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

  1. Oct 23, 2015 #1
    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
     
  2. jcsd
  3. Oct 25, 2015 #2

    ogg

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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...
     
  4. Oct 25, 2015 #3
    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.
     
  5. Oct 25, 2015 #4
    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.
     
  6. Oct 26, 2015 #5
    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 [Broken], in the presence of an acid catalyst, has been found to be more efficient at converting ketones to thione.
     
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