Tetrachlorocyclopropene is a chemical compound with the formula C<sub>3</sub>Cl<sub>4</sub>. A colorless liquid, the compound is a reagent used to prepare acetylene derivatives and in organic synthesis. It was first reported by Tobey and West. It is prepared by addition of dichlorocarbene to trichloroethylene and then further treating the resultant pentachlorocyclopropane with base to perform dehydrochlorination.
Treatment with a good chloride-accepting Lewis acid affords trichlorocyclopropenium () salts, a carbocation that is aromatic like other cyclopropenium ions, as predicted by Hückel's rule. Aluminum trichloride is most commonly used as the Lewis acid, giving the tetrachloroaluminate salt:
The structure of trichlorocyclopropenium has been confirmed by X-ray crystallography of this salt. Salts of hexachloroantimonate (), tetrachloroferrate (), and tetrachlorogallate () have also been prepared from other Lewis acids. Tetrachlorocyclopropene appears inert towards boron trifluoride and trichloride, but reacts with boron tribromide to give tetrabromocyclopropene, with gaseous boron trichloride leaving solution:
Arylpropiolate esters may be prepared from trichlorocyclopropenium by reacting with arenes, followed by hydrolysis in alcohols. This can be done without isolating the trichlorocyclopropenium ion.
Trichlorocyclopropene is also used to prepare arylcyclopropanones by a two-step, one-pot procedure beginning with a Friedel-Crafts-like arylation, with trichlorocyclopropenium as an intermediate prepared in situ:
The aryltetrachlorocyclopropenes are then hydrolyzed to give the keto-alcohol: