unome
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Drying alcohols with Sodium Glycoxide
Attached please find a paper on the drying of alcohols using sodium glycoxide (the sodium alkoxide of ethylene glycol), which can apparently be formed
by thermal dehydration of a solution of sodium hydroxide in ethylene glycol.
Now, what I'm wondering is, if an equilibrium mixture of NaOH + EtOH <==> NaOEt + H2O, what happens when ethanol (99% dry ethanol, ie. virtually
absolute) is added to the sodium glycoxide in glycol/ethanol? What would be the pKa of the glycoxide?
The pKa of ethanol is 15.9 & that of ethylene glycol = 14.2, so that would make ethanol a weaker acid, so I'm guessing that EtOH would not
displace the glycol, but if that is the case (and the basicity of an alkoxide apparently increases with size - I'm not at all sure about this shit
but ), what would be the pKa of the glycoxide? Would it be a stronger base than
sodium methoxide?
Attachment: Dehydration of Alcohols with Alkali Metal Alcoholates.pdf (392kB)
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Attachment: php8AsQpX (542kB)
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[Edited on 4-1-2010 by unome]
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Nicodem
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I do not know the pKa of ethylene glycol in ethanol, but I would estimate that by dissolving NaOCH2CH2OH in ethanol you would achieve a similar
concentration of NaOEt as when dissolving an equivalent amount of NaOH. I doubt it would give a higher concentration.
What is your goal anyway? If you want to make a solution of NaOEt in ethanol then it is obviously much more practical to dissolve NaOH in absolute
ethanol. This would gives a solution containing mostly ethoxide with some hydroxide (the ratio depending on the amount of NaOH dissolved and can be
calculated from the pKa's of H2O and EtOH in ethanol - the result being accurate only for the more diluted solutions). You can make it a solution of
pure sodium ethoxide in ethanol by using one of the methods for removing water from ethanol (azeotropic distillation or molecular sieves) or by
isolating pure NaOEt as a solid and then redissolving it in another batch of absolute ethanol. The other method is to dry a saturated solution of NaOH
in EtOH by a few batches of solid NaOH. Or you can just react sodium with excess absolute ethanol. All these methods are described in several threads
in the forum. The easiest and safest way is to just buy solid NaOEt, which is very cheap, and then dissolve it in absolute ethanol.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
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unome
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The idea was that if one dissolved the glycoxide in ethanol, then the ethoxide would be generated without the concomitant formation of water, thus
serving to fuck up the alkoxide again. The glycoxide (wouldn't it be NaOCH2CH2ONa (or Na2C2H4O2)?) appears to be an 'easily' generated solid (although
unstable), which can be thermally dehydrated (cf ethoxide). It just seemed that if this were so, then the generation of it thermally would beat the
azeotropic route...
As to the concentration - the following is from the paper (above, p.2/3):
Material charged - 496 g of glycol, 333 g of 48.1% NaOH (Solution presumably) and 19,775g of 92% (by weight) ethanol
Material obtained - 541 g of sodium ethylene glycoxide residue, 1,591g water fraction, 849g middle alcohol fraction, 17, 494 g strong alcohol fraction
97% (by weight).
That suggests, especially considering they acidify the glycoxide residue to get their glycol back, that the concentration of glycoxide post
distillation is pretty much the entirety of the residue. Thus the extractive distillation of the water, alcohol (and some glycol) leaves only the
glycoxide behind.
Dissolving that in EtOH should give a higher concentration of NaOEt for mine, there is no water generated... The equilibrium would be entirely
glycoxide/ethoxide (providing the alcohol was dry), wouldn't it?
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rrkss
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I just use a 3A molecular sieve. Its very cheap and can easily be regenerated in your oven. Leaves pretty much absolute alcohol without a fuss.
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unome
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Attaching 2 papers, both studies on the H_ acidity / pKa of sodium glycoxide/glycol solution... Interestingly enough, they put the pKa of a 3M
solution of the glycoxide in glycol ~17.2
Given that is about the range of other better known alkoxides, (potassium t-butoxide for one) why couldn't it be used 'as is' for condensation reactions?
Attachment: Aiyar.Datta.Kundu.Determination.H.Acidity.Na.Glycoxide.Glycol.pdf (605kB)
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Attachment: Kundu.Aiyar.H.Acidity.Fx.Li.Na.K.Glycoxides.pdf (473kB)
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[EDIT] This glycoxide (the sodium one) was implicated (more to the point, its runaway exothermic decomposition) in the Seveso accident/disaster.
[Edited on 6-1-2010 by unome]
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Nicodem
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You can not compare the pKa of glycol in glycol (~17.2) with the pKa of glycol in water (~14). pKa values are solvent dependent, so that such a
difference in values is completely normal. You can only compare trends of pKa's of a series of compounds from one solvent to the other. And even here
the order might not remain the same. For example, I would expect glycols to be more acidic in nonpolar solvent that normal alcohols because the anion
is stabilized by the intramolecular H-bonding while in normal alcohols this is not possible. In water this effect is more or less irrelevant because
water molecules compete for H-bonding so that ethylene glycol is just somewhat more acidic than ethanol (about 10 times more acidic).
| Quote: | | The idea was that if one dissolved the glycoxide in ethanol, then the ethoxide would be generated without the concomitant formation of water, thus
serving to fuck up the alkoxide again. The glycoxide (wouldn't it be NaOCH2CH2ONa (or Na2C2H4O2)?) appears to be an 'easily' generated solid (although
unstable), which can be thermally dehydrated (cf ethoxide). It just seemed that if this were so, then the generation of it thermally would beat the
azeotropic route... |
Even though no equilibrium containing water is obtained if you dissolve the glycoxide in ethanol, you nevertheless get the equilibrium with ethylene
glycol which in ethanol is probably more or less just as acidic as water (probably more). That is why I said that I think you would obtain a similar
concentration of NaOEt as if you would dissolve an equivalent of NaOH. Since the ratio of NaOEt/NaOH obtained by dissolving NaOH in absolute ethanol
are up to 9:1 and more (depending on the concentration), this is not really important. It is however important for the reactions where it is important
to have anhydrous conditions, like for the Claisen condensation for example. But reactions like this one proceed very poorly in ethanolic NaOEt. They
give best results when solid NaOEt or some other alcoxide is used in a nonpolar aprotic solvent. So it would be pointless to waste a glycoxide by
dissolving it in ethanol when it would work better as such. Besides, I think you are wrong. Preparing solid NaOEt is very much simpler than the
process you describe for this sodium glycoxide. It is even simpler to prepare sodium isopropoxide.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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sonogashira
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If you have the time would you briefly describe what you did? I recall reading the patent on this method and I believe heat was sometimes used? Is it
necessary, or did you just add molecular sieve... or excess of hydroxide? Thanks 
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Nicodem
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I never prepared sodium isopropoxide (these alkoxides are too cheap to waste time preparing unless for amateur reasons). But as you must have noticed
from the now many reports that you can use molecular sieves to remove water from the solutions of NaOH in alcohols.
For other methods look in Vogel under the methods for drying alcohols. It states that alkoxides can not be used as drying agents because
alcohols/water azeotropes still distil first leaving the alkoxide in dry alcohol behind. The ethanol/water azeotrope has a too low water content so
that there is a need to either add toluene or to use a very efficient distillation column. But the isopropanol/water azeotrope (88:12) is just perfect
as it has a larger water content so that it is not even that necessary to use a distillation column.
So all it takes is to prepare a saturated solution of NaOH in isopropanol, dry it over NaOH (which removes most water anyway) and then rotavap the
supernatant to dryness to remove the last traces of water. This should leave a fairly pure sodium isopropoxide (which should be treated with due care
as alkoxides can go into flames if heated in air).
Slightly more problematic simple alkoxide is the methoxide. As far as I know the only way to prepare NaOMe from NaOH is using molecular sieves (as
already described in some other threads). Though the azeotropic removal of water with added toluene might also work with methanol. Solid NaOEt is
simple to make as all it takes is to prepare a saturated NaOH/EtOH solution, precipitate out the NaOEt with acetone, isolate the solids by filtration,
wash with toluene and dry in a desiccator (there is a patent describing the process, certainly already cited in some other thread). I tried this years
ago and it indeed works well. Or maybe even simpler is the use of NaOH as drying agent to dry NaOH/EtOH into NaOEt/EtOH.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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sonogashira
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Thanks very much!
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hinz
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Does the molecular sieve thing really work? Its well known that a hydroxide solution in water or alcohol attacks borosilicate glass if you leave it
around at room temperature for some days.
So I wonder how can those incredible fine pores (average size in 3A is 0,3 nm, or 3 Ångström, hence it´s name) can survive the hydroxide. I would
expect that those molecular sieve prills get quickly eaten by the alkoxide solution even at room temperature because they have such a big surface
area.
Probably another nice nice way of preparing sodium isopropoxide is adding NaOH to a solution of aluminium isopropoxide, precipitating Al(OH)3 and
leaving a solution of Na isopropoxide behind.
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