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Topic: Molecular sieves and refluxing toluene?  (Read 1093 times)

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Offline rolnor

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Molecular sieves and refluxing toluene?
« on: March 01, 2019, 01:30:45 PM »
I want to use ground 4Å molecular sieves in a refluxing toluene solution. Is there a problem with this, does the sieves release part of the absorbed water at 110°C?

Offline TheUnassuming

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Re: Molecular sieves and refluxing toluene?
« Reply #1 on: March 01, 2019, 02:12:51 PM »
You can and I've done so before, but they aren't the most effective at elevated temp for exactly the reason you point out. Technically sieves have water in a equilibrium with the environment and this equilibrium shifts towards release as the temp increases.  You can always just add a large excess and hope for the best. 

I've had better luck with running this sort of reaction under cycling nitrogen and periodic additions of additional dry solvent to help drive the reaction forward.  It seems to work well with toluene if I remember right because it makes such a good azeotrope with water.
When in doubt, avoid the Stille coupling.

Offline pgk

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Re: Molecular sieves and refluxing toluene?
« Reply #2 on: March 01, 2019, 03:23:44 PM »
Yes, they do.
Molecular sieves absorb water in their cavity by a physical way and thus, they excrete absorbed water at elevated temperatures.
Note that the “apparent” boiling point of water in this case, is not 100oC but it is 84oC, which corresponds to the toluene/water azeotrope.
By the way, you can preliminary dry toluene by azeotropy, using a Dean-Stark trap and then run your reaction in the same flask. 
« Last Edit: March 01, 2019, 03:40:34 PM by pgk »

Offline rolnor

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Re: Molecular sieves and refluxing toluene?
« Reply #3 on: March 02, 2019, 05:27:12 AM »
OK, I have thought of a Dean Stark-trap but I need to eliminate water very fast and it takes some time for this apparatus to start, if there is water present for just a few minutes the reaction gives byproducts. I am going to try calcium hydride and also large excess ground molecular sieves. What I have tried so far is bis-trimethylsilyl acetamide and this works fine. I can not use P2O5 because my compound can react with this drying-agent. Thanx.

Offline rolnor

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Re: Molecular sieves and refluxing toluene?
« Reply #4 on: March 04, 2019, 04:23:19 PM »
The reaction worked poorly with the sieves, probably because the are weakly acidic, only 26% yield. With calcium hydride I got 77%, very nice.

Offline TheUnassuming

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Re: Molecular sieves and refluxing toluene?
« Reply #5 on: March 04, 2019, 06:35:51 PM »
That's great!

Should have asked it before, for archive/later search purposes if nothing else.  What was the reaction you were running?
When in doubt, avoid the Stille coupling.

Offline wildfyr

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Re: Molecular sieves and refluxing toluene?
« Reply #6 on: March 04, 2019, 10:32:11 PM »
I thought sieves were weakly basic?

Offline clarkstill

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Re: Molecular sieves and refluxing toluene?
« Reply #7 on: March 05, 2019, 06:19:25 AM »
Agreed - isnt this why you shouldn't use them to dry acetone?

Offline rolnor

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Re: Molecular sieves and refluxing toluene?
« Reply #8 on: March 05, 2019, 07:29:54 AM »
I am not sure, the formula given by wikipedia for 4Å is
NaO2*Al2O3*2SiO2*9/2H2O, is this basic?

Online AWK

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Re: Molecular sieves and refluxing toluene?
« Reply #9 on: March 05, 2019, 08:44:43 AM »
https://www.sigmaaldrich.com/chemistry/chemical-synthesis/learning-center/technical-bulletins/al-1430/molecular-sieves.html
But note - you are not using water solution (5 % slurry - pH ~10.5) hence these values are rather useless.
AWK

Offline rolnor

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Re: Molecular sieves and refluxing toluene?
« Reply #10 on: March 05, 2019, 10:37:12 AM »
Thanx, they are clearly not acidic, good reading.
Also, they only work up to 90°C. I give you a "molsnack"!
I dont want to tell the forum wich reaction it is, I am going to publish this.
« Last Edit: March 05, 2019, 10:58:57 AM by rolnor »

Offline pgk

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Re: Molecular sieves and refluxing toluene?
« Reply #11 on: March 06, 2019, 10:42:20 AM »
The chemical formula is not NaO2.Al2O3.2SiO2.9/2H2O but it is Na2O.Al2O3*2SiO2*9/2H2O and which indicates a strongly basic, co-crystalline compound due to the presence of Na2O.
However, molecular sieves are moderately basic and thus, the stoichiometric formula Na12Al12Si12O48·27H2O (zeolite type A) is more successful as indicating hydrate co-crystals of sodium silicate and sodium aluminate that are basic after hydrolysis.
The confusion about acidity starts from the amphoteric nature of Al2O3 (if adopting the mixed oxides formula) that may produce salts with both acids (e.g. AlCl3) and bases (e.g. Na2Al2O4). Anyway, molecular sieves are basic and not acidic.
But although molecular sieves are moderately basic in water, their basicity increases in organic solvents and thus, they are not compatible with strong acids, aliphatic halides (E2 elimination), etc.
« Last Edit: March 06, 2019, 11:28:45 AM by pgk »

Offline rolnor

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Re: Molecular sieves and refluxing toluene?
« Reply #12 on: March 06, 2019, 12:45:52 PM »
Thanx pgk, I wrote the formula wrong. I give you a "molsnack"!

Offline pgk

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Re: Molecular sieves and refluxing toluene?
« Reply #13 on: March 06, 2019, 01:01:18 PM »
Obviously, it was a typing error but note that NaO2 (sodium superoxide) also exists.
Thanks for the "molsnack".

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