Sciencemadness Discussion Board

Permanganate oxidation of methanol

ampakine - 20-7-2011 at 02:40

The mechanism:
CH3OH + KMnO4 => HCOOH + H2O + MnO + K2O

I recently tried this reaction. I reacted pure methanol with potassium permanganate and when I filtered out the manganese(II) oxide I was left with a clear yellow solution. Is the yellow tint caused by unfiltered MnO? My objective is to obtain formic acid so I'm going to have to distill it but I'm wondering what will be left in the boiling flask. If I'm not mistaken this is what will happen to the H2O and K2O:

H2O + K2O -> 2KOH

so all that should be left in the distilling flask is KOH but whatever is turning the solution yellow will most likely be left behind too.

ItalianChemist - 20-7-2011 at 03:27

Maybe MnO reacted with formic acid to form manganese(II) formate?
My advice is to distill out the unreacted methanol and add conc. sulfuric acid! Then distill formic acid

theflickkk - 20-7-2011 at 06:02

Methanoic acid can be further oxidised by KMnO4. The yellow tint is likely to be small particles of MnO2 since you conducted the oxidation in the presence of alkali.

[Edited on 20-7-2011 by theflickkk]

kryss - 20-7-2011 at 11:09

Don't forget Conc H2SO4 can dehydrate formates and form carbon monoxide, which is very poisoness and odourless.

blogfast25 - 20-7-2011 at 12:55

No MnO is formed here: MnO2 instead.

But permanganate oxidation of alcohols is a very poor way of making carboxylic acids: runaway reactions, difficult work-up and further oxidation to CO2/CO are all possible problems. K2Cr2O7 may be slightly easier to handle as an oxidiser here. Note that such reactions are highly exothermic and it really ISN'T recommended to treat the pure alcohols with such strong oxidisers: permanganate makes glycerol catch fire!

I would search for another method to oxidise methanol to formic acid, if I was you...

ampakine - 21-7-2011 at 02:05

Quote: Originally posted by blogfast25  
No MnO is formed here: MnO2 instead.

But permanganate oxidation of alcohols is a very poor way of making carboxylic acids: runaway reactions, difficult work-up and further oxidation to CO2/CO are all possible problems. K2Cr2O7 may be slightly easier to handle as an oxidiser here. Note that such reactions are highly exothermic and it really ISN'T recommended to treat the pure alcohols with such strong oxidisers: permanganate makes glycerol catch fire!


I have never dealt with MnO but I've dealt with MnO2 before and this black/brown solid I filtered out is a lot like MnO2 so I'll take your word for it that MnO2 is the product. This stuff is messy as hell, I usually try to avoid it. Good thing I used glass jars rather than my good beakers for this because cleaning MnO2 off glassware is a nightmare. Dichromates might be more effect but they're far more toxic so I only use them when theres no alternative. I recommend you try this reaction for yourself. In my experience this is not a very exothermic reaction. It is nothing like the permanganate oxidation of glycerol.

Quote: Originally posted by blogfast25  

I would search for another method to oxidise methanol to formic acid, if I was you...

Any suggestions besides chromates/dichromates? I wonder if H2O2 would do the trick? Thats my favorite oxidiser due to the lack of unwanted byproducts.

blogfast25 - 21-7-2011 at 04:58

Using dichromates is not very troublesome: they are present in the pantry of most laboratories w/o problems.

As regards the exotherm of these oxidations, I carried out the oxidation of ethanol (well, methylated spirits) in acid conditions in a compensation calorimeter, with KMnO4 and then with K2Cr2O7, to find that they proceeded very quickly and with much generation of heat: about 400 kJ/mol of heat released per mol of acid oxidised.

See here:

http://www.sciencemadness.org/talk/viewthread.php?tid=15856#...

So if you didn’t get much heat I wonder what your conditions were.

MnO2 is a nuisance to clean up but there’s one thing that gets rid of it real easily: conc. HCl:

MnO2(s) + 4 HCl(aq) === > MnCl2(aq) + Cl2(g) + 2 H2O(l)

More dilute HCl also does it too but takes longer/heating.

Other oxidisers? HNO3 does the trick on primary alcohols (e.g. oxidation of propane-1,3-diol to malonic acid (reported here: http://www.sciencemadness.org/talk/viewthread.php?tid=12832#... with high heat generation). Not sure about CH3OH though…

kryss - 21-7-2011 at 11:22

Persulphate any use?

Retard-3000 - 22-7-2011 at 05:31

Wouldn't using potassium dichromate first oxidize the methanol to formaldehyde ? formaldehyde has a boiling point of -19C, would the formaldehyde stay dissolved in the methanol during reflux ?

blogfast25 - 22-7-2011 at 05:55

In my experiments I used an excess of alcohol to prevent that. Oxidation of primary alcohols with dichromate is a standard test but also the basis of a quantative determination of the ABV of alcoholic drinks. If carried out properly the alcohol is oxidised to its corresponding carboxylic acid (and no further). But recovery of the acid is probelematic, so it's rarely uesed in synthesis.

Waffles SS - 1-3-2012 at 00:14

Really there is no easy and cheap way for oxidation of Methanol to formic acid?

Hexavalent - 5-3-2012 at 13:07

I'm certain there is, it's just a matter of thinking logically and basic experimentation. Perhaps a primary alcohol oxidation using acidified dichromate, with a dry ice cooled condenser would do the trick. To get it out, perhaps just using stoichiometic quantities of everything needed and looking into the possibility of distillation/phase separation would work.