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Author: Subject: Making sodium isopropoxide - Sodium is not dissolving
SplendidAcylation
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mad.gif posted on 30-6-2020 at 17:36
Making sodium isopropoxide - Sodium is not dissolving


Hi

I have recently acquired some sodium metal, with which I intend to make an anhydrous superbase for use in the Stevens rearrangement.

"Sodium isopropoxide looks like fun", I thought, as I have 99.9% isopropanol;

5g of isopropanol was placed in a beaker (the beaker and pipette were oven dried beforehand)

1g of sodium was removed from oil, and dried with a paper towel to remove the oil

The sodium was then cut into thin slices, which were then put in a dry, sealed container for a short period (so that I did not have to add it all at once)

A piece of sodium was added to the beaker, and a very slow reaction began
Very small fine bubbles emerged from the sodium
I allowed it some time to get started, as the reaction with methanol is supposedly quite exothermic, and I expected this to be similar.

After 15 minutes, the reaction was still going very slowly, so I put the rest of the sodium in.

That was about 6 hours ago; The reaction has now slowed down considerably, the sodium appears to be coated with a uniform white layer, and the bubbles are almost invisible

The beaker, sealed with cling film, has been placed in a sealed jar containing CaCl2.2H2O pellets

I'm not exactly sure what's happening;
At first glance it seems like the sodium has been passivated by a thin layer of hydroxide or oxide, formed due to a small amount of dissolved oxygen/water...
However, sodium oxide should react with the alcohol just as sodium does, and the hydroxide is also soluble in alcohols as far as I know.

So either way, there should be no insoluble passivation layer, as far as I can see :P

Any ideas would be much appreciated

Also, if I do succeed in dissolving the sodium, I am planning to vacuum distil off the excess isopropanol, followed by rapidly transferring the solid sodium isopropoxide to a sealed container while still hot

As far as I can tell, this should work, but there isn't much data on sodium isopropoxide, so I'm basing my assumptions on the assumed similarities to the methoxide

Do you guys think this would work?

Many thanks

Edit: I cringed at how many times I said "as far as I can tell" or similar phrase

[Edited on 1-7-2020 by SplendidAcylation]
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draculic acid69
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[*] posted on 30-6-2020 at 22:16


What if u heat it a little bit?
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[*] posted on 30-6-2020 at 22:50


With increasing length of the alcohol chain, the reaction with sodium becomes slower. Methanol reacts violently, ethanol still fairly vigorously, apparently isopropyl alcohol reacts very slowly. I did indeed expect a faster reaction, but sometimes reality surprises.

Just an interesting test for you. Add some NaOH to your isopropyl alcohol. Does any of that dissolve? Solubility of NaOH in alcohols also very rapidly decreases with increasing chain length.




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[*] posted on 30-6-2020 at 23:41


Sodium isopropoxide is not that soluble in IPA, I'm pretty sure the white substance you see is your product.

From here:
Quote:
For example, sodium isopropoxide is only soluble up to about 2 wt % in isopropanol


From here:
Quote:
The reaction was completed by heating and the mixture was refluxed
with continous stirring for a total of four hours. A light brown solution was
formed which on distillation at 80°C expelled excess isopropanol and light
brown crystals were obtained as product, yield about 85%


where they use 2.4 grams of sodium in 100 ml IPA. Apparently you also don't need vacuum.

[Edited on 1-7-2020 by Tsjerk]
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[*] posted on 1-7-2020 at 04:10


You need to apply heat in the end, I always have when making both sodium (m)ethoxide.
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[*] posted on 1-7-2020 at 13:02


I made sodium isopropoxide several times and the worst thing one can do is refluxing IPA and Na.
During heating, Na melts and gives damned molten balls, turning around and floating for hours and very slowly dissapearing....
The best way to obtain suspension of the isopropoxide is to use Na in form of "sodium sand" (from toluene, xylene etc... see Vogel) without allowing to melt it.
BTW. the easiest "superbase" to make is probably sodium-naphthalene radical salt. It can even (slowly) deprotonate THF, unfortunately it gives hydrogenated naphthalene as by product and often it is just impractical... But it is easy to make :P

[Edited on 1-7-2020 by kmno4]




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SplendidAcylation
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[*] posted on 1-7-2020 at 13:37


Thanks for the replies everyone!


Quote: Originally posted by woelen  
With increasing length of the alcohol chain, the reaction with sodium becomes slower. Methanol reacts violently, ethanol still fairly vigorously, apparently isopropyl alcohol reacts very slowly. I did indeed expect a faster reaction, but sometimes reality surprises.

Just an interesting test for you. Add some NaOH to your isopropyl alcohol. Does any of that dissolve? Solubility of NaOH in alcohols also very rapidly decreases with increasing chain length.


Yes, it surprised me too.
Funny how, when you want to store sodium without reaction, it seems to react with everything, but when you want it to react... Haha

Since it reacts so slowly with the alcohol, the isopropoxide must be a strong base, I am hoping.

The pKas of methanol, ethanol, and isopropanol, are quite close together, so I don't suppose the basicity will vary too much though


Quote: Originally posted by Tsjerk  
Sodium isopropoxide is not that soluble in IPA, I'm pretty sure the white substance you see is your product.

From here:
Quote:
For example, sodium isopropoxide is only soluble up to about 2 wt % in isopropanol


From here:
Quote:
The reaction was completed by heating and the mixture was refluxed
with continous stirring for a total of four hours. A light brown solution was
formed which on distillation at 80°C expelled excess isopropanol and light
brown crystals were obtained as product, yield about 85%


where they use 2.4 grams of sodium in 100 ml IPA. Apparently you also don't need vacuum.

[Edited on 1-7-2020 by Tsjerk]


Yes! You were indeed spot on

Thanks for those links, it really helped to see an actual write up on it

So here's what happened;

Having left it overnight, the white layer had become thicker and somewhat fluffy, but no bubbles were visible, and a lot of the sodium remained

So I added more isopropanol, to match the ratios given in the document you sent, and I swirled it a bit

Despite this, the white stuff didn't seem to be dissolving, however some of it flaked off the sodium, leaving shiny sodium metal which still didn't seem to react at all.

Then I applied heat, and the reaction sped up quite significantly, with most of the white solid appearing to dissolve.

Even at the boiling point of IPA, though, the reaction was by no means vigorous; it was similar to the reaction between lithium and water at room temperature.

As the end approached, a few little bits of sodium remained undissolved, and it was necessary to maintain the solution boiling for the remainder of the reaction, as soon as the temperature dropped a little, the reaction seemed almost to stop.

Once everything was dissolved, I was left with a slightly cloudy solution with no undissolved material.

As soon as this solution began to cool a little, beautiful needle crystals began to form
At one point I allowed it to cool to room temperature, and the whole solution solidified!

I set up a short path distillation apparatus with a 500mL RBF into which was poured the solution, boiling hot to avoid crystals forming.
As soon as the stuff had been poured from the beaker, it began to solidify on the spout of the beaker

Next time I'll make the solution less concentrated!.

Vacuum distillation then commenced, and I ended up with most (but not all) of the isopropanol recovered, and lovely white snowy crystals of the alkoxide!

Picture to follow
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[*] posted on 1-7-2020 at 13:45


Quote: Originally posted by SplendidAcylation  


Since it reacts so slowly with the alcohol, the isopropoxide must be a strong base, I am hoping.

The pKas of methanol, ethanol, and isopropanol, are quite close together, so I don't suppose the basicity will vary too much though


Again, kinetics vs. thermodynamics. The pKas of the various alcohols aren't very different, so the basicities won't vary very much (that's a thermodynamic property). How quickly the isopropanol reacts with the sodium is determined by kinetic factors.




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[*] posted on 1-7-2020 at 15:16


I totally relate to this! Took me 3 days of refluxing 5-7 hours a day to make like 100 grams of isopropoxide. It was just as laughably slow as you describe. Me with a flask of sodium, adding a mL and waiting with stirring, then adding another mL, and adding heat, and thinking it was going to take off anything second. Then 3 days later finding out that my flask is etched.

[Edited on 7/1/2020 by BromicAcid]




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SplendidAcylation
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[*] posted on 1-7-2020 at 17:47


Quote: Originally posted by DraconicAcid  
Quote: Originally posted by SplendidAcylation  


Since it reacts so slowly with the alcohol, the isopropoxide must be a strong base, I am hoping.

The pKas of methanol, ethanol, and isopropanol, are quite close together, so I don't suppose the basicity will vary too much though


Again, kinetics vs. thermodynamics. The pKas of the various alcohols aren't very different, so the basicities won't vary very much (that's a thermodynamic property). How quickly the isopropanol reacts with the sodium is determined by kinetic factors.


Interesting again :D :D


There is a correlation though, right?

Quote: Originally posted by BromicAcid  
I totally relate to this! Took me 3 days of refluxing 5-7 hours a day to make like 100 grams of isopropoxide. It was just as laughably slow as you describe. Me with a flask of sodium, adding a mL and waiting with stirring, then adding another mL, and adding heat, and thinking it was going to take off anything second. Then 3 days later finding out that my flask is etched.

[Edited on 7/1/2020 by BromicAcid]



I know, right!
I wish I had that much sodium to turn into superbases :P

I suppose your flask is etched because hot sodium alkoxides dissolve glass similarly to NaOH?

Quite a few of my flasks are etched yet I can recall no reactions involving NaOH, HF, or H3PO4; its very curious.

The isopropoxide seems to have a lot of volume; I should only have 3g but it looks more like 30g

I don't think its fully dry, but I can't imagine traces of alcohol being an issue; Still, I'll probably end up trying to dry it further

Here's a pic, sorry about the low resolution

IMG_20200702_001559.jpg - 926kB

[Edited on 2-7-2020 by SplendidAcylation]
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[*] posted on 2-7-2020 at 07:09


Try potassium. That works.

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