Kolbe electrolysis

The Kolbe electrolysis is a chemical reaction in which by electrolysis of carboxylic acids or their salts, the carboxylates are coupled two residues together. The Kolbe electrolysis was named after its discoverer Hermann Kolbe (1818–1884). In this way, for example, he represented ethane from acetic acid (R = H):

Reaction mechanism

The salt of the carboxylic acid (carboxyl anion) is oxidized to the carboxylate radical at the anode . This radical splits off carbon dioxide , creating another radical. Two of these radicals combine ( dimerize ) to form the alkane:

At the same time, hydrogen is produced at the cathode:

${\displaystyle \mathrm {2\ H_{2}O{\xrightarrow {Kathode}}2\ OH^{\,-}+H_{2}} }$

If DC voltage is applied, the positively charged metal cations migrate to the cathode (the negative pole) and the negatively charged carboxylates to the anode, where they react. Alkali or alkylammonium cations are usually used as counterions to the carboxylate , as these do not affect the reaction. Other metal ions such as Fe ²⁺ , Mn ²⁺ and Co ²⁺ reduce the yield.

yield

For symmetrical n-alkanes, a yield of up to 90% can be achieved, depending on parameters such as current density , solvent, salt concentration and temperature. If different carboxylates are used (mixed Kolbe electrolysis), products with an odd number of carbon atoms are also accessible. The yields decrease, however, as product mixtures are mostly formed:

3 R ¹ COO ^- + 3 R ² COO ^- → R ¹ −R ¹ + R ¹ −R ² + R ² −R ² + 6 CO ₂ + 6 e ^-

Platinum has proven itself as an anode material .

The yields of the reaction depend on various factors such as the substrate, the electrolysis conditions , the current densities and the solvents used. The reaction is usually carried out at moderate temperatures.

example

An example reaction is the synthesis of 2,7-dimethyl-2,7-dinitrooctane from 4-methyl-4-nitrovalerian (nitropentanoic) acid in methanol on platinum electrodes:

Example of a synthesis step with Kolbe electrolysis

Historical

Michael Faraday had already reported in 1834 that the electrolysis of potassium acetate can produce a hydrocarbon, which he did not investigate further. In 1848 Hermann Kolbe reported on the electrolysis of valeric acid (pentanoic acid) and butyric acid (butanoic acid), whereby he correctly recognized the chain length of the hydrocarbons formed. In 1849 he described these reactions and those of acetic acid in more detail, so that he is considered the discoverer of this CC-linking electrolysis.

literature

• Hans J. Schäfer : CC links on anode and cathode . In: Angewandte Chemie . tape 93 , no. 11 . WILEY-VCH, November 1981, p. 978-1000 , doi : 10.1002 / anie.19810931107 .
• Thomas Laue, Andreas Plagens: Name and catchword reactions in organic chemistry . Teubner study books chemistry. Ed .: Ch. Elschenbroich, F. Hensel, H. Hopf. 5th edition. BG Teubner, GWV, Wiesbaden 2006, ISBN 3-8351-0091-2 , p. 209-211 .
• Barry M. Trost, Ian Fleming: Comprehensive organic synthesis: selectivity, strategy, and efficiency in modern organic chemistry . 1st edition. Pergamon Press, Oxford, England 1991, pp. 633-658 ( knovel.com ).

Individual evidence

1. ↑ ^{a b} Hermann Kolbe : Decomposition of valeric acid by the electric current . In: Friedrich Wöhler, Justus Liebig (Ed.): Annals of Chemistry and Pharmacy . tape 64 , no. 3 . CF. Winter, Heidelberg 1848, p. 339–341 , doi : 10.1002 / jlac.18480640346 ( online at the Bayerische Staatsbibliothek BSB ).
2. ↑ ^{a b} Hermann Kolbe: Investigations into the electrolysis of organic compounds . In: Friedrich Wöhler, Justus Liebig (Ed.): Annals of Chemistry and Pharmacy . tape 69 , no. 3 . CF Winter, Heidelberg 1849, p. 257–294 , doi : 10.1002 / jlac.18490690302 ( online in the Internet Archive [accessed on July 31, 2016]): "To obtain important information about their chemical constitution through the electrolytic decomposition of organic compounds."
3. ^ Z. Wang: Comprehensive Organic: Name Reactions and Reagents. Wiley Verlag, 2009, ISBN 978-0-471-70450-8 , pp. 1656-1659.
4. ↑ Hans J. Schäfer: Sustainable synthesis through electrolysis. Week 34. In: The current newsreel. Society of German Chemists, GDCh Section Sustainable Chemistry, August 20, 2008, accessed on August 6, 2016 . 
5. ↑ WH Sharkey, CM Langkammerer: 2,7-Dimethyl-2,7-dinitrooctane In: Organic Syntheses . 41, 1961, p. 24, doi : 10.15227 / orgsyn.041.0024 ; Coll. Vol. 5, 1973, p. 445 ( PDF ).
6. ↑ Michael Faraday : Seventh series of experimental investigations on electricity . About the primary and secondary character of the substances developed at the electrodes. In: Johann Christian Poggendorff (Ed.): Annals of Physics and Chemistry . 33 ( volume 109 of the Complete Annals of Physics), No. 28 . Johann Ambrosius Barth, Leipzig 1834, p. 433–451 , doi : 10.1002 / andp.18341092307 ( online at Gallica BnF [accessed on August 5, 2016] English: Experimental Researches in Electricity. Seventh Series . 1834. Translated by Johann Christian Poggendorff, Elektrolse von Kaliumacetat (“acetic acid potash”) ) on page 438, section 749, online ): "... less gas developed at the anode than at the cathode, and it now contained hydrocarbons ..."