Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: gradstudent0114 on September 02, 2015, 03:27:16 PM
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Hey,
I am working on mechanistic studies of an organic reaction and I need to run the rxn in NMR tubes. Does anyone know an adequate way to mix, given inability to stir, the mixture between NMR experiments? There was a post on in-situ NMR some time ago that did not address this issue.
If you read Advanced Practical Organic Chemistry, (see: https://books.google.com/books?id=FXvSBQAAQBAJ&pg=PA254&lpg=PA254&dq=how+shake++running+reaction+NMR+tube&source=bl&ots=X5TlXbs1cC&sig=ugIbObDT-WwaXlkOs12JABy19i4&hl=en&sa=X&ved=0CCMQ6AEwAWoVChMIoaOrmu7YxwIVze-ACh0W9ga6\#v=onepage&q=how%20shake%20%20running%20reaction%20NMR%20tube&f=false)
They suggest sonication or shaking by hand. I've got a sonicater and figured this route out, however, my advisor suggested that unwanted chemistry may occur via sonification. To be honest this assertion seems highly speculative and unlikely.
So, can someone explain off the top of their head how this could possibly be true? Should I waste my energy convincing him that this is the only way in the immediate future without purchasing new equipment?
Otherwise there a number of shaker tables in the department that do not suffice due to the 50mm diameter of the tubes which renders gravitational mixing necessary. A compact rocker table available from fisher would work but we do not have any of those and again, in my department, this is going to be a hard sell due to funding limitations.
Any thoughts on a simple solution amenable to basic, underfunded labs would be greatly appreciated or otherwise gumption to challenge a professors hand-waving assertions.
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Briefly, sonication for long periods of time is known to break bonds that aren't broken thermally. You run the risk of degradation that you can't even explain. Jury rigging something with the standard microbiology lab shaker is probably your best bet for large numbers of samples.
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If the solution is homogeneous then it does not need to be shaken because diffusion will make the reaction happen. Probably a good idea to do so at first when mixing the reagents, but you can see by comparing the yield I really doubt you will see much deviation from the yield of the batch reaction. And if there is a statistically significant difference in yield, then the procedure needs to become more convoluted. Fortunately, you can deduce whether you need to or not.
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I would have some concerns about breaking the NMR tube, which is thin glass.