DFliyerz
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Kolbe Electrolysis
Lately, the subject of Kolbe Electrolysis has been very interesting for me. I understand that electrolysis of acetic acid results in ethane and carbon
dioxide, but am confused how it works with straight-chain saturated carboxylic acids with more carbon atoms. For example, propionic acid: it seems as
though it could make ethane, ethylene, or even propane. So my question is, what determines how radicals bond with each other?
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Nicodem
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Thread Moved
3-3-2015 at 14:05 |
Molecular Manipulations
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The reaction as best I can figure out is like this:
2 CH3R1COOH --> R2 + 2 CO2 + H2.
Where R2 = 2 R1 + 2 CH3.
Eg: 2 CH3(CH2)4COOH --> C10H22 + 2 CO2 + H2.
Did you read this?
[Edited on 4-3-2015 by Molecular Manipulations]
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We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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woelen
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I have done quite some experimenting with this and I have written a web page about it. It seems that the reaction of acetate is quite unique. Other
similar acids do not necessarily lead to equivalent reactions.
http://woelen.homescience.net/science/chem/exps/precision_el...
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Molecular Manipulations
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Quote: Originally posted by woelen  | It seems that the reaction of acetate is quite unique. Other similar acids do not necessarily lead to equivalent reactions.
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Well with acetic acid the reaction you got was: 2 CH3COOH → H2 + H3C-CH3 + 2 CO2
This is perfectly equivalent to the reaction I wrote earlier, where R1 = 0 and R2 = 2 R1 (0) + 2 CH3· or H3C-CH3! It is the
same: 2 CH3R1COOH --> R2 + 2 CO2 + H2. Nice to see the consistency!
It seems like other acids with at least two carbons do lead to equivalent reactions.
I'm impressed you were able to find that reaction out yourself.
With formic acid you got: HCOOH → H2 + CO2, which makes sense cause there's only one carbon, and it's in the third oxidation
state, but anything with two or more seems to follow the basic: 2 CH3R1COOH --> R2 + 2 CO2 +
H2 reaction.
[Edited on 4-3-2015 by Molecular Manipulations]
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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deltaH
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What happens when you have a hydroxyl group adjacent to the carboxylic acid, e.g. lactic acid, would you then make 2,3-butanediol?
EDIT: Nevermind, Google answered it for me, seems like acetaldehyde is produced with acrylic acid and acetic acid as by-products.
http://pubs.acs.org/doi/abs/10.1021/ie101839r
Now I have a new question to Google, does this mean tartaric acid would make glyoxal (and by-products)?
Dratz... wrong again, Wiki says you form dihydroxymalonic acid 
http://en.wikipedia.org/wiki/Dihydroxymalonic_acid#cite_note...
That's weird, not what I would have thought.
Take home message: it's not that simple with adjacent heteroatoms 
[Edited on 4-3-2015 by deltaH]
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woelen
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With longer chains, or with H-atoms substituted with other atoms or groups, the dimerization, after splitting off of CO2 does only occur partially and
many side reactions occur.
E.g. with propionic acid/propionate, in an ideal situation you would expect CO2 and n-butane. However, in practice, indeed some butane is formed, but
also other products. With longer chains, or with H-atoms replaced by Cl, OH or other atoms, the results become even more unpredictable.
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DFliyerz
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So, electrolysis of propionic acid can result in ethane, ethylene, propane, and butane, and the yield of each depends on factors such as current
density?
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woelen
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Electrolysis of propionic acid can result in production of n-butane, carbon dioxide and many other products at the anode and production of hydrogen at
the cathode. The "many other products" I cannot specify, they most likely will not be propane, ethane or alkenes, but I can imagine that they can be
different hydrocarbons with partial replacement of hydrogen by oxygen or hydroxyl groups.
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