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Author: Subject: Activating metals post-addition
Newton2.0
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[*] posted on 13-4-2021 at 12:40
Activating metals post-addition


I am performing a reduction of benzoin to deoxybenzoin using Zn grains in GAA. The issue I am facing is that I have forgotten to activate the Zn grains. I fear the oxide layer prevents the reduction from occurring so I am wondering if it is possible to activate the Zn in situ.

I am wondering if I can add a small amount of HCl to the reaction without damaging the product.

Has anyone had this problem?
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njl
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[*] posted on 13-4-2021 at 13:04


I would first take a smaple of your reaction mixture, add HCl, and look for any change. Some reactions like this are sensitive to different acids. To address your question, yes, HCl is added sometimes to activate metals in situ. However I would expect the GAA to attack the oxide just as well albeit more slowly.



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Newton2.0
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[*] posted on 13-4-2021 at 13:50


I do worry that the HCl might attack any deoxybenzoin that forms, either lowering the yield or forming unwanted side products.

Normally the Zn is consumed within 1.5 hours or so when it is properly activated.

I will follow your suggestion and see how the reaction mixture reacts in small amounts. Based on that, I might add more HCl in dropwise. I may also just have to run the reaction longer or perhaps add additional GAA? So much to learn from this!

Thanks for responding!

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zed
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[*] posted on 14-4-2021 at 02:57


A crystal of Iodine?
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njl
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[*] posted on 14-4-2021 at 04:50


Zed I was thinking that too but I'm not sure by what mechanism it would help here. In Grignard reactions it's just turning the aklyl halide reagent into a more active alkyl iodide, right? When forming aluminum alkoxides it produces AlI3.



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digga
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[*] posted on 14-4-2021 at 06:00


I have seen people use a tiny amount of iodine or a mercury salt to remove passivation in situ. Would use the former before the latter.
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zed
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[*] posted on 14-4-2021 at 07:07


Mechanisms are not always certain. We often do things, because they work. Why they work, is subject to debate.
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[*] posted on 16-4-2021 at 15:33


Have you tried using ammonium chloride with tetrahydrofuran? Tetrahydrofuran forms a unique complex with Zn that allows it to reduce Diketones.
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mr_bovinejony
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[*] posted on 16-4-2021 at 17:15


Hcl won't do anything to the reaction. I reduced benzlin with tin and hcl and it worked just fine
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