2,5-dichloro-2,5-dihydrothiophene 1-oxide: determination of the complete stereochemistry.

The title five-membered heterocycle, C4H4Cl2OS, adopts an envelope conformation with the S atom at the tip of the flap. All three ring substituents, viz. the sulfoxide O atom and the two Cl atoms, are cis to each other. The two C atoms alpha to the sulfoxide group are also bonded to chlorine. The electron-withdrawing chlorine substituents give rise to weak C-H...O hydrogen bonds with the sulfoxide O atom of a symmetry-related molecule [H...O = 2.44 (2) and 2.61 (2) A, C...O = 3.143 (3) and 3.302 (2) A and C-H...O = 129.9 (19) and 135.1 (19) degrees ]. There is also a possible weak C-H...Cl interaction. Chains of molecules held together by these weak interactions run parallel to the a axis.