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Author: Subject: benzyl alcohol oxidation
kmno4
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[*] posted on 27-4-2008 at 07:27


"The acid is the solvent for the alcohol and is in great excess. Everything should be stirred well for several hours (that is probably too long). The dense lower layer is collected and separated with a funnel to remove any of the acid solution. It is then dried with a suitable drying agent and is ready to use for many purposes."

At this conditions - dense lower layer is HCl(aq). I do not know what are "many purposes" but making benzyl cyanide from this crude product was very unsuccessful. Besides, I always wanted to do some quantitative measurements of reaction: benzyl alcohol+ HCl(aq)... And now I am going to do that (If I have time :D ).
Sorry for starting offtopic.

By Nicodem:

Assuming x = 0.5 as a realistic value we get:
k1/k2 ≈ e^(6.05) ≈ 423

(...)Yet, taking those approximations, the calculation indicates the oxidation of BnOH might even be a few hundred times faster than the oxidation of PhCHO (assuming the same order of reaction).


I agree with it. But look at Table 1 in ACS paper:
BDE(C-H) for ØCH2OH is 79 (or ~84) kcal/mol
BDE(C-H) for ØCHO is 86 kcal/mol

and
"Normalized Second-Order Rate Constant of H-Abstraction from H-Donor Substrates by BTNO" in MeCN:
k<sub>H</sub> for ØCH2OH is 0,94 (1/Ms)
k<sub>H</sub> for ØCHO is 0,8 (1/Ms)

so k<sub>1</sub>/k<sub>2</sub> &#8776 1
Besides there is large acceleration of rate in protic, H-bonding solvent (AcOH) and order is this time reverted: hydrogen is abstracted from aldehyde faster than from alcohole.



[Edited on 27-4-2008 by kmno4]
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[*] posted on 28-4-2008 at 07:48


Indeed I agree. Assuming the Evans-Polanyi relation for the two reactions to be of a similar value (calculated as equal) is the weakest assumption in my calculations. This factor depends highly on the one electron oxidant used and other conditions and I have no idea how the persulfate influences it. Therefore x=y was the only assumption I could take without using too much imagination.
With some transition metal one electron oxidants even the oxidation of toluene to benzaldehyde most commonly completely stops at PhCHO stage (talking about the numerous and well studied Co/Cu/Mn/etc catalyzed oxidations of toluene with oxygen). Often no PhCOOH can be detected. Considering that the BDE of Bn-H is 89 kcal/mol, thus even more than for PhCO-H, I can't think of anything else but that the E<sub>a</sub> for radical oxidations of PhCHO is commonly quite higher (but obviously not always as demonstrated by the kinetic measurement you cite).
Anyway, those calculations were plain exercise since they can not give a reliable number given the bold assumptions used. It was more about showing the difference in the BDE values of the two substrates and showing how small absolute differences can give rise to huge differences in the kinetics.
Anyway, if anybody has another explanation for the reaction stopping at the PhCHO stage, I would be glad to hear it. Radical oxidations are not something I'm familiar with.
Quote:
Originally posted by JohnWW
Please download and post here those two articles, Nicodem.

Done.

Attachment: BDE_values_for_BnOH_and_BzH_references.rar (1.2MB)
This file has been downloaded 281 times





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[*] posted on 28-4-2008 at 23:16


I found a good source of otc benzyl alcohol the other day I would like to share.
concrete floor stripper. usualy around 20 + %

hehe (I am now a hazard to others look out world) :D

[Edited on 29-4-2008 by Ephoton]




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[*] posted on 29-4-2008 at 09:12


Take your BzOH, place in diethyl ether with an equivalent of sodium hydroxide and molecular sieves to form sodium benzylate.

React with BnCl ala Williamson ether synthesis to dibenzyl ether (DBE).

Oxidize the DBE with dilute nitric acid catalized with sodium nitrite.

http://www.sciencemadness.org/talk/viewthread.php?action=att...

Reported yields as high as 80% based on DBE with the reaction sucessfully scaled up to 1 mole with similar results.

[Edited on 29-4-2008 by evil_lurker]




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[*] posted on 2-5-2008 at 14:24


Offtopically about reaction of benzyl alcohol(=BA) and HCl(aq)(=AC).
On the picture: 5 test-tubes with with mixtures BA and AC, shaken
many times to reach equilibrium or close to it (at least I think so).
Test tube 1: 2g BA/2g AC [ 75 minutes]
Test tube 2: 2g BA/3g AC [ 50 minutes]
Test tube 3: 2g BA/4g AC [ 30 minutes]
Test tube 4: 2g BA/5g AC [ less than 30 minutes]
Test tube 5: 2g BA/6g AC [ less than 30 minutes]
I also made 2g BA/1g AC but this mixture did not split into separate layers ( I waited 8 hours). For the rest of mixtures, time of separating given in [ ].
To estimate degree of conversion BA into benzyl chloride I titrated (twice) remaining water-acid layer.
Results:
Test tube 1: 2g BA/2g AC -------------
Test tube 2: 2g BA/3g AC 44%
Test tube 3: 2g BA/4g AC 52%
Test tube 4: 2g BA/5g AC 67%
Test tube 5: 2g BA/6g AC 73%
Layer from test tube 1 was not investigated, because water+acid was not separated completly from organic layer and it would cause too big errors.
(from 2g BA/1g AC mixture it did not separete at all)
In no case density of organic layer is larger than water layer, as can be seen. To convert alcohol into chloride quantitatively, large excess of acid is needed. Use of CaCl2, ZnCl2 etc..., would be more convenient.

All values given by me are approximations. It would be good if someone could repeat these measurements.
End of offtopic :D
I have found interesting article about oxidation of benzyl alcohol with persulfate.
Aqueous Media Oxidation of Alcohols with Ammonium Persulfate
Chinese Journal of Chemistry, 2007, 25, 836—838
If someone is interested, link to article: http://mihd.net/egpdyq2

[Edited on 3-5-2008 by kmno4]

alc+bezn.JPG - 42kB
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[*] posted on 2-5-2008 at 16:14


Thanks, kmno4, for posting that journal article. I will be trying their method for making n-butyraldehyde soon.
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[*] posted on 2-5-2008 at 16:30


Well I think I have me a project for this summer.

1. Partially convert BnOH to BzCl as close to 1:1 ratio within reason. Start by adding the appropriate amount of 37% HCl to the alchol, heat and agitate to 80íC till it reaches equilibrium.

2. Drain water/acd, add in small quantity of NaOH (enough to remove HCl from previous reaction) then add in concentrated NaOH solution with either a tiny bit of triethylamine or tri-n-butylamine to form the ether.

3 Drain NaOH solution, rinse with a little nitric acid to remove alkali, drain again, then add in dilute HN03/NaNO2 catalyst solution and stirr for several hours.

If one used a reaction flask with bottom drain it could damn near be "one pot" with no solvents, use fairly cheap and easy to get reagents, and be completed in 5-6 hours with no need for isolation of intermediate products.




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[*] posted on 3-5-2008 at 02:11


Kmno4, that was some nice looking and strict scientific experimentation of which I wish we had posted more and more often.

Thanks also for the paper. It is interesting to see that no overoxidation products are detected. But what is most confusing to me is that butanol reacts efficiently already at 30°C, while for the benzylic, allylic and secondary alcohols they used 75-85°C. Looks like a reversed reactivity order of what is expected for a radical oxidation (and the authors don't even bother giving any hypothesis to account for the differences).

Magpie, you better don't use this method as is on a larger scale since it was developed on a 1 mmol scale. Scaling it up by multiplication could get you in troubles of the type you encountered in your latest butanol oxidation trial. In my experience similar mmol reactions (of which Tetrahedron letter and Synthesis abounds) scaled up usually end up in runaways.

Evil_lurker, you might want to first try out that Bn2O oxidation with HNO3/NaNO2 on BnOH itself before wasting time with its etherification.

[Edited on 3/5/2008 by Nicodem]




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[*] posted on 3-5-2008 at 02:23


Quote:
Originally posted by Nicodem
Evil_lurker, you might want to first try out that Bn2O oxidation with HNO3/NaNO2 on BnOH itself before wasting time with its etherification.


I had considered that... did I miss something in the paper amongst all the tecnical info?




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[*] posted on 3-5-2008 at 03:07


You forgot that in 15% HNO3 the media is acidic enough to cleave Bn2O to BnOH. You also forgot the context of the article – the authors were looking for an economical use of Bn2O which is a worthless side product in the industrial preparation of BnOH. Cited from the paper:

Quote:
During the oxidation of DBE, BnOH is formed and oxidised to BzH.[ref 7] Benzoic acid is formed by oxidation of benzaldehyde. Therefore, with the progress of the reaction the yield of BnOH decreases and that of BzOH increases. Presence of BnOH in the final product may not be a serious problem as it can be recovered from the reaction mixture by fractional distillation and is a value-added product.


In short, the treatment of BnOH with refluxing HNO3 gives BzOH just like when starting with toluene (BnH), but at milder conditions (90°C) you can actually get viable yields of BzH with BzOH being the major side product. The presence of nitrite works in your favor, assumingly as a source of NO2 to start the radical chain reaction.
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[*] posted on 3-5-2008 at 03:43


KMnO4, the mixtures of benzyl alcohol and conc HCl have to be heated to effect the reaction! Try it again, and this time heat the test tubes in a boiling water bath.

I have very successfully made benzyl chloride many times from conc aqueous HCl and benzyl alcohol. At a certain temperature, the reaction spntaneously takes place and is over in a few minutes, with complete separation of phases. HCl is used in at least threefold excess. No other reagents are added.
Look at Rhodiums methods for benzyl chloride from -alcohol.




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[*] posted on 3-5-2008 at 09:43


Hi,

Really interesting stuff here. But I am not old enough to buy the pure alcohol from any distributors, can anyone help with any info on OTC source of BzOH in the US?
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[*] posted on 3-5-2008 at 09:49


There's probably no useful OTC source of BnOH (it's Bn not Bz) in the US. You can get it from pharmacies but when I did that it cost me like $18 for 100mL (I think a couple of other places quoted $40 a quart). Definitely not a practical source unless you work with VERY small amounts like Woelen.



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[*] posted on 3-5-2008 at 09:59


www.lemelange.com $45 gallon USP grade (repacked under non USP conditions so it can't be labelled as such due to FDA regs).

[Edited on 3-5-2008 by evil_lurker]

[Edited on 3-5-2008 by evil_lurker]




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[*] posted on 3-5-2008 at 13:39


Hey! They used to be $35 a gallon! Or are you including shipping?



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[*] posted on 3-5-2008 at 14:21


They went up on it I reckon. Need to order a gallon or so.



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[*] posted on 3-5-2008 at 20:10


pitty you don't live in australia i could of given you a gallon i use it for work as a addative solvent.
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[*] posted on 3-5-2008 at 20:52


Just mail it. Even with shipping it might be cheaper and it's certainly not HAZMAT.



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[*] posted on 4-5-2008 at 09:25


Quote:
Originally posted by garage chemist
KMnO4, the mixtures of benzyl alcohol and conc HCl have to be heated to effect the reaction! Try it again, and this time heat the test tubes in a boiling water bath.
(...)

I do not say no. My experiments were conducted at room temperature, concentration of HCl(aq) is 35.4%, only to prove
that Fleaker's post says untruth.
Besides, heating of concentrated HCl somehow scares me....
I do not like gaseous HCl :P
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[*] posted on 7-5-2008 at 02:07


Quote:
Originally posted by Nicodem
In short, the treatment of BnOH with refluxing HNO3 gives BzOH just like when starting with toluene (BnH), but at milder conditions (90°C) you can actually get viable yields of BzH with BzOH being the major side product. The presence of nitrite works in your favor, assumingly as a source of NO2 to start the radical chain reaction.

Original paper (if already posted, delete this post, please):
Kinetics of Oxidation of Benzyl Alcohol with Dilute Nitric Acid
Ind. Eng. Chem. Res. 2005, 44, 325-333

Attachment: ie0303911.pdf (176kB)
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[*] posted on 7-5-2008 at 10:37


Dude, the information that paper contains is sweeet.

If it is indeed true, we may now finally have a viable method for the home manufacture of BnH from BnOH.




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[*] posted on 7-5-2008 at 12:29


Why would you want to make toluene out of BnOH? What a waste!



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[*] posted on 7-5-2008 at 12:41


Quote:
Originally posted by MagicJigPipe
Why would you want to make toluene out of BnOH? What a waste!


Ummm did you miss something here?

BnH is benzaldehyde, BnOH is benzyl alcohol.




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[*] posted on 7-5-2008 at 13:02


I just knew this would happen as it always happens every time abbreviations and acronyms are used on this forum (like the TCCA vs. TCT confusion coming up all the time).

BnH is toluene since Bn stands for benzyl. Benzaldehyde would be BzH since Bz stands for benzoyl. Therefore it would be preferable to simply use PhCHO for benzaldehyde to avoid further confusion among those who are unfamiliar with such organic shorthand conventions as Bz which are rarely used anyway.
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[*] posted on 7-5-2008 at 13:12


Yes, evil lurker. Abbv. like that are based on a certain structure/moeity and not just an abbv. of a certain word.

Bn = Ph-CH2 Therefore, like Nicodem pointed out, a benzyl group with a hydrogen is toluene. Another example: Ph is C6H5 so benzene would be PhH (I've also seen benzene represented as "Ph" but that is technically incorrect) and phenol is PhOH. I can understand BzH. If that was used I would have realized he was speaking of benzaldehyde.

I just find BzH easier than typing PhCHO.


[Edited on 5-7-2008 by MagicJigPipe]




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