Thiobenzophenone is an organosulfur compound with the formula (C₆H₅)₂CS. It is the prototypical thioketone

Thioketone

Thioketones are organosulfur compounds related to conventional ketones. Instead of the formula R2C=O, thioketones have the formula R2C=S. Unhindered alkylthioketones are typically unstable; such compounds tend to form polymers or rings.-Preparative methods:Thiones are usually prepared from ketones...

. Unlike other thioketones that tend to dimerize to form rings and polymers, thiobenzophenone is quite stable, although it photoxidizes in air to form benzophenone and sulfur. Thiobenzophenone is deep blue and dissolves readily in many organic solvents.

Structure

According to the double bond rule, the C=S double bond of most thioketones is unstable with respect to dimerization making the stability of thiobenzophenone valuable for studying C=S chemistry. The energy difference between the p orbitals of sulfur and carbon is greater than that between oxygen and carbon in ketones. The relative difference in energy and diffusity of the atomic orbitals of sulfur compared to carbon results in poor overlap of the orbitals and the energy gap between the HOMO and LUMO is thus reduced for C=S relative to C=O. The striking blue appearance of thiobenzophenone is due to π→ π* transitions upon the absorption of light with a wavelength of 314.5 nm. 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 dihydral angle SC-CC is 36°. 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

Sodium hydrosulfide

Sodium hydrosulfide is the chemical compound with the formula NaHS. This compound is the product of the half neutralization of hydrogen sulfide with sodium hydroxide. NaHS is a useful reagent for the synthesis of organic and inorganic sulfur compounds. It is a colorless solid that typically smells...

 and benzophenone dichloride:

Ph₂CCl₂ + 2 NaSH → Ph₂C=S + 2 NaCl + H₂S

A method adapted from the sequence above that has been found to be more convenient and gives more reproducible results involves the use of benzophenone as the starting material:

Ph₂C=O + H₂S → Ph₂C=S + H₂O

In the above reaction scheme, a mixture of gaseous hydrogen chloride and hydrogen sulfide

Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 are passed into a cooled solution of benzophenone in ethanol. Thiobenzophenone can also be produced by a Friedel-Crafts reaction

Friedel-Crafts reaction

The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. This reaction type is a form of electrophilic aromatic substitution...

 of thiobenzoyl chloride and benzene.

Reactivity

Due to the weakness of the C=S bond, thiobenzophenone is more reactive than its C=O benzophenone counterpart. Thiobenzophenone as well as other thioketones are considered to be superdipolarophiles and dienophiles that rapidly combine 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.