frogfot - 22-5-2004 at 07:05
I'm a bit confused here.. I've looked through some oxidations of sidechains in an aromatic ring with KMnO4, and every synthesis uses huge
ammounts of water 
why?!?
Heres some examples..
500 g o-nitrotoluene is oxidised in 25 l water
5 g toluene needs 350 ml water
I've tested the second synthesis with only 100 ml water, and it gave same yield, in same time!
Dammit, why? I've seen this mentioned in only one book, they only said "oxidations with KMnO4 is usually carried out with 3-5%
solutions"
Only reason I can see is the bad solubility of the aromatic compound.. But shouldn't it react at a great rate anyway, if reaction mix is properly
agitated?..
BromicAcid - 22-5-2004 at 07:17
Just as you thought, these reactions are occasionally called to be run in a great excess of water because reaction can only occur at the liquid-liquid
interface due to the intense insoulubility of the permanganate anion in non-polar solvents. By increasing the volume of water you increase the
surface area of reaction, assuming you have a large enough vessel, in addition it increases the KMnO4 availible to the reaction. However I don't
know any specifics to the reaction myself. Maybe the water is just a heat sink to prevent a run away oxidation.
[Edited on 5/22/2004 by BromicAcid]
frogfot - 23-5-2004 at 02:59
Then I'd guess reaction rate can be controlled by stirring speed. I dunno, runaway seems to be improbable, since reaction goes slow..
In worst scenario I thoat that low permanganate conc was needed to avoid some "by-oxidations"..
Anyway, I gonna try to oxidise a mix of o- & p- (65 and 35%, respectively) nitrotoluenes, 5 g in only 100ml water.
Books tells that reaction temp. with p-isomer should be kept at 70*C during KMnO4 addition (about 4h) and then additional 2 h at 85*C.
While o-isomer however, is refluxed for 24 h with about half of the KMnO4, and after, rest of KMnO4 is added and mix is refluxed for another 24 h.
So, the o-isomer seems to require harder conditions. The good part is that second synth tells following: "..in the same way one can oxidise
p-nitrotoluene..". So I guess a mix of isomers could be used here too.. and since I'll use more conc permanganate soln. and a vigurous
stirring, I'd expect reaction time to be much shorter.. Do you think smelling reaction mix would be a good indication of the end of reaction?
thunderfvck - 23-5-2004 at 12:29
So you're trying to prepare nitrobenzoic acids? Just wanting to make sure...
Hmm. I don't think smell would be a great indication of the END of a reaction. It would be a sure test for a reaction though. But as for the
reaction being finished 100%...I don't see the harm in over oxidizing though...Could be wrong on this, but all that KMnO4 is going to do is
oxidize those methyl groups, so whatever, let it reflux baby.