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I am conducting following electrochemical experiment on Au wires in organic medium:

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I suppose the anodic reaction goes through a benzaldehyde intermediate:

enter image description here

The cathodic reaction is hydrogen evolution, so we are dealing with a disproportionation reaction. I suppose the hydrogen evolution comes from the methanol, so methanolate is regenerated in situ.

To confirm that this conversion is indeed possible, we wanted to take a NMR sample of out reaction solution in acetone-d6. However, as we conducted the experiment, the reaction volume turned yellow and cloudy, even in our second run of the experiment. Now our fear is that due to the high applied voltage also the phenyl rest of the benzaldehyde or the benzoic acid methyl ester gets reduced.

Does anyone have an idea what kind of side reaction could be responsible for this kind of discoloration and the reasons behind it and how it could be prevented?

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  • $\begingroup$ Which voltage and current did you use? On which basis did you choose your specific settings? How big is your electrode area? NMR would be a good start, also with reduced reaction time. Also from start material. $\endgroup$
    – raptorlane
    Feb 6 at 22:03
  • $\begingroup$ Voltage was 3V, current around 40 mA. Electrode area is pretty big, can’t calculate it rn. Gold surface oxide formation was around 1.4 V vs SHE and hydrogen evolution around -2V vs SHE on Au. That’s why the voltage was chosen at 3 V so not to form the gold oxide but maintaining high current $\endgroup$
    – Mäßige
    Feb 6 at 22:42
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    $\begingroup$ You are likely forcing your anode into reaction. Starting from 1.5 V oxygen mediated gold dissolution will take place. To make sure, you could weigh your electrodes before a test and then apply voltage just in the medium without starting material and see if you see the same precipitation. On any case you can compare the weight before and after. If it was become lighter, you know that it is not related to the intended reaction. In that case maybe Pt could be worth a shot. pubs.rsc.org/en/content/articlehtml/2013/ra/c3ra42684j#imgfig7 $\endgroup$
    – raptorlane
    Feb 6 at 23:50
  • $\begingroup$ The source you mentioned performed this experiment in sulfuric acid though. I don’t think the gold oxide formation is responsible as if gold oxidation should happen, at the other electrode the counter hydrogen evolution needs to happen. We are in a two electrode setup, so the decomposition voltage of H+/H2//Au/AuO should be around 3.5 V, because in a CV the hydrogen evolution occurs in methanol at -2.5 V and Au oxidation at +1.4 vs SHE $\endgroup$
    – Mäßige
    Feb 7 at 6:58
  • $\begingroup$ Okay update, I believe the hemiacetal is formed with NaOMe [methoxy(phenyl)methanol], which remains deprotonated due to its high acidity (found on pubchem). This compound has a red color. $\endgroup$
    – Mäßige
    Feb 13 at 18:47

1 Answer 1

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According to the web search results, one possible side reaction that could cause the discoloration and cloudiness of your reaction solution is the oxidation of the gold anode by oxygen. This could result in the formation of gold oxide and gold chloride, which are yellow and brown compounds, respectively. These compounds could precipitate in the organic phase and make it turbid.

To prevent this side reaction, you could try to lower the applied voltage to avoid the oxygen evolution at the anode, or use a different anode material that is more resistant to oxidation, such as platinum. You could also weigh your electrodes before and after the experiment to check if there is any loss of mass due to the anode dissolution.

Reference: (1) organic chemistry - Electrolysis of benzyl alcohol in methanol .... Electrolysis of benzyl alcohol in methanol. (2) Selective oxidation of benzyl alcohol by two phase electrolysis using .... https://pubs.rsc.org/en/content/articlehtml/2012/cy/c2cy00424k. (3) Ether synthesis by etherification (alkylation) - Organic Chemistry Portal. https://www.organic-chemistry.org/synthesis/C1O/ethers.shtm. (4) Solvolysis of benzyl phenyl ether in high-temperature aqueous methanol .... https://pubs.rsc.org/en/content/articlelanding/2021/gc/d0gc04008h. (5) en.wikipedia.org. https://en.wikipedia.org/wiki/Benzyl_alcohol.

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