Thienothiophene

Thienothiophene usually refers to any of three structurally related derivatives of thiophene with the formula C₆H₄S₂. In order of importance, they are: thieno(3,2-b)thiophene, thieno(2,3-b)thiophene, and thieno(3,4-b)thiophene. Other isomers feature S(IV) and are less stable. Thieno[2,3-b]thiophene was the first member of the series to be isolated. It was obtained in very low yield upon heating citric acid, a source of a six-carbon linear chain, with P₄S₁₀. More efficient syntheses of this and the other two stable thienothiophenes involve cyclization reactions of substituted thiophenes.

Three thienothiophenes, being aromatic and bicyclic, are often compared with naphthalene. They are the topic of academic research. They have no commercial applications nor are they or their derivatives found naturally.

• Isomers of Thienothiophene
• 
  Thieno[3,2-b]thiophene
  CAS RN 251-41-2
  m.p. 56.0-56.5 °C
  white solid.
• 
  Thieno[2,3-b]thiophene
  CAS RN 250-84-0
  b.p. 102 °C at 16 mmHg
  colorless oil.
• 
  Thieno[3,4-b]thiophene
  CAS RN 250-65-7
  m.p. 7.0-7.5 °C
  colorless oil.
• 
  2λ4δ2-Thieno[3,4-c]thiophene
  CAS RN 24976-21-4
  not isolated.