Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
 Pages:  1  ..  3    5
Author: Subject: benzyl alcohol oxidation
JJay
International Hazard
*****




Posts: 3336
Registered: 15-10-2015
Member Is Offline


[*] 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.




View user's profile View All Posts By User
Melgar
Anti-Spam Agent
*******




Posts: 2004
Registered: 23-2-2010
Location: Connecticut
Member Is Offline

Mood: Estrified

[*] 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.

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.




The first step in the process of learning something is admitting that you don't know it already.

I'm givin' the spam shields max power at full warp, but they just dinna have the power! We're gonna have to evacuate to new forum software!
View user's profile View All Posts By User
thermochromic
Harmless
*




Posts: 2
Registered: 17-1-2018
Member Is Offline

Mood: No Mood

[*] 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 :)


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.
View user's profile View All Posts By User
Melgar
Anti-Spam Agent
*******




Posts: 2004
Registered: 23-2-2010
Location: Connecticut
Member Is Offline

Mood: Estrified

[*] 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.




The first step in the process of learning something is admitting that you don't know it already.

I'm givin' the spam shields max power at full warp, but they just dinna have the power! We're gonna have to evacuate to new forum software!
View user's profile View All Posts By User
Magpie
lab constructor
*****




Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline

Mood: Chemistry: the subtle science.

[*] 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.




The single most important condition for a successful synthesis is good mixing - Nicodem
View user's profile View All Posts By User
Chemi Pharma
National Hazard
****




Posts: 328
Registered: 5-5-2016
Location: Latin America
Member Is Offline

Mood: Quarantined

[*] 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 480 times

[Edited on 10-2-2018 by Chemi Pharma]
View user's profile View All Posts By User
Magpie
lab constructor
*****




Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline

Mood: Chemistry: the subtle science.

[*] 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




The single most important condition for a successful synthesis is good mixing - Nicodem
View user's profile View All Posts By User
RadicallyStabilized
Harmless
*




Posts: 22
Registered: 3-10-2018
Member Is Offline


[*] posted on 7-3-2020 at 01:20


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]




Everyday consciousness classifies and subordinates, coerces under patterns of easy manipulation and disregards the essential.
View user's profile View All Posts By User
Chemi Pharma
National Hazard
****




Posts: 328
Registered: 5-5-2016
Location: Latin America
Member Is Offline

Mood: Quarantined

[*] posted on 7-3-2020 at 12:20


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.
View user's profile View All Posts By User
RadicallyStabilized
Harmless
*




Posts: 22
Registered: 3-10-2018
Member Is Offline


[*] posted on 8-3-2020 at 05:54


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).




Everyday consciousness classifies and subordinates, coerces under patterns of easy manipulation and disregards the essential.
View user's profile View All Posts By User
RadicallyStabilized
Harmless
*




Posts: 22
Registered: 3-10-2018
Member Is Offline


[*] posted on 10-3-2020 at 10:58


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 :cool:


ba_adduct.jpg - 39kB




Everyday consciousness classifies and subordinates, coerces under patterns of easy manipulation and disregards the essential.
View user's profile View All Posts By User
Chemi Pharma
National Hazard
****




Posts: 328
Registered: 5-5-2016
Location: Latin America
Member Is Offline

Mood: Quarantined

[*] posted on 10-3-2020 at 13:13


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 59 times
View user's profile View All Posts By User
RadicallyStabilized
Harmless
*




Posts: 22
Registered: 3-10-2018
Member Is Offline


[*] posted on 11-3-2020 at 03:29


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]




Everyday consciousness classifies and subordinates, coerces under patterns of easy manipulation and disregards the essential.
View user's profile View All Posts By User
Chemi Pharma
National Hazard
****




Posts: 328
Registered: 5-5-2016
Location: Latin America
Member Is Offline

Mood: Quarantined

[*] posted on 11-3-2020 at 11:14


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]
View user's profile View All Posts By User
Mr.StinkyGas
Harmless
*




Posts: 21
Registered: 7-10-2010
Member Is Offline

Mood: Content

[*] posted on 11-3-2020 at 11:25


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?
View user's profile View All Posts By User
RadicallyStabilized
Harmless
*




Posts: 22
Registered: 3-10-2018
Member Is Offline


[*] posted on 18-3-2020 at 07:38


@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 44 times

Benzyl_alcohol_layer.png - 153kB




Everyday consciousness classifies and subordinates, coerces under patterns of easy manipulation and disregards the essential.
View user's profile View All Posts By User
clearly_not_atara
International Hazard
*****




Posts: 1810
Registered: 3-11-2013
Member Is Offline

Mood: Big

[*] posted on 18-3-2020 at 09:07


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.




[Edited on 04-20-1969 by clearly_not_atara]
View user's profile View All Posts By User
RadicallyStabilized
Harmless
*




Posts: 22
Registered: 3-10-2018
Member Is Offline


[*] posted on 18-3-2020 at 09:31


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?




Everyday consciousness classifies and subordinates, coerces under patterns of easy manipulation and disregards the essential.
View user's profile View All Posts By User
clearly_not_atara
International Hazard
*****




Posts: 1810
Registered: 3-11-2013
Member Is Offline

Mood: Big

[*] posted on 18-3-2020 at 14:02


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]




[Edited on 04-20-1969 by clearly_not_atara]
View user's profile View All Posts By User
RadicallyStabilized
Harmless
*




Posts: 22
Registered: 3-10-2018
Member Is Offline


[*] posted on 19-3-2020 at 00:09


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)?




Everyday consciousness classifies and subordinates, coerces under patterns of easy manipulation and disregards the essential.
View user's profile View All Posts By User
 Pages:  1  ..  3    5

  Go To Top