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A thioketone (or thione) is any molecule with the group R2C=S. It is like a ketone with the oxygen atom changed with a sulfur one. The sulfur atom has a double bond with the carbon atom. It also keeps two lone pairs.
Thioketones are very reactive. This is because they are good electrophiles. Many of them are unstable. They want to form rings. They do that with cycloaddition reactions, similar to the Diels-Alder reaction.