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JJay
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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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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.
| Quote: | | 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.
| Quote: | | 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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thermochromic
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Hello, i have lurked around on SM for a while now but finally decided to make an account and contribute
| Quote: | | 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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| 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.
The first step in the process of learning something is admitting that you don't know it already.
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Magpie
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Please see: https://www.sciencemadness.org/whisper/viewthread.php?tid=26...
This is my prep in Prepublication of benzaldehyde from benzyl alcohol using persulfate.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Chemi Pharma
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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 1677 times
[Edited on 10-2-2018 by Chemi Pharma]
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Magpie
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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
The single most important condition for a successful synthesis is good mixing - Nicodem
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RadicallyStabilized
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I wonder whether the method outlined by Chemi Pharma is really going to work.
First, if using TBAB, wouldn't the hypochlorite oxidise the bromide to bromine which would probably brominate the substrate or cause problems with the
aldehyde group? (also see https://chemistry.stackexchange.com/questions/70893/does-br2...) Maybe that's why the paper uses TBA hydrogen sulfate instead.
Second, the paper seems a bit sloppily done. In the first paragraph, "cyclohexane" instead of "cyclohexanone". Then, they first specify sodium
hydrogen sulfite to neutralize unreacted hypochlorite whereas further below they use "sodium hydrogen sulfate".
@Chemi Pharma: Did you really follow this procedure to make benzaldehyde with calcium hypochlorite? Which PTC did you use?
[Edited on 7-3-2020 by RadicallyStabilized]
[Edited on 7-3-2020 by RadicallyStabilized]
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Chemi Pharma
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| Quote: Originally posted by RadicallyStabilized | I wonder whether the method outlined by Chemi Pharma is really going to work.
First, if using TBAB, wouldn't the hypochlorite oxidise the bromide to bromine which would probably brominate the substrate or cause problems with the
aldehyde group? (also see https://chemistry.stackexchange.com/questions/70893/does-br2...) Maybe that's why the paper uses TBA hydrogen sulfate instead.
Second, the paper seems a bit sloppily done. In the first paragraph, "cyclohexane" instead of "cyclohexanone". Then, they first specify sodium
hydrogen sulfite to neutralize unreacted hypochlorite whereas further below they use "sodium hydrogen sulfate".
@Chemi Pharma: Did you really follow this procedure to make benzaldehyde with calcium hypochlorite? Which PTC did you use?
[Edited on 7-3-2020 by RadicallyStabilized]
[Edited on 7-3-2020 by RadicallyStabilized] |
I agree with you the text has some typos, but nothing so extraordinary. For sure where you read "cycloxane" the right term is "cycloxenanone. Where
you read "sodium hydrogen sulfate" to neutralize eventual chlorine produced the right term is "sodium hydrogen sulfite).
About the PTC, the Author recommends TBAHS use, but I did this reaction in my lab a few years ago to produce benzaldehyde from benzyl alcohol using
TBAB, that's cheaper. At those times benzaldehyde were strict controled to be sold and I had not opened my company yet, so purchasing some reagents
were more difficult than today.
PTC usually is any tetra alkyl ammonium salt. I don't know if the bromide were oxydated in the reaction to bromine, leaving the chloride but we have
to keep in mind that the quantity of TBAB used is minimum, just as a catalyser, and even in the occurrence of side reactions with it, the job in doing
the interaction between solid calcium hypochlorite and the alcohol/solvent liquid phase had been done, as I have seen in my case, giving me pure and
crystalline almost 50ml of benzaldehyde, after bissulfite adduct purification.
I don't clear remember the quantity of reagents I have used, but were based upon this paper, scaled and adjusting the weight of the catalyser
employed, TBAB instead TBAHS.
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RadicallyStabilized
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Chemi Pharma, thanks for the reply. Sounds reassuring.
I will give this a try and report back (though benzaldehyde is nowadays OTC and even cheaper than benzyl alcohol).
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RadicallyStabilized
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OK, I tried this and got a spectacularly shitty yield.
Lab notes start:
32.8 g (0.15 mol) of solid calcium hypochlorite (pool chlorinating agent, 65% available chlorine) were placed in a 300 ml Erlenmeyer flask. 10.8 g
(0.1 mol) of benzyl alcohol ("puriss. p. a." but giving off a very slight smell of benzaldehyde) were mixed with about 150 ml of DCM. This mixture was
added to the solids. 1.7 g of commercial TBAB were added, followed by a few ml of water. The water floated on top, forming a layer of about 3 mm
width; the solutions were clear and colorless with the solids sitting at the bottom.
When magnetic stirring was turned on the mixture instantly turned yellow. A reflux condenser was fitted and the reaction mix was refluxed for one
hour. Towards the end of the reaction chlorine gas could be smelled at the open end of the condenser.
The milky, slightly yellow suspension was cooled with a water bath and allowed to settle. The precipitate looked very fine and difficult to filter. A
separate aqueous layer could not be observed.
After half an hour it became obvious that waiting for the solids to settle would last too long. The mixture was filtered using a glass fritte and a
layer of celite. Along with the fine precipitate some granules of unreacted hypochlorite could be seen. The filtrate was a greenish-yellow liquid
smelling of DCM, chlorine and benzaldehyde. A spatula of sodium metabisulfite was added and shaken vigorously but nothing happened. As an experiment
some sodium thiosulfate was added as well with no result. A few ml of water were then added, followed my more thiosulfate. On shaking the mixture
turned brown. Adding more metabisulfite and heavy swirling eventually caused it to turn almost colorless.
The organic phase was isolated using a separatory funnel and dried using Na2SO4. The solvent was then rotovapped off, leaving behind about 10.5 g of a
deep yellow oil that smelled of benzaldehyde.
The bisulfite adduct procedure was then prepared according to the instructions of Eleusis:
Assuming 0.1 mole of aldehyde the bisulfite solution should contain 0.2 moles of sodium bisulfite corresponding to 0.1 mole of sodium metabisulfite
(19 g).
20 g of Na2S2O5 were dissolved in the minimum amount of water (about 30 ml). 25 ml of 96% EtOH were added. The turbidity was dissolved by adding more
water. The total volume of the solution was about 80 ml. The crude product was slowly added to the bisulfite solution which became turbid. It was
stirred for one hour. After filtration and washing with 2 x 10 ml of 99% EtOH the result was 8.35 g of a wet white mass that was left to dry in the
air.
The next day the weight was determined to be about 4.5 g. This stuff wasn't even fully dry and still smelled a little of benzyl alcohol. Assuming that
it's the pure adduct (189.2 g/mol) the total yield is 23.8 mmol or 23.8 % of the theoretical.
Lab notes end.
Recovery of the pure aldehyde will probably incur losses of about 10%, bringing the yield of benzaldehyde down to maybe 20% or so.
What did I do wrong?
- Used 1.7 g of TBAB without calculating the molar equivalent for TBAHS? Probably not that big of a deal.
- Used more than a few drops of water for the aqueous phase?
- Reaction temperature too low? The solution was boiling but not very strongly. Reflux seemed fine though.
- Didn't decant the solids but filtered them off? I doubt that the paper authors waited for the stuff to settle. They claim it can be done within
three hours which doesn't seem feasible to me. And how should this affect the yield. I washed the solids with DCM of course.
- Problems with the thiosulfate? This is a reducing agent used to quench bromine. And it wasn't very much anyway. I'm not sure how this could affect
the yield this much.
- Too much bisulfite to reduce chlorine? Maybe the adduct formed in the DCM and dissolved in the aqueous layer. Then again, adduct formation usually
takes quite a long time. Still I don't like to use metabisulfite for this (that's why I tried the thiosulfate).
- 10.5 g of crude... first I thought, ok, not so bad. But my guess is that much of this was still unreacted benzyl alcohol.
Any idea where I might have f..ed up? As of now, I'm not convinced 
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Chemi Pharma
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Nice write up!
Sorry for your failure.
My observations:
1 - In my case I have used 10% more TBAB than the amount predicted to TBAHS. Eg, Instead 1,7 grs I used 1,87 grs and have used just a few drops of
water, not a few milliliters;
2 - I refluxed the flask in an oil bath, cause benzyl alcohol BP is 205ºC, benzaldehyde BP is 178ºC and CH2Cl2 BP is only 40ºC. If you use just a
water bath the maximum temperature you can achieve is near 100ºC and this way only DCM will be refluxed;
3 - The time for reaction given in the paper (01 hour) is for 1-Phenyl Ethanol conversion to acetophenone. They don't brought a table in that study
with "substrate X time X ºC X moles reagents X yield". When I produced benzaldehyde I remember had refluxed the mixture for 2 hours, based in the
half time the reaction takes place at room temperature I have seen at another paper about alcohol oxidation to aldehyde with solid mix made with
calcium hypochlorite and Alumina that I'm attaching below;
4 - I have not used thiosulfate and have not followed the work up the paper say strictly. What I did was (in that order) wait the reaction to cool
--> filter the solids with a vacuum pump --> wash the solids with a little more DCM --> distill the DCM --> take the left oily liquid in
the flask and started to proceed the bisulfite aduct purification process. I thought that the step of neutralization of chlorine with bisulfite is
beyond unnecessary, harmfull, cause some amount of aduct could be formed and wasted among the filtration residues. Also have judged unnecessary the
treatment with anhydrous sodium sulfate to dry any water remained before the DCM distillation cause I did immediately the bisulfite aduct purification
process (I used the Eleusys method too) and it uses water.
5 - My Yield was around 60% if I remember well, but just like I've said, I don't remember the right amounts of reagents I've used in that opportunity.
I'm pretty sure I achieved around 50 ml of benzaldehyde at the end.
Attachment: alcohol oxidation by CaOCL to aldehydes.pdf (161kB)
This file has been downloaded 559 times
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RadicallyStabilized
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Thanks for the clarification! I agree with your ideas about the workup. I only dried the organic phase because I wanted to weigh the crude without
water. The chlorine in the DCM bothered me though.
I have never done a PTC reaction before and only got my TBAB recently, so this can be regarded as a noob question... but what's the point of a PTC if
you don't actually have an aqueous phase? The water that I have added has been totally absorbed (maybe there are some impurities like CaCO3 or so in
the pool chlorinator that can bind water). So where's the hypochlorite dissolving in?
Clayden (2nd ed., p. 195) states that hypochlorites "may oxidize primary alcohols all the way to carboxylic acids, especially in water." So perhaps
this was the cause of my poor yield.
I'm not sure that heating more would actually increase the temperature of the mix by much. Maybe it would be better to use chloroform (bp = 61 °C),
that could speed up the reaction time by about 4. But then oxidation of the benzaldehyde by air might become a problem.
I don't know how to proceed. This reaction sadly doesn't seem to be fool-proof ;-) Maybe I'll try again when I have ideas for improvement.
[Edited on 11-3-2020 by RadicallyStabilized]
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Chemi Pharma
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Better than explain PTC action for you is to read about that at Wiki:
https://en.wikipedia.org/wiki/Phase-transfer_catalyst
I suggest you to try this reaction again at room temperature, with stirring, for 04 (four) hours, instead 01 (one) hour at reflux, using the
information brought by the last paper I have attached.
May be mixing the two methods the yield could be better. May be using Calcium hypochlorite with Al2O3 in the solid phase, the alcohol in DCM as liquid
phase and TBAB as PTC you could achieve the 99% yield the last paper claims.
[Edited on 11-3-2020 by Chemi Pharma]
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ChemichaelRXN
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There is also Cerium Oxide nanoparticles that can be used; https://www.nature.com/articles/srep46372
...CeO2 NPs is highly active towards the selective oxidation of benzyl alcohol to benzaldehyde at low temperature.
(if you are willing to make the nanoparticles, which is a bit timely.)
What is the most successful method so far?
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RadicallyStabilized
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@Chemi Pharma: Thanks. Unfortunately I don't have any Al2O3 right now. Still I don't get how TBAB is supposed to work with solid phases like in the
case of calcium hypochlorite.
@Mr. StinkyGas: I don't have any experience making nano particles...
I found another paper (see attachment) and thought to give it one last try, even though that paper has its fair share of errors as well. They claim a
yield of over 90%. I didn't have 5% bleach though (I used 2.8%), so maybe that was the cause for my failure. My experience with other reactions
(brominations) is that the absolute bleach concentration is not critical, but that may not be the case here.
Lab notes start:
11 g (0.1 mol) of benzyl alcohol were mixed with 50 ml of ethyl acetate in a 300 ml beaker. 3.2 g (0.01 mol) of TBAB were added and dissolved with
magnetic stirring. 100 ml of fresh supermarket bleach (2.8% sodium hypochlorite) were added. The mixture turned greenish-yellow. It was covered with a
watch glass and vigorously stirred for one hour at room temperature (18.5 °C).
The upper organic phase was separated and the aqueous phase extracted with 2 x 50 ml of EtOAc. 11.45 g of NaHCO3 were dissolved in about 140 ml of
water (pH = 8.3). The organic phase was once washed with 30 ml of this solution (which was probably unnecessary as the aqueous phase had a pH of 10.5,
so any acid formed would have been in this solution anyway). The organic phase was then washed with brine and the EtOAc rotovapped off. The crude
product, a yellow liquid, smelled like benzaldehyde and weighed 14.77 g.
The bisulfite adduct procedure was then done according to the instructions of Eleusis:
20 g (0.105 mol) of Na2S2O5 were dissolved in the minimum amount of water (about 30 ml). 25 ml of 96% EtOH were added. The turbidity was dissolved by
adding a little more water. The total volume of the solution was about 80 ml. The crude benzaldehyde (without further drying) was slowly added to the
bisulfite solution which became turbid. This time it was stirred for longer time. After two hours there was still not very much precipitate. A little
(400 mg) of TBAB was added as recommended by https://www.sciencemadness.org/whisper/viewthread.php?tid=61... but there was no appreciable effect after a few minutes. The formed precipitate
was filtered off and stirring was continued for six hours in total. No additional precipitate had formed. After standing for some time a separate
layer formed on top, presumably benzyl alcohol/EtOAc (see image).
The filtered-off substance was washed with EtOAc and air-dried. The yield was 2.02 g of adduct (211.19 g/mol) = 9.6 mmol (9.6 % of the theoretical).
Lab notes end.
It seems that the hypochlorite oxidation doesn't work for me (as advertised, at least). Then again, I tried the persulfate method some time ago
without getting too great of a yield either so perhaps it's just my bad luck with this reaction. Anyhow, I have enough adduct to make my experiments
now. Should I find a way that works well (for me) I'll post it here.
Attachment: Selective Synthesis of Benzaldehydes.pdf (372kB)
This file has been downloaded 542 times
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clearly_not_atara
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In order to achieve selectivity in this oxidation you need to reduce the proportion of benzaldehyde that exists in the aqueous phase vs. the organic
phase. Using a smaller volume of water naturally limits this. Also, EtOAc mixes with water much more than benzene, so it may allow more phase mixing
than you actually want in this reaction.
I would suggest using a more hydrophobic solvent for the organic phase, like toluene or DCM, and a smaller amount of water.
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RadicallyStabilized
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OK, thank you, clearly_not_atara.
Please see my post from 2020-3-10 where I used DCM and very little water and still got only about 24%. Do you have an idea what might have gone wrong?
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clearly_not_atara
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Frankly I'm not sure; where I've seen this reaction before it's used for sensitive substrates and extensive precautions are taken.
But I do notice two things:
- TBAHS is an acid, TBAB is not. Small amounts of protons significantly increase the reactivity of hypochlorites. As such the rxn may have been too
slow and the conversion incomplete.
- You noted in your post on 10 March that you used more water than the original procedure recommended. Have you forgotten this?
[Edited on 18-3-2020 by clearly_not_atara]
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RadicallyStabilized
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No, I haven't forgotten, but the amount of water in my first try was much less than what was used in the sodium hypochlorite oxidation (by a factor of
20 or more). And after the reaction the water layer was gone anyway. I don't know how the calcium hypochlorite is supposed to dissolve under these
conditions. There needs to be a water phase for the TBA* to work, doesn't it?
The bleach I used has a pH of about 10.5. I'm not sure whether a relatively small amount of TBAHS would be able to significantly change this.
Do you think it would make sense to buffer the solution to be slightly acidic (ph = 6 or so)?
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dawt
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Below you'll find my notes on the oxidation of benzyl alcohol to benzaldehyde with 10 % H2O2 and a quaternary ammonium octamolybdate catalyst, as per
Green Chem. 9 (5), 421–423. DOI: 10.1039/B700534B (see attachment). I opted to go with DDAC instead of making the butylpyridine bromide for
the catalyst used in the paper, since it's cheap and readily available to the amateur.
Preparation of the catalyst (didecyldimethylammonium octamolybdate)
Bardac 22 (50 % DDAC, 14.50 g, 20 mmol, 1 eq.) and 75 mL water were place in a 250 mL Erlenmeyer flask and heated to 70–72 °C under stirring. A
solution of sodium molybdate dihydrate (9.70 g, 40 mmol, 2 eq.) in 40 mL water, to which aq. HCl (37 %, 5.90 g, 60 mmol, 3 eq.) had been added, was
poured into an addition funnel and dripped into the stirred DDAC solution at approx. 1 drop per 1–3 s, whereupon a colorless precipitate immediately
formed. When the addition was complete the mixture was stirred for a further 20 min before cooling to room temperature. The resulting colorless solid
was vacuum filtered and washed with a small amount of water. Filtration was difficult due to the product turning into a thick paste on the filter as
water was removed.
Don't have a yield yet as it's still drying in the desiccator, but the wet catalyst can also be used, as below. In a previous attempt I was a bit
sloppy, temperature was in the 60s °C, the molybdate solution was added via pipette at a much higher rate and I ended up with an extremely fine
precipitate that was nearly impossible to filter. In that attempt I also used a great excess of DDAC as in the ref, which likely contributed to the
difficulties and just seemed unnecessary. Also note that the molybdate solution will not mix with the DDAC unless the latter is sufficiently diluted.
Fun fact: At 2490 g/mol the catalyst (DDAC)4Mo8O26 is the heaviest substance I've ever made!
Oxidation of benzyl alcohol to benzaldehyde
To wet didecyldimethylammonium octamolybdate (0.70 g, 0.3 mmol, 0.3 mol%) in a 100 mL round-bottom flask fitted with a condenser were added benzyl
alcohol (10.80 g, 100 mmol, 1 eq.) and hydrogen peroxide (10 %, 35.40 g, 104 mmol, 1.04 eq.), and the mixture heated to reflux under stirring. The
catalyst turned yellow, dissolved into the organic phase and darkened over the course of the reaction, while a change to green indicated the hydrogen
peroxide had been consumed after 45 min. After cooling the apparatus was set up for distillation, and the product collected alongside water over a
range of 96–98 °C, while the residue in the pot took on a deep blue color and was kept for a second run. The water was removed from the distillate
via pipette, the crude product dried over MgSO4, gravity filtered through a pipette plugged with cotton and stored in the refrigerator under argon.
Yield of crude benzaldehyde: 6.80 g (64 % of theoretical 10.61 g).
A significant amount of benzaldehyde likely remained in the sump and some was also lost during the removal of the cloudy water layer from the product
in the final step. I'm currently performing a second run with the recycled catalyst and will combine the crude products before purification.
Attachment: Ming-Lin, Hui-Zhen 2007 - Selective oxidation of benzyl alcohol.pdf (280kB)
This file has been downloaded 487 times
  
Edit: So I performed the reaction again in the same RBF including the residue from the distillation and without adding fresh catalyst. However, I
increased the amount of benzyl alcohol by 33 % (14.35 g, 133 mmol, 1 eq.) and used 1.2 eq. of H2O2 (10 %, 54.30 g, 160 mmol), leading to a reaction
time of 4 h. After distillation the water was removed in a separatory funnel. Everything else was performed exactly as above, yielding 10.50 g (74 %
of theoretical 14.11 g) of crude benzaldehyde.
A significant amount of dark green/blue oil remained in the sump after distillation, which solidified fairly quickly upon cooling. Guess this will
mostly be benzoic acid.
I didn't bother with extracting the aqeous layer in the sep funnel, but I'll likely do that when I scale the reaction up.
So far I like this reaction. Aside from the messy filtration of the catalyst I can't think of a way to screw this up. All the reagents are easily
accessible and cheap as well. I'll see if I can clean up the catalyst for future batches, but I might just save it as is.
[Edited on 2021-2-7 by dawt]
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arkoma
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lovely...................working on nitroethane ATM wink wink
"We believe the knowledge and cultural heritage of mankind should be accessible to all people around the world, regardless of their wealth, social
status, nationality, citizenship, etc" z-lib
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dawt
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Ha! I'm headed in a different direction though. Wanna make some benzhydrol via benzoin (as per http://orgsyn.org/demo.aspx?prep=CV1P0089), but I need to scale up this reaction first.
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arkoma
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does TBAB work? (have some is why i ask......)
"We believe the knowledge and cultural heritage of mankind should be accessible to all people around the world, regardless of their wealth, social
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dawt
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Well the reason I did the reaction in the first place was to see what role the cation played and if using an alkylpyridinium was necessary. I don't
see a reason why it wouldn't work with TBAB. Seems to me like the quat. amine's role is simply to make the octamolybdate dissolve in the benzyl
alcohol, so my hypothesis is that any quat. amine PTC should work. I'd be interested to see whether the TBA octamolybdate is easier to handle and
won't form a sticky sludge on the filter.
[Edited on 2021-2-7 by dawt]
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