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Author: Subject: Photochemical oxidation of alcohols to aldehydes using uranyl nitrate
CRUSTY
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[*] posted on 18-3-2023 at 20:47
Photochemical oxidation of alcohols to aldehydes using uranyl nitrate


I just successfully replicated the results of the paper listed below and oxidized ethanol to acetaldehyde using UV light and uranyl nitrate. The radioactivity of the UO2(NO3)2 has nothing to do with its photochemical activity, uranyl nitrate is actually quite accessible in many countries, and this reaction is very relevant to organic chem, so I figured I would post it here for those interested. Hopefully it's okay that I did so; admins/mods please move this thread to General Chemistry or Radiochemistry if this is not the appropriate place to post it.

This work is based on the paper: Sakuraba, S.; Matsushima, R. Photochemical Reactions of Uranyl Ions with Organic Compounds. II. the Mechanism of the Photo-Oxidation of Alcohols by Uranyl Ions. Bulletin of the Chemical Society of Japan 1970, 43 (8), 2359–2363. The authors report the oxidation of several alcohols to their corresponding aldehydes when mixed with uranyl nitrate and exposed to light in the 400-450 nm range. I decided I would try this myself with ethanol and instead use 365 nm light.

My small-scale test procedure:
Approximately 50 mg uranium trioxide was dissolved in 0.5 mL nitric acid with a pH of ~2 (I have not titrated this nitric acid yet) to form uranyl nitrate. The uranium trioxide can be substituted for an equimolar amount of any uranium oxide, including the raw uranium mineral uraninite. The solution was diluted with 2.5 mL distilled water. 0.5 mL ethanol was then added and the entire solution was thoroughly mixed. The entire solution was transferred to a quartz cuvette and was observed to have a pale yellow color. The cuvette was capped and covered with Parafilm, and then exposed to about 10 watts of 365 nm UV light for 1.5 hours.

Upon completion of the reaction, the cuvette was opened and a strong smell of acetaldehyde was immediately noticed. I currently do not have the proper reagents to chemically test for the presence of acetaldehyde but I have dealt with acetaldehyde many times before in the lab and the odor was unmistakable. I plan to post an update once I obtain an aldehyde test reagent.

In the near future I will perform this reaction on a larger scale and attempt to isolate the acetaldehyde product and remove the uranyl nitrate. I would also like to try this reaction with longer-chain aliphatic alcohols. Until then, thanks for reading!


[Edited on 19-3-2023 by CRUSTY]




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[*] posted on 23-3-2023 at 18:08


This is a really interesting reaction. Is it like a spinoff of the Jones oxidation? I'm seeing a lot of similarities, although I believe the Jones reagent tends to oxidize the aldehydes further, to carboxylic acids.

I found a nice pdf of the paper, although I don't really understand it. How's the stoichiometry work out? What oxidizing agent is consumed?
Quote:
// air oxidation?
2C₂H₅OH + O₂ -> 2CH₃CHO + 2H₂O //catalyzed by UO₂²⁺

// oxidation by excess nitric acid?
C₂H₅OH + UO₂(NO₃)₂ + 2HNO₃ -> 2CH₃CHO + U(NO₃)₄ + 2H₂O

The paper suggests that uranium(IV) is a product of the reaction, so I presume that some variation of the latter formula occurs.




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[*] posted on 23-3-2023 at 19:28


Uranyl nitrate is the oxidizing agent. Specifically, uranyl ions they have been excited by UV light.

UO22+ + RCH2OH —> UO2+ + RCHO + H2O




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[*] posted on 24-3-2023 at 08:32


Quote: Originally posted by Texium  
Uranyl nitrate is the oxidizing agent. Specifically, uranyl ions they have been excited by UV light.

UO22+ + RCH2OH —> UO2+ + RCHO + H2O

Oh, so uranium(IV) also forms an oxycation. What would you call that one? Can't find any information on it.

Wikipedia suggests the existence of a UO2+ chloride salt as well as regular UCl4. They might form analogous nitrate salts.




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[*] posted on 24-3-2023 at 09:17


Probably just uranyl(IV), as opposed to the more commonplace uranyl(VI).

Regardless, the important thing is that it’s U(VI) doing the oxidation, and nitrate is just a spectator ion. The reaction should proceed with any soluble uranyl(VI) salt.

Edit: I did a literature search, and from what I’ve read, I suspect that UO2+ is unstable and disproportionates to UO2 and U4+, but I haven’t found a clear confirmation of if that occurs in aqueous solution. If I do, I’ll post it.

[Edited on 3-24-2023 by Texium]




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[*] posted on 25-3-2023 at 13:47


Quote: Originally posted by SnailsAttack  
What oxidizing agent is consumed?
Quote:
// air oxidation?
2C₂H₅OH + O₂ -> 2CH₃CHO + 2H₂O //catalyzed by UO₂²⁺

// oxidation by excess nitric acid?
C₂H₅OH + UO₂(NO₃)₂ + 2HNO₃ -> 2CH₃CHO + U(NO₃)₄ + 2H₂O

The paper suggests that uranium(IV) is a product of the reaction, so I presume that some variation of the latter formula occurs.


https://pubs.acs.org/doi/pdf/10.1021/ic00128a014 , like other publications, explains the mechanism as follows: a uranyl cation (UO₂²⁺) is excited by a photon, then the excited state is quenched by an alcohol, aldehyde, or alkane through one of several mechanisms (hydrogen abstraction, e- transfer, etc), yielding UO₂⁺, an organic radical, and a proton. The radical is oxidized by O2 and then UO₂⁺ can be oxidized back to UO₂²⁺ by molecular oxygen.

This mechanism appears to be valid because when I run the reaction in oxygen-free conditions the solution turns from yellow to lime green, indicating a buildup of the reduced uranium (IV) cation. As I mentioned above uranyl nitrate should also be able to oxidize alkanes and aldehydes so its uses could be even greater than I originally discussed. The paper I linked mentioned the oxidation of cyclopentane to cyclopentanone. I also found that when I run my ethanol oxidation for a few more hours the solution starts to develop an odor of acetic acid, presumably due to the further oxidation of acetaldehyde.

[Edited on 25-3-2023 by CRUSTY]




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[*] posted on 25-3-2023 at 16:58


Uranium chemistry for the win! good work!



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