Sciencemadness Discussion Board

The Kolbe Electrolysis and Organic Electrosynthesis thread

sakshaug007 - 16-8-2010 at 14:19

For a while now I have been interested in organic synthesis as well as electrochemistry and I have been researching any and all techniques in which to integrate the two. Perhaps the best and most well known way of doing this involves what is known as Kolbe electrolysis which can be generally summarized using the following reaction equation: 2RCOO- => 2CO2 + 2e- + R-R
Although this process may not be the most efficient for producing many of the chemicals and reagents we are familiar with it is a simple and safe method that can be carried out at home for producing small quantities of a desired organic compound.

With that in mind, alkyl halides are often the most difficult chemicals for the amateur chemist to obtain easily for at home experiments as many of you may agree. In light of this, I seek to take Kolbe electrolysis a different route and apply it to the synthesis of alkyl halides.

Based on the literature I've come across, the most "efficient" electrolytic way of producing alkyl halides may not in fact be Kolbe electrolysis but rather the Hofer-Moest reaction (non-Kolbe electrolysis) in which the alkyl radicals are further oxidized to their carbonium cation counterparts and therefore react more readily with the halide anion present in solution. The carbonium ion products are often preferred over the Kolbe dimer products when the alkyl radical has an ionization potential energy <8eV. The carbonium ion products can also be induced if porous carbon anodes are used in place of platinum and under aqueous-low current densities rather than nonaqueous-high current density conditions of Kolbe electrolysis.

I am in the process of devising an experiment for the synthesis of methyliodide using the techniques stated above, in which sodium acetate and potassium iodide being the constituent reagents will be electrolyzed in a divided cell (to prevent methyliodide reduction) of water using a porous graphite anode and steel cathode. Also, I will likely keep the cell in an ice bath to help reduce volatility loss of methyliodide.

Anode reactions:
CH3OO- => CH3+ + CO2 + 2e- ;E = 2.24 volts
3I- => I3- + 2e- ;E = -0.53 volts
2H2O => O2 + 4H+ + 4e- ;E = -1.23 volts

Cathode reactions:
I2 + 2e- => 2I- ;E = 0.54 volts
H+ + 2e- => H2 ;E = 0 volts

Overall reaction:
CH3COO- + I- => CH3I + CO2

Since the anodic potential of iodide is considerably less than that of acetate it will likely oxidize first so I have considered adding the sodium acetate to the solution after the iodide has been allowed to oxidize. Some things I'm not certain about, however, include the overpotential necessary for the oxidation to proceed, whether the solution should be stirred or kept still during electrolysis, and the spacing between electrodes (which may not matter in a divided cell). Does anyone have experience with this technique? Is anyone capable of carrying out this experiment?

I want this thread to be dedicated to any and all things Kolbe electrolysis and electrosynthesis related. I encourage anyone to post their comments, experiences, and references related to these topics.

Attachment: Kolbe potentials.pdf (1.5MB)
This file has been downloaded 1254 times

More references to follow

[Edited on 16-8-2010 by sakshaug007]

sakshaug007 - 17-8-2010 at 11:37

Here are a couple more references to this topic.

Nicodem - 21-8-2010 at 07:06

I wish I had the time to study all that literature, but I don't. I only rapidly checked a few pages and from that I could draw a couple of tentative conclusions:

- Coupling occurs at a monolayer of ions on the electrode surface and the alkyl radicals do not diffuse anywhere away from there. This means that increasing the concentration of the triiodide would not increase the yield of MeI (also because then almost no acetate would get oxidized). Optimally you might expect better results at the [I<sub>3</sub><sup>-</sup>] vs. [MeCOO<sup>-</sup>] ratio where their redox potentials are equal (the corresponding concentrations can be roughly calculated using the Nerst equation). At this ratio they would be oxidizing at the anode at comparable rate (though not really, as this does not include ion diffusion rates, competition at the anode surface and lots of other parameters).

- If the conditions are such that the alkyl radicals are oxidized further to carbocations, it is unlikely that any alkyl iodide would form. Carbocations react with the first nucleophile they encounter and it is unlikely they would meet a iodide anion on the surface of an anode prior to runing into other anions or solvent molecules.

PS: Using electrolysis for something that can be made so easily from methanol and iodides can not be anything but of scientific interest, but this reaction was already reported in the article you attached in the Short questions thread (BTW, not linking to previous posts on the subject is a huge strategic mistake!). However, you seem to show interest in the preparative aspects of such electrolysis. This is a bit strange, actually very strange. Even more strange is that you report no experimental results. It is a terribly simple thing to put two electrodes in a solution of potassium acetate and KI in water and describe your observations. It would not demonstrate much without detecting MeI in the outgoing gas with GC or whatever expensive equipment, but it is a viable experiment to anyone. My advice is to try to be innovative instead. For example, make experiments in the electrolysis of sodium benzoate and iodide in water. Use a graphite anode and a benzoate vs. iodide ratio of about 10 : 1. Then describe what goes on. I have reasons to believe this might be either more efficient than the acetate + triiodide electrolysis, or not working at all. For one, the Kolbe electrolysis of benzoates does not work and this is a good sign in my opinion. And iodobenzene is not that volatile, has a higher MW and thus can be easily isolated. It would not require any fancy equipment for the confirmation of its formation. If you isolate at least 5 mg, you can ask someone to do you an IR or NMR analysis and if you isolate more than 1 mL you can eventually purify it with microdistillation and then measure the bp.

Nicodem - 7-11-2012 at 08:18

I encountered a discussion on a related topic on the Hyperlab where this interesting reference was cited and thought to post it in this old thread:

Synthese von Monocarbonsäuren der Fettreihe auf elektrochemischem Wege
W. v. Miller, H. Hofer
Ber. 1895, 28, 2427–2439.
DOI: 10.1002/cber.18950280322 Link to the article at gallica.

It describes experiments of electrolysis of carboxylates where the alkyl radicals are apparently captured by iodine or NO2 produced at the anode from iodides or nitrites. This way iodoethane or 1,2-diiodoethane form from the electrolysis of aqueous EtCOONa or potassium succinate correspondingly in the presence of KI. Similarly nitroethane is formed from EtCOONa and NaNO2. Cross-Kolbe coupling between glycolate and acetate to give ethanol and several other interesting Kolbe reactions are also described.
Apparently the strategy used to prevent selective oxidation at the anode (discussed upthread) was to put the KI in the cathode chamber and the carboxylate in the anode chamber. The iodide must therefore travel trough the membrane and its over-potential is thus kept higher. But unfortunately my understanding of German is not good enough to understand the details.

Other posts on the topic (for some reason sakshaug007 failed to review the topic): and the replies

sakshaug007 - 10-11-2012 at 20:20

Lol so I see you're still harping on this Nicodem, that's funny. I'm beyond this now but it's nice to know you care and thanks for the interesting reference none the less. Btw, in regards to your contemptuous comment I had already reviewed the responses to my questions in the aforementioned threads hence why I haven't responded to this can let it go now.

[Edited on 11-11-2012 by sakshaug007]

tetrahedron - 11-11-2012 at 16:49

Quote: Originally posted by sakshaug007  
Lol so I see you're still harping on this Nicodem, that's funny. I'm beyond this now but it's nice to know you care and thanks for the interesting reference none the less. Btw, in regards to your contemptuous comment I had already reviewed the responses to my questions in the aforementioned threads hence why I haven't responded to this can let it go now.

i actually find your post undeservedly contemptuous and ungrateful. nicodem's comments are to the point, if you've lost interest just move on, someone else is going to find the collected information useful.

tyro - 1-2-2023 at 10:42

Apologies for resurrecting an absolutely ancient thread.

I wanted to share some information I came across regarding the electrolysis of sodium propionate and sodium nitrite referenced by Nicodem above.

Seems that some researchers tried to replicate the work of W. v. Miller and H. Hofer, and found that the formed products were various alkyl nitrites, and no nitroethane. Their investigation into the matter is pretty interesting.

Fr. Fichter 1939 Trans. Electrochem. Soc. 75 309
DOI 10.1149/1.3498380

Hadn't seen this mentioned elsewhere or in any of the various nitroethane threads. Figured some might find it of use.