for your sake and for the sake of others, if you're going to be working with Hg salts, get some of these:
https://www.amazon.com/dp/B0049WLGFI/
edit: What may be complicating things is that acetic acid has more nonpolar character than most other, stronger acids. Amines are necessarily polar
as well, and the solubility of amines would be non-zero in a sodium acetate solution. Even if the solubility is low, having large amounts of solution
could cause the amount in that phase to be significant. I'd imagine you could crystallize the NaAc trihydrate and rinse, just to reduce the volume of
your solution, but I've never done that myself to know if it's actually possible.
[Edited on 5/12/17 by Melgar]
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JJay
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The threshold dielectric constant for polar/nonpolar solvents is defined here: http://web.chem.ucla.edu/~harding/IGOC/D/dielectric_constant...
Your point is well taken, though - it is possible to salt ethanol out of aqueous solution with potassium carbonate. This is an exception to the rule,
though. I'm actually not 100% certain if this is due to the ionic strength of a solution of potassium carbonate solution being so high that ethanol is
effectively nonpolar or if it is due to unrelated solubility issues.
[Edited on 12-5-2017 by JJay]
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Texium
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Thread Moved
18-5-2017 at 08:45 |
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