[Bagnar]

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

[Archer]

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

[AlphaScent]

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.

[Bagnar]

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

[stewie griffin]

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.

[discodermolide]

I've used barium manganate with good success, in dichloromethane.
Unfortunately I have mis-placed the reference.

[AlphaScent]

The above link has some great little tricks.  Great for a chemist still (always) learning like myself.

Cheers to that!

[Bagnar]

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%

[408]

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!!