Sciencemadness Discussion Board

Kolbe Electrolysis

DFliyerz - 3-3-2015 at 13:53

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?

Molecular Manipulations - 4-3-2015 at 09:38

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]

woelen - 4-3-2015 at 10:26

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...

Molecular Manipulations - 4-3-2015 at 11:15

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.

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]

deltaH - 4-3-2015 at 11:43

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 :o

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 :mad:

[Edited on 4-3-2015 by deltaH]

woelen - 4-3-2015 at 13:19

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.

DFliyerz - 5-3-2015 at 10:47

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?

woelen - 5-3-2015 at 23:58

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.