Hydrogenolysis

Hydrogenolysis is a chemical reaction whereby a carbon–carbon or carbon–heteroatom single bond is cleaved or undergoes lysis (breakdown) by hydrogen.<ref name="ConnorAdkins">Ralph Connor, Homer Adkins. Hydrogenolysis Of Oxygenated Organic Compounds. J. Am. Chem. Soc.; 1932; 54(12); 4678–4690. {{#invoke:doi|main}}</ref> The heteroatom may vary, but it usually is oxygen, nitrogen, or sulfur. A related reaction is hydrogenation, where hydrogen is added to the molecule, without cleaving bonds. Usually hydrogenolysis is conducted catalytically using hydrogen gas.

History

The term "hydrogenolysis" was coined by Carleton Ellis in reference to hydrogenolysis of carbon–carbon bonds.<ref name="ConnorAdkins" /><ref>Carleton Ellis. Hydrogenation of Organic Substances, 3rd ed., Van Nostrand Company, New York, 1930, p. 564 (as referred by Connor and Adkins).</ref> Earlier, Paul Sabatier had already observed the hydrogenolysis of benzyl alcohol to toluene,<ref>Sabatier and Murat. Ann. Chim. [9] 4, 258, (1915), according to Connor and Adkins.</ref> and as early as 1906, Padoa and Ponti had observed the hydrogenolysis of furfuryl alcohol.<ref>Furfuryl alcohol hydrogenation is accompanied by hydrogenolysis into 2-methylfuran, which gives 2-methyltetrahydrofuran, and further hydrogenolysis opens the ring to give 2-pentanol. Original: Padoa and Ponti. Atti. R. accad. Lincei, 15, [5] 610 (1906); Gazz. chim. ital. 37, [2] 105 (1907), according to Kaufmann: W. E. Kaufmann, Roger Adams. The Use Of Platinum Oxide As A Catalyst In The Reduction Of Organic Compounds. Iv. Reduction Of Furfural And Its Derivatives. J. Am. Chem. Soc.; 1923; 45(12); 3029–44. {{#invoke:doi|main}}</ref> Homer Burton Adkins and Ralph Connor were the first to call the carbon–oxygen bond cleavage "hydrogenolysis".<ref name="ConnorAdkins" />

In the petrochemical industry

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} In petroleum refineries, catalytic hydrogenolysis of feedstocks is conducted on a large scale to remove sulfur from feedstocks, releasing gaseous hydrogen sulfide (H₂S). The hydrogen sulfide is subsequently recovered in an amine treater and finally converted to elemental sulfur in a Claus process unit. In those industries, desulfurization process units are often referred to as hydrodesulfurizers (HDS) or hydrotreaters (HDT). Catalysts are based on molybdenum sulfide containing smaller amounts of cobalt or nickel. Hydrogenolysis is accompanied by hydrogenation.<ref>Topsøe, H.; Clausen, B. S.; Massoth, F. E., Hydrotreating Catalysis, Science and Technology, Springer-Verlag: Berlin, 1996.</ref>

Another hydrogenolysis reaction of commercial importance is the hydrogenolysis of esters into alcohols by catalysts such as copper chromite.

The hydrodeoxygenation reaction used in the Neste Renewable Diesel process, the vegetable oil refining process of largest production capacity, is a hydrogenolysis of triglycerides into alkanes.

In the laboratory

In the laboratory, hydrogenolysis is used in organic synthesis. Debenzylation is most common and involves the cleavage of benzyl ethers:<ref>For example, Organic Syntheses, Coll. Vol. 7, p. 386 (1990); Vol. 60, p. 92 (1981). http://orgsynth.org/orgsyn/pdfs/CV7P0386.pdf. For an example of C-N scission, see Organic Syntheses, Coll. Vol. 8, p. 152 (1993); Vol. 68, p. 227 (1990). http://orgsynth.org/orgsyn/pdfs/CV8P0152.pdf</ref>

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Thioketals undergo hydrogenolysis using Raney nickel in the Mozingo reduction.

Laboratory hydrogenolysis is operationally similar to hydrogenation, and may be accomplished at atmospheric pressure by stirring the reaction mixture under a slight positive pressure of hydrogen gas, having flushed the apparatus with more of this gas. The hydrogen may be provided by attaching a balloon to a needle, filling it from a bottle, and inserting the needle into the reaction flask via a rubber septum. At high pressure, a hydrogenation autoclave (i.e., a Buchi or Parr hydrogenator) or similar piece of equipment is required.