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Author: Subject: Sodium Benzoate and KMnO4
TGWS
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[*] posted on 12-1-2020 at 04:46
Sodium Benzoate and KMnO4


So I recently acquired some Potassium Permanganate from eBay and have been trying lots of different experiments. I know that carboxylic acids can be oxidised under pushing conditions, releasing Carbon Dioxide and forming an alkane. I wondered what would happen if you mixed Sodium Benzoate and KMnO4, which I know is a strong oxidising agent.

I did this in neutral solution, so it's not to be as effective an oxidising agent as in acidic solution. I used dilute KMnO4 (0.1N) solution and created a saturated solution of Sodium Benzoate. I wasn't expecting anything due to the fact that I'm not aware of any reaction that should happen, and the KMnO4 solution is very dilute (3.3gdm-3) I mixed the two together and observed a slow browning of the solution to a final orange/yellow colour, which I think suggests the reduction of the Manganese to Mn(IV).

My question is: Is this actually a redox reaction, and if so what has happened to the Sodium Benzoate? Or have I just messed up and impurities from the air or relics from previous experiments embedded deep into the beakers I used messing up the reaction?

Much thanks and any help would be appreciated, even if to tell me I'm an idiot. :)
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[*] posted on 12-1-2020 at 05:59


Quote: Originally posted by TGWS  

My question is: Is this actually a redox reaction, and if so what has happened to the Sodium Benzoate? Or have I just messed up and impurities from the air or relics from previous experiments embedded deep into the beakers I used messing up the reaction?


It's probably a redox reaction. I'm not sure what happened to the benzoate, but as a control you could leave out the benzoate and see if the KMnO4 still decomposes.




As below, so above.
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[*] posted on 13-1-2020 at 00:04


KMnO4 is capable of breaking down the carbon skeleton of many organic compounds. The breakdown may be at any place in the molecule, leading to a whole bunch of reaction products. It can go as far as complete breakdown to carbon dioxide (in acidic solution) or carbonate (in alkaline or neutral solution). This makes it less useful for changing organic compounds if the overall structure of the compound must be preserved. For this purpose, the use of dichromate or chromium trioxide in acidic solutions is preferred. That kind of oxidizer is milder.



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[*] posted on 13-1-2020 at 07:46


Hi, thanks for the replies!

So I assume it's not possible to ascertain the products and separate them if it could be a range of organic molecules.

I have several questions if you don't mind:

1) Is the orange colour due to a reduced form of Manganese (i.e. Mn(IV) ), or an oxidation product of the sodium benzoate?

2) If I do it in strongly alkaline media, would that weaken the destructive capability, and ensure just CO2 is abstracted? Or is still strong enough to attack at any point?

The reason I ask is that it's difficult to get your hands on any dichromate or chromium trioxide in my country.

Additionally, in terms of basic mechanism, as far as I know, water attacks the carbon centre in an alcohol or aldehyde, and the MnO4- abstracts an H+ from the carbon centre, removing HMnO4-, and the molecule sorts itself out into the most stable form (I know it's more complicated than that but I can't draw it, sorry :( ).

3) Where else can it attack in, say, Sodium Benzoate, that would allow it to act as an oxidising agent? Surely the resonance hybrids introduce stability surrounding it, hence limiting the possible attack centres?
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[*] posted on 14-1-2020 at 00:05


1) The orange/brown color is due to very finely suspended hydrous MnO2. If the material becomes more concentrated, it will turn darker brown.
2) Permanganate is destructive always, both in acid and in alkalies. In acid it is reduced all the way down to very pale pink Mn(2+) ions, in alkalies it is reduced down to brown hydrous MnO2.
3) If permanganate acts as a too strong oxidizer, then it can attack an organic molecule at any place. The mix of products you get is completely unpredictable, unless oxidation is complete (to CO2 at low pH, to CO3(2-) at high pH). It is just as with burning an organic molecule. E.g. if an alcohol is burned, then the molecule is broken apart, and there is not a clean attack at the hydroxyl group. Only if burning is complete you get predicatable molecules (CO2 and H2O), otherwise you get all kinds of smaller organics, CO and C.




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[*] posted on 14-1-2020 at 03:06


Quote: Originally posted by TGWS  
So I recently acquired some Potassium Permanganate from eBay and have been trying lots of different experiments. I know that carboxylic acids can be oxidised under pushing conditions, releasing Carbon Dioxide and forming an alkane. I wondered what would happen if you mixed Sodium Benzoate and KMnO4, which I know is a strong oxidising agent.


Wrong. Carbon dioxide elimination:
R-CO2-H -> R-H+CO2
is not a redox reaction.

Permanganate is used for some selective goals. Like, benzoate C6H5-COOH, is commonly end result of selective oxidations... because permanganate is commonly used to selectively oxidize side chain, while leaving aromatic ring intact.
What went wrong? Wrong conditions, where permanganate did oxidize the aromatic ring? Or was benzoate unreacted, and permanganate simply slowly oxidized water oxygen?
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[*] posted on 14-1-2020 at 08:57


Thanks very much woelen, that makes a lot of sense! :)
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[*] posted on 14-1-2020 at 09:13


Quote: Originally posted by chornedsnorkack  

Wrong. Carbon dioxide elimination:
R-CO2-H -> R-H+CO2
is not a redox reaction.

Wait, why isn't it a redox reaction? Isn't the carbon in the COOH group being oxidised? In the COOH group, I think it should be in the +3 oxidation state, but in CO2 it's +4, meaning it's been oxidised?

Quote: Originally posted by chornedsnorkack  

Wrong conditions, where permanganate did oxidize the aromatic ring?

According to woelen, this is highly likely as the KMnO4 is highly destructive, wherein I assume no change in conditions can stop it on its hell-bent journey to be reduced.

Quote: Originally posted by chornedsnorkack  

Or was benzoate unreacted, and permanganate simply slowly oxidized water oxygen?

I'm not sure if this one is possible or not, I followed this link here:
Oxidation of Water by Permanganate
And the author calculates that the reaction would, ignoring kinetics, be possible at concentrations above 0.393M, but my KMnO4 solution I made up was 0.1N (I'm not sure I understand normality, but I used 3.3g of KMnO4 in 1dm3 water, and a quick calculation tells me that it's concentration is ~0.02M), which is well below the concentration required to adequately permit said reaction.
I do understand that that would be a possibility, as the oxygen in water can be oxidised to form elemental oxygen, but surely under quick pressing conditions, more so that at STP using a very dilute solution of KMnO4.

I think you're definitely right with the first option though, and woelen backs that up. Thanks a lot for your help. :)
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