Thioketone

In organic chemistry, thioketones (from Ancient Greek θεῖον (theion) 'sulfur';[1] also known as thiones or thiocarbonyls) are organosulfur compounds related to conventional ketones in which the oxygen has been replaced by a sulfur.[2] Instead of a structure of R2C=O, thioketones have the structure R2C=S, which is reflected by the prefix "thio-" in the name of the functional group. Thus the simplest thioketone is thioacetone, the sulfur analog of acetone. Unhindered alkylthioketones typically tend to form polymers or rings.[3][4]

General formula of a thioketone

Structure and bonding

The C=S bond length of thiobenzophenone is 1.63 Å, which is comparable to 1.64 Å, the C=S bond length of thioformaldehyde, measured in the gas phase. Due to steric interactions, the phenyl groups are not coplanar and the dihedral angle SC-CC is 36°.[5] Unhindered dialkylthiones polymerize or oligomerize but thiocamphor is well characterized red solid.[6]

Consistent with the double bond rule, most alkyl thioketones are unstable with respect to dimerization.[7] The energy difference between the p orbitals of sulfur and carbon is greater than that between oxygen and carbon in ketones.[8] The relative difference in energy and diffusity of the atomic orbitals of sulfur compared with carbon results in poor overlap of the atomic orbitals and the energy gap between the HOMO and LUMO is thus reduced for C=S molecular orbitals relative to C=O.[5] The striking blue appearance of thiobenzophenone is attributed to π→ π* transitions upon the absorption of red light.[8] Thiocamphor is red.

Preparative methods

Thiones are usually prepared from ketones using reagents that exchange S and O atoms. A common reagent is phosphorus pentasulfide[9] and the related reagent Lawesson's reagent. Other methods uses a mixture of hydrogen chloride combined with hydrogen sulfide. Bis(trimethylsilyl)sulfide has also been employed.[3][10]

Solution and solid samples of thiobenzophenone. The blue color is consistent with a small HOMO–LUMO gap associated with the thiocarbonyl functional group.

Thiobenzophenone [(C6H5)2CS] is a stable deep blue compound that dissolves readily in organic solvents. It photooxidizes in air to benzophenone and sulfur. Since its discovery, a variety of related thiones have been prepared.[11]

Thiosulfines

Thiosulfines, also called thiocarbonyl S-sulfides, are compounds with the formula R2CSS. Although superficially appearing to be cumulenes, with the linkage R2C=S=S, they are more usefully classified as 1,3-dipoles and indeed participate in 1,3-dipolar cycloadditions. Thiosulfines are proposed to exist in equilibrium with dithiiranes, three-membered CS2 rings. Thiosulfines are often invoked as intermediates in mechanistic discussions of the chemistry of thiones. For example, thiobenzophenone decomposes upon oxidation to the 1,2,4-trithiolane (Ph2C)2S3, which arises via the cycloaddition of Ph2CSS to its parent Ph2CS.[12]

See also

References
  1. θεῖον Archived 2017-05-10 at the Wayback Machine, Henry George Liddell, Robert Scott, A Greek–English Lexicon
  2. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Thioketones". doi:10.1351/goldbook.T06356
  3. Kuhn, N.; Verani, G. (2007). "Chalcogenone C=E compounds". Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium. Royal Society of Chemistry. doi:10.1039/9781847557575-00107.
  4. E. Champaigne (1966). "Thioketones". In Saul Patai (ed.). The Carbonyl Group. PATAI'S Chemistry of Functional Groups. Vol. 1. John Wiley & Sons Ltd. p. 916. doi:10.1002/9780470771051.ch17. ISBN 9780470771051.
  5. Sustmann, R.; Sicking, W.; Huisgen, R. "A Computational Study of the Cycloaddition of Thiobenzophenone S-Methylide to Thiobenzophenone". J. Am. Chem. Soc. 2003, 125, 14425-14434. doi:10.1021/ja0377551
  6. Brunelli, M.; Fitch, A. N.; Mora, A. J. (2002). "Crystal Structures of R-Thiocamphor". Zeitschrift für Kristallographie - Crystalline Materials. 217 (2): 83–87. Bibcode:2002ZK....217...83B. doi:10.1524/zkri.217.2.83.20627. S2CID 97935620.
  7. Organosulfur Chemistry I: Topics in Current Chemistry, 1999, Volume 204/1999, 127-181, doi:10.1007/3-540-48956-8_2
  8. Fisera, L.; Huisgen, R.; Kalwinsch, I.; Langhals,E.; Li, X.; Mloston, G.; Polborn, K.; Rapp, J.; Sicking, W.; Sustmann, R. "New Thione Chemistry". Pure Appl. Chem., 1996, 68, 789-798. doi:10.1351/pac199668040789
  9. Polshettiwar, Vivek; Kaushik, M. P. (2004). "A new, efficient and simple method for the thionation of ketones to thioketones using P4S10/Al2O3". Tetrahedron Letters. 45 (33): 6255–6257. doi:10.1016/j.tetlet.2004.06.091.
  10. Mcgregor, W. M.; Sherrington, D. C. (1993). "Some recent synthetic routes to thioketones and thioaldehydes". Chemical Society Reviews. 22 (3): 199. doi:10.1039/CS9932200199.
  11. Okazaki, R.; Tokitoh, N. (2000). "Heavy ketones, the heavier element congeners of a ketone". Accounts of Chemical Research. 33 (9): 625–630. doi:10.1021/ar980073b. PMID 10995200.
  12. Rolf Huisgen; J. Rapp (1997). "1,3-Dipolar Cycloadditions. 98. The Chemistry of Thiocarbonyl S-Sulfides". Tetrahedron. 53 (3): 939–960. doi:10.1016/S0040-4020(96)01068-X.
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