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Magpie
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[*] posted on 1-3-2012 at 15:15
benzoin


I debated whether or not it would be of any interest posting the results of my recent benzoin synthesis. But here's a picture of the product. I used 4 year old homemade benzaldehyde and 38% purity KCN as catalyst. The beaker holds 6.3g and the yield was 60.0%. Once you get the precursor and catalyst the synthesis is quick and easy. This is often the case.

This synthesis is often referred to as the "benzoin condensation." This is a misnomer as it is actually a dimerization.



benzoin.JPG - 128kB




The single most important condition for a successful synthesis is good mixing - Nicodem
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[*] posted on 2-3-2012 at 00:50


I always appreciate another chemist's experience with a lab, so thanks for this...

...but could you detail this a little better? I mean, what setup did you use, what general conditions, just a classic write-up, you know? I don't mean to nag, but these are the things that make me want to read and congratulate all the more.

It may not be particularly useful to most people, but I often find that someone will eventually want to know how this is done, and your experience in detail helps...




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[*] posted on 2-3-2012 at 05:42


Interestingly the reaction is also catalysed by thiamin.
A procedure is detailed here;

http://courses.chem.psu.edu/chem35/Syn%20Sp06/35Exp12.pdf

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[*] posted on 2-3-2012 at 08:59


Interesting that it worked just as well with such an impure KCN.
Quote: Originally posted by Hohenheim  
...but could you detail this a little better? I mean, what setup did you use, what general conditions, just a classic write-up, you know?

You will hardly find anything more classic than what is already written about it in the year 1921:
http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv1...
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Magpie
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[*] posted on 2-3-2012 at 09:26


Quote: Originally posted by Hohenheim  

...but could you detail this a little better? I mean, what setup did you use, what general conditions, just a classic write-up, you know?


Yes, I will describe the experiment in a little more detail. But, as I said, it is very simple. For the home chemist the real challenge is just getting the precursors.

I followed the procedure in Brewster (1960). 35mL of ethanol ( I used anhydrous), 10.1 mL benzaldehyde (my old homemade stuff), and 1.5g of KCN (mine was contaminated with degradation products, I presume KOCN and K2CO3). Since I had titrated it using the Liebig method at 36.7%, I used 4.1g of the crude reagent.

I mixed all 3 in a 250mL RBF and set up for reflux at a low boil for 40 minutes. This goes quite smoothly except for an occaisional bump. Maybe I should have added a mag stirrer bar to prevent this. I would mix occaisionally by shaking the whole apparatus. The outlet of the condenser was led to an inverted funnel in a 600mL beaker containing ~150mL of 10% NaOH. The funnel mouth is placed a slight distance above the liquid level. As Brewster says there is no HCN generated but just to be cautious a trap is recommended. This way you don't even have to worry about it.

When 40 minutes had elapsed the heat is turned off and the The RBF is stoppered and place in an ice bath. The product has a strong orange color. I let this set overnight for my convenience.

The next day I scraped out the semi-solid product using a bent spatula onto a 7cm Buchner funnel. The product was then washed successively with 150mL water, 30mL denatured alcohol, and 20 mL of 25% ether in heptane (I didn't have any pure ether). The product was spread on a piece of paper where it air dried very quickly.

I treated the filtrate in the Buchner flask as very poisonous, as the cyanide is just a catalyst. It was carefully disposed, wearing full safety apparel.

This is the Brewster procedure. I took a quick glance at the one in OrgSyn and it seemed very similar.

I will get a melting point today and report it later. I also plan to make benzil from this, and then benzilic acid, per Brewster.

@ScienceSquirrel
Yes, I found out about the use of thiamine from entropy51 also (after my synthesis). That's a very interesting paper. Thanks for posting it.



[Edited on 2-3-2012 by Magpie]




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[*] posted on 2-3-2012 at 13:36


I recrystallized a small portion of my benzoin from boiling denatured alcohol. Nice needles of a very white crystal resulted. The mp was 133-134°C. My 49th ed CRC (1968-1969) gives the following mp's for benzoin: d =133-134°; l =133-134°; dl =137°. Interesting.

The obvious question: Is one of the stereoisomers favored over the other?

[Edited on 2-3-2012 by Magpie]




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[*] posted on 3-3-2012 at 02:16


Quote: Originally posted by Magpie  
The mp was 133-134°C. My 49th ed CRC (1968-1969) gives the following mp's for benzoin: d =133-134°; l =133-134°; dl =137°. Interesting.

The obvious question: Is one of the stereoisomers favored over the other?

No. Whenever you have a symmetric left side of the reaction the results on the right side are also symmetric (symmetric in the concept of reactions means a medium where all possible chiral configurations are represented by same amounts). To achieve asymmetric synthesis (aka enantioselective synthesis) there has to be some initial asymmetry present (an enantiomeric excess of some reactant, reagent, catalyst or solvent). This is why symmetric reactions can only give a diastereomeric excess, but no enantiomeric excess.
The reason why the racemate can have a different mp than each of the two enantiomers is in that some racemates form racemic compounds. These have different physical properties resulting from their different crystal structure (molecular connectivity is the same as for each enantiomer, but the crystal structure and conformerism is different). The phenomenon is similar to polymorphs where each can have different physical properties resulting from different crystal structures and molecular conformation even though the molecular configuration is the same in-between all polymorphs of the same compound (imagine that the mp can differ even for tens of K in some such polymorphs and the dissolution kinetics in the same solvent can differ for magnitudes!). Thus, there are even cases of dimorphism where a compound can be either a liquid or a crystalline solid at the same conditions.
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[*] posted on 4-3-2012 at 21:55


Today I took the 6g of benzoin I had left and oxidized it to benzil. This was done using con nitric acid (70%). This turns a white alpha-hydroxy ketone into a yellow diketone:

ChemSketch3.bmp - 221kB

This is another simple preparation. The benzoin and 20mL of acid were added to a 250mL flask and heated for 11 minutes on a steam bath. Per the recommendation of Brewster I set up an NO2 trap using an inverted funnel over a 10% NaOH solution. This worked quite well. Its effectiveness was easily monitored due to the strong brown color of the NO2. I could smell just a trace of NO2. During cleanup, however the hood fan was handy for removing the NO2 trapped in the apparatus.

The tubing shown is Tygon. I know this is not the best material for this application but it was handy and cheap enough to be disposable. Glass would have been ideal.

The benzil was isolated on a 7cm Buchner funnel, washed with a little water, then a little denatured alcohol, then air dried. The yield was 4.7g, and the %yield was 75.2%.

oxidation of benzoin.JPG - 105kB benzil.JPG - 94kB

[Edited on 5-3-2012 by Magpie]

[Edited on 5-3-2012 by Magpie]



[Edited on 6-3-2012 by Magpie]




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[*] posted on 5-3-2012 at 10:51


If anyone is interested, I have tried the benzoin condensation with thiamine as catalyst, following one of the many procedures found on the internet. It worked very well.

Later I oxidized a portion of it to benzil with copper sulfate/water/NaOH and in situo rearrangement to benzilic acid. (With less NaOH benzil can be isolated instead). An easy procedure and all chemicals are cheap and easy to get. And good yields.

See http://pubs.acs.org/doi/abs/10.1021/ja01268a017 for the article on it. There is also details on a lot of reductions of benzil in it, you can for example reduce it to desoxybenzoin with Zn/NH4OH.
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[*] posted on 5-3-2012 at 11:02


<strong>Magpie</strong>, a full ScienceMadness.org member publication of the process from <a href="http://en.wikipedia.org/wiki/Benzaldehyde" target="_blank">benzaldehyde</a> <img src="../scipics/_wiki.png" /> through <a href="http://en.wikipedia.org/wiki/Benzoin" target="_blank">benzoin</a> <img src="../scipics/_wiki.png" /> to <a href="http://en.wikipedia.org/wiki/Benzil" target="_blank">benzil</a> <img src="../scipics/_wiki.png" /> would be a nice addition to the existing collection. Organic chemistry hasn't been sufficiently represented.

I do agree that the process is simple, and there's plenty of literature, but we tend to put our own 'amateur' mark on things.

[Edited on 7/9/13 by bfesser]




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[*] posted on 5-3-2012 at 12:23


I agree. We had these same benzaldehyde->benzoin->benzil reactions as lab practice in the undergraduate studies, so that it brings back some nice memories. That already is reason enough for a contribution to the Prepublication section. Magpie and/or Bronstein, perhaps when you have the time to do so you could compile some nice report.
Quote: Originally posted by Bronstein  
See http://pubs.acs.org/doi/abs/10.1021/ja01268a017 for the article on it. There is also details on a lot of reductions of benzil in it, you can for example reduce it to desoxybenzoin with Zn/NH4OH.

The Clemmensen reduction of benzil gives trans-stilbene.
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[*] posted on 5-3-2012 at 14:51


<strong>Microscale Organic Experiments</strong> by Kenneth L. Williamson (<a href="http://www.bookfinder.com/dir/i/Microscale_Organic_Experiments/0669149225/" target="_blank">ISBN 0-669-14922-5</a> <img src="../scipics/_ext.png" />;) has a series of reactions in a section titled "<Strong>Derivatives of 1,2-Diphenylethane&mdash;A Multistep Synthesis</strong>." In this section, the experiments are as follows:

Quote:
<em><ol start="54"><li>The Benzoin Condensation: Cyanide Ion and Thiamine Catalyzed&nbsp;&nbsp;&nbsp;&nbsp;414</li><li>Nitric Acid Oxidation. Preparation of Benzil from Benzoin. Synthesis of a Heterocycle: Diphenylquinoxaline&nbsp;&nbsp;&nbsp;418</li><li>Borohydride Reduction of a Ketone: Hydrobenzoin from Benzil&nbsp;&nbsp;&nbsp;&nbsp;422</li><li>1,4-Addition: Reductive Acetylation of Benzil&nbsp;&nbsp;&nbsp;&nbsp;426</li><li>Synthesis of an Alkyne from an Alkene. Bromination and Dehydrobromination: Stilbene and Diphenylacetylene&nbsp;&nbsp;&nbsp;&nbsp;429</li><li>The Perkin Reaction: Synthesis of &alpha;-Phenylcinnamic Acid&nbsp;&nbsp;&nbsp;&nbsp;435</li><li>Decarboxylation: Synthesis of cis-Stilbene&nbsp;&nbsp;&nbsp;&nbsp;438</li></ol></em>


By the way, <strong>Magpie</strong>, I love your copper steam bath. If only I had funds when those pop up on eBay...

[editing to include 3<sup>rd</sup> ed. & Pavia <em>et al.</em> to come later]

[Edited on 7/9/13 by bfesser]




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[*] posted on 5-3-2012 at 15:16


Quote: Originally posted by Nicodem  
We had these same benzaldehyde->benzoin->benzil reactions as lab practice in the undergraduate studies, so that it brings back some nice memories. That already is reason enough for a contribution to the Prepublication section.


I agree. My goal is to work all the Brewster procedures that we never had time to do in 1961. ;) I'll do a writeup for Prepublication after I finish making benzilic acid.

Quote: Originally posted by bfesser  

By the way, Magpie, I love your copper steam bath.


Yes, I love it too, and use it whenever possible, even though it is more trouble to set up with the steam generator and all.




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[*] posted on 10-3-2012 at 16:33


Well done!:)

I did the thiamin catalysed version some time ago (small scale, because I bought the thiamin in a local drugstore and it wasn´t cheap at all there): Benzoin Synthesis.

The oxidation was done with copper(II)-sulfate in pyridine:Benzil synthesis
Orgsyn claims, that with nitric acid oxidation is uncomplete, which can be proven by the Fehling's test (Orgsyn synthesis). Do you have still some benzil left to do a Fehling's test?
There are also other ways to perform the oxidation: With copper(II)-acetat and ammonium nitrate, with vanadium(V)-oxychloride and oxygen, ...

Here some interesting synthesis (apart from benzilic acid) that can be done with benzoin-benzil:

Synthesis of the anticonvulsant phenytoin: Phenytoin synthesis

Tetraphenylcyclopentadienone: Tetraphenylcyclopentadienone

A nice demonstration of the formation of a radical anion: Radical anion (If it doesnt work, add a bit (or a lot) more of NaOH. I tried the demonstration, and with the indicated concetration of base, no colour could be observed. I added more NaOH and then it worked like a charm.)
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[*] posted on 10-3-2012 at 17:03



Very mpressive. Your investigations put mine to shame, as mine were only cursory.

Quote: Originally posted by Heuteufel  
Do you have still some benzil left to do a Fehling's test?


I did this test and it gave the positive brown with a little red showing the insoluble reduction product Cu2O. Wanting to see what a strong positive for this test looks like I tested a little fructose. Now that gives a beautiful brick red!

I had just a little benzil left and thought I should get a melting point on it. But when I dissolved it in hot ethanol it seems to have "oiled out."




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13-3-2012 at 05:11
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[*] posted on 14-3-2012 at 21:35


I have researched this reaction quite a bit. (researched with chemicals and elbow grease, not internet and carpel tunnel)

I just did the thiamine catalyzed version. I may publish some time if i get enough time to finish. I made so many flasks of substituted benzaldehydes.... It's surprising how few of them will give a solid product. I also did some work with the photospectometer on trying to find the concentration of the yellow deprotinated product of thiamine in different solvents.


I suggest that you oxidize the benzoin to benzil by the base(?) catalyzed reaction of benzoin with benzil in atmospheric oxygen. It turns a really cool black/purple from the formation of a diradical anion.



Also I believe the optimal synthesis was something like 5ml h20, 15ml HOEt, NaOh to get it to pH 9 (to high and ox, to low and no product) maybe a half gram of thiamine and 5ml benzaldehyde. The amount of thiamine was way way over kill. The water/EtOH system works really well because it dissolves benzaldehyde, but benzoin is practically insoluble.


On a side not, one of the derivatives smelled like centipedes and I was very displeased with this. I hate those things. The inch-inch and a half red ones that snake around really creepily. they also bite.




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