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Topic: "wet" solvents as sources of impurities  (Read 2777 times)

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Offline RaInBowDaSh1488

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"wet" solvents as sources of impurities
« on: July 13, 2013, 08:36:26 PM »
I am doing multistep synthesis in which later reactions are VERY sensitive to impurities. Generally, my starting materials are aldehydes or acids which I reduce using LAH or borane. Obviously a very simple reaction. However, NMR has shown that impurities are being introduced during this first step which cause me much trouble later on.

I am very confident SM is pure, my glassware is clean and my dry solvent is not contaminated. I am getting somewhat suspicious of "wet" solvents I sometimes use later on. For example, if I am using LAH, I will quench the reaction and filter it through filter paper to remove the precipitate. Sometimes I will use wet THF stored in a tinted bottle to wash the precipitate after all the solvent in the reaction flask has been filtered.

My best guess is that the wet THF is the problem. I am becoming very suspicious of many of the solvents I commonly use. Many of the bottles are quite old and are stored with just the cap on.

I don't have a lot of experience, so I am hoping to get a bit of advice here. Are wet or shelf grade solvents common sources of difficult to remove impurities? Obviously impurities that can be removed by vacuum aren't going to be any sort of problem.

If my solvents might be the source of my troubles, are there any solvents which might be better to use then others ie: less likely to contain any problematic impurities.

I realize I am the only one who can actually figure out why I am getting crap in my reactions, but I would appreciate or suggestions as to what might be going wrong.

Offline orgopete

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Re: "wet" solvents as sources of impurities
« Reply #1 on: July 13, 2013, 10:44:34 PM »
I don't know your reaction, nor the kinds of impurities you are getting, but I urge caution in thinking the reactions themselves may not also be leading to impurities. Often, this is simple to check, purify some solvent and run the reaction again. Now solvent impurities have been removed, what's the result? From what you describe, I suspect it won't be an impurity in the solvent used to work up your reaction. if impurities are quenching your reagents, this could be a simple explanation.
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Offline 408

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Re: "wet" solvents as sources of impurities
« Reply #2 on: July 14, 2013, 04:33:57 AM »
Always do NMR on your solvents if you are worried about this.

But like orgopete, if after I reaction I have more peaks in the NMR than I wanted, my first response is not "oh s#*$, my solvents were not pure", it is "byproducts".  Chemistry is not as nice as texts often indicate, and minor (or major) side products are very common.

Offline gritch

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Re: "wet" solvents as sources of impurities
« Reply #3 on: July 14, 2013, 11:28:30 AM »
I'm not really an organic chemist but this doesn't sound like a issue with your solvents but rather with your reaction itself. You could try to identify the steps you consider most problematic and test them with NMR. If impurities are such an issue as you've stated it might be necessary to isolate and purify the product from an intermediate reaction before proceeding.

Personally I'd try to separate and identify what byproducts are actually being formed and see if it would be possible to modify your initial reaction to limit byproduct formation.

Offline RaInBowDaSh1488

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Re: "wet" solvents as sources of impurities
« Reply #4 on: July 14, 2013, 04:07:29 PM »
Thanks for the advice folks. In most cases, my first assumption would be that side products are being produced. It's just the fact that these impurities are being introduced during something as simple as a reduction that has me sort of grasping here.

I can't imagine what sort of side reactions could possibly occur in something as straight forward as the reduction of a pure compound.


Offline kriggy

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Re: "wet" solvents as sources of impurities
« Reply #5 on: July 16, 2013, 05:49:35 AM »
If you are reducing an acid you can get aldehyde, alcohol and unreduced acid in your mixture. More -COOH groups in you molecule gives you more room for unwanted combinations. You could try to separate them by chromatography

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