One of my references includes a procedure using excess amount(5eq.) of hydrogen peroxide(H2O2). (with NaOH 0.5eq. in methanol)
I usually quench it during workup with sat. sodium thiosulfate(Na2S2O3) solution, but this reference didn't include this reductive quenching.
As this quenching is pretty exothermic, this process might not be necessary if most of H2O2 stay in aqueous layer unlike acetic acid dimerization.
In addition, my substrate doesn't possess any other labile functional groups to H2O2, and the only potential problem may occur here is decomposition due to heat.
(I also observed that there was white powder at the bottom of the workup funnel(aqueous layer) generating heat and dissolving into the layer when sodium thiosulfate solution was poured.)
Do you guys use Na2S2O3 whenever using excess H2O2? (especially in gram-scale)
I wonder how others handle that.
Thanks in advance!
In your reference the excess may just mean enough amount or equivalence of H2O2. So no need to quench. Do you have an estimation how much is the excess in your work?
@Yuehui Zhou It was 5 equivalents of H2O2 to 1eq. substrate. (I used the same amount of it) I think there must be remaining H2O2 in their workup funnel or even in their crude mixture.
it is generally advisable to quench H2O2 before disposal to prevent unwanted reactions. Check this link as to how TATP was "accidentally" prepared from acetone and H2O2.
http://cenblog.org/the-safety-zone/2017/02/how-a-student-unintentionally-made-an-explosive-at-u-bristol/
Back to the question, I think you can quench H2O2 before or after the workup. H2O2 will generally stay only in the aqueous layer and the quenching is more for disposal purposes.
Myungjo J,
It's obvious too much. You may just try 1.5-2 equivalence. Old methods tend to use more starting material to accelerate the reaction. H2O2 is a strong reagent, it doesn't need such a large excess. Your reference may not have described the same trouble they had. Good that you try to have a smooth after-treatment. Good luck!
You need to be aware that application of Na2S2O3 may affect formation of by-products. One need to use NaNO2 or bovine liver catalyse (enzyme).
It depends on objectives of your studies.
In the past, I have used a small amount of potassium iodide to decompose excess H2O2 catalytically. This is convenient because it doesn't matter how much H2O2 is left over--I can add a few mg of KI, and once the solution stops bubbling, I know it is safe to dispose of. Also the reaction rate can be controlled by changing the concentration of catalyst (smaller amount of catalyst leads to slower reaction and lower temperatures).
Potential drawbacks include the possibility that I2 (intermediate on the catalytic cycle) could react with your desired product.
You can add the thio solution slowly in an ice bath to minimize heat formation
Thanks for your valuable answers, everyone! I'll keep these in mind!
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