CrimpJiggler
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Getting your product out of 1-octanol
I have products which are soluble in intermediate polarity solvents, highly soluble in ethyl acetate, fairly insoluble in hexane, and insoluble in
water but soluble in DMF and DMSO. I need to get these products out of octanol, and rotavapping isn't an option because time is limited.
I've been brainstorming to see if I can figure out a way. Heres the best idea I have so far: DMF is immiscible with pentane, cyclopentane and
cyclohexane (they are readily available for me, but if there are better non polars for this, I'll try obtain them), if I were to attempt a
liquid-liquid extraction with DMF and a big excess of pentane, would the n-octanol get pulled into the pentane layer, leaving my products behind in
the DMF?
Are there ways to enhance the likelihood of this happening, as in something I could add to the pentane to make the octanol more soluble in it, without
increasing the solubility of the products? The products are pretty stable to bases. They can't be converted to salts unfortunately. The product runs
extremely slowly in a 100% hexane mobile phase and the octanol in there makes the spots appear as boomerang shapes. The products are pretty
concentrated in the octanol, but I tried crashing them out with pentane and that didn't work.
I'm not sure if the products are soluble in acetonitrile, I'll run a TLC with 100% acetonitrile and see if the products stick to the baseline, if they
do then maybe an acetonitrile/pentane extraction might work better?
Another idea is to add sodium hydride to the octanol solution to see if I can turn it into a salt which (I hope) should crash out. The alkoxide salt
could cause a side reaction with my products but only if they are solvated in something, so if the alkoxide salt is completely insoluble and crashes
out, that shouldn't be an issue, right?
Are any of these viable ways to get your intermediate polarity compounds out of long chain alcohols like octanol?
[Edited on 26-4-2014 by CrimpJiggler]
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forgottenpassword
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Quote: Originally posted by CrimpJiggler  | | The product runs extremely slowly in a 100% hexane mobile phase and the octanol in there makes the spots appear as boomerang shapes. The products are
pretty concentrated in the octanol, but I tried crashing them out with pentane and that didn't work. |
Dilute
the octanol solution with hexane and pour it onto a small column of silica. Wash the octanol through with a little hexane. You can extract your
product from the silica once the excess hexane has evaporated.
[Edited on 26-4-2014 by forgottenpassword]
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DraconicAcid
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Is the product an acid or base that can be converted to a salt and then extracted with water?
ETA: Sorry- you already said they weren't. I missed it.
[Edited on 26-4-2014 by DraconicAcid]
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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subsecret
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What exactly are the products?
Fear is what you get when caution wasn't enough.
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jwpa17
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Seems to me that your easiest route is to add a solvent that is a poor solvent for your products but miscible with 1-octanol. I believe hexane meets
these criteria. You should be able to crystallize the products in that fashion, if they're solids at room temperature. If they're liquids, you might
get an emulsion, so don't try it on your entire reaction batch.
Why use 1-octanol for your solvent? Very high boiling point, makes it awkward to remove.
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Dr.Bob
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I have rotovaped n-octanol before, it does require a very good vacuum pump (a diaphragm pump even worked for me) and somewhat higher bath temps, about
70C, but it would remove the vast majority of your octanol in about 30 minutes, which is pretty fast to me. Then I used a silica gel column, the
remaining traces of octanol acted as a solvent modifier to speed up the movement of the desired product (an n-octyl ester), but I was able to use a
very slow hexane to ethyl acetate gradient to separate them, in my case my product was less polar than the octanol, so it came out before it.
It will be very hard to extract away the octanol, as it is partly soluble in many solvents, and unless your product is able to be crystallized from
some octanol mixture, any other thing I can thing of is much harder than rotovaping it away. And DMF is likely to make a horrible mess or emulsion,
based on my experiences trying to extract things from it, which is difficult.
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CrimpJiggler
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forgottenpassword: Cheers. I need to run a hexane column to separate out the isomers anyway, I just wasn't sure if the octanol would drag the products
out with it or not. Well, I'm still not sure about that, the products are pretty soluble in octanol.
jwpa17: Yeah, hexane moves the products up a TLC plate at snail pace, they only make it about 1/2 centimeter past the baseline before the solvent runs
off the plate, and like I said, they're boomerang shaped spots so it could just be the octanol dragging them. I tried adding pentane to the solution,
nothing crashed out, but I'll try that again because there was way too much volume in there, no way all of it was octanol, I'll rotavap longer to get
rid of any ethyl acetate left in there. I'm not a big fan of octanol myself either, but its necessary in this case. No way around it unfortunately.
Dr.Bob: I tried rotavapping on the high vac, I even left it over night. Maybe my product is a liquid after all. God I hope not. I need to keep the
heat low because theres a side reaction that can and probably will happen. I've had a few of these horrible emulsions with DMF myself too. Would
staining a TLC plate with permanganate give me an idea of how well the product will separate from octanol with a hexane mobile phase? My products are
all UV active, so I'll know exactly where they are on the plate and if a brown smear appears mid way up the plate, its probably octanol. I have some
permanganate stain handy so I may as well give it a try.
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DraconicAcid
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Is there any other way you can derivatize your product to make it octanol-insoluble? What kind of molecule is it?
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Dr.Bob
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Or if the product is completely unreactive, add some diazomethane to make the methyl ether of the octanol or some scetic acid to make the ester.
Those would both be much more volatile. If your desired product is volatile, then those are problems, but most final (of my) products are higher BP
than solvent.
If there is not too much octanol, or if the reaction is small enough, then silica gel column alone might work, I had about 10 times more octanol than
product, so that would have required a much bigger column, but if this is a small scale, then it might be practical. Are you making an ester from the
alcohol?
[Edited on 28-4-2014 by Dr.Bob]
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DraconicAcid
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| Quote: Originally posted by Dr.Bob | | Or if the product is completely unreactive, add some diazomethane to make the methyl ether of the octanol or some scetic acid to make the ester.
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Acetic anhydride would react more rapidly to form the ester. Then you have to extract the acetic acid with a basic aqueous solution (hopefully the
product doesn't dissolve in this). The product's solubility would surely be different in octyl acetate than in octanol.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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CrimpJiggler
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The molecule has a nitrile group, but the whole process of converting it to a carboxylic acid salt, then back to the nitrile would be tricky. I tried
separating with a silica column, but no luck. My skills with the column are very basic though, I didn't even manage to separate the two isomers which
move close together in 10:90 ethyl acetate:hexane, the only thing I separated was the colored, polar byproduct(s) which stuck to the surface.
Another thing I tried was flooding the octanol solution with hexane to see if I could crash the product out, what happened initially was two phases
formed, the octanol settled at the bottom, but on shaking they mixed and became a homogenous solution.
The TLC plates I ran with 100% acetonitrile only showed smearing on the faster moving spot (unlike pure hexane which showed smearing on both spots),
is this evidence that the acetonitrile moves the octanol up the silica plate faster than the products? I would have expected it to be the other way
around with acetonitrile mobile phase. The colored, polar product moved faster than everything else, as expected. I stained the plate with
permanganate afterwards, and a new spot appeared, behind (slower moving) the two products but not too far behind, so I'll need to hone my columning
skills before I can use 100% acetonitrile as the mobile phase.
| Quote: Originally posted by Dr.Bob |
If there is not too much octanol, or if the reaction is small enough, then silica gel column alone might work, I had about 10 times more octanol than
product, so that would have required a much bigger column, but if this is a small scale, then it might be practical. Are you making an ester from the
alcohol?
[Edited on 28-4-2014 by Dr.Bob] |
I'm making octyl ethers not esters, but the products are complex with numerous reaction sites so I have to keep the conditions mild. Diazomethane
would attack multiple sites. Anhydrides on the other hand, they should be okay. Octyl acetate boils at 211C so acetic anhydride and I think the
products might be even more soluble in it, but another ester might be easier to separate. Derivatising the products with an anhydride in this case
isn't a problem, so this might be a good solution. Thanks for the ideas.
[Edited on 1-5-2014 by CrimpJiggler]
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CrimpJiggler
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| Quote: Originally posted by Dr.Bob | Or if the product is completely unreactive, add some diazomethane to make the methyl ether of the octanol or some scetic acid to make the ester.
Those would both be much more volatile. If your desired product is volatile, then those are problems, but most final (of my) products are higher BP
than solvent.
If there is not too much octanol, or if the reaction is small enough, then silica gel column alone might work, I had about 10 times more octanol than
product, so that would have required a much bigger column, but if this is a small scale, then it might be practical. Are you making an ester from the
alcohol?
[Edited on 28-4-2014 by Dr.Bob] |
I'm making octyl ethers via nucleophilic substitutions but octanol is such a weak nucleophile, I was having trouble driving the reaction to
completion. I had to use a deficiency of octanol to get an octanol free product, but I still want to know how to separate the octanol from these kinda
compounds, because to make some of the isomers, I need more octanol and the reaction doesn't go to completion.
Diazomethane is too reactive, but converting the alcohol to an ester sounds like a good idea if I can do it without destroying the products. The
products are kind of complicated molecules with multiple reaction sites, so I have to keep the conditions mild.
EDIT: There isn't a great deal of octanol in there, but the hexane column I ran didn't seem to separate it out. Octanol isn't extremely soluble in
hexane, takes some stirring or shaking to get them to mix into a homogenous solution, whereas it mixes readily with acetonitrile and DMF so I'm
guessing its more of an intermediate polarity compound. Converting it to an ester would be perfect if I can do that in mild conditions. Acetic
anhydride I think should be fine.
[Edited on 1-5-2014 by CrimpJiggler]
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forgottenpassword
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Octanol is the archetypal hydrophobic liquid. There is no way that you should have failed to separate it from products which "run extremely slowly
with 100% hexane" on a silica column. Did you recombine the fractions or something? What were the Rf values of the products for the TLC with hexane?
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zed
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Dunno. Is it possible to freeze your octanol out?
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madscientist
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I'd just go with the rotavap, you're going to spend way more time fiddling around with these other methods than you'd lose just pulling it off. Maybe
you can azeotrope it off with hexanes, heptanes, toluene, or a lower boiling alcohol. Many alcohols form useful azeotropes with hydrocarbon solvents.
I weep at the sight of flaming acetic anhydride.
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