Thiophene is a heterocyclic compound with the formula C4H4S. Consisting of a planar five-membered ring, it is aromatic as indicated by its extensive substitution reactions. It is a colorless liquid with a benzene-like odor. In most of its reactions, it resembles benzene. Compounds analogous to thiophene include furan (C4H4O) selenophene (C4H4Se) and pyrrole (C4H4NH), which each vary by the heteroatom in the ring.
Thiophene was discovered as a contaminant in benzene. It was observed that isatin (an indole) forms a blue dye if it is mixed with sulfuric acid and crude benzene. The formation of the blue indophenin had long been believed to be a reaction of benzene itself. Viktor Meyer was able to isolate thiophene as the actual substance responsible for this reaction.
Thiophene and especially its derivatives occur in petroleum, sometimes in concentrations up to 1–3%. The thiophenic content of oil and coal is removed via the hydrodesulfurization (HDS) process. In HDS, the liquid or gaseous feed is passed over a form of molybdenum disulfide catalyst under a pressure of H2. Thiophenes undergo hydrogenolysis to form hydrocarbons and hydrogen sulfide. Thus, thiophene itself is converted to butane and H2S. More prevalent and more problematic in petroleum are benzothiophene and dibenzothiophene.
Thiophene derivatives have been detected at nanomole levels in 3.5 billions year old Martian soil sediments (Murray Formation, Pahrump Hills) by the rover Curiosity at Gale crater (Mars) between 2012 and 2017. It represents an important milestone for the mission of the Mars Science Laboratory (MSL) in the long and elusive quest of organic matter on the red planet. Heating at high temperature (500° to 820 °C) of lacustrine mudstone samples by the Sample Analysis at Mars (SAM) instrument allowed gas chromatography-mass spectrometry (GC-MS) analyses of the evolved gases and the detection of aromatic and aliphatic molecules including several thiophene compounds