DocX
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Anethole extraction issues
Sorry for spamming with troubleshooting threads, but I finally got time to do some chemistry again, and that means
jumpstarting 3-4 sleeping projects simultaneously. While being sort of rusty.
Anyway: I'm doing an extraction of anethole from anise seeds.
Issue 1: using a Soxhlet extractor running methanol: cotton, seeds, reflux. For some reason, after a few cycles, the drain function
doesn't seem to work. The fluid reaches the upper curvature of the siphon tube, but never passes it- only drips over the edge more or less at the same
speed as the reflux of methanol enter the extractor.
If I open the extractor up and pour in some cl:s of extra methanol it promptly overflows and drains.
WTF? Any ideas?
That's issue nr 1.
Issue nr 2: getting rid of the methanol.
Naturally, I went for simple distillation. At first a fine, crystal-clear distillate (incruciatingly) slowly drips over, but soon it becomes apparent
that the distillate is actually yellow. Slightly tinted, but still. I hate it when they're yellow.
I'm thinking two things: some of the product gets steam-distilled over along with the methanol, AND/OR reeeallly, really low-boiling turpentines/other
impurities co-distills with the alcohol.
I can loose the methanol even if it would be nice to clean it. But I don't want to loose product.
If it's a steam distillation problem- what can be done?
If it's a co-distillation problem- what then? (less of a problem, since I can loose the methanol and don't care for turps)
Any other ideas?
Issue nr 3: even if I succeed in all of the above, I will still have a dark-brown product. A dirty oil. Composed mainly of anethole,
sure, but I want a fine, clean isolate of it. Yellow-to-transparent oil.
Naturally, I have done the Filtration. Didn't really clear it up much.
Next I'll do the Steam Distillation- mix the oil with many times its own weight in dH2O, muse at the ouzo effect, and run it over.
Then extract the oil with some ether or toluene, using the same on the water left in the distilling flask.
What then? If that's not sufficient, what's the next step?
Vacuum filtration?
Some sort of crystallisation/recryst?
Fractional freezing?
Please don't say column chromatography.
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Texium
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Steam distillation works better than Soxhlet extraction for anethole. It will inevitably be contaminated with a significant amount of p-anisaldehyde
and trace amounts of other impurities, but it comes out as a nice pale yellow oil and smells wonderful. Granted, I have only done this with star
anise, not anise seed, so the constituents may be a little different in your case. I guarantee it will yield a better quality product if done
correctly though.
I’ve realized that Soxhlet extraction is often misused in this community, likely because it uses fun glassware and people don’t seem to know how
to properly run a steam distillation.
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DocX
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Interesting. I can't help but wonder why? And possibly where that information comes from? I've been looking for sources for anethole extraction from
plant material, and I see both methods being used. Granted, most sources use steam distillation. But then there's this:
https://www.tandfonline.com/doi/abs/10.1080/01496395.2011.57...
| Quote: |
Foeniculum vulgare (fennel) and Pimpinella anisum (anise) extracts were obtained by Soxhlet, cold percolation, ultrasound assisted extraction, and
centrifugal extraction using ethanol as solvent; anise extracts were also obtained by steam distillation. Soxhlet presented the highest yields for
both fennel and anise seed (16.8% and 23.3%, respectively). The highest anethole content among ethanolic extracts was obtained for centrifugal
extraction (6.8 mg/g and 143 mg/g for fennel and anise extracts, respectively). Steam distillation presented low yield (0.26%), but high anethole
content (68–98%, area).
| Quote: | "Extraction Kinetics and Anethole Content of Fennel (Foeniculum vulgare) and Anise Seed (Pimpinella anisum).
Patricia F et al, Sep Sci and Tech, 2010 |
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So unless you do a literature review, how do you know if you're "misusing" the Soxhlet extractor?
I love using it because it's an automated process (when it works like it should) that you can just switch on and leave for a couple of hours to do
it's work. It won't boil to dryness, it won't burn, and. unless you're extracting something that gets contaminated if you run it too long, it's better
the longer you go.
So I would choose that over other extraction methods whenever I can. But not when I can't.
[Edited on 20232323/4/11 by DocX]
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yobbo II
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..
....Steam distillation presented low yield (0.26%), but high anethole content (68–98%, area).
What does it mean when they say 'area' ?
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Texium
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From personal
experience. I have run this steam distillation many times. In fact, as TA for an organic chemistry lab, I got dozens of undergrads with practically 0
lab skills to successfully run it. It isn't difficult. The product was a pleasant smelling pale yellow oil, and its NMR spectrum consisted of about
90% anethole and 10% p-anisaldehyde with no other significant components. Once again though, this was for ground star anise, not aniseed.
Quote: Originally posted by DocX  | I've been looking for sources for anethole extraction from plant material, and I see both methods being used. Granted, most sources use steam
distillation. But then there's this:
https://www.tandfonline.com/doi/abs/10.1080/01496395.2011.57...
So unless you do a literature review, how do you know if you're "misusing" the Soxhlet extractor?
I love using it because it's an automated process (when it works like it should) that you can just switch on and leave for a couple of hours to do
it's work. It won't boil to dryness, it won't burn, and. unless you're extracting something that gets contaminated if you run it too long, it's better
the longer you go.
So I would choose that over other extraction methods whenever I can. But not when I can't. |
I read
over the article. It appears that they used what I would call "dry" steam distillation with whole anise seeds. I have used "wet" steam distillation,
with ground star anise added directly to water and distilled, though they refer to this technique as hydrodistillation. You stopped quoting a little
too soon, see:
| Quote: | | Anise essential oil yield obtained by hydrodistillation was 3.1% with 80% of anethole content... This clearly indicates that although yields obtained
by steam distillation are lower than yields obtained by other extractive techniques, steam distillation is a highly selective method, leading to
products with high concentration of the target compound. The extract obtained by organic solvent processing still has to go through a purification
process before achieving the purity of the steam distillated extract. Purification processes may be long and expensive, thus, an evaluation of the
whole picture should be done before deciding the best extraction method for a determined raw material. |
This
gets to the meat of the issue. Quantity vs quality. Soxhlet extraction will yield the largest amount of material extracted, with over 20% of the mass
put in, however, only 46 mg/g, or 4.6% of the extract, is actually anethole. Certainly far from the "mainly anethole" that you presumed to have.
Meanwhile, dry steam distillation yields only 0.2% of the mass but in up to 98% purity. Multiply those yields by purity, and you find that you extract
0.9% of the starting mass as anethole with soxhlet extraction. Now that doesn't look nearly that much better than the 0.2% you get from steam
distillation, especially considering the Soxhlet extract will require further purification if pure anethole is what you're after. Most likely you'd
need to perform vacuum distillation or hydrodistillation on your Soxhlet extract, so there goes the convenience and time you saved.
My personal opinion is that hydrodistillation, or wet steam distillation, is a happy medium between the two. 3.1% yield with 80% purity translates to
2.5% of the mass obtained as anethole. That's significantly higher than either Soxhlet extraction or dry steam distillation, and it's far easier to
purify something that is 80% pure than something that is 5% pure. These reported results are also in line with my personal experience, though the
purity I observed from the star anise distillation was even higher. If you are trying to isolate volatile compounds, dry or wet steam distillation
will always be more selective than Soxhlet extraction. Soxhlet extraction will inevitably yield a very crude mixture as it contains many non-volatile
compounds as well as the target volatile ones.
| Quote: Originally posted by yobbo II | ...Steam distillation presented low yield (0.26%), but high anethole content (68–98%, area).
What does it mean when they say 'area' ? |
Area under the curve on the GC chromatogram.
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DocX
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| Quote: Originally posted by Texium | You stopped quoting a little too soon, see:
| Quote: | | Anise essential oil yield obtained by hydrodistillation was 3.1% with 80% of anethole content... This clearly indicates that although yields obtained
by steam distillation are lower than yields obtained by other extractive techniques, steam distillation is a highly selective method, leading to
products with high concentration of the target compound. The extract obtained by organic solvent processing still has to go through a purification
process before achieving the purity of the steam distillated extract. Purification processes may be long and expensive, thus, an evaluation of the
whole picture should be done before deciding the best extraction method for a determined raw material. |
|
Oh yes I big fat did! Thank you for finding this. I didn't notice the discrepancy of "dry" (whatever that is? Steam produced in a separate vessel?)
and wet steam distillation. This is what comes from sloppy reading.
And yes, you are quite right. I now have the problem with an impure product to deal with- and the issue of the product being really easily steam
distilled: I can say for sure that the product has been steam distilled along with the methanol when I was trying to get rid of the solvent.
I have about 500 ml of methanol, all yellowish, and when I dilute some of that 3 times with H2O I get a very distinct ouzo effect, indicating
a substantial presence of anethole. Possibly all of my anethole, actually, since the remaining oil seems all tarish and oxidised.
Now, when I screw up royally I tend to play along with my almost-ruined material, since I have nothing to loose. So I decided to try some
liquid-liquid extraction using a non polar solvent to pull the anethole out of the milky water/methanol phase.
Toluene was the most available/cheap np I have (I would have preferred chloroform, but I have none), and I've been trying to shake the now pretty
voluminous mixture (500 ml methanol + 3-4 x added water to crash out anethole + about 2-300 ml toluene... it's a large can) off and on during the day,
hoping for the milky layer to clear up as the anethole gets pulled into the top layer.
But no. It almost seems more densely white than before. I might have created a stubborn emulsion rather than any real separation, although I
do have a fine, clear top layer also.
So now I have an extraction mixture consisting of three solvents: methanol, toluene and water. Whichever I try to distill off will probably carry at
least part of the anethole in it's vapour pressure.
So I'm finally f***ed, right?
I should abandon ship, discard the mess I made, never talk about it again unless I'm drunk, and then start all over using steam distillation. Right?
| Quote: |
Most likely you'd need to perform vacuum distillation or hydrodistillation on your Soxhlet extract, so there goes the convenience and time you saved.
|
Yeah. I can't even successfully extract my extract, so QED.
[Edited on 20232323/4/11 by DocX]
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Texium
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| Quote: Originally posted by DocX |
So I'm finally f***ed, right?
I should abandon ship, discard the mess I made, never talk about it again unless I'm drunk, and then start all over using steam distillation. Right?
|
That would probably be the wise course of action. 
The procedure I ran with my students consisted of adding 8 grams of ground star anise and 100 mL water to a 250 mL flask, set up for simple
distillation, and distilling until about 50 mL of distillate had come over. This could easily be scaled up proportionally, and distilling for a longer
time/adding more water to the still pot throughout the distillation could potentially increase yield. The essential oil was then extracted from the
aqueous emulsion using diethyl ether. Other water-immiscible organic solvents should work fine too, though I wouldn’t personally use toluene due to
its high boiling point. Diethyl ether, ethyl acetate, DCM, or chloroform would be my preference. I don’t remember what the yields were (and
wouldn’t trust students to get accurate yields, especially since they were limited to “stop distillation when you have 50 mL of distillate,” due
to time constraints), but I think it was generally around 1-3% of the starting mass, which would be consistent with the paper that we discussed.
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DocX
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Ok, that's next on the agenda. Or no, correction: I might first procure or synth some ether and/or chloroform. I'm out of good non-polars, and
toluene, as you say, has such a high boiling point that you risk oxidising the product.
But how did your students strip the ether after extraction? By distillation? Or evap? Seems like any distillation risks bringing product along with
it?
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Texium
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Ether was removed on a rotovap.
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Osmiridium
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| Quote: Originally posted by DocX |
Issue 1: using a Soxhlet extractor running methanol: cotton, seeds, reflux. For some reason, after a few cycles, the drain function
doesn't seem to work. The fluid reaches the upper curvature of the siphon tube, but never passes it- only drips over the edge more or less at the same
speed as the reflux of methanol enter the extractor.
If I open the extractor up and pour in some cl:s of extra methanol it promptly overflows and drains.
WTF? Any ideas?
That's issue nr 1.
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I had this very annoying problem too a while ago.
It occurs mainly with bigger soxhlets.
The straight forward explanation is that there's not enough solvent flow to obtain the necessary hydrostatic pressure. You basically need to heat
more. This is somewhat limited by the particular solvent, heat source and flask volume combination.
So if it doesn't work and your soxhlet doesn't want to cycle or cycles not reliably you need a bigger flask and/or possibly a stronger heat source.
For example with a 250ml soxhlet you should use at least a 1l flask.
There might be differences between soxhlets from different manufacturers and between different solvents regarding their viscosity and so on.
Another reason might be overstuffing the extractor so the to high flow resistance hinders it from cycling somehow.
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DraconicAcid
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I think that problem would just be the width of the draining tube and lack of surface tension with the solvent.
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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DocX
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Yes, I think I might have filter packed it too tight, actually.
But an update on the liquid-liquid extraction: due to circumstances, the suspension was left standing for the past week, and has now separated well. I
now have a clear top layer of toluene containing product AND a clear water layer. So I have trapped the product in toluene.
But the problem still remains: if I try to distill the toluene to isolate the product, I guess I will steam distill it over to the collecting flask
anyway. What would be the best course of action now? Give up or attempt evaporation? Naturally, I don't have a rotovap.
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Fery
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Isn't it a pity to contaminate your natural product with chemical solvent? Did you try what Texium suggested you - steam distillation of the plant? If
you collect a lot of water phaseduring hydrodistillation of your plant then you can distill that water phase again and collect less volume of
distillate in which will be almost all your product which was formerly dissolved in water. But solubility of anethole in water is quite low 111 mg / 1
L https://pubchem.ncbi.nlm.nih.gov/compound/Anethole#section=S...
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DocX
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Well, if you read the beginning of the thread you see what landed me here. It wasn't my intention, it just came to this.
But now that I'm here I'm really curious to see if anything can be done.
No, but I will. I'm just trying to salvage any product possible from this first run.
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DocX
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Actually, is there a way to transiently convert the anethole to something that will stay in solution while distilling off the toluene? Like one could
with a salt-forming cation.
Stupid question, I'm sure, but I'll gladly take the shame of my revealed incompetence as payment for knowledge
[Edited on 20232323/5/9 by DocX]
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