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Terpenes TBR OChem Example 4.15

Discussion in 'MCAT Study Question Q&A' started by AwayFromReality, Sep 4, 2014.

  1. AwayFromReality

    2+ Year Member

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    Hey guys

    I'm having difficulty understanding one of the questions about Terpenes

    Which of the following compounds requires the formation of three new carbon-carbon bonds when synthesized from isopentenyl pyrophosphate? (See attached photo)

    photo 1.JPG

    The answer is A. The solution states "In choice A, there are ten carbons and two rings. One new carbon-carbon bond is necessary to make a ten-carbon species and two more new carbon-carbon bonds are required to form the bicyclic system, so Choice A requires three carbon-carbon bonds when formed from 5-carbon isoprene derivatives."

    I don't really understand what they said in the solution. If we start with a 5-carbon isoprene, then 3 additional carbon-carbon bonds will result in an 11 carbon structure (2 carbons per carbon-carbon structure times 3= 6 carbons. 5 carbons isoprene plus 6 carbons = 11 carbons).

    Terpenes are pretty difficult subject for me so any help would be much appreciated.
     
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  3. justadream

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    @AwayFromReality

    It only takes 1 carbon-carbon bond to connect one 5-carbon-length thing to another 5-carbon-length-thing.

    Ok so now you have a 10 carbon thing with ONE extra C-C bond. So far, you have NO rings.

    If you look at choice A), you notice that there are TWO rings. This requires 2 units of unsaturation which comes from forming 2 more C-C bonds.
     
    PlsLetMeIn21 likes this.
  4. AwayFromReality

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    That makes sense, thanks for your explanation
     
  5. Sarahka74

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    But the question is around which forms 3 new carbon carbon bonds, not two....am I missing the third one here? Also is the point of the question just to identify 3 C-C bonds/rings? I think I am misunderstanding something here, I would really appreciate it if someone could clarify this question.
     
  6. PlsLetMeIn21

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    isoprene is 5 carbons long, so the first C to C bond makes a 10 carbon linear terpene. Then it forms two new C to C bonds in making the two rings of the bicyclic structure. This results in three new C to C bonds total.
     
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