Thiones

Thiones are a class of organosulfur compounds that feature a thiocarbonyl group, a carbon atom double-bonded to sulfur (C=S). They are the sulfur analogs of carbonyl compounds and are commonly encountered as thioketones (R2C=S) and thioaldehydes (RCHO replaced by RCH=S). The C=S bond is generally longer and less polar than C=O, leading to distinct reactivity and spectroscopic properties. In infrared spectroscopy, the thiocarbonyl stretch typically appears around 1050–1200 cm−1, lower in energy than the carbonyl band.

Preparation and interconversion: Thiones are often prepared by thionation of carbonyl compounds. Reagents such as Lawesson’s reagent or phosphorus pentasulfide (P4S10) convert aldehydes and ketones into their thiocarbonyl counterparts. Some thiones can also be generated by desulfurization of dithioketones or by equilibration with thiols and sulfurs under appropriate conditions. Thiones can exhibit thione–thiol tautomerism in certain contexts, depending on substituents and solvents.

Examples and scope: The simplest thione is thioformaldehyde (CH2=S). A representative thioketone is benzophenone thione, also called diphenylthioketone (Ph2C=S). Thiones appear in various organic syntheses and as intermediates in the preparation of sulfur-containing heterocycles and polymers.

Reactivity and applications: Compared with carbonyls, thiones are generally less reactive toward nucleophilic addition, though they participate in typical thiocarbonyl chemistry, including cycloadditions and metal coordination. They are used in organic synthesis to replace carbonyl groups, in ligation chemistry, and as probes or intermediates in photochemical and materials applications. See also thiols, thioureas, and related sulfur-containing functional groups.