Thioketone (thione): the sulfur analogues of ketones

Structure and bonding

The C=S unit resembles a carbonyl in having a double bond between carbon and a chalcogen, but sulfur is larger and less electronegative than oxygen. As a result the C=S bond has poorer p-orbital overlap, is typically longer and more polarizable than a C=O bond, and gives rise to different spectroscopic signatures and colors in conjugated systems. The carbon of the thiocarbonyl is strongly electrophilic, while sulfur retains lone pairs that influence coordination and reactivity.

Stability and preparation

Many simple thioketones are more reactive and less persistent than their oxygen analogues; they can dimerize or undergo cycloaddition. Some substituted thioketones are stable enough to isolate and study—thiobenzophenone (diphenylthioketone) is a well-known example with a distinctive color. Common laboratory methods to convert carbonyls into thiocarbonyls include use of thionating reagents such as Lawesson's reagent or phosphorus pentasulfide (P4S10), which replace oxygen with sulfur under controlled conditions.

Typical reactions and uses

Thioketones are excellent electrophiles and participate in a variety of addition and cycloaddition reactions. They undergo [4+2] and [2+2] cycloadditions with dienes and alkenes, sometimes rapidly at room temperature, and can form stable adducts or polymerize. These reactions are exploited in synthetic organic chemistry to build sulfur-containing heterocycles and complex frameworks. Thioketones also serve as intermediates in the preparation of thiols, sulfides and other sulfur functional groups.

Related compounds and notable distinctions

If one substituent is hydrogen (RCH=S), the compound is called a thial, the sulfur analogue of an aldehyde; thials are typically even more reactive and often malodorous. Thioketones differ from thiols (RSH) and sulfides (R–S–R) by the presence of the double bond to carbon. In heterocyclic and organosulfur chemistry the balance between thione and thiol tautomeric forms can be important for reactivity and biological activity.

Further reading and references

• Overview of thiocarbonyl chemistry
• Structural comparisons of C=O and C=S bonds
• Ketone vs thioketone: functional group relationships
• Electronegativity and chalcogen effects
• Physical properties of sulfur-containing organics
• Bonding and lone pairs on sulfur
• Reactivity of carbon–sulfur double bonds
• Lone pair effects in thiocarbonyls
• Thioketones as electrophiles
• Cycloaddition examples with thioketones
• Diels–Alder type reactions with C=S
• Definition and behaviour of thials
• Aldehyde analogues in sulfur chemistry
• Further experimental techniques and safety notes