Thiobenzophenone is an organosulfur compound with the formula (C6H5)2CS. It is the prototypical thioketone. Unlike other thioketones that tend to dimerize to form rings and polymers, thiobenzophenone is quite stable, although it photoxidizes in air back to benzophenone and sulfur. Thiobenzophenone is deep blue and dissolves readily in many organic solvents.
Structure 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°.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. A variety of thiones with structures and stability related to thiobenzophenone have also been prepared.
Synthesis One of the first reported syntheses of thiobenzophenone involves the reaction of sodium hydrosulfide and diphenyldichloromethane:H. Staudinger, H. Freudenberger "Thiobenzophenone" Organic Syntheses, 1931, Vol. 11, p.94. Ph2CCl2 + 2 NaSH → Ph2C=S + 2 NaCl + H2S
An updated method involves sulfiding of benzophenone:B. F. Gofton and E. A. Braude "Thiobenzophenone" Org. Synth. 1955, vol. 35, 97. Ph2C=O + H2S → Ph2C=S + H2O
In the above reaction scheme, a mixture of gaseous hydrogen chloride and hydrogen sulfide are passed into a cooled solution of benzophenone in ethanol. Thiobenzophenone can also be produced by a Friedel-Crafts reaction of thiobenzoyl chloride and benzene.Horst Viola, Steffen Scheithauer and Roland Mayer "Organische Schwefelverbindungen,97. Friedel-Crafts-Reaktionen mit Thiosäurechloriden" Chem. Ber. 1968, volume 101, pages 3517–3529.
Reactivity Due to the relative weakness of the C=S bond, thiobenzophenone is more reactive than benzophenone. Thiobenzophenone (as well as other thioketones) is a dipolarophiles and dienophiles. For example, it combines with 1,3-dienes in Diels-Alder cycloadditions. The rate of thioketones in cycloadditions is related but not limited to the size of the small HOMO/LUMO energy gap of the π-MOs of the C=S double bond. Reactions between thiobenzophenone and most dienes yield Diels-Alder adducts whereas reactions with monoolefins yield bicyclic compounds.
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