Electrolytic reduction of pyridine to piperidine

This is a preliminary result but it worked and was in my opinion interesting enough to be shared. Pyridine can be reduced to piperidine, for example with sodium in ethanol or with hydrogen on platinum group metal catalysts. In this experiment I followed german patent DE104664A on the electrochemical reduction using lead electrodes in dilute sulfuric acid.

It should be noted that the patent states that iron and copper ions are detrimental to the reaction and most other electrodes do not work.

Chemicals:
Pyridine (distilled from copper chromite and niacin per DougsLab)
Sulfuric acid (food grade, ~16%, easily made from higher concentrations by dilution)
Water
Sodium hydroxide
(Ethyl acetate)

Electrodes:
Lead cathode (sheet, 10x15 cm, rolled into an approximately 5 cm diameter and 10 cm tall cylindrical electrode)
Lead anode (rod, 8 mm diameter, 20 cm long, placed centrally inside the cathode)

The lead was bought for electrochemical purposes and claims to be 99.95% pure.

Cell design:
This first experiment was performed without separation of anode and cathode compartments as the patent stated this is not required, but using a split cell with a diaphragm is something I plan on eventually trying. The electrodes were slotted into a polypropylene food container and fit both well into a 150 mL beaker placed inside the box. The lid of the box contains a small hole to relieve pressure. To make this temporary, the electrodes were sealed with hot glue, but I recommend epoxy as the heat softens the glue quite a bit.

Preparation:
22 g (216 mmol) of 96% sulfuric acid was carefully diluted in 113 g of water, making up for 135 g of a 15-16% solution. To this was added 10 g (126 mmol) of pyridine. The solution was mixed, left to cool to room temperature and placed inside the beaker. The electrodes were lowered into the solution and 5.1 A of current were passed through it. The voltage ranged from 4 to 5 V during the reaction. Immediately changes occur to the electrodes: The cathode goes blank and shiny while the anode turns black. Gas evolution begins as well. The solution was electrolysed for 8 hours (theoretical 4 hours, it was planned for 6 hours, but some delay occured). The temperature rose and it was felt necessary to keep the temperature down with an ice/water bath. The solution hovered between 20-50 °C for the electrolysis. The solution acquired a somewhat rancid smell and a brown color.



Workup:
The workup of this preparation still needs to be improved. After the solution was made basic with a solution of 25 g (625 mmol) of NaOH in 40 g of water and 10 g of ice (this melted), one could solvent extract with ethyl acetate, then reextract with hydrochloric acid, boil the solution down and free the piperidine with more NaOH. This would likely prevent some of the problems that were encountered during the steam distillation. The basic solution was then distilled off. During this simple steam distillation water (> 100 mL in portions) and some more NaOH (~10 g) were added into the flask. In total the steam distillation took 4 h and 220 mL of distillate was collected. After this time the distillate coming over was still strongly basic to pH-paper while the residue in the flask had coagulated into a sticky brown mess. The distillate was acidified with 2.87 g (28 mmol) of 96% sulfuric acid and boiled almost to dryness. After placing it in the freezer, a brownish syrup containing small crystals was obtained.



This was treated with 4 g (100 mmol) of NaOH dissolved in 6 g of water. This caused phase separation and the top piperidine layer was collected using a separatory funnel.



The yield was 3.90 g (46 mmol) of brownish piperidine, which should be redistilled (due to the small amount I have postponed this for now). If pure this would be around a 36% yield.

The residue in the distilling flask still smelled of piperidine, however extraction with ethyl acetate, reextraction into acid, evaporation to near dryness and treatment with NaOH as before only gave 0.25 g of very discolored piperidine and thus it was discarded. The extended heating seemed to cause a lot of impurities to form - if the piperidine is extracted without being excessively heated with base before I suspect a cleaner product will be obtained in less time.

Chemical tests:
The presence of piperidine in the mother liquor, the steam distillate and the final product can be detected using Reinecke's salt NH₄[Cr(SCN)₄(NH₃)₂], which can be prepared fairly easily and which is a rewarding inorganic preparation in its own right.
The sample is brought into an excess of ammonia solution (the mother liquor was partially neutralized with NaOH first). To this, a solution of reineckes salt in dilute ammonia is added. Piperidine produces a precipitate which depending on the color of the solution may range from light purple to beige. Pyridine produces no precipitate under these conditions.

This test is positive for mother liquor (pic 1), steam distillate (pic 2) and final product, and negative for the starting material (pic 3).



Discussion:
For a first test with many flaws it worked quite well. The brown discoloration can be avoided by using a split cell, according to the patent. Sulfuric acid may be replaced with hydrochloric acid. I have not tried this.
Extraction might be a better method for recovering product. The product should be redistilled. I think I electrolyzed for too long as well.

The patent states that traces of copper and iron ions are detrimental to the reaction and therefore care should be taken to exclude these metals or their ions.

Overall I think this is a promising way, despite the low yield on this first run, to produce piperidine from pyridine without the use of pressurized hydrogen, precious metals or large amounts of sodium.

Sources:

German patent DE104664A
Attachment: DE000000104664A_all_pages.pdf (269kB)
This file has been downloaded 291 times

L. Kum-Tatt, Quantitative Separation of Nitrogenous Organic Bases using Ammonium Reineckate. Nature 1958, 182, 655–656. https://doi.org/10.1038/182655a0
Attachment: kum-tatt1958.pdf (188kB)
This file has been downloaded 274 times

https://www.youtube.com/watch?v=FNsqYwzm40M (Doug's Lab video on pyridine from niacin)

[Edited on 21-6-2022 by Diachrynic]

Quote: Originally posted by Triflic Acid

Have you tried using other pyridine derivatives like niacin directly? If the conditions are right, I would hope that you might be able to decarboxylate in the acidic solution with enough heat and get a one-pot synthesis of piperidine.

Use of lead electrodes instead of something more exotic makes this quite accessible.
Please do write this up for prepub. It is lovely work.

Quote: Originally posted by Hexabromobenzene

You got a low yield probably due to the lack of a diaphragm in the electrolyte

Quote: Originally posted by PirateDocBrown

I've had good results using a terra-cotta pot a a diaphragm for electrolysis.

Quote: Originally posted by PirateDocBrown

I've had good results using a terra-cotta pot a a diaphragm for electrolysis. They can be had as small as 50mm in diameter. Just fill the bottom hole with some silicone sealant, or epoxy.

Quote: Originally posted by Hexabromobenzene

Pyridine can dissolve PVC

Can it still in dilute, highly acidic solution? It will be pyridinium sulfate after all.

Thanks kmno4 for the added information, quite useful. I agree that the characterization, apart from forming the reineckate, was lacking. I decided to postpone the purification by distillation for when I retry.
I looked into TLC and it might be possible to separate pyridine and piperidine that way, as another purity check. Potentially even as their reineckates.
Melting point of a salt is also a good idea.

I tried the analytical test for pyridine on my starting material and confirmed that it is a very nice method for detecting pyridine.

Here I used a 10 mM copper solution, 2 eq. of thiocyanate and 2 eq. of pyridine. It's a lovely, rich, dark green, flocculent precipitate. For fun I also recrystallized from hot water after filtration and took some microscope pictures. Enjoy:

Quote: Originally posted by Diachrynic

Electrodes: Lead cathode (sheet, 10x15 cm, rolled into an approximately 5 cm diameter and 10 cm tall cylindrical electrode) Lead anode (rod, 8 mm diameter, 20 cm long, placed centrally inside the cathode) The lead was bought for electrochemical purposes and claims to be 99.95% pure.

Quote: Originally posted by Corrosive Joeseph

Would you bee so kind as to link me to your lead source please.....?