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 The Lycaeum Forums » Integrate » The Alchemist's Den (Moderator: tryp2fun)Topic:
 Wacker Oxidations

[Moriarty]

Well I've made some observations on the wacker oxidation that might be of use to any of you who are crafty enough to get a hold of anything more than what is available at the local hardware store.  First I started noticing some things so I made some modifications and have had a new bee to the trade try out some of my ideas.  One is that there is simply no need to use so much benzoquinone.  Second is that this is an exothermic reaction, so raising the heat on it does  little to increase the yield if anything at all.  Here's what I've posted at another forum so far:

"1. They Purified a kilogram of brown camphor oil (sassafras oil) by crystallization alone. Don't do this. Without first distilling only ~600 grams was obtained. From past experience it is known that at least 800 grams exist in this oil. Plus it had to be crystallized and melted and recrystallized like 5 times before it was considered pure.

2. Density was checked. I had this fledgling bee buy decent graduated cylinders and test their accuracy with distilled water. The 100 grams of distilled water was just above the 100ml mark so a new mark was placed on the graduated cylinder and the suspected olefin was measured out. 100ml weighted 109.6 grams at 25C. I believe the density is 1.10 at 25C and 1.096 at 20C so there was some confusion but the sample was considered pure.

3. Five (5) grams of palladium chloride was added to a 6-liter flat bottomed flask with 350 grams of 1,4-benzoquinone.

4. 1200ml of methanol and 150ml of DH2O was added and stirring began.

5. Stirring was allowed for 2 hours although I can almost assure you this is not necessary.

6. 600 grams of some olefin that has a specific gravity of 1.096 is weighted out and ~200ml of methanol is added so as if to make ~750 of olefin/methanol solution.

7. This was added in 150ml portions every 12 minutes until gone.

0:00 Hours 150ml of the solution is added, some discoloration noted but no real heat

0:12 Hours another 150ml of the solution is added and a small amount of heat is noticed

0:24 Hours another 150ml of solution is added and it begins to get warm

0:36 Hours another 150ml of solution is added and reflux is obvious. This reaction has achieved at least the 60C boiling point of methanol.

0:48 Hours another 150ml of solution is added and reflux gets hot and heavy almost like an Al/Hg amalgamation with nitromethane

1:00 Hours things are not refluxing anymore but the flask is still hot.

2:00 Hours a noticeable decrease in heat has been observed.

3:00 Hours cooled off almost completely. I asked him to let it stir for the evening with no heat.

Tomorrow I will ask him to extract 3 rimes with 500ml of DCM, Wash and distill. Hopefully the yield is good."

Yields were around 84%.  That's a whole lot of ketone to aminate.  Anyway, I had the same experimenter do the same reaction but this time I asked him to only use 4.5 grams PdCl2.  These are the initial results:

0:00 Hours 150ml of the solution is added, some discoloration noted but no real heat

0:12 Hours another 150ml of the solution is added and there is continued discoloration but no heat

0:24 Hours another 150ml of solution is added.  This "looks like a typical wacker but there is no real heat noticed

0:36 Hours another 150ml of solution is added and no real heat noticed.

0:48 Hours another 150ml of solution is added and there is no real evidence of an exothermic reaction.

1:00 Barely notice any real exothermic heat.

2:00 Hours a noticeable increase in heat has been observed.

3:00 Hours it's still warm but no reflux. I asked him to let it stir for the evening with no heat.

We will see what the yield is tomorrow.  We were trying to determine the amount of PdCl2 needed as we were able to get good results with less before.  I might add that the dynamic and mechanics of this catalyst and reaction seem to change from journal article to journal article every 5 to 10 years.  So when Bright Star said, "Don't skimp on the catalyst", I wasn't sure he knew what he was talking about but initially the speed of the reaction does seem to slow as the catalyst decreases.  I guess that should have been obvious.  Will post on yields tomorrow.

[SaTaN]

That is pretty interesting that is not very much p-Benz at all. Amazed it still worked as well though I must admit. The slack on the catalyst didn't surprise me at all though seeing how the main purpose of them is to speed  up the reaction. Still sweet for someone to be taking the time to see if they can refine the process and help people conserve chemicals. Any report of the yields yet on the second batch??? Now do you believe in the stirring of the catalyst for 2 hours before adding the p-Benz like MM states or not??? I will agree that the reaction of the PdCl2 and the p-Benz doesn't need to take place more than 2 hours an hour should be more than sufficient. Overall very interesting and would like to hear more.

[Moriarty]

Yeah, I just never reported the results because it didn't seem as if anyone was interested and there is a sort of reservation I have about reporting results for experiments I didn't carry out myself in the first place.  Regardless, based on what I was told the yield held at 84%.  The reaction time was extended because the lack of catalyst seemed to slow the reaction quit a bit.  In the first series of adaptations I recommended for the experimenter the reaction reached a reflux quickly.  The second did reach reflux on it's own but not until hour 2 or 3.  The speed of the reaction seems to slow by lowering the amount of catalyst but extended reaction time still gave the same results.  I might add that the run using 4.5 grams PdCl2 was only 1/2 a gram less than the one prior that reached refux quickly and 5 grams PdCl2 for 600 grams olefin is still substantially less than what Brightstar or Methyl Man used by ratio.

The lower amounts of quinone were meant to test a few theories but didn't really shed much light on either of them.  What is understood is that only stoichiomtric amounts of quinone were required.  What isn't known how much that is.  Yes, we can all do math well enough to calculate mole per mole but how much oxygen does 1,4-benzoquinone really provide?  One might surmise from the fact that 1,4-benzoquinone is said to be reduced to hydroquinone that possibly 1/2 a mole might be required.  However, recently a group in New Zealand (or maybe it was Australia, I can't remember) recreated known pathways to popular precursors and reactions where MDMA and MDA were the desired product.  In the wacker oxidation they recreated the reaction in methanol (did I ever tell you how the DEA was monitoring the hive and new syntheses- new to the clandestine world anyway- were reported from the hive in the DEA's then classified Microgram newsletter?).  Anyway, rhodium and the hive were no doubt where they ascertained what all the kids were doing now days.  Well, they found many bi-products.  Hydroquinone was one of them but so were many other like 4-methoxyphenol.  I think that means only one of the double bonded oxygens were made available for oxidation.  They listed 5 bi-products.  I'm sure hydroquinone was the main constituent.  Anyway, by my own admission the wacker oxidation with 1,4-benzoquinone is one of the few mechanism I don't understand.  I have read and read about it but I still don't know where the carbonyl (double bonded oxygen) functional group comes from.  Remember, hydroquinone retains both it's oxygens despite being the reduction product in this redox (oxidation/reduction).  The alkene is the obvious oxidation substrate and the resultant ketone the oxidation product.  I just don't personally understand it.  I have read the reactions mechanism but still am lost.  Possibly understanding this more might enlighten me as to whether 1/2 mole of quinone is required or a 1:1 ration but more than that is certainly unnecessary.

A third experiment was done.  I asked the experimenter to use 7.5 grams of PdCl2, a *stoichiometric amount of 1,4-benzoquinone was used which, believe it or not, was 666 grams per the kilogram of olefin (this olefin had a molecular mass of ~162 grams).  So catalyst amount was the same ratio as the 4.5 reaction in the scaled up reaction but the quinone was a *stoichiometric amount.  The reaction proceeded much the same but the observer had some difficulties in the work-up.  In other words, there were mechanical losses.  I believe the same yield was reached based on his description but I don't know.  I just have the things he told me to go off of and no matter well someone describes something to you, you are still lacking what they left out but saw with their own eyes.  Even if I was there I think there were errors so I couldn't do more than guess in any event.

"Now do you believe in the stirring of the catalyst for 2 hours before adding the p-Benz like MM states or not???"

No, not at all.  When I was doing all this back in 2002-2003 I knew far less so I stirred for at least an hours just for superstition's sake but even then I thought it, like much of his other recommendations, not necessary.  Many of them even killed the yield due to mechanical loses.

"The slack on the catalyst didn't surprise me at all though seeing how the main purpose of them is to speed  up the reaction."

Well, the reaction is not speed PdCl2 in that since alone.  It participates in the reaction.  Without it the yield would be 0%.  The reaction is exothermic so less catalyst probably means it just took longer, that is, unless the optimal heat for activation energy was less than room temperature (which it isn't or it wouldn't heat up all on it's own from room temperature to, as Strike said, "clearly exothermic").  Some reactions produce less side products and are more efficient at a certain temperature.  Since I don't know if this is the case I can't really say but I suspect conversion rate at higher temperature, like those that might cause reflux, might happen faster but my inclination is to believe it also might cause side products at a greater rate.  Probably not much if this occurs at all.

Anyway, I have more notions on this reaction but I think I am done.  I fear a government agency might argue conspiracy if they knew you supplied idea like these knowing someone IS going to do something illegal.  I don't mind trying to answer questions but I am a retired bee and won't be doing anymore experimenting of my own in this lifetime.  I had my fill in the early part of the last decade.

* How much 1,4-benzoquinone is a stoichiometric amount is still unknown to me.