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Topic: extracting an aldehyde into the aqueous layer?  (Read 633 times)

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Offline Babcock_Hall

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extracting an aldehyde into the aqueous layer?
« on: March 05, 2020, 09:12:22 AM »
It looks as if an aldehyde starting material has a similar Rf on silica TLC, relative to a vinyl sulfone that we made via a Knoevenagel reaction of the aldehyde.  It occurs to me that we might be able to extract the aldehyde into water if we make a water soluble derivative and thus make our purification easier.  I have seen papers in which aldehydes are converted into a derivative with bisulfite for purification and storage IIUC.  However, I do not recall seeing this done for the sake of getting rid of an unwanted aldehyde.  Thoughts?
« Last Edit: March 05, 2020, 09:31:57 AM by Babcock_Hall »

Offline pgk

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Re: extracting an aldehyde into the aqueous layer?
« Reply #1 on: March 05, 2020, 12:05:51 PM »
You can easily get read of an unwanted aldehyde, by washing (extracting) your organic solution several times (say 4-5 times) with an equal volume of a saturated aqueous sodium bisulfite solution. I have done this and it works perfectly.
Hint 1: Each washing (extraction) must take 5 minutes, at least.
Hint 2: If your product has the tendency to pass into the aqueous phase, you use lower amounts of saturated aqueous sodium bisulfite solution and wash for 3 times.
Hint 3: Saturated aqueous sodium bisulfite solution (NaHSO3) is prepared by adding sodium metabisulfite powder (Na2S2O5) into water until precipitation.
But attention! Solvation of NaHSO3 and Na2S2O5 in water, are endothermic and cool the solution, which decreases their solubility. Thus, precipitated NaHSO3 solutions that are quickly prepared, are not really saturated though seeming to be.
In order to avoid this, you add slowly small portions of Na2S2O5 into water and wait until the temperature equilibrates with the ambient one (by putting your hand on the flask). Then, repeat it several times until a precipitation that doesn’t get colder, any more.
It demands patience. 
Hint 4: Do not warm (Le Chatelier principle, etc.) during preparation because 2 NaHSO3 thermally decompose to Na2SO3 + SO2 + H2O.
« Last Edit: March 05, 2020, 12:27:42 PM by pgk »

Offline phth

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Re: extracting an aldehyde into the aqueous layer?
« Reply #2 on: March 08, 2020, 10:32:18 PM »
You can easily get read of an unwanted aldehyde, by washing (extracting) your organic solution several times (say 4-5 times) with an equal volume of a saturated aqueous sodium bisulfite solution. I have done this and it works perfectly.
Hint 1: Each washing (extraction) must take 5 minutes, at least.
Hint 2: If your product has the tendency to pass into the aqueous phase, you use lower amounts of saturated aqueous sodium bisulfite solution and wash for 3 times.
Hint 3: Saturated aqueous sodium bisulfite solution (NaHSO3) is prepared by adding sodium metabisulfite powder (Na2S2O5) into water until precipitation.
But attention! Solvation of NaHSO3 and Na2S2O5 in water, are endothermic and cool the solution, which decreases their solubility. Thus, precipitated NaHSO3 solutions that are quickly prepared, are not really saturated though seeming to be.
In order to avoid this, you add slowly small portions of Na2S2O5 into water and wait until the temperature equilibrates with the ambient one (by putting your hand on the flask). Then, repeat it several times until a precipitation that doesn’t get colder, any more.
It demands patience. 
Hint 4: Do not warm (Le Chatelier principle, etc.) during preparation because 2 NaHSO3 thermally decompose to Na2SO3 + SO2 + H2O.
I was gonna say this. 10.1021/acs.oprd.7b00231

Offline pgk

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Re: extracting an aldehyde into the aqueous layer?
« Reply #3 on: March 09, 2020, 01:45:10 PM »
The recently reported protocol is more effective, as being faster and obviously leading to higher yields.
Besides, the use of a minerally-saturated aqueous solution, permits the extractions with a water-miscible solvent.
But on the other hand and although it worths a try, my personal experience is rather poor results whenever I have tried the said kind of extractions with THF or THF/toluene.

« Last Edit: March 09, 2020, 02:28:36 PM by pgk »

Offline Babcock_Hall

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Re: extracting an aldehyde into the aqueous layer?
« Reply #4 on: May 28, 2020, 01:44:34 PM »
I tried the protocol in the paper, making the modification of using 60% EtOAc/40% hexanes, because I suspected that my product solubility would be too low in 10% EtOAc.  I took a TLC today and it looks as if the aldehyde is either gone or nearly gone.

Offline wildfyr

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Re: extracting an aldehyde into the aqueous layer?
« Reply #5 on: May 28, 2020, 09:31:48 PM »
What do you mean gone? Degraded or went into aq layer?

Offline Babcock_Hall

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Re: extracting an aldehyde into the aqueous layer?
« Reply #6 on: May 29, 2020, 11:41:56 AM »
Presumably the aldehyde has gone into the aqueous layer.  There is one spot by TLC, and it has a smaller Rf than the aldehyde.  I expect that it is the desired product, but I will take an NMR shortly and find out.  So far I have not been able to locate my notes on the TLC prior to extraction, but I recall seeing a substantial spot at the same Rf as the aldehyde prior to the extraction in the presence of NaHSO3

Offline Babcock_Hall

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Re: extracting an aldehyde into the aqueous layer?
« Reply #7 on: June 04, 2020, 03:34:57 PM »
The NMR confirmed almost complete removal of the aldehyde.  Given that the product and the aldehyde were so close in Rf values, I suspect that a silica column would not have separated them.  The silica chromatography step was, however, necessary to remove other stuff.  I tried recrystallization but in my hands it was not successful.

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