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Author: Subject: benzyl alcohol oxidation
JJay
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[*] posted on 13-8-2017 at 19:23


Oh, no, it's ok... I certainly don't know everything, but I'm not as ignorant as I appear :)

If I were to try to make benzaldehyde, I'd probably go with manganese dioxide oxidation. There are lots of other ways to do it, but that method looks easy and doesn't involve harsh conditions or especially dangerous chemicals.




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Melgar
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[*] posted on 18-8-2017 at 19:09


Quote: Originally posted by Chemi Pharma  
Melgar, please, give it a try! I've never read at nowhere, this reaction isn't scalable. Don't blame the authors if they only did a small batch run and ommited about the scalability of the reaction.

They did it on a ~100 mg scale. And I knew that this would be the case before even reading the paper. Somehow, after reading enough papers on the oxidation of benzyl alcohols, you learn to anticipate these things.

Oxidation of benzyl alcohols seems to be a popular topic for authors that need to get something, anything published, when it's not all that important what it is. I'm not sure why this is, but it seems to be the case.

Hypochlorite oxidation of benzyl alcohol is nothing new. I did it a few years ago, but with 10% NaOCl. The water moderates the temperature in this case, and since benzyl alcohol is more water-soluble than benzaldehyde, the alcohol is preferentially oxidized, while the benzaldehyde mostly stays in the nonpolar layer. Of course, workup is annoying, and you need a very large vessel to react a significant amount, but it does work. Yields are average.

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I think some of us could run a 500 ml benzyl alcohol batch and tell about the results. I really don't expect a runaway! As i just have written to JJay, I think the Alumina presence makes the reaction milder than with hypochlorite alone.

I think its main purpose is to increase surface area, which is probably not something you want to do if your goal is to avoid a thermal runaway.

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Ok, and what about the yields of benzaldehyde in this KMNO4 oxidation? I have my doubts if it not results in a mixture with near equal quantities of benzaldehyde and benzoic acid due the powerfull oxidating properties of permanganate.

You have to account for the fact that KMnO4 can perform multiple oxidations of benzyl alcohol to benzaldehyde. Initially, KMnO4 concentration is high, but the only thing to oxidize is the alcohol. By the time aldehyde concentration is high enough that KMnO4 oxidation to the acid becomes likely, manganese dioxide is the main oxidizer present, which is very selective for oxidizing alcohol to aldehyde. Stirring has to be very good, of course, and once the purple-colored permanganate isn't visible in the aqueous solution, you need some heat to increase the alcohol solubility in the aqueous layer, or it takes a very long time.

Could you use only manganese dioxide? Sure, but you'd need a lot, and at least for me, it's somewhat easier to get potassium permanganate than it is to get manganese dioxide. You can always oxidize the resulting sludge from KMnO4 oxidation, using nitric acid and hydrogen peroxide, but I never bothered. Yields tended to be 75-80% with a half-assed workup, and that was okay for my purposes.




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[*] posted on 17-1-2018 at 23:46


Hello, i have lurked around on SM for a while now but finally decided to make an account and contribute :)


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Ok, and what about the yields of benzaldehyde in this KMNO4 oxidation? I have my doubts if it not results in a mixture with near equal quantities of benzaldehyde and benzoic acid due the powerfull oxidating properties of permanganate.


I have successfully completed several solvent-less partial oxidations of benzyl alcohol with KMnO4, using a slight excess of benzyl alcohol, usually a molar ratio of 1:1.5 (KMnO4:BnOH). No benzoic acid has ever been detected. Benzyl alcohol is one of a diverse group of organic molecules that can act as an inhibitor of benzaldehyde oxidation, hence using the slight excess. It can be removed by fractional distillation under vacuum if desired, but leaving it in acts as a very effective stabilizer.

In fact, it is so effective that it protects the benzaldehyde in some cases at temperatures as high as 433 K in the presence of 10 bar O2 pressure with metal catalysts present. No need to worry too much about over-oxidation. It has been shown that benzyl alcohol quantities as low as 2% were sufficient to inhibit the appearance of benzoic acid crystals when benzaldehyde was left sitting out for 90 min so I would assume that even the residual BnOH left in benzaldehyde after a fractioinal distillation might offer decent pretection (depending on how good your fraction column is).

Benzaldehyde is usually auto-oxidized by a free radical chain process. The benzyl alcohol selectively removes one of the radicals normally present during autoxidation, thereby inhibiting the chain reaction and preventing any significant amount of benzoic acid from forming.

https://www.nature.com/articles/ncomms4332.pdf

Meenakshisundaram Sankar, Ewa Nowicka, Emma Carter, Damien M. Murphy, David W. Knight, Donald Bethell & Graham J. Hutchings. 25 February 2014. The benzaldehyde oxidation paradox explained by the interception of peroxy radical by benzyl alcohol. Nature Communications 5, Article number: 3332 (2014). https://www.nature.com/articles/ncomms4332. doi:10.1038/ncomms4332.
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Melgar
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[*] posted on 2-2-2018 at 00:39


Quote: Originally posted by thermochromic  
Hello, i have lurked around on SM for a while now but finally decided to make an account and contribute :)

I have successfully completed several solvent-less partial oxidations of benzyl alcohol with KMnO4, using a slight excess of benzyl alcohol, usually a molar ratio of 1:1.5 (KMnO4:BnOH). No benzoic acid has ever been detected. Benzyl alcohol is one of a diverse group of organic molecules that can act as an inhibitor of benzaldehyde oxidation, hence using the slight excess. It can be removed by fractional distillation under vacuum if desired, but leaving it in acts as a very effective stabilizer.

In fact, it is so effective that it protects the benzaldehyde in some cases at temperatures as high as 433 K in the presence of 10 bar O2 pressure with metal catalysts present. No need to worry too much about over-oxidation. It has been shown that benzyl alcohol quantities as low as 2% were sufficient to inhibit the appearance of benzoic acid crystals when benzaldehyde was left sitting out for 90 min so I would assume that even the residual BnOH left in benzaldehyde after a fractioinal distillation might offer decent pretection (depending on how good your fraction column is).

Benzaldehyde is usually auto-oxidized by a free radical chain process. The benzyl alcohol selectively removes one of the radicals normally present during autoxidation, thereby inhibiting the chain reaction and preventing any significant amount of benzoic acid from forming.

Huh. An excellent finding. This actually fits with all my observations so far. I've learned via experiment that stoichiometry is very important for getting good yields from benzyl alcohol and KMnO4; you can't just dump the two in a vessel together and hope for the best.

I'm wracking my brain trying to remember how/if I controlled pH, but I lost my notes from back then. It was long enough ago that my memory of the reaction isn't very good. I suspect that conditions would have been acidic to avoid the formation of benzoic acid, and I would have expected myself to assume that acid/salt formation would be promoted by basic conditions. And those potassium ions would have to react with something during the conversion of KMnO4 to MnO2.

It seems reckless to do the reaction solventless, but if you've gone and done it, then more power to you. I just assumed that the solubility difference between benzyl alcohol and benzaldehyde in water could be exploited as yet another means of suppressing benzoic acid formation, and so did it over time, stirred, in two phases. But if it turns out that that was overkill, perhaps it's high time to develop a new, better procedure here.




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Magpie
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[*] posted on 2-2-2018 at 08:49


Please see: https://www.sciencemadness.org/whisper/viewthread.php?tid=26...

This is my prep in Prepublication of benzaldehyde from benzyl alcohol using persulfate.




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[*] posted on 10-2-2018 at 08:13


Quote: Originally posted by Magpie  
Please see: https://www.sciencemadness.org/whisper/viewthread.php?tid=26...

This is my prep in Prepublication of benzaldehyde from benzyl alcohol using persulfate.


Sorry @Magpie, with all respect, but like I've also said at this thread in the past, persulfate oxidation don't give an acceptable yield and also produces benzoic acid.

I think it's better the oxidation of a benzylic alcohol with Calcium hypochlorite, much more OTC and prone to give better yields.

@Melgar at all claims for permanganate oxidations. I disagre and think besides the lower yield, more than acceptable quantities of benzoic acid will be produced, whatever they tell you about concentration of reagents, Ph, time or temperature to improve the reaction.

I will keep Calcium hypochlorite as my best choice cause the yields are above 90% all the times I've made benzaldehyde from benzyl alcohol and no benzoic acid was produced at all.

I'm attaching another study about using calcium hypochlorite to get benzylic alcohol oxidation using TBAB and methylene chloride as a solvent, to prove this reaction is secure even if you do that with moles of reagent, instead of milimoles. Just dilute the reagents with an aprotic solvent like dichlorometane and do the reaction next to 0ºC, to avoid any kind of runaway.

Attachment: Alcohols oxydation to aldehydes with Calcium hypochlorite and TBAB.pdf (712kB)
This file has been downloaded 84 times

[Edited on 10-2-2018 by Chemi Pharma]
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Magpie
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[*] posted on 10-2-2018 at 10:09


ChemiPharma,

I agree, your procedure gives a much better yield and uses more OTC materials. Well done. I will note this in my procedure in Prepublication.

Magpie




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