subsecret
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Isolation of Formic Acid - Questions
Recently, I decided to make formic acid via oxidation of methanol via permanganate in water. Stoichiometric amounts of potassium permanganate and
methanol were mixed, with the permanganate being dissolved in an arbitrary amount of water. The mixture was stirred in a 500 mL RBF for about 8 hours,
during which the contents of the flask had turned brown due to manganese dioxide precipitating. After this, 1-2 mL of methanol were added to react
with any remaining permanganate, and the mixture was left to sit. The solution should now contain potassium formate.
Here is my question: Will a significant amount of carbon monoxide be produced if the formate solution is mixed with dilute sulfuric acid and
distilled?
I've read that mineral acids will produce CO when reacted with formic acid, but I've read elsewhere (somewhere on SM) that weak mineral acids tend not
to produce CO. I've included the link for the first bit of information. It is a download, and I apologize for not referencing the thread it came from;
I could not find it.
www.sciencemadness.org/talk/files.php?pid=252844&aid=192...
Any help is appreciated. My fume hood is currently out of service, but even so, I don't particularly want to be poisoned.
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Chemosynthesis
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No. See Fowne's p385.
Also substantiated here: https://www.sciencemadness.org/whisper/viewthread.php?tid=45...
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subsecret
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That's the thread I was looking for. Shame I didn't see it... Thank you.
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Chemosynthesis
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No problem! We all know the search function can be a bit quirky here unless you know exactly what you're looking for. Please report back with your
approximate concentration and results.
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subsecret
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About that... I realize now that I neglected to write down the amount of potassium permanganate I used. Something like 8.2 or 8.3 grams. I don't
remember the hundredths figure, but even so, I may be wrong. The calculation will be based on 8.30 g of potassium permanganate.
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bbartlog
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I don't think you need to worry about CO, but do note that any excess methanol present may form an ester with the formic acid, especially if sulfuric
acid is present in excess. I might have skipped the addition of the extra methanol at the end for this reason.
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S.C. Wack
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Quote: Originally posted by Awesomeness  | | Stoichiometric amounts of potassium permanganate and methanol were mixed...The solution should now contain potassium formate. |
Who says this is so?
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subsecret
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I say it is so, because the potassium ions would've been present in solution. The reaction does happen to produce H+ (according to balanced redox
equation), so some formic acid would be present. Did I miss anything?
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bbartlog
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I think he's questioning the assumption of neat quantitative oxidation of the methanol to formic acid. Your MnO<sub>2</sub> precipitate
certainly suggests that something got oxidized, but it's a leap from that to assuming you now have formic acid.
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subsecret
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I see... Do you suggest that a considerable amount of formaldehyde is left in the reaction flask?
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blogfast25
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Half reactions in neutral/alkaline conditions
KMnO4 + 4 H(+) + 3 e- === > MnO2 + K(+) + 2 H2O
CH3OH + H2O === > HCOOH + 4 H(+) + + 4 e-
Balance electrons and eliminate surplus H(+) and H2O:
4 KMnO4 + 4 H(+) + 3 CH3OH === > 4 MnO2 + 5 H2O + 3 HCOOH + 4 K(+)
In alkaline conditions (add 8 OH(-) to the left) that would have yielded 3 K formate and 1 KOH. But you didn't add any alkali, now did you?
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subsecret
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No, alkali was not added. I represented the generic form of the alcohol with ROH and the permanganate with MnO4-. Thanks for the tips.
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S.C. Wack
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I'm asking for references. There are AFAIK no literature preparations of formic/formate from methanol, likely for a good reason. Ethanol and acetate,
fine, but formate isn't acetate.
BTW IIRC I'm not so sure that the precipitate from permanganate oxidations is exactly MnO2, at least until it's very dried.
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Metacelsus
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I thought that permanganate oxidized methanol all the way to carbon dioxide.
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HgDinis25
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| Quote: Originally posted by S.C. Wack | I'm asking for references. There are AFAIK no literature preparations of formic/formate from methanol, likely for a good reason. Ethanol and acetate,
fine, but formate isn't acetate.
BTW IIRC I'm not so sure that the precipitate from permanganate oxidations is exactly MnO2, at least until it's very dried. |
What you mean by "until dried"? At the moment, I can't think of an oxidation with the Permanganate ion that doesn't yeald an immediate precipitate of
Manganese Dioxide.
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S.C. Wack
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What's the pH now? Alkaline permanganate oxidizes formate completely.
| Quote: Originally posted by HgDinis25 | | What you mean by "until dried"? At the moment, I can't think of an oxidation with the Permanganate ion that doesn't yeald an immediate precipitate of
Manganese Dioxide. |
IIRC the actual product of aq. KMnO4 reduction is somewhere between the hydroxide and dioxide.
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HgDinis25
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| Quote: Originally posted by S.C. Wack | What's the pH now? Alkaline permanganate oxidizes formate completely.
| Quote: Originally posted by HgDinis25 | | What you mean by "until dried"? At the moment, I can't think of an oxidation with the Permanganate ion that doesn't yeald an immediate precipitate of
Manganese Dioxide. |
IIRC the actual product of aq. KMnO4 reduction is somewhere between the hydroxide and dioxide. |
I am pretty sure that the precipitate in most aquous oxidations with KMnO4 is plain Manganese(IV) Ooxide. I can't even figure out a mechanism for the
formation of the Hydroxide. Besides, Manganese Hydroxide (wich is actualy the complex [Mn(H2O)4(OH)2]) would be oxidized to Manganese(III) Oxide, so
no MnO2 would be formed.
Here is a proposed reaction mechanism:
http://www.dartmouth.edu/~chem52/handouts/OxidHand.htm
[Edited on 20-11-2014 by HgDinis25]
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subsecret
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I checked with blue litmus (the only pH indicator I currently have), and the pH is not acidic.
I'm particularly bad about doing research before doing an experiment. If it works with ethanol, it should work with methanol, right?
Not really...
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blogfast25
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Evidence/reference?
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Praxichys
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"Kinetics of the Permanganate Oxidation of Formic Acid and Formate Ion in Aqueous Solution"
DOI: 10.1021/ja01521a004
"The Mechanisms of Permanganate Oxidation. II. The Oxidation of Formate Ion"
DOI: 10.1021/ja01587a034
Permanganate oxidizes formate completely, forming MnO2, regardless of pH.
http://www.organic-chemistry.org/chemicals/reductions/formic...
Formate is fragile and easily oxidized, which is why it is used as a reducing agent.
[Edited on 20-11-2014 by Praxichys]
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blogfast25
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| Quote: Originally posted by Praxichys | "Kinetics of the Permanganate Oxidation of Formic Acid and Formate Ion in Aqueous Solution"
DOI: 10.1021/ja01521a004
"The Mechanisms of Permanganate Oxidation. II. The Oxidation of Formate Ion"
DOI: 10.1021/ja01587a034
Permanganate oxidizes formate completely, forming MnO2, regardless of pH. |
Interesting and news to me.
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