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Topic: Need to keep a reaction dry that produces water as byproduct.  (Read 826 times)

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Offline 408

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Need to keep a reaction dry that produces water as byproduct.
« on: October 18, 2019, 04:33:55 PM »

I am doing a reaction in DMSO using a water sensitive reagent.

The reaction also happens to produce water.  Said water reacts with the reagent producing a base, which kills our compound.

I need some method of removing the water or neutralizing the base before it destroys my compound.

I'll be trying the trivial things like mol sieves, but I'm looking for suggestions that are DMSO-compatible drying agents, that don't produce anything basic as their byproduct.

I have several bad ideas, (thionyl chloride) but no good ideas. I also can't have any acidic protons present in my reaction :S

Any suggestion?

Offline kriggy

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Re: Need to keep a reaction dry that produces water as byproduct.
« Reply #1 on: October 19, 2019, 12:15:15 PM »
Sieves work fine or you can add a scavenger such as 2,2-DMP that reacts with water to produce acetone and methanol

Offline 408

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Re: Need to keep a reaction dry that produces water as byproduct.
« Reply #2 on: October 19, 2019, 04:54:27 PM »
Thanks!  Which do you think has faster kinetics?

Offline rolnor

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Re: Need to keep a reaction dry that produces water as byproduct.
« Reply #3 on: October 20, 2019, 02:17:53 AM »
It would be good with more details on your compound and the conditions, one possibility would be hexamethyldisilazane or N,O-bistrimethylsilylacetamide. Dimethoxypropane needs acid to react with water?

Offline hypervalent_iodine

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Re: Need to keep a reaction dry that produces water as byproduct.
« Reply #4 on: October 20, 2019, 08:45:30 PM »
I believe we use the rapid method in Armarego (Purification of Laboratory Chemicals 5th ed, 2003, pp 219) for dry DMSO.

Quote
Dimethyl sulfoxide (DMSO) [67-68-5] M 78.1, m 18.0-18.5°, b 75.6-75.8°/12mm, 190°/760mm, d 1.100, n 1.479. Colourless, odourless, very hygroscopic liquid, synthesised from dimethyl sulfide. The main impurity is water, with a trace of dimethyl sulfone. The Karl-Fischer test is applicable. It is dried with Linde types 4Å or 13X molecular sieves, by prolonged contact and passage through a column of the material, then distd under reduced pressure. Other drying agents include CaH2, CaO, BaO and CaSO4. It can also be fractionally crystd by partial freezing. More extensive purification is achieved by standing overnight with freshly heated and cooled chromatographic grade alumina. It is then refluxed for 4h over CaO, dried over CaH2, and then fractionally distd at low pressure. For efficiency of desiccants in drying dimethyl sulfoxide see Burfield and Smithers [J Org Chem 43 3966 1978; Sato et al. J Chem Soc, Dalton Trans 1949 1986.

Rapid purification: Stand over freshly activated alumina, BaO or CaSO4 overnight. Filter and distil over CaH2 under reduced pressure (~ 12 mm Hg). Store over 4Å molecular sieves.

Offline hollytara

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Re: Need to keep a reaction dry that produces water as byproduct.
« Reply #5 on: October 21, 2019, 12:37:48 AM »
Mol Sieves are good - they are aluminosilicates (zeolites) however...  have some acid/ base - but usually basic properties.  If you store aldehydes or ketones over mol sieves (acetone for example), you get some aldol chemistry occurring.  So depending on your reaction....   

Activated alumina is good, as is activated silica gel; but these can also act as catalysts. 

There aren't many reagents that react with water and dont make acid or base!  Things like calcium hydride will make calcum hydroxide - pretty insoluble but definitely a base.  Thionyl chloride will make SO2 and HCl (phosgene is actually a great dehydrating reagent - makes CO2 and HCl, but I dont recommend it!)  Isocyanates react to make carbamic acids that decompose to CO2 and amines - but they are dangerous as well.

Offline rolnor

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Re: Need to keep a reaction dry that produces water as byproduct.
« Reply #6 on: October 21, 2019, 12:55:24 PM »
The N,O-bistrimethylsilylacetamide reacts very fast with water and does not give acid or base.

Offline kamiyu2550

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Re: Need to keep a reaction dry that produces water as byproduct.
« Reply #7 on: October 21, 2019, 08:39:12 PM »
The drying of DMSO itself is a challenge. There is a paper in JOC, probably 3 decades ago which studied the efficiency of different drying methods for hydroscopic solvents. It turned out activated alumina is the best.

Alumina is available in many forms, i.e. basic, neutral and acidic.

Offline 408

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Re: Need to keep a reaction dry that produces water as byproduct.
« Reply #8 on: November 09, 2019, 11:38:20 AM »
Sieves got yield to 98% :)

Offline JSUCY232

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Re: Need to keep a reaction dry that produces water as byproduct.
« Reply #9 on: November 18, 2019, 01:52:18 PM »
If sieves don't work, you can try silica gel. I have used it under reactions successfully, but not ones this sensitive. What temp are you running this at?

Donna

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