Sciencemadness Discussion Board

thf and cobalt

soma - 9-5-2011 at 17:55

I was told that adding cobalt (on molecular sieve) to ether would prevent formation of peroxide but that the ether would form some ethanol from it.

I was wondering what thf would form?

smuv - 10-5-2011 at 04:30

Apparently Indicating Molecular sieves (which use unspecified cobalt salts, probably chloride) inhibit peroxidation of ethers, including THF.

http://www.chem.tamu.edu/rgroup/gladysz/documents/nmr10.pdf

Edit: didn't read your question fully. It is tricky actually, and your friend is not telling the whole story about ether. Here is the equation for reduction of a peroxide (mechanism by which Co(II) is working):

For Ethyl Ether: EtOCH2(OOH)CH3 + electron --> OH- + EtOCH2(O*)CH3 (radical)

The fate of this radical is complex, but by in large, this radical goes on to produce ethanol and acetaldehyde.

So for THF, I suppose analogously 4-hyroxybutanal would be produced.


[Edited on 5-10-2011 by smuv]

soma - 14-5-2011 at 01:30

I haven't been able to find much information on 4-hydroxybutanal (4-hyroxybutyraldehyde). I'm wondering about the pH.

Would you know how much of this would be produced over a set period of time and how it would affect a reaction using LAH?

Thanks for the info.

vulture - 14-5-2011 at 03:46

For LAH reductions or other reactions needing dry THF, the THF is stored under inert gas anyway, so the formation of peroxides is a non-issue.

aonomus - 14-5-2011 at 04:58

This is intriguing though because I've used both THF in sureseal bottles, inhibited with BHT and under inert atmosphere for reactions, as well as HPLC grade, uninhibited THF. 1 guess as to which causes more problems with chemistry and chromatography.

smuv - 14-5-2011 at 08:26

pH of ethers with peroxides will always be acidic, in this case due to further oxidation of 4-hydroxybutanal to 4-hydroxybutyric acid. As long as there are not tons of peroxides the acidity should be a non-issue.

DJF90 - 14-5-2011 at 08:41

The cobalt inhibiting peroxide formation is new to me, and I thank you for bringing it to my attention. I assume saoking some mol sieves in a saturated CoCl2 solution and then dehydrating/activating them will give the desired indicating and peroxide inhibiting desiccant?

Regards the THF, its always best to distil from LAH what you need prior to the reaction - this way its guarenteed anhydrous and peroxide free. Chances are you won't have a dedicated solvent still, but its worth the effort setting up a distillation rig to get your pure solvent before doing the reaction.

smuv - 14-5-2011 at 10:47

I don't know exactly what Co salt they are using (in my quick search of indicating MS no manufacturers specified). One thing that comes to mind, is what is the solubility of CoCl2 in THF. But yes DJF90 I think that would work (but I don't know how much it will contaminate your ether).

Keep in mind, this method to reduce peroxides is an old trick. Classically ferrous sulfate is used, more recently Sn(II) salts. It is possible that the copper wire trick to reduce peroxides, really involves oxidation of Cu(0)/Cu(I) at the surface of the wire. At any rate, with these methods, it is important to keep in mind that while there are low concentrations of peroxide in your ether, if the ether has been exposed to air, over time there will be a bunch of other shit (reduction products of the peroxide).

vulture - 16-5-2011 at 14:20

Quote:

This is intriguing though because I've used both THF in sureseal bottles, inhibited with BHT and under inert atmosphere for reactions, as well as HPLC grade, uninhibited THF. 1 guess as to which causes more problems with chemistry and chromatography.


I'm not sure what you're getting at, but I assume you had problems with the stabilized THF? Sureseal from Aldrich is a bunch of crap, their septa are worthless. They are good if you use them ONCE and transfer the whole content of the bottle immediately. We used to buy our Grignards from Aldrich, they would be going cloudy after a month because the septa goes to shit after two punctures. Now we switched to acros. I recently had an experience with hydrazine in THF actually reacting with a sureseal and eating it to bits. Not something you like to see when you open a bottle. If I have to use a sureseal bottle these days, I take a rubber septum for grounded glass joints and pull it over the sureseal upside down.

I'm aware that BHT can cause a problems, we had our fair share.

But I'm also a bit apprehensive about using Co, unless you distill the THF from it. It's a good catalyst for a number of things, already at low loadings, which might lead to nasty surprises, certainly with organometallics.

[Edited on 16-5-2011 by vulture]