Thiophene synthesis using H2S, HCl, and 1,4 diketone

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Discussion Overview

The discussion revolves around the synthesis of thiophene using hydrogen sulfide (H2S) and hydrochloric acid (HCl) in conjunction with a 1,4 diketone. Participants explore the efficiency and methodology of this synthesis compared to traditional methods involving phosphorus pentasulfide (P2S5) or Lawesson's reagent. The conversation includes references to historical context and alternative approaches.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Historical

Main Points Raised

  • One participant expresses confusion over their synthesis approach involving hydration of a carbonyl and a backside attack by H2S, which was marked incorrect on a midterm.
  • Another participant suggests that a chemistry library with specialized texts on thiophenes would be beneficial for understanding the synthesis better.
  • A participant mentions finding information on the "Paal thiophene synthesis" and discusses the historical use of P2S5, noting that it produced furan as a byproduct.
  • There is a correction regarding the timeline of Lawesson's reagent, with a participant questioning the date mentioned in a book and providing the actual year of its creation.
  • A later reply asserts that the approach using H2S and HCl may be more efficient than using P2S5 or Lawesson’s reagent, as the latter can lead to furan contamination in the final product.

Areas of Agreement / Disagreement

Participants express differing views on the efficiency and correctness of the synthesis methods discussed. There is no clear consensus on the best approach or the historical details surrounding the reagents used.

Contextual Notes

Participants reference various sources and historical contexts, but there are unresolved questions regarding the efficiency of the proposed synthesis and the accuracy of historical claims about the reagents.

Lotic7
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I had this synthesis question on my last midterm and got it wrong. I was wondering if someone here could point me in the right direction. It is a thiophene synthesis that starts off with a 1,4 diketone, but instead of P2S5 or Lawesson's reagent, H2S and HCl are used. I tried to hydrate one of the carbonyls and have H2S do a back side attack. This was marked wrong and I only got 1 mark out of 12 for this wonderful synthesis. What is the proper way to do this synthesis. How efficient is this synthesis when hydrogen sulphide and hydrochloric acid are used instead of P2S5.

Thanks for the assistance
 
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Lesson 1. The WWW is not well suited to be a specialist's library. I suggest your chemistry library should have many tomes on thiophenes, for instance Vol 39 of Topics in Hetrocyclic Chemistry.
I'm not a organic chemist, so I'd be guessing, but 1,4 diketone + H2S and follow that with H(+)...ah where is my March when I need it? I'd guess it'd take about 30 seconds to answer this with any half-decent Advanced Organic Text...
 
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I was able to get some answers after doing a search on google for "Paal thiophene synthesis". There was a link to a book preview in google books https://books.google.ca/books?id=1N-MZVSesTcC&pg=PA206&lpg=PA206&dq=paal+thiophene+synthesis&source=bl&ots=ujRapYla8u&sig=hSttJT3hl_K4dExVDelpN7uOSXg&hl=en&sa=X&ved=0CGsQ6AEwDGoVChMIlKjv78TeyAIVRsVjCh2uUwjK#v=onepage&q=paal%20thiophene%20synthesis&f=false
The books title is "Name reactions in hetrocyclic chemistry" by Jie Jack Li. Phosphorous pentasulphide was originally used in this sysnthesis (1885), but furans were produced as a byproduct. Using Lawesson's reagent increases the thiophene yield, but was not developed until 1978.
 
Lawesson's reagent was first created in 1956 ( J. Am. Chem. Soc., 1956, 78 (19), pp 5018–5022), so I am not sure why the book mentions 1978. Maybe that was the first time it was used specifically for thiophene synthesis.
 
Lotic7 said:
[...] It is a thiophene synthesis that starts off with a 1,4 diketone, but instead of P2S5 or Lawesson's reagent, H2S and HCl are used. I tried to hydrate one of the carbonyls and have H2S do a back side attack. This was marked wrong and I only got 1 mark out of 12 for this wonderful synthesis. What is the proper way to do this synthesis. How efficient is this synthesis when hydrogen sulphide and hydrochloric acid are used instead of P2S5.
Your approach is more correct and better in thiophene synthesis compared to that being done with either phosphorus pentasulphide or Lawesson’s reagents because the latter also introduces the presence of furan in the final product.
http://www.organic-chemistry.org/namedreactions/paal-knorr-thiophene-synthesis.shtm
http://www.researchgate.net/publication/266057818_Synthesis_properties_and_biological_activity_of_thiophene_A_review , in the presence of an acid catalyst, has been found to be more efficient at converting ketones to thione.
 
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