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Topic: Activated or not MnO2  (Read 9902 times)

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Offline Bagnar

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Activated or not MnO2
« on: December 02, 2013, 11:58:02 AM »
Good afternoon everybody !

Please, apoligize all my mistakes, I'm non an english-spoken person and I just try my best to speak & write it properly.

So.

I'm playing with a benzoic acid, reduced in the corresponding primary alcohol ( 1.05 eq LiAlH4 in THF (Dry, of course), stirring at RT overnight, 99% yield ! *feel happy* )
Since a few days, I try to oxydize it into the aldehyde. Problem, I used some MnO2... A normal one I think.
I did this reaction in DCM, THF, mixture, acetone, refluxed or not... Nothing happened. (5 eq of MnO2)
Finally I get a nice bottle of "activated" MnO2 and my oxydation runned in 45 min ! Awesome isn't it ?

Now, I just simple question. What is the difference between the activated and the non-activated oxydazing agent?
Oxydation state (IV) is the same.
Even if the solid if less brillant it is still a black powder...

Do you have any idea ? Thank !

Bagnar
I get

Offline Archer

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Re: Activated or not MnO2
« Reply #1 on: December 02, 2013, 12:05:44 PM »
I believe that the presence of water de-activates MnO2 for oxidations.

J. Org. Chem., 1969, 34 (6), pp 1979–1981 DOI: 10.1021/jo01258a106
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Offline AlphaScent

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Re: Activated or not MnO2
« Reply #2 on: December 02, 2013, 07:08:33 PM »
It is as I am concerned water that is the major player in being activated or not.

Tonight I am "activating" basic (pH 9) alumina (Al2O3) under vacuum at 200°C for a reaction tomorrow morning where I rearrange an allene to an E,Z system.

Consulting my colleague, the activation is the the removal of water.
If you're not part of the solution, then you're part of the precipitate

Offline Bagnar

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Re: Activated or not MnO2
« Reply #3 on: December 03, 2013, 03:58:17 AM »
I will test it asap.

I put in the oven 2g of deactivated MnO2, 180 degrees. I will test it tomorrow on a lambda reaction (Probably oxydation of ethanol ;) )

Thks for this answer ! I will dig in this direction and try to find why water inhibite reactions ;)

Offline stewie griffin

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Re: Activated or not MnO2
« Reply #4 on: December 03, 2013, 12:54:46 PM »
Check out orgo prep daily's comments on MnO2 here http://orgprepdaily.wordpress.com/2007/03/16/notes-on-lousy-reactions/
He concurs it is the presence of water.
For what it's worth, the MnO2 oxidation I used to do went much faster (2 hrs vs 24+ hrs) in refluxing benzene vs dicholormethane.

Offline discodermolide

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Re: Activated or not MnO2
« Reply #5 on: December 03, 2013, 12:58:14 PM »
I've used barium manganate with good success, in dichloromethane.
Unfortunately I have mis-placed the reference.
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Offline AlphaScent

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Re: Activated or not MnO2
« Reply #6 on: December 03, 2013, 03:18:32 PM »
The above link has some great little tricks.  Great for a chemist still (always) learning like myself.

Cheers to that!
If you're not part of the solution, then you're part of the precipitate

Offline Bagnar

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Re: Activated or not MnO2
« Reply #7 on: December 10, 2013, 05:02:53 AM »
Refluxing benzene ? O.M.G !

I am really shocked to read this... Time is money, I agree, but do any reaction overnight in DCM in a thousand times saver than using benzene. Whatever.

A few notes:
Activation: 12hrs, 180 degrees in oven
Reaction: Oxydation of benzylic alcohol in DCM (100 mg of starting material)
Day 1: Completion in 5 hrs
Day 2: Completion in 5 hrs
Day 3: Completion in 5 hrs
Day 5: Completion in 5 hrs
Day 7: Completion in 5 hrs

MnO2 stored at room temperature in the lab. Approx humidity 15-20%

Offline 408

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Re: Activated or not MnO2
« Reply #8 on: December 10, 2013, 10:46:27 PM »
Wow!  Thanks guys!  I had always thought that the difference was calcined-vs-non-calcined MnO2, so I had either been buying the activated stuff or preparing my own.  Had no idea that a bit of heat would activate MnO2!!

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