16.6: Hydrogenolysis

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Hydrogenolysis is a chemical reaction whereby a carbon–carbon or carbon–heteroatom single bond is cleaved or undergoes "lysis" by hydrogen.^[1] 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.^[1]^[2] Earlier, Sabatier had already observed the hydrogenolysis of benzyl alcohol to toluene,^[3] and as early as 1906, Padoa and Ponti had observed the hydrogenolysis of furfuryl alcohol.^[4] Adkins and Connors were the first to call the carbon–oxygen bond cleavage "hydrogenolysis".^[1]

Petrochemical industry

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.^[5]

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

In the laboratory

In the laboratory, hydrogenolysis is used in organic synthesis. Debenzylation is most common and involves the cleavage of benzyl ethers:^[6]

\[ROCH_2C_6H_5 + H_2 \rightarrow ROH + CH_3C_6H_5\]

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 Parr hydrogenator) or similar piece of equipment is required.

Desulfurization

Raney nickel is used in organic synthesis for desulfurization. For example, thioacetals will be reduced to hydrocarbons in the last step of the Mozingo reduction:^[14]^[15]

Thiols,^[16] and sulfides^[17] can be removed from aliphatic, aromatic, or heteroaromatic compounds. Likewise, Raney nickel will remove the sulfur of thiophene to give a saturated alkane.^[18]

References

Ralph Connor, Homer Adkins. Hydrogenolysis Of Oxygenated Organic Compounds. J. Am. Chem. Soc.; 1932; 54(12); 4678–4690. doi:10.1021/ja01351a02
Carleton Ellis. Hydrogenation of Organic Substances, 3rd ed., Van Nostrand Company, New York, 1930, p. 564 (as referred by Connor and Adkins).
Sabatier and Murat. Ann. Chim. [9] 4, 258, (1915), according to Connor and Adkins.
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. doi:10.1021/ja01665a033
Topsøe, H.; Clausen, B. S.; Massoth, F. E., Hydrotreating Catalysis, Science and Technology, Springer-Verlag: Berlin, 1996.
For example Organic Syntheses, Coll. Vol. 7, p.386 (1990); Vol. 60, p.92 (1981). http://orgsynth.org/orgsyn/pdfs/CV7P0386.pdf. For 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
Solomons, T.W. Graham; Fryhle, Craig B. (2004). Organic Chemistry. Wiley. ISBN 0-471-41799-8.
Jonathan Clayden; Nick Greeves; Stuart Warren (2012). Organic Chemistry (2 ed.). Oxford University Press. ISBN 9780199270293.
Graham, A. R.; Millidge, A. F.; Young, D. P. (1954). "Oxidation products of diisobutylene. Part III. Products from ring-opening of 1,2-epoxy-2,4,4-trimethylpentane". Journal of the Chemical Society (Resumed): 2180. doi:10.1039/JR9540002180
Gassman, P. G.; van Bergen, T. J. (1988). "Indoles from anilines: Ethyl 2-methylindole-5-carboxylate". Org. Synth.; Coll. Vol. 6, p. 601
Hoegberg, Hans Erik; Hedenstroem, Erik; Faegerhag, Jonas; Servi, Stefano (1992). "Bakers' yeast reduction of thiophenepropaenals. Enantioselective synthesis of (S)-2-methyl-1-alkanols via bakers' yeast mediated reduction of 2-methyl-3-(2-thiophene)propenals". The Journal of Organic Chemistry 57 (7): 2052–2059. doi:10.1021/jo00033a028

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